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Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095 Prepared by: DN Reviewed by: RPG Approved By: AKS Rev: 01 Date: 09-11-2004 Pages: 1 of 83 INDIAN PETROCHEMICAL S CORPORATION LTD NAGOTHANE TRAINING MODULE FOR NON DESTRUCTIVE TESTINGS LEARNING CENTRE NC
83
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Page 1: NDT

Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095

Prepared by: DN Reviewed by: RPG Approved By: AKS

Rev: 01 Date: 09-11-2004 Pages: 1 of 83

INDIAN PETROCHEMICAL S CORPORATION LTD

NAGOTHANE

TRAINING MODULE

FOR

NON DESTRUCTIVE TESTINGS

LEARNING CENTRE

NC

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Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095

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OBJECTIVE: Non destructive testing (NDT) is one of the important topic in day today life. Though

NDT techniques are used in industries, certain techniques like X-ray, ultrasonic

testing is used in medical field. It is very interesting to know that X-rays were first

used in medical field, later in industry. In this module various NDTs / NDE are listed

out but NDTs, which are most commonly used are explained in little detail to familiar

with NDTs.

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MODULE IMPLEMENTATION PLAN TOPIC: NON-DESTRUCTIVE TESTING CODE NO: IPCLDSMEC173 FOR: NDT DATE : 09-11-2004 REV:0 SITE: IPCL-NC

SR NO

CONTENTS AUTHOR

RESOURCES AVAILABLE ( Y/N)

LEARNING VALIDATION

1 Introduction DN ASM NDT Handbook

Y

2 Techniques of NDT

DN ASM NDT Handbook / ASME hand book

Y

3 Liquid Penetrant Test

DN ASM NDT Handbook / ASME hand book

Y

4 Magnetic Particle Testing

DN ASM NDT Handbook / ASME hand book

Y

5 Radiography DN ASM NDT Handbook / ASME hand book

Y

6 Ultrasonic Testing

DN ASM NDT Handbook / ASME hand book

Y

8 Hrs. Quiz

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INDEX

CHAPTER NO.

DESCRPTION PAGE NO.

1 INTRODUCTION TO NDT TECHNIQUES 6

2 LIQUID PENETRANT TESTING

2.1 Introduction

2.2 Principle

2.3 Basic steps of Liquid Penetrant Testing

2.4 Quality control of Penetrant

2.5 Quality control of Developer

2.6 Selection of Penetrant Technique

2.7 Process control of Temperature

2.8 Common uses of Liquid Penetrant Testing

2.9 Nature of Defects

2.10 Advantages & Disadvantages of LPT

2.11 Health & Safety Precautions in LPT

12

3 RADIOGRAPHIC TESTING

3.1 History of Radiography

3.2 Natural Radioactivity

3.3 Inverse Square Law

3.4 Absorption

3.5 Radiographic Technique

3.6 Sharpness of Radiographic Images

3.7 Filters in Radiography

3.8 Controlling Radiographic Quality

3.9 Film Processing

3.10 Viewing Radiographs

3.11 Image considerations

3.12 Radiographic Interpretation

3.13 Discontinuities

21

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4 MAGNETIC PARTICLE TESTING

4.1 Introduction

4.2 Principle

4.3 Magnetising Current

4.4 Lighting

4.5 Particle Concentration & Condition

4.6 Magnetic Field Indicators

4.7 Quantitative Quality Indicators

4.8 Pie Gage

4.9 Slotted Strips

48

5 ULTRASOINIC TESTING

5.1 Introduction

5.2 Wave Propagation

5.3 Wavelength Frequency & Velocity

5.4 Sound Propagation in Elastic Material

5.5 Material Affect on Speed & Sound

5.6 Acoustic Impedance

5.7 Ultrasonic Wave Generation

5.8 Refraction & Snell’s Law

5.9 Calibration Methods

5.10 Introduction to Common Standards

5.11 The IIW Type Calibration Blocks

5.12 Couplant

5.13 Normal Beam Inspections

5.14 Angle Beams Inspection

5.15 Weldments (Weld Joints)

5.16 Distance Amplitude Correction (DAC)

5.17 Wavelength & Defect Detection

61

7 Bibliography 83

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CHAPTER – 1 INTRODUCTION

NONDESTRUCTIVE TESTING

The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that

plays a critical role in assuring that structural components and systems perform their

function in a reliable and cost effective fashion. NDT techniques that locate and

characterize material conditions and flaws that might otherwise result in failure of

pressure vessels, pipelines or machinery components. These tests are performed

in a manner that does not affect the future usefulness of the object or material. In

other words, NDT allows parts and materials to be inspected and measured without

damaging them. Because it allows inspection without interfering with a product's

final use, NDT provides an excellent balance between quality control and cost-

effectiveness. Generally speaking, NDT applies to industrial inspections. While

technologies are used in NDT that are similar to those used in the medical industry,

typically nonliving objects are the subjects of the inspections.

NONDESTRUCTIVE EVALUATION

Nondestructive Evaluation (NDE) is a term that is often used interchangeably with

NDT. However, technically, NDE is used to describe measurements that are more

quantitative in nature. NDE method would not only locate a defect, but it would also

be used to measure something about that defect such as its size, shape, and

orientation. NDE may be used to determine material properties such as fracture

toughness, formability, and other physical characteristics.

NDT / NDE METHODS

The number of NDT methods that can be used to inspect components and make

measurements is large and continues to grow. There are six NDT methods that are

used most often. These methods are visual inspection, penetrant testing, magnetic

particle testing, electromagnetic or eddy current testing, radiography, and ultrasonic

testing. These methods and a few others are briefly described below.

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1. VISUAL OR OPTICAL TESTING (VT)

Visual inspection involves using an inspector's eyes to look for defects. The

inspector may also use special tools such as magnifying glasses, mirrors,

boroscopes or fibroscopes to gain access and more closely inspect the subject area.

Visual examination involves procedures that range from simple to very complex.

2. LIQUID PENETRANT TESTING (LPT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is then

removed from the surface, and a developer is applied. The developer acts as

blotter, drawing trapped penetrant out of imperfections open to the surface. With

visible dyes, vivid color contrasts between the penetrant and developer make "bleed-

out" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleed-

out fluoresce brightly, thus allowing imperfections to be readily seen.

3. MAGNETIC PARTICLE TESTING (MPT)

This NDT method is accomplish by inducing a magnetic field in a ferromagnetic

material and then dusting the surface with iron particles (either dry or suspended in

liquid). Surface and near-surface imperfections distort the magnetic field and

concentrate iron particles near imperfections, previewing a visual indication of the

flaw.

4. ELECTROMAGNETIC (ET) OR EDDY CURRENT TESTING

Electrical currents are generated in a conductive material by an induced alternating

magnetic field. The electrical currents are called eddy currents because they flow in

circles at and just below the surface of the material. Interruptions in the flow of eddy

currents, caused by imperfections, dimensional changes, or changes in the

material's conductive and permeability properties, can be detected with the proper

equipment.

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5. RADIOGRAPHIC TESTING (RT)

Radiography involves the use of penetrating gamma or X-radiation to examine parts

and products for imperfections. An X-ray generator or radioactive isotope is used as

a source of radiation. Radiation is directed through a part and onto film or other

imaging media. The resulting shadowgraph shows the dimensional features of the

part. Possible imperfections are indicated as density changes on the film in the same

manner as a medical X-ray shows broken bones.

6. ULTRASONIC TESTING (UT)

Ultrasonic testing uses transmission of high-frequency sound waves into a material

to detect imperfections or to locate changes in material properties. The most

commonly used ultrasonic testing technique is pulse echo, wherein sound is

introduced into a test object and reflections (echoes) are returned to a receiver from

internal imperfections or from the part's geometrical surfaces.

7. ACOUSTIC EMISSION TESTING (AET)

When a solid material is stressed, imperfections within the material emit short bursts

of acoustic energy called "emissions." As in ultrasonic testing, acoustic emissions

can be detected by special receivers. Emission sources can be evaluated through

the study of their intensity, rate, and location.

8. LEAK TESTING (LT)

Several techniques are used to detect and locate leaks in pressure containment

parts, pressure vessels, and structures. Leaks can be detected by using electronic

listening devices, pressure gauge measurements, liquid and gas penetrant

techniques, and / or a simple soap-bubble test.

In this module most commonly and widely used NDTs explained in detail as under:

1. Liquid Penetrant Testing

2. Radiographic Testing

3. Magnetic Particle Testing

4. Ultrasonic Testing

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CHAPTER – 2

LIQUID PENETRANT TESTING (LPT)

2.1 INTRODUCTION:

Liquid Penetrant testing (LPT) is one of the Non Destructive Testing (NDT)

methods of inspection to locate discontinuities those are open to the surface.

LPT can be used on any material except those are extremely porous & irregular

surface. Discontinuities such as cracks, porosities etc. those are open to the

surface are detected by `blotting action' after the surface has been treated with

penetrant. This method is used as an effective NDT in welding fabrication /

maintenance / condition monitoring / quality control.

2.2 PRINCIPLE:

In LPT, a liquid penetrant (contrast colour dye or fluorescent) is applied over the

thoroughly cleaned and dry surface, which is having flows(discontinuities) those

are open surface due to capillary action. Sufficient time is allowed so that the

penetrant can enter in narrow discontinuities. Excess penetrant is removed by

cleaning and developer (a fluffy chalk like powder) is applied over the surface.

Due to blotting nature of the developer, entrapped penetrant in the discontinuities

flows out and gives an indication, which can be viewed either in normal light for

contrast dye or in “black light” (UV light) for fluorescent dye. The indication is

always greater than the discontinuity due to diffusion of the penetrant in the

developer.

2.3 BASIC STEPS OF A LIQUID PENETRANT INSPECTION

2.3.1 SURFACE PREPARATION

One of the most critical steps of a liquid penetrant inspection is the surface

preparation. The surface must be free of oil, grease, water, or other contaminants

that may prevent penetrant from entering flaws. The sample may also require

etching if mechanical operations such as machining, sanding, or grit blasting

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have been performed. These and other mechanical operations can smear the

surface of the sample, thus closing the defects.

2.3.2 PENETRANT APPLICATION

Once the surface has been thoroughly cleaned and dried, penetrant material is

applied either by spraying, brushing or immersing the parts in a penetrant bath.

2.3.3 PENETRANT DWELL

The penetrant is left on the surface for a sufficient time, to allow as much

penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell

time is the total time that the penetrant is in contact with the part surface. Dwell

times are usually recommended by the penetrant producers or required by the

specification being followed. The times vary depending on the application,

penetrant materials used, the material, the form of the material being inspected,

and the type of defect being inspected. Minimum dwell times typically range from

5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell

time as long as the penetrant is not allowed to dry. The ideal dwell time is often

determined by experimentation and is often very specific to a particular

application.

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DWELL TIME FOR SOME OF THE MATERIALS (As per ASTM E 165, Table 2)

Minimum Dwell time (minutes)

Material Form Type of discontinuity

Penetrant Developer

Aluminium, Magnesium, Steel, Steel, Brass and Bronze, Titanium and High temp. alloys

Cast- castings and welds

Wrought- Extrusions, forgings, Plate

Cold shuts, Porosity, Lack of fusion, Laps, Cracks all forms)

5

10

7

7

Carbide tipped tools

Lack of fusion, Porosity, Cracks

5 7

Plastics All forms Cracks 5 7

Glass All forms Cracks 5 7

Ceramics All forms Cracks 5 7

2.3.4 EXCESS PENETRANT REMOVAL

This is a most delicate part of the inspection procedure because the excess

penetrant must be removed from the surface of the sample while removing as

little penetrant as possible from defects. Depending on the penetrant system

used, this step may involve cleaning with a solvent, direct rinsing with water, or

first treated with an emulsifier and then rinsing with water.

2.3.5 DEVELOPER APPLICATION

A thin layer of developer is then applied to the sample to draw penetrant trapped

in flaws back to the surface where it will be visible. Developers come in a variety

of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet

developers).

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2.3.6 INDICATION DEVELOPMENT

The developer is allowed to stand on the part surface for a period of time

sufficient to permit the extraction of the trapped penetrant out of any surface

flaws. This development time is usually a minimum of 10 minutes and

significantly longer times may be necessary for tight cracks.

2.3.7 INSPECTION

Inspection is then performed under appropriate lighting to detect indications from

any flaws which may be present.

2.3.8 CLEAN SURFACE

The final step in the process is to thoroughly clean the part surface to remove the

developer from the parts that were found to be acceptable.

2.4 QUALITY CONTROL OF PENETRANT

The quality of a penetrant inspection is highly dependent on the quality of the

penetrant materials used. The performance of a penetrant can be affected by

contamination and aging. Contamination by another liquid will change the surface

tension and contact angle of the solution, and virtually all organic dyes deteriorate

over time resulting in a loss of color or fluorescent response. Therefore, regular

checks must be performed to insure that the material performance has not degraded.

When the penetrant is first received from the manufacturer, a sample of the fresh

solution should be collected and stored as a standard for future comparison. The

standard specimen should be stored in an opaque glass or metal, sealed container.

Penetrants that are in-use should be compared regularly against the standard

specimen to detect changes in color, odor and consistency. When using fluorescent

penetrants, a brightness comparison per the requirements of ASTM E 1417 is also

often required. This check involves placing a drop of the standard and the in-use

penetrants on a piece of Whatman #4 filter paper and making a side by side

comparison of the brightness of the two spots under UV light.

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Additionally, the water content of water washable penetrants must be checked

regularly. When water contaminates oil-based penetrants, the surface tension and

contact angle of the mixture will increase since water has a higher surface tension

than most oil-based penetrants in self-emulsifiable penetrants, water contamination

can produce a gel break or emulsion inversion when the water concentration

becomes high enough. The formation of the gel is an important feature during the

washing processes but must be avoided until the stage in the process. Data

indicates that the water contamination must be significant (greater than 10%) for gel

formation to occur. Most specification limit water contamination to around 5% to be

conservative. Non-water-based, water washable penetrants are checked using the

procedure specified in ASTM D95 or ASTM E 1417. Water-based, water washable

penetrants are checked with a refractometer. The rejection criteria are different for

different penetrants so the requirements of the qualifying specification or the

manufacturer's instructions must be consulted.

2.5 QUALITY CONTROL OF DEVELOPER

The function of the developer is very important in a penetrant inspection. It must

draw out of the discontinuity a sufficient amount of penetrant to form an indication,

and it must spread the penetrant out on the surface to produce a visible indication. In

a fluorescent penetrant inspection, the amount of penetrant brought to the surface

must exceed the dye's thin film threshold of fluorescence of the indication will not

fluoresce. Additionally, the developer makes fluorescent indications appear brighter

than indications produced with the same amount of dye but without the developer.

In order to accomplish these functions, a developer must adhere to the part surface

and result in a uniform, highly porous layer with many paths for the penetrant to be

moved due to capillary action. Some developers are applied wet and other dry, but

the desired end result is always a uniform, highly porous, surface layer. Since the

quality control requirements for each of the developer types is slightly different, they

will be covered individually.

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2.5.1 DRY POWDER DEVELOPER

A dry powder developer should be checked daily to ensure that it is fluffy and not

caked. It should be similar to fresh powdered sugar and not granulated like powered

soup. It should also be relatively free from specks of fluorescent penetrant material

from previous inspection. This is checking is performed by spreading out a sample

of the developer and examining it under UV light. If there are ten or more fluorescent

specks in a 10 cm diameter area, the batch should be discarded.

Apply a light coat of the developer by immersing the test component or dusting the

surface. After the development time, excessive powder can be removed by gently

blowing on the surface with air not exceeding 35 kPa or 5 psi.

2.5.2 WET SOLUBLE / SUSPENDIBLE DEVELOPER

Wet soluble developer must be completely dissolved in the water and wet

suspendible developer must be thoroughly mixed prior to application. The

concentration of powder in the carrier solution must be controlled in these

developers. The concentration should be checked at least weekly using a

hydrometer to make sure it meets the manufacturer's specification. To check for

contamination, the solution should be examined weekly using both white light and

UV light. If a scum is present or the solution fluoresces, it should be replaced. Some

specification require that a clean aluminum panel be dipped in the developer, dried,

and examined for indications of contamination by fluorescent penetrant materials.

These developers are applied immediately after the final wash. A uniform coating

should be applied by spraying, flowing or immersion of the component. They should

never be applied with a brush. Care should be taken to avoid a heavy accumulation

of the developer solution in crevices and recesses. Prolonged contact of the

component with the developer solution should be avoided in order to minimize

dilution or removal of the penetrant from discontinuities.

2.5.3 SOLVENT SUSPENDIBLE

Solvent suspendible developers are typically supplied in an sealed aerosol spray

can. Since the developer solution is in a sealed vessel, direct check of the solution

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are not possible. However, the way that the developer is dispensed must be

monitored. The spray developer should produce a fine, even coating on the surface

of the part. Make sure the can is well shaken and apply a thin coating to a test

article. If the spray produces spatters or other an uneven coating the can should be

discarded.

When applying a solvent suspendible developer, it is up to the inspector to control

the thickness of the coating. When a visible penetrant system, the developer coating

must be thick enough to provide a white contrasting background but not heavy

enough to mask indications. When using a fluorescent penetrant system, a very light

coating should be used. The developer should be applied under white light condition

and should appear evenly transparent.

2.5.4 DEVELOPMENT TIME

Part should be allowed to develop for a minimum of 10 minutes and no more than 2

hours before inspecting.

2.6 SELECTION OF A PENETRANT TECHNIQUE

When sensitivity is the primary consideration for choosing a penetrant system, the

first decision that must be made is whether to use fluorescent dye penetrant, or

visible dye penetrant. Fluorescent penetrants are generally more capable of

producing a detectable indication from a small defect because the human eye is

more sensitive to a light indication on a dark background and the eye is naturally

drawn to a fluorescent indication. When a dark indication on a light background is

further reduced in size, it is no longer detectable even though contrast is increased.

Furthermore, with a light indication on a dark background, indications down to 0.003

mm (0.0001 inch) were detectable when the contrast between the flaw and the

background was high enough.

Since visible dye penetrants do not require a darkened area for the use of an

ultraviolet light, visible systems are more easy to use in the field. Solvent removable

penetrants, when properly applied can have the highest sensitivity and are very

convenient to use but are usually not practical for large area inspection or in high-

volume production settings.

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Another consideration in the selection of a penetrant system is whether water

washable, post-emulsifiable or solvent removable penetrants will be used. Post-

emulsifiable systems are designed to reduce the possibility of over-washing, which is

one of the factors known to reduce sensitivity. However, these systems add another

step, and thus cost, to the inspection process.

2.7 PROCESS CONTROL OF TEMPERATURE

The temperature of the penetrant materials and the part being inspected can have

an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in

the literature to produce optimal results. Many specifications allow testing in the

range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be

touched for an extended period of time without burning the skin are generally below

52oC (125oF).

Since the surface tension of most materials decrease as the temperature increases,

raising the temperature of the penetrant will increase the wetting of the surface and

the capillary forces. Of course, the converse is also true and lowing the temperature

will have a negative effect on the flow characteristics. Raising the temperature will

also raise the speed of evaporation of penetrants, which can have a positive or

negative effect on sensitivity. The impact will be positive if the evaporation serves to

increase the dye concentration of the penetrant trapped in a flaw up to the

concentration quenching point and not beyond. Higher temperatures and more rapid

evaporation will have a negative effect if the dye concentration is caused to exceed

the concentration quenching point or the flow characteristics are changed to the

point where the penetrant does not readily flow.

The method of processing a hot part was once commonly employed. Parts were

either heated or processed hot off the production line. In its day, this served to

increase inspection sensitivity by increasing the viscosity of the penetrant. However,

the penetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on

the market in the 1960's and 1970's. Heating the part prior to inspection is no longer

necessary and no longer recommended.

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2.8 COMMON USES OF LIQUID PENETRANT TESTING

Liquid penetrant Testing (LPT) is one of the most widely used nondestructive

evaluation (NDE) method. Its popularity can be attributed to two main factors, which

are its relative ease of use and its flexibility. LPT can be used to inspect almost any

material provided that its surface is not extremely rough or porous. Materials that are

commonly inspected using LPT include the following:

• Metals (aluminum, copper, steel, titanium, etc.)

• Glass

• Many ceramic materials

• Rubber

• Plastics

LPT offers flexibility in performing inspections because it can be applied in a large

variety of applications ranging from automotive spark plugs to critical aircraft

components. Penetrant material can be applied with a spray can or a cotton swab to

inspect for flaws known to occur in a specific area or it can be applied by dipping or

spraying to quickly inspect large areas.

Liquid penetrant inspection is used to inspect of flaws that break the surface of the

sample. Some of these flaws are listed below:

• Fatigue cracks

• Quench cracks

• Grinding cracks

• Overload and impact fractures

• Porosity

• Laps

• Seams

• Pin holes in welds

• Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that

flaws must be open to the surface.

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2.9 NATURE OF THE DEFECT

The nature of the defect can have a large affect on sensitivity of a liquid penetrant

inspection. Sensitivity is defined as the smallest defect that can be detected with a

high degree of reliability. Typically, the crack length at the sample surface is used to

define size of the defect. A survey of any probability-of-detection curve for penetrant

inspection will quickly lead one to the conclusion that crack length has a definite

affect on sensitivity. However, the crack length alone does not determine whether a

flaw will be seen or go undetected. The volume of the defect is likely to be the more

important feature. The flaw must be of sufficient volume so that enough penetrant

will bleed back out to a size that is detectable by the eye or that will satisfy the

dimensional thresholds of fluorescence.

2.10 ADVANTAGES AND DISADVANTAGES OF LPT

Like all nondestructive inspection methods, liquid penetrant inspection has both

advantages and disadvantages. The primary advantages and disadvantages when

compared to other NDE methods are summarized below.

PRIMARY ADVANTAGES

• The method has high sensitive to small surface discontinuities.

• The method has few material limitations, i.e. metallic and nonmetallic,

magnetic and nonmagnetic, and conductive and nonconductive materials may

be inspected.

• Large areas and large volumes of parts/materials can be inspected rapidly

and at low cost.

• Parts with complex geometric shapes are routinely inspected.

• Indications are produced directly on the surface of the part and constitute a

visual representation of the flaw.

• Aerosol spray cans make penetrant materials very portable.

• Penetrant materials and associated equipment are relatively inexpensive.

PRIMARY DISADVANTAGES

• Only surface breaking defects can be detected.

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• Only materials with a relative nonporous surface can be inspected.

• Precleaning is critical as contaminants can mask defects.

• Metal smearing from machining, grinding, and grit or vapor blasting must be

removed prior to LPT.

• The inspector must have direct access to the surface being inspected.

• Surface finish and roughness can affect inspection sensitivity.

• Multiple process operations must be performed and controlled.

• Post cleaning of acceptable parts or materials is required.

• Chemical handling and proper disposal is required.

2.11 HEALTH AND SAFETY PRECAUTIONS IN LPT

When proper health and safety precautions are followed, liquid penetrant inspection

operations can be completed without harm to inspection personnel. However, there

is a number of health and safety related issues that must be addressed. Since each

inspection operation will have its own unique set of health and safety concerns that

must be addressed, only a few of the most common concerns will be discussed here.

CHEMICAL SAFETY

Whenever chemicals must be handled, certain precautions must be taken as

directed by the material safety data sheets (MSDS) for the chemicals. Before

working with a chemical of any kind, it is highly recommended that the MSDS be

reviewed so that proper chemical safety and hygiene practices can be followed.

Some of the penetrant materials are flammable and, therefore, should be used and

stored in small quantities. They should only be used in a well-ventilated area and

ignition sources avoided. Eye protection should always be worn to prevent contact

of the chemicals with the eyes. Many of the chemicals used contain detergents and

solvents that can dermatitis. Gloves and other protective clothing should be warn to

limit contact with the chemicals.

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ULTRAVIOLET LIGHT SAFETY

Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths

ranging from 180 to 400 nanometers. These wavelengths place UV light in the

invisible part of the electromagnetic spectrum between visible light and X-rays. The

most familiar source of UV radiation is the sun and is necessary in small doses for

certain chemical processes to occur in the body. However, too much exposure can

be harmful to the skin and eyes. Excessive UV light exposure can cause painful

sunburn, accelerate wrinkling and increase the risk of skin cancer. UV light can

cause eye inflammation, cataracts, and retinal damage.

Because of their close proximity, laboratory devices, like UV lamps, deliver UV light

at a much higher intensity than the sun and, therefore, can cause injury much more

quickly. The greatest threat with UV light exposure is that the individual is generally

unaware that the damage is occurring. There is usually no pain associated with the

injury until several hours after the exposure. Skin and eye damage occurs at

wavelengths around 320 nm and shorter which is well below the 365 nm wavelength,

where penetrants are designed to fluoresce. Therefore, UV lamps sold for use in

LPT application are almost always filtered to remove the harmful UV wavelengths.

The lamps produce radiation at the harmful wavelengths so it is essential that they

be used with the proper filter in place and in good condition.

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CHAPTER – 3

RADIOGRAPHIC TESTING

3.1 HISTORY OF RADIOGRAPHY

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was

a Professor at Wuerzburg University in Germany. Working with a cathode-ray tube

in his laboratory, Roentgen observed a fluorescent glow of crystals on a table near

his tube. He concluded that a new type of ray was being emitted from the tube. This

ray was capable of passing through the heavy paper covering and exciting the

phosphorescent materials in the room. He found the new ray could pass through

most substances casting shadows of solid objects. Roentgen also discovered that

the ray could pass through the tissue of humans, but not bones and metal objects.

Prior to 1912, X-rays were used little outside the realms of medicine, and dentistry,

though some X-ray pictures of metals were produced. The reason that X-rays were

not used in industrial application before this date was because the X-ray tubes (the

source of the X-rays) broke down under the voltages required to produce rays of

satisfactory penetrating power for industrial purpose.

In 1922, industrial radiography took another step forward with the advent of the

200,000-volt X-ray tube that allowed radiographs of thick steel parts to be produced

in a reasonable amount of time. In 1931, General Electric Company developed

1,000,000 volt X-ray generators, providing an effective tool for industrial radiography.

That same year, the American Society of Mechanical Engineers (ASME) permitted

X-ray approval of fusion welded pressure vessels that further opened the door to

industrial acceptance and use.

3.2 NATURAL RADIO ACTIVITY

Shortly after the discovery of X-rays, another form of penetrating rays was

discovered. In 1896, French scientist Henri Becquerel discovered natural

radioactivity.

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It was Henri Becquerel who discovered this phenomenon while investigating the

properties of fluorescent minerals. Becquerel was researching the principles of

fluorescence, certain minerals glow (fluoresce) when exposed to sunlight. He utilized

photographic plates to record this fluorescence.

While working in France at the time of Becquerel's discovery, Polish scientist Marie

Curie became very interested in his work. She suspected that a uranium ore known

as pitchblende contained other radioactive elements. Marie and her husband, a

French scientist, Pierre Curie started looking for these other elements. In 1898, the

Curies discovered another radioactive element in pitchblende, they named it

'polonium' in honor of Marie Curie's native homeland. Later that year, the Curie's

discovered another radioactive element, which they named 'radium', or shining

element. Both polonium and radium were more radioactive than uranium. Since

these discoveries, many other radioactive elements have been discovered or

produced.

Radium became the initial industrial gamma ray source. The material allowed

radiographing castings up to 10 to 12 inches thick. During World War II, industrial

radiography grew tremendously as part of the Navy's shipbuilding program. In 1946,

manmade gamma ray sources such as cobalt and iridium became available. These

new sources were far stronger than radium and were much less expensive. The

manmade sources rapidly replaced radium, and use of gamma rays grew quickly in

industrial radiography.

X-rays and Gamma rays are electromagnetic radiation of exactly the same nature as

light, but of much shorter wavelength. Wavelength of visible light is of the order of

6000 angstroms while the wavelength of x-rays is in the range of one angstrom and

that of gamma rays is 0.0001 angstrom. This very short wavelength is what gives x-

rays and gamma rays their power to penetrate materials that light cannot. These

electromagnetic waves are of a high energy level and can break chemical bonds in

materials they penetrate.

Strength of source is measured in Curie (Ci). 1 Curie is equivalent to 3.7x1010

disintegrations (nuclear decays) per second. Intensity of radiation is expressed in

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roentgens meter hour (RHM). It is the amount of received by the material at distance

of 1 meter from 1curie source.

Half-life is time required to reduce the source strength to half of its original value.

3.3 INVERSE SQUARE LAW

Any point source which spreads its influence equally in all directions without a limit to

its range will obey the inverse square law. This comes from strictly geometrical

considerations. The intensity of the influence at any given radius (d) is the source

strength divided by the area of the sphere.

Where, I1 & I2 are intensities of sources at distance d1 & d2 .

All measures of exposure will drop off by the inverse square law.

Sources Used in Industrial Radiography and its properties are given below:

Source Half Life Energy(MeV) RHM Useful thickness range(mm)

Ir-192 74 Days 0.4 0.5 12 - 65 Co-60 5.26 Years 1.17, 1.33 1.3 50 - 200 Cs-137 30 Years 0.66 0.32 20 - 90 Tu-170 127 Days 0.08 0.009 2.5 – 12.5

3.4 ABSORPTION

Absorption characteristics of materials are important in the development of contrast

in a radiograph. Absorption characteristics will increase or decrease as the energy

of the x-ray is increased or decreased. A radiograph with higher contrast will provide

greater probability of detection of a given discontinuity. An understanding of the

relationship between material thickness, absorption properties, and photon energy is

fundamental to producing a quality radiograph. An understanding of absorption is

also necessary when designing x- and gamma ray shielding, cabinets, or exposure

vaults.

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Attenuation of x-rays in solids takes place by several different mechanisms, some

due to absorption, others due to the scattering of the beam. Thompson scattering

and Compton Scattering were introduced in the material titled "Interaction Between

Penetrating Radiation and Matter" and "Compton Scattering." This needs careful

attention because a good radiograph can only be achieved if there is minimum x-ray

scattering.

1. Thomson scattering (R) (also known as Rayleigh, coherent, or classical

scattering) occurs when the x-ray photon interacts with the whole atom so that

the photon is scattered with no change in internal energy to the scattering

atom, nor to the x-ray photon.

2. Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is

absorbed resulting in the ejection of electrons from the outer shell of the atom,

resulting in the ionization of the atom. Subsequently, the ionized atom returns

to the neutral state with the emission of an x-ray characteristic of the atom.

3. Compton Scattering (C) (also known a incoherent scattering) occurs when

the incident x-ray photon ejects an electron from an atom and a x-ray photon

of lower energy is scattered from the atom.

4. Pair Production (PP) can occur when the x-ray photon energy is greater than

1.02 MeV, when an electron and positron are created with the annihilation of

the x-ray photon (absorption).

5. Photodisintegration (PD) is the process by which the x-ray photon is captured

by the nucleus of the atom with the ejection of a particle from the nucleus

when all the energy of the x-ray is given to the nucleus (absorption).

3.5 RADIOGRAPHIC TECHNIQUE

Radiographs shall be made with a single source of radiation centered as near as

practical with respect to the length and width of the portion of the weld being

examined. The source to subject distance shall not be less than the total length of

film being exposed in a single plane. This provision does not apply to panoramic

exposures.

The source to subject distance shall not be less than seven times the thickness of

weld plus reinforcement and backing ,if any , then the radiation shall penetrate any of

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the weld represented in the radiograph at an angle greater than 26.5 deg from a line

normal to the weld surface. Welded joints shall be radiographed and the film indexed

by methods that will provide complete and continous inspection of the joint within the

limits specified to be examined. Joints limits shall show clearly in the radiographs.

Short film, short screen, excessive undercut by scattered radiation, or any other

process that obscures portions of the total weld length shall render the radiograph

unacceptable. Film shall have sufficient length and shall be placed to produce at

least 0.5" film, exposed to direct radiation from the source, beyond each free edge

where the weld is terminated.

3.5.1 SINGLE WALL TECHNIQUE

In the single wall technique, the radiation passes through only one wall of the weld

which is viewed for acceptance on the radiograph .A single-wall technique shall be

used for radiography whenever practical. When it is not practical to use a single wall

technique, a double wall technique shall be used. An adequate number of exposures

shall be made to demonstrate that the required coverage had been obtained.

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.

3.5.2 DOUBLE WALL TECHNIQUE

For materials and welds in pipe and tube 3.5" or less in nominal outside diameter, a

technique may be used in which the radiation passes through two radiation walls and

the weld in both walls is viewed for acceptance on the same film. For welds, the

radiation beams may be offset from the plan of the weld at an angle sufficient to

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separate the images of the source side and film side portion of the weld so that there

is no overlap of the areas to be interpreted, in which case a minimum of two

exposures taken at 90deg to each other shall be made for each joint. As an

alternate, the weld may be radiographed with the radiation beam positioned so that

the image of both walls are superimposed, in which case at least three exposure

shall be made at60deg to each other.

Double wall technique, single wall viewing –for material and welds in pipe and tubes

with a nominal outside diameter greater than 3.5" radiographic examination shall be

performed for single wall viewing only. An adequate number of exposures shall be

taken to ensure complete coverage.

For welds in pipe and tubes with a nominal outside diameter 0.5 or less, single wall

viewing may be used provided the source is offset from the welds. As a minimum,

three exposures 120 degrees apart shall be required.

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3.6 SHARPNESS OF RADIOGRAPHIC IMAGE:

Geometric unsharpness limitation - geometric unsharpness of radiograph shall not

exceed the following.

Geometric unsharphness of the radiograph shall be determined in accordance with:

Ug = Fd /D

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Where

• Ug = geometrical unsharpness

• F = source size in mm

• D = distance in mm from the source of the radiation to the weld or object being

radiographed

• d = distance in inches from the source side of the weld or object being radiographed

to the film.

3.7 FILTERS IN RADIOGRAPHY

At radiation energies, filters consist of material placed in the useful beam to absorb,

preferentially, radiations based on energy level or to modify the spatial distribution of

the beam. The use of filters produces a cleaner image by absorbing the lower

energy x-ray photons that tend to scatter more.

For industrial radiography, the filters added to the x-ray beam are most often

constructed of high atomic number materials such as lead, copper, or brass. The

thickness of filter materials is dependent on atomic numbers, and the desired

filtration factor.

Gamma radiography produces relatively high energy levels at essentially

monochromatic radiation, therefore filtration is not a useful technique and is seldom

used.

3.8 CONTROLLING RADIOGRAPHIC QUALITY

One of the methods of controlling the quality of a radiograph is through the use of

image quality indicators (IQI). IQIs provide a means of visually informing the film

interpreter of the contrast sensitivity and definition of the radiograph. The IQI

indicates that a specified amount of material thickness change will be detectable in

the radiograph, and that the radiograph has a certain level of definition so that the

density changes are not lost due to unsharpness. Without such a reference point,

consistency and quality could not be maintained and defects could go undetected.

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Image quality indicators take many shapes and forms due to the various codes or

standards that invoke their use. In the United States two IQI style are prevalent; the

placard, or hole-type and the wire IQI. IQIs come in a variety of material types so

that one with radiation absorption characteristics similar to the material being

radiographed can be used.

3.8.1 HOLE-TYPE IQIS ASTM Standard E1025 gives detailed requirements for the design and material

group classification of hole-type image quality indicators. E1025 designates eight

groups of shims based on their radiation absorption characteristics. A notching

system is incorporated into the requirements allowing the radiographer to easily

determine if the penetrameter is the correct material type for the product. The

thickness in thousands of an inch is noted on each pentameter by a lead number

0.250 to 0.375 inch wide depending on the thickness of the shim. Military or

Government standards require a similar penetrameter but use lead letters to indicate

the material type rather than notching system as shown on the left in the image

above.

Image quality levels are typically designated using a two part expression such as 2-

2T. The first term refers to the IQI thickness expressed as a percentage of the region

of interest of the part being inspected. The second term in the expression refers to

the diameter of the hole that must be revealed and it is expressed as a multiple of

the IQI thickness. Therefore, a 2-2T call-out would mean that the shim thickness

should be two percent of material thickness and that a hole that is twice the IQI

thickness must be detectable on the radiograph. This presentation of a 2-2T IQI in

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the radiograph verifies that the radiographic technique is capable of showing a

material loss of 2% in the area of interest.

It should be noted that even if 2-2T sensitivity is indicated on a radiograph, a defect

of the same diameter and material loss might not be visible. The holes in the

penetrameter represent sharp boundaries, and a small thickness change.

Discontinues within the part may contain gradual changes, and are often less visible.

The penetrameter is used to indicate quality of the radiographic technique and not to

be used as a measure of size of cavity that can be located on the radiograph.

3.8.2 WIRE PENETRAMETERS ASTM Standard E747 covers the radiographic examination of materials using wire

penetrameters (IQIs) to control image quality. Wire IQIs consist of a set of six wires

arranged in order of increasing diameter and encapsulated between two sheets of

clear plastic. E747 specifies four wire IQIs sets, which control the wire diameters.

The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number

in the lower left corner indicates the material group. The same image quality levels

and expressions (i.e. 2-2T) used for hole-type IQIs are typically also used for wire

IQIs.

The wire sizes that correspond to various hole-type quality levels can be found in a

table in E747 or can be calculated using the following formula.

3.8.3 PLACEMENT OF IQIS IQIs should be placed on the source side of the part over a section with a material

thickness equivalent to the region of interest. If this is not possible, the IQI may be

placed on a block of similar material and thickness to the region of interest. When a

block is used, the IQI should the same distance from the film as it would be if placed

directly on the part in the region of interest. The IQI should also be placed slightly

away from the edge of the part so that atleast three of its edges are visible in the

radiograph.

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3.9 FILM PROCESSING

Processing film is a science governed by rigid rules of chemical concentration,

temperature, time, and physical movement. Whether processing is done by hand or

automatically by machine, excellent radiographs require the highest possible degree

of consistency and quality control.

3.9.1 MANUAL PROCESSING & DARKROOMS

Manual processing begins with the darkroom. The darkroom should be located in a

central location, adjacent to the reading room and a reasonable distance from the

exposure area. For portability darkrooms are often mounted on pickups or trailers.

Film should be located in a light, tight compartment, which is most often a metal bin

that is used to store and protect the film. An area next to the film bin that is dry and

free of dust and dirt should be used to load and unload the film. While another area,

the wet side, will be used to process the film. Thus protecting the film from any water

or chemicals that may be located on the surface of the wet side.

Each of step in film processing must be excited properly to develop the image, wash

out residual processing chemicals, and to provide adequate shelf life of the

radiograph. The objective of processing is two fold. First to produce a radiograph

adequate for viewing, and secondly to prepare the radiograph for archival storage. A

radiograph may be retrieved after 5 or even 20 years in storage.

One must bear in mind that radiographic contrast and definition are not dependent

upon the same set of factors. If detail in a radiograph is originally lacking, then

attempts to manipulate radiographic contrast will have no effect on the amount of

detail present in that radiograph

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To understand how the image on a radiograph is formed, the characteristics of the

film are very important. There are three important parts to a radiographic film. These

include the base, the emulsion, and the protective coating.

3.9.2 THE BASE All radiographic film consists of a base for which the other materials are applied. The

film base is usually made from a clear, flexible plastic such as cellulose acetate. This

plastic is similar to what you might find in a wallet for holding pictures. The principle

function of the base is to provide support for the emulsion. It is not sensitive to

radiation, nor can it record an image.

The clarity or transparency of the film base is an important feature. Radiographic film

must be capable of transmitting light. Once a film has been processed chemically, it

is subject to interpretation. This is commonly done by using a film illuminating device,

which is usually a high intensity light source.

3.9.3 THE EMULSION

The film emulsion and protective coating comprise the other two components and

are essentially made from the same material. They are applied to the film during

manufacturing and usually take on a pale yellow color with a glassy appearance.

Although they are made from the same material, they offer two distinct features to

the film. These features are separated into the image layer of the emulsion, and the

protective layer.

3.9.4 THE PROTECTIVE LAYER

The protective layer has the important function of protecting the softer emulsion

layers below. It is simply a very thin skin of gelatin protecting the film from scratches

during handling. It offers very important properties to film manufacturers, which

include shrinkage (during drying that forms glassy protective layers) and dissolving in

warm water. It will absorb the water and swell if it is dissolved in cold water.

The softer layers of the gelatin coating are technically known as the emulsion. An

emulsion holds something in suspension. It is this material in suspension that is

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sensitive to radiation and forms the latent image on the film. During manufacturing

of the film, silver bromide is added to the solution of dissolved gelatin. When the

gelatin hardens the silver bromide crystals are held in suspension throughout the

emulsion. Upon exposure of the film to radiation, the silver bromide crystals become

ionized in varying degrees forming the latent image. Each grain or crystal of silver

bromide that has become ionized can be reduced or developed to form a grain of

black metallic silver. This is what forms the visible image on the radiograph. This

visible image is made up of an extremely large number of silver crystals each is

individually exposed to radiation but working together as a unit to form the image.

Once a film has been exposed to radiation and possesses the latent image, it

requires chemical development. The purpose of developing the film is to bring the

latent image out so that it can be seen visibly. There are three processing solutions

that must be used to convert an exposed film to a useful radiograph. These are the

developer, stop bath, and the fixer. Each of these solutions is important in

processing the image so that it may be viewed and stored over a period of time.

3.9.5 THE PROCESS OF DEVELOPING FILM

1. To begin the process of converting the latent image on the radiograph to a

useful image we first expose the film to the developer solution. The

developer’s purpose is to develop, and to make the latent image visible. A

special chemical within the developer solution acts on the film by reducing the

exposed silver bromide crystals to black metallic silver. This process of

developing is actually a multi-step process. Recall the characteristics of the

film manufacturing mentioned earlier, they become important in the

development process. Before the developer can change the silver crystals it

must penetrate the protective coating of the film. Keep in mind that the

protective coating of the film is made of gelatin and is sensitive to temperature

and water. The developer solution is comprised of a combination of

chemicals, consisting of alkali and metol or hydroquinone mixed with water.

The purpose of the alkali is to penetrate the protective coating allowing the

metol to reduce the exposed silver bromide to black metallic oxide.

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2. The second step in the development process is the stop bath. This bath is

comprised of a glacial acetic acid and water. It is important to recognize that

alkali’s and acid’s neutralize each other. The function of the stop bath is to

quickly neutralize any excessive development of the silver crystals. Over

development of the silver crystals results in a radiographic image that is

virtually impossible to interpret.

3. The third step in development is the fixer. Its function is to permanently fix the

image on the film. This is also a multi-step process. The fixer must first

remove any unexposed silver crystals and then harden the remaining crystals

in the emulsion. It is this process that is used to preserve the radiographic

image over time.

Once the film has been properly developed, it is then rinsed in water and dried so

that it may be visually examined.

3.10 VIEWING RADIOGRAPHS

Radiographs (developed film exposed to x-ray or gamma radiation) are generally

viewed on a light-box. However, it is becoming increasingly common to digitize

radiographs and view them on a high resolution monitor. Proper viewing conditions

are very important when interpreting a radiograph. The viewing conditions can

enhance or degrade the subtle details of radiographs.

Before beginning the evaluation of a radiograph, the viewing equipment and area

should be considered. The area should be clean and free of distracting materials.

Magnifying aids, masking aids, and film markers should be close at hand. Thin

cotton gloves should be available and worn to prevent fingerprints on the radiograph.

Ambient light levels should be low. Ambient light levels of less than 2 fc are often

recommended, but subdued lighting, rather than total darkness, is preferable in the

viewing room. The brightness of the surroundings should be about the same as the

area of interest in the radiograph. Room illumination must be arranged so that there

are no reflections from the surface of the film under examination.

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Film viewers should be clean and in good working condition. There are four groups

of film viewers. These include: strip viewers, area viewers, spot viewers, and a

combination of spot and area viewers. Film viewers should provide a source of

defused, adjustable, and relativity cool light as heat from viewers can cause

distortion of the radiograph. A film having a measured density of 2.0 will allow only

1.0 percent of the incident light to pass. A film containing a density of 4.0 will allow

only 0.01 percent of the incident light to pass. With such low levels of light passing

through the radiograph the delivery of a good light source is important.

The radiographic process should be performed in accordance with a written

procedure or code, or as required by contractual documents. The required

documents should be available in the viewing area and referenced as necessary

when evaluating components. Radiographic film quality and acceptability, as

required by the procedure, should first be determined. It should be verified that the

radiograph was produced to the correct density on the required film type, and that it

contains the correct identification information. It should also be verified that the

proper image quality indicator was used and that the required sensitivity level was

met. Next, the radiograph should be checked to ensure that it does not contain

processing and handling artifacts that could mask discontinuities or other details of

interest. The technician should develop a standard process for evaluating the

radiographs so that details are not overlooked.

Once a radiograph passes these initial checks it is ready for interpretation.

Radiographic film interpretation is an acquired skill combining, visual acuity with

knowledge of materials, manufacturing processes, and their associated discontinues.

If the component is inspected while in service, an understanding of applied loads and

history of the component is helpful. A process for viewing radiographs, left to right

top to bottom etc., is helpful and will prevent overlooking an area on the radiograph.

This process is often developed over time and individualized. One part of the

interpretation process, sometimes overlooked, is rest. The mind as well as the eyes

need to occasionally rest when interpreting radiographs.

When viewing a particular region of interest, techniques such as using a small light

source and moving the radiograph over the small light source, or changing the

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intensity of the light source will help the radiographer identify relevant indications.

Magnifying tools should also be used when appropriate to help identify and evaluate

indications. Viewing the actual component being inspected is very often helpful in

developing an understanding of the details seen in a radiograph.

Interpretation of radiographs is an acquired skill that is perfected over time. By using

the proper equipment and developing consistent evaluation processes, the

interpreter will increase his or her probability of detecting defects.

3.11 IMAGE CONSIDERATIONS

The most common detector used in industrial radiography is film. The high sensitivity

to ionizing radiation provides excellent detail and sensitivity to density changes when

producing images of industrial materials. Image quality is determined by a

combination of variables: radiographic contrast and definition. Many variables

affecting radiographic contrast and definition are summarized below and addressed

in following sections.

3.11.1 RADIOGRAPHIC CONTRAST

Radiographic contrast describes the differences in photographic density in a

radiograph. The contrast between different parts of the image is what forms the

image and the greater the contrast, the more visible features become. Radiographic

contrast has two main contributors: subject contrast and detector or film contrast.

Subject contrast is determined by the following variables:

- Absorption differences in the specimen

- Wavelength of the primary radiation

- Scatter or secondary radiation

Film contrast is determined by the following:

- Grain size or type of film

- Chemistry of film processing chemicals

- Concentrations of film processing chemicals

- Time of development

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- Temperature of development

- Degree of mechanical agitation (physical motion)

Exposing the film to produce higher film densities will generally increase contrast. In

other words, darker areas will increase in density faster than lighter areas because in

any given period of time more x-rays are reaching the darker areas. Lead screens in

the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy

levels below 150, 000 volts. Above this point they will emit electrons to provide more

exposure of the film to ionizing radiation thus increasing the density of the

radiograph. Fluorescent screens produce visible light when exposed to radiation and

this light further exposes the film.

3.11.2 DEFINITION

Radiographic definition is the abruptness of change in going from one density to

another. There are a number of geometric factors of the X-ray equipment and the

radiographic setup that have an effect on definition. These geometric factors include:

- Focal spot size, which is the area of origin of the radiation. The focal

spot size should be as close to a point source as possible to produce

the most definition.

- Source to film distance, which is the distance from the source to the

part. Definition increases as the source to film distance increase.

- Specimen to detector (film) distance, which is the distance between the

specimen and the detector. For optimal definition, the specimen and

detector should be as close together as possible.

- Abrupt changes in specimen thickness may cause distortion on the

radiograph.

- Movement of the specimen during the exposure will produce distortion

on the radiograph.

- Film graininess, and screen mottling will decrease definition. The grain

size of the film will affect the definition of the radiograph. Wavelength of

the radiation will influence apparent graininess. As the wavelength

shortens and penetration increases, the apparent graininess of the film

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will increase. Also, increased development of the film will increase the

apparent graininess of the radiograph.

3.11.3 RADIOGRAPHIC DENSITY

Film speed, gradient, and graininess are all responsible for the performance of the

film during exposure and processing. As these combine with processing variables a

final product or the radiograph is produced. In viewing the radiograph, requirements

have been established for acceptable radiographs in industry. The density of a

radiograph in industry will determine if further viewing is possible.

Density considerations date back to early day radiography. Hurder and Drifield have

been credited with developing much of the early information on the characteristic

curve and density of a radiograph. Codes and standards will typically require

densities of a radiograph to be maintained between 1.8 to 4.0 H&D (Hurder and

Drifield) for acceptable viewing. As density increases, contrast will also increase.

This is true above 4.0 H&D, however as density reaches 4.0 H&D an extremely

bright viewing light is necessary for evaluation.

Density, technically should be stated "Transmitted Density" when associated with

transparent-base film. This density is the log of the intensity of light incident on the

film to the intensity of light transmitted through the film. A density reading of 2.0 H&D

is the result of only 1 percent of the transmitted light reaching the sensor. At 4.0 H&D

only 0.01% of transmitted light reaches the far side of the film.

3.12 RADIOGRAPH INTERPRETATION - WELDS

Interpretation of radiographs takes place in three basic steps, which are (1)

detection, (2) interpretation, and (3) evaluation. All of these steps make use of the

radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in

an image. The ability of an individual to detect discontinuities in radiography is also

affected by the lighting condition in the place of viewing, and the experience level for

recognizing various features in the image. The following material was developed to

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help students develop an understanding of the types of defects found in weldments

and how they appear in a radiograph.

3.13 DISCONTINUITIES

Discontinuities are interruptions in the typical structure of a material. These

interruptions may occur in the base metal, weld material or "heat affected" zones.

Discontinuities, which do not meet the requirements of the codes or specification

used to invoke and control an inspection, are referred to as defects.

3.13.1 GENERAL WELDING DISCONTINUITIES

The following discontinuities are typical of all types of welding.

Cold lap is a condition where the weld filler metal does not properly fuse with the

base metal or the previous weld pass material (interpass cold lap). The arc does not

melt the base metal sufficiently and causes the slightly molten puddle to flow into

base material without bonding.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take

many shapes on a radiograph but often appears as dark round or irregular spots or

specks appearing singularly, in clusters or rows. Sometimes porosity is elongated

and may have the appearance of having a tail. This is the result of gas attempting to

escape while the metal is still in a liquid state and is called wormhole porosity. All

porosity is a void in the material it will have a radiographic density more than the

surrounding area.

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.

Cluster porosity is caused when flux coated electrodes are contaminated with

moisture. The moisture turns into gases when heated and becomes trapped in the

weld during the welding process. Cluster porosity appear just like regular porosity in

the radiograph but the indications will be grouped close together.

Slag inclusions are nonmetallic solid material entrapped in weld metal or between

weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the

weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld

metal fails to penetrate the joint. It is one of the most objectionable weld

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discontinuities. Lack of penetration allows a natural stress riser from which a crack

may propagate. The appearance on a radiograph is a dark area with well-defined,

straight edges that follows the land or root face down the center of the weldment.

Incomplete fusion is a condition where the weld filler metal does not properly fuse

with the base metal. Appearance on radiograph: usually appears as a dark line or

lines oriented in the direction of the weld seam along the weld preparation or joining

area.

Internal concavity or suck back is condition where the weld metal has contracted

as it cools and has been drawn up into the root of the weld. On a radiograph it looks

similar to lack of penetration but the line has irregular edges and it is often quite wide

in the center of the weld image.

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Internal or root undercut is an erosion of the base metal next to the root of the

weld. In the radiographic image it appears as a dark irregular line offset from the

centerline of the weldment. Undercutting is not as straight edged as LOP because it

does not follow a ground edge.

External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Offset or mismatch is terms associated with a condition where two pieces being

welded together are not properly aligned. The radiographic image is a noticeable

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difference in density between the two pieces. The difference in density is caused by

the difference in material thickness. The dark, straight line is caused by failure of the

weld metal to fuse with the land area.

Inadequate weld reinforcement is an area of a weld where the thickness of weld

metal deposited is less than the thickness of the base material. It is very easy to

determine by radiograph if the weld has inadequate reinforcement, because the

image density in the area of suspected inadequacy will be more (darker) than the

image density of the surrounding base material.

Excess weld reinforcement is an area of a weld that has weld metal added in

excess of that specified by engineering drawings and codes. The appearance on a

radiograph is a localized, lighter area in the weld. A visual inspection will easily

determine if the weld reinforcement is in excess of that specified by the engineering

requirements.

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Cracks can be detected in a radiograph only when they are propagating in a

direction that produces a change in thickness that is parallel to the x-ray beam.

Cracks will appear as jagged and often very faint irregular lines. Cracks can

sometimes appear as "tails" on inclusions or porosity.

3.13.2 DISCONTINUITIES IN TIG WELDS

The following discontinuities are peculiar to the TIG welding process. These

discontinuities occur in most metals welded by the process including aluminum and

stainless steels. The TIG method of welding produces a clean homogeneous weld

which when radiographed is easily interpreted.

Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the

electrode in tungsten inert gas welding. If improper welding procedures are used,

tungsten may be entrapped in the weld. Radiographically, tungsten is more dense

than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on

the radiograph.

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Oxide inclusions are usually visible on the surface of material being welded

(especially aluminum). Oxide inclusions are less dense than the surrounding

materials and, therefore, appear as dark irregularly shaped discontinuities in the

radiograph.

3.13.3 DISCONTINUITIES IN GAS METAL ARC WELDS (GMAW)

The following discontinuities are most commonly found in GMAW welds.

Whiskers are short lengths of weld electrode wire, visible on the top or bottom

surface of the weld or contained within the weld. On a radiograph they appear as

light, "wire like" indications.

Burn-Through results when too much heat causes excessive weld metal to

penetrate the weld zone. Often lumps of metal sag through the weld creating a thick

globular condition on the back of the weld. These globs of metal are referred to as

icicles. On a radiograph, burn through appears as dark spots, which are often

surrounded by light globular areas (icicles).

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CHAPTER – 4

MAGNETIC PARTICLE TESTING

4.1 INTRODUCTION

Magnetic particle testing (MPT) is a nondestructive testing method used for defect

detection. MPT is a fast and relatively easy to apply and part surface preparation is

not as critical as it is for some other NDT methods. These characteristics make MPT

one of the most widely utilized nondestructive testing methods.

MPT uses magnetic fields and small magnetic particles, such as iron filings to detect

flaws in components. The only requirement from an inspectability standpoint is that

the component being inspected must be made of a ferromagnetic material such iron,

nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can

be magnetized to a level that will allow the inspection to be effective.

The method is used to inspect a variety of product forms such as castings, forgings,

and weldments. Many different industries use magnetic particle inspection for

determining a component's fitness-for-use. Some examples of industries that use

magnetic particle inspection are the structural steel, automotive, petrochemical,

power generation, and aerospace industries. Underwater inspection is another area

where magnetic particle inspection may be used to test items such as offshore

structures and underwater pipelines.

4.2 PRINCIPLE

Magnetic particle testing (MPT) is a relatively simple concept. It can be considered

as a combination of two nondestructive testing methods: magnetic flux leakage

testing and visual testing. Consider a bar magnet. It has a magnetic field in and

around the magnet. Any place that a magnetic line of force exits or enters the

magnet is called a pole. A pole where a magnetic line of force exits the magnet is

called a north pole and a pole where a line of force enters the magnet is called a

south pole.

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When a bar magnet is broken in the center of its length, two complete bar magnets

with magnetic poles on each end of each piece will result. If the magnet is just

cracked but not broken completely in two, a north and south pole will form at each

edge of the crack. The magnetic field exits the north pole and reenters the at the

south pole. The magnetic field spreads out when it encounter the small air gap

created by the crack because the air cannot support as much magnetic field per unit

volume as the magnet can. When the field spreads out, it appears to leak out of the

material and, thus, it is called a flux leakage field.

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to

and cluster not only at the poles at the ends of the magnet but also at the poles at

the edges of the crack. This cluster of particles is much easier to see than the actual

crack and this is the basis for magnetic particle testing.

The first step in a magnetic particle inspection is to magnetize the component that is

to be inspected. If any defects on or near the surface are present, the defects will

create a leakage field. After the component has been magnetized, iron particles,

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either in a dry or wet suspended form, are applied to the surface of the magnetized

part. The particles will be attracted and cluster at the flux leakage fields, thus forming

a visible indication that the inspector can detect

4.3 MAGNETIZING CURRENT

Electric current is often used to establish the magnetic field in components during

magnetic particle inspection. Alternating current and direct current are the two basic

types of current commonly used. Current from single phase 110 volts, to three

phase 440 volts are used when generating an electric field in a component. Current

flow is often modified to provide the appropriate field within the part. The type of

current used can have an effect on the inspection results so the types of currents

commonly are explained.

4.3.1 DIRECT CURRENT

Direct current (DC) flows continuously in one direction at a constant voltage. A

battery is the most common source of direct current. As previously mentioned,

current is said to flow from the positive to the negative terminal when in actuality the

electrons flow in the opposite direction. DC is very desirable when performing

magnetic particle inspection in search of subsurface defects because DC generates

a magnetic field that penetrates deeper into the material. In ferromagnetic materials,

the magnetic field produced by DC generally penetrates the entire cross-section of

the component; whereas, the field produced using alternating current is concentrated

in a thin layer at the surface of the component.

4.3.2 ALTERNATING CURRENT

Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second.

Since AC is readily available in most facilities, it is convenient to make use of it for

magnetic particle inspection. However, when AC is used to induce a magnetic field

in ferromagnetic materials the magnetic field will be limited to narrow region at the

surface of the component. This phenomenon is known as "skin effect" and it occurs

because induction is not a spontaneous reaction and the rapidly reversing current

does not allow the domains down in the material time to align. Therefore, it is

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recommended that AC be used only when the inspection is limited to surface

defects.

4.3.3 RECTIFIED ALTERNATING CURRENT

The skin effect limits the use of AC since many inspection applications call for the

detection of subsurface defects. However, the convenient access to AC, drive its

use beyond surface flaw inspections. AC can be converted to current that is very

much like DC through the process of rectification. With the use of rectifiers, the

reversing AC can be converted to a one-directional current. The three commonly

used types of rectified current are described below.

4.3.4 HALF WAVE RECTIFIED ALTERNATING CURRENT (HWAC)

When single-phase alternating current is passed through a rectifier, current is

allowed to flow in only one direction. The reverse half of each cycle is blocked out so

that a one directional, pulsating current is produced. The current rises from zero to a

maximum and then returns to zero. No current flows during the time when the

reverse cycle is blocked out. The HWAC repeats at same rate as the unrectified

current (50 or 60 hertz typical). Since half of the current is blocked out, the amperage

is half of the unaltered AC.

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This type of current is often referred to as half wave DC or pulsating DC. The

pulsation of the HWAC helps magnetic particle indications form by vibrating the

particles and giving them added mobility. This added mobility is especially important

when using dry particles. The pulsation is reported to significantly improve inspection

sensitivity. HWAC is most often used to power electromagnetic yokes.

4.3.5 FULL WAVE RECTIFIED ALTERNATING CURRENT (FWAC)

Full wave rectification inverts the negative current to positive current rather than

blocking it out. This produces a pulsating DC with no interval between the pulses.

Filtering is usually performed to soften the sharp polarity switching in the rectified

current. While particle mobility is not as good as half-wave AC due to the reduction in

pulsation, the depth of the subsurface magnetic field is improved.

4.3.6 THREE PHASE FULL WAVE RECTIFIED ALTERNATING

CURRENT

Three phase current is often used to power industrial equipment because it has more

favorable power transmission and line loading characteristics. This type of electrical

current is also highly desirable for magnetic particle testing because when it is

rectified and filtered, the resulting current very closely resembles direct current.

Stationary magnetic particle equipment wired with three phase AC will usually have

the ability to magnetize with AC or DC (three phase full wave rectified), providing the

inspector with the advantages of each current form.

4.4 LIGHTING

Magnetic particle inspection predominately relies on visual inspection to detect any

indications that are formed. Therefore, lighting is a very important element of the

inspection process. Obviously, the lighting requirements are different for an

inspection conducted using visible particles than they are for an inspection

conducted using fluorescent particles. The lighting requirements for each of these

techniques, as well as how light measurements are made, is discussed below.

4.4.1 LIGHT REQUIREMENTS WHEN USING VISIBLE PARTICLES

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Magnetic particle inspections conducted using visible particles can be conducted

using natural lighting or artificial lighting. When using natural lighting, it is important

to keep in mind that daylight varies from hour to hour. Inspector must stay constantly

aware of the lighting conditions and make adjustments when needed. To improve

uniformity in lighting from one inspection to the next, the use of artificial lighting is

recommended. Artificial lighting should be white whenever possible and white flood

or halogen lamps are most commonly used. The light intensity is required to be 100

foot-candles at the surface being inspected. It is advisable to choose a white light

wattage that will provide sufficient light, but avoid excessive reflected light that could

distract from the inspection.

4.4.2 LIGHT REQUIREMENTS WHEN USING FLUORESCENT

PARTICLES

4.4.2.1 ULTRAVIOLET LIGHTING

When performing a magnetic particle inspection using fluorescent particles, the

condition of the ultraviolet light and the ambient white light must be monitored.

Standards and procedures require verification of lens condition and light intensity.

Black lights should never be used with a cracked filter as output of white light and

harmfulness black light will be increased. The cleanliness of the filter should also be

checked as a coating of solvent carrier, oils, or other foreign materials can reduce

the intensity by up to as much as 50%. The filter should be checked visually and

cleaned as necessary before warm-up of the light.

For UV lights used in component evaluations, the normally accepted intensity is

1000 microwatts per square centimeter when measured at 15 inches from the filter

face (requirements can vary from 800 to 1200). The required check should be

performed when a new bulb is installed, at startup of the inspection cycle, if a change

in intensity is noticed, or every eight hours if in continuous use. Regularly checking

the intensity of UV lights is very important because bulbs loose intensity over time. In

fact, a bulb that is near the end of its operating life will often have an intensity of only

25 percent of its original output. Black light intensity will also be affected by voltage

variations, so it is important to provide constant voltage to the light. A bulb that

produces acceptable intensity at 120 volts will produce significantly less at 110 volts.

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4.4.2.2 AMBIENT WHITE LIGHTING

When performing a fluorescent magnetic particle inspection, it is important to keep

white light to a minimum as it will significantly reduce the inspectors ability to detect

fluorescent indications. Light levels of less than 2 fc are required by most procedures

with some procedures requiring less than 0.5 fc at the inspection surface. When

checking black light intensity at 15 inches a reading of the white light produced by

the black light may be required to verify white light is being removed by the filter.

4.4.2.3 WHITE LIGHT FOR INDICATION CONFIRMATION

While white light is held to a minimum in fluorescent inspections, procedures may

require that indications be evaluated under white light. The white light requirements

here are the same as when performing an inspection with visible particles. The

minimum light intensity at the surface being inspected must be 100 foot-candles.

4.4.3 LIGHT MEASUREMENT

Light intensity measurements are made using a radiometer. A radiometer is an

instrument that translates light energy into an electrical current. Light striking a

silicon photodiode detector causes a charge to build up between internal layers.

When an external circuit is connected to the cell, an electrical current is produced.

This current is linear with respect to incident light. Some radiometers have the ability

to measure both black and white light, while others require a separate sensor for

each measurement. Whichever type used, the sensing area should be clean and

free of any materials that could reduce or obstruct light reaching the sensor.

Radiometers are relatively unstable instruments and readings often change

considerably over time. Therefore, they must be calibrated regularly. They should be

calibrated at least every six months. A unit should be checked to make sure its

calibration is current before taking any light readings.

Ultraviolet light measurements should be taken using a fixture to maintain a

minimum distance of 15 inches from the filter face to the sensor. The sensor should

be centered in the light field to obtain and record the highest reading. UV spot lights

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are often focused so intensity readings will vary considerable over a small area.

White lights are seldom focused and depending on the wattage, will often produce in

excess of the 100 fc at 15 inches. Many specifications do not require the white light

intensity check to be conducted at a specific distance.

4.5 PARTICLE CONCENTRATION AND CONDITION

4.5.1 PARTICLE CONCENTRATION

The concentration of particles in the suspension is a very important parameter in the

inspection process and must be closely controlled. The particle concentration is

checked after the suspension is prepared and continued regularly as part of the

quality system checks. ASTM E-1444-01 requires concentration checks to be

performed every eight hours or ever shift change.

The standard process used to perform the check requires agitating the carrier for a

minimum of thirty minutes to ensure even particle distribution. A sample is then taken

in a pear-shaped 100 ml centrifuge tube having a stem graduated to 1.0 ml in 0.05

ml increments for fluorescent particles, and graduated to 1.5 ml. in 0.1 ml increments

for visible particles. The sample is then demagnetized so that the particles do not

clump together while settling. The sample must then remain undisturbed for a

minimum of 60 minutes for a petroleum-based carrier or 30 minutes for a water-

based carrier, unless shorter times have been documented to produce results similar

to the longer settling times. The volume of settled particles is then read. Acceptable

ranges are 0.1 to 0.4 ml for fluorescent particles and 1.2 to 2.4 ml for visible

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particles. If the particle concentration is out of the acceptable range, particles or the

carrier must be added to bring the solution back in compliance with the requirement.

Particle loss is often attributed to "dragout". Dragout occurs because the solvent

easily runs off components and is recaptured in the holding tank. Particles, on the

other hand, tend to adhere to components, or be trapped in geometric features of the

component. These particles will be "drug out" or lost to the system, and will

eventually need to be replaced.

4.5.2 PARTICLE CONDITION

After the particles have settled, they should be examined for brightness and

agglomeration. Fluorescent particles should be evaluated under ultraviolet light and

visible particles under white light. The brightness of the particles should be evaluated

weekly by comparing the particles in the test solution to those in an unused

reference solution that was saved when the solution was first prepared. The

brightness of the two solutions should be relatively the same. Additionally, the

particles should appear loose and not lumped together. If the brightness or the

agglomeration of the particles is noticeably different from the reference solution, the

bath should be replaced

4.6 MAGNETIC FIELD INDICATORS

Determining whether a magnetic field is of adequate strength and in the proper

direction is critical when performing magnetic particle testing. As discussed

previously, knowing the direction of the field is important because the field should be

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as close to perpendicular to the defect as possible and no more than 45 degrees

from normal. Being able to evaluate the field direction and strength is especially

important when inspecting with a multidirectional machine, because when the fields

are not balanced properly a vector field will be produced that may not detect some

defects.

There is actually no easy to apply method that permits an exact measurement of

field intensity at a given point within a material. In order to measure the field strength

it is necessary to intercept the flux lines. This is impossible without cutting into the

material and cutting the material would immediately change the field within the part.

However, cutting a small slot or hole into the material and measuring the leakage

field that crosses the air gap with a Gauss meter is probably the best way to get an

estimate of the actual field strength within a part. Nevertheless, there are a number

of tools and methods available that are used to determine the presence and direction

of a field surrounding the component.

4.6.1 GAUSS METER OR HALL EFFECT GAGE

A Gauss meter with a Hall Effect probe, is commonly used to measure the tangential

field strength on the surface of the part. As discussed in some detail on the

"Measuring Magnetic Fields" page, the Hall effect is the transverse electric field

created in a conductor when placed in a magnetic field. Gauss meters, also called

Tesla meters, are used to measure the strength of a field tangential to the surface of

the magnetized test object. The meters measure the intensity of the field in the air

adjacent to the component when a magnetic field is applied.

The advantages of Hall effect devices are; they provide a quantitative measure of the

strength of magnetizing force tangential to the surface of a test piece, they can be

used for measurement of residual magnetic fields, and they can be used repetitively.

Their main disadvantages are that they must be periodically calibrated, and they

cannot be used to establish the balance of fields in multidirectional applications.

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4.7 QUANTITATIVE QUALITY INDICATOR (QQI)

The Quantitative Quality Indicator (QQI) or Artificial Flaw Standard is often the

preferred method of assuring proper field direction and adequate field strength. The

use of QQIs is also the only practical way of ensuring balanced field intensity and

direction in multiple-direction magnetization equipment. QQIs are often used in

conjunction with a Gauss meter to establish the inspection procedure for a particular

component. They are used with the wet method only and, as other flux sharing

devices, they can only be used when continuous magnetization is used.

The QQI is a thin strip of either 0.002 or 0.004 inch thick AISI 1005 steel. A photo-

etch process is used to inscribe a specific pattern, such as concentric circles or a

plus sign. QQIs are nominally 3/4 inch square, but miniature shims are also

available. QQIs must be in intimate contact with the part being evaluated. This is

accomplished by placing the shim on a part etched side down, and taping or gluing it

to the surface. The component is then magnetized and particles applied. When the

field strength is adequate, the particles will adhere over the engraved pattern and

provide information about the field direction. When a multidirectional technique is

used, a balance of the fields is noted when all areas of the QQI produce indications.

Some of the advantages of QQIs are: they can be quantified and related to other

parameters; they can accommodate virtually any configuration with suitable

selection; and they can be reused with careful application and removal practices.

Some of the disadvantages are: the application process is somewhat slow, the parts

must be clean and dry; shims cannot be used as a residual magnetism indicator as

they are a flux sharing device; they can be easily damaged with improper handling

and will corrode if not cleaned and properly stored.

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Above left is a photo of a typical QQI shim. The photo on the right, shows the

indication produced by the QQI when it is applied to the surface a part and a

magnetic field is established that runs across the shim from right to left.

4.8 PIE GAGE

The pie gage is a disk of highly permeable material divided into four, six, or eight

sections by nonferromagnetic material. The division serves as artificial defects that

radiate out in different directions from the center. Diameter of the gage is ¾ to 1

inch. The divisions between the low carbon steel pie sections are to be no greater

than 1/32 inch. The sections are furnace brazed and copper plated. The gage is

placed on the test piece copper side up, and the test piece is magnetized. After

particles are applied, and excess removed, the indications provide the inspector the

orientation of the magnetic field.

The principal application is on flat surfaces such as weldments or steel castings

where dry powder is used with a yoke or prods. The pie gage is not recommended

for precision parts with complex shapes, for wet-method applications, or for proving

field magnitude. The gage should be demagnetized between readings.

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Several of the main advantages of the pie gage are: it is easy to use and it can be

used indefinitely without deterioration. The pie gage has several disadvantages,

which include: it retains some residual magnetism so indications will prevail after

removal of the source of magnetization, it can only be used in relatively flat areas,

and it cannot be reliably used for determination of balanced fields in multidirectional

magnetization.

4.9 SLOTTED STRIPS

Slotted strips, also known as Burmah-Castrol Strips, are pieces of highly permeable

ferromagnetic material with slots of different widths. They are placed on the test

object as it is inspected. The indications produced on the strips give the inspector a

general idea of the field strength in a particular area.

Advantages of these strips are: they are relatively easily applied to the component;

they can be used successfully with either the wet or dry method when using the

continuous magnetization; they are repeatable as long as orientation to the magnetic

field is maintained and they can be used repetitively. Disadvantages include: they

cannot be bent to complex configuration; and they are not suitable for

multidirectional field applications since they indicate defects in only one direction.

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CHAPTER – 5

ULTRASONIC TESTING 5.1 INTRODUCTION

Sound in the range of 20 Hz to 18000 Hz is in audible ranges of human ear. Sound

beyond this range cannot be heard by human and called as ultrasonic sound.

However, some mammals can hear well above this. For example, bats and whales

use echo location that can reach frequencies in excess of 100,000Hz.

5.2 WAVE PROPAGATION

Ultrasonic testing is based on time-varying deformations or vibrations in materials,

which is generally referred to as acoustics. All material substances are comprised of

atoms, which may be forced into vibrational motion about their equilibrium positions.

Many different patterns of vibrational motion exist at the atomic level, however, most

are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles

that contain many atoms that move in unison to produce a mechanical wave. When

a material is not stressed in tension or compression beyond its elastic limit, its

individual particles perform elastic oscillations. When the particles of a medium are

displaced from their equilibrium positions, internal (electrostatic) restoration forces

arise. It is these elastic restoring forces between particles, combined with inertia of

the particles, that leads to oscillatory motions of the medium.

In solids, sound waves can propagate in four principle modes that are based on the

way the particles oscillate. Sound can propagate as longitudinal waves, shear

waves, surface waves, and in thin materials as plate waves. Longitudinal and shear

waves are the two modes of propagation most widely used in ultrasonic testing. The

particle movement responsible for the propagation of longitudinal and shear waves is

illustrated below.

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In longitudinal waves the oscillations occur in the longitudinal direction or the

direction of wave propagation. Since compressional and dilational forces are active

in these waves, they are also called pressure or compressional waves. They are also

sometimes called density waves because their particle density fluctuates as they

move. Compression waves can be generated in liquids, as well as solids because

the energy travels through the atomic structure by a series of comparison and

expansion (rarefaction) movements.

In the transverse or shear wave the particles oscillate at a right angle or transverse

to the direction of propagation. Shear waves require an acoustically solid material for

effective propagation and, therefore, are not effectively propagated in materials such

as liquids or gasses. Shear waves are relatively weak when compared to

longitudinal waves In fact, shear waves are usually generated in materials using

some of the energy from longitudinal waves.

5.3 WAVELENGTH, FREQUENCY AND VELOCITY

Among the properties of waves propagating in isotropic solid materials are

wavelength, frequency, and velocity. The wavelength is directly proportional to the

velocity of the wave and inversely proportional to the frequency of the wave. This

relationship is shown by the following equation.

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5.4 SOUND PROPAGATION IN ELASTIC MATERIALS

Sound waves propagate due to the vibrations or oscillatory motions of particles

within a material. An ultrasonic wave may be visualized as an infinite number of

oscillating masses or particles connected by means of elastic springs. Each

individual particle is influenced by the motion of its nearest neighbor and both inertial

and elastic restoring forces act upon each particle.

5.5 MATERIAL AFFECT ON SPEED OF SOUND

Sound travels at different speeds in different materials. This is because the mass of

the atomic particles and the spring constants are different for different materials.

The mass of the particles is related to the density of the material, and the spring

constant is related to the elastic constants of a material. The general relationship

between the speed of sound in a solid and its density and elastic constants is given

by the following equation:

Where V is the speed of sound, C is the elastic constant, and p is the material

density. This equation may take a number of different forms depending on the type

of wave (longitudinal or shear) and which of the elastic constants that are used. The

typical elastic constants of materials include:

• Young's Modulus, E: proportionality constant between uniaxial stress and

strain.

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• Poisson's Ratio, n: the ratio of radial strain to axial strain

• Bulk modulus, K: a measure of the incompressibility of a body subjected to

hydrostatic pressure.

• Shear Modulus, G: also called rigidity, a measure of substance's resistance to

shear.

• Lame's Constants, l and m: material constants that are derived from Young's

Modulus and Poisson's Ratio.

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson's

Ratio are commonly used. When calculating the velocity of a shear wave, the shear

modulus is used. It is often most convenient to make the calculations using Lame's

Constants, which are derived from Young's Modulus and Poisson's Ratio.

It must also be mentioned that the subscript ij attached to C in the above equation is

used to indicate the directionality of the elastic constants with respect to the wave

type and direction of wave travel. In isotropic materials, the elastic constants are the

same for all directions within the material. However, most materials are anisotropic

and the elastic constants differ with each direction. For example, in a piece of rolled

aluminum plate, the grains are elongated in one direction and compressed in the

others and the elastic constants for the longitudinal direction are different than those

for the transverse or short transverse directions.

Examples of approximate compressional sound velocities in materials are:

• Aluminum - 0.632 cm/microsecond

• 1020 steel - 0.589 cm/microsecond

• Cast iron - 0.480 cm/microsecond.

Examples of approximate shear sound velocities in materials are:

• Aluminum - 0.313 cm/microsecond

• 1020 steel - 0.324 cm/microsecond

• Cast iron - 0.240 cm/microsecond.

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When comparing compressional and shear velocities it can be noted that shear

velocity is approximately one half that of compressional. The sound velocities for a

variety of materials can be found in the ultrasonic properties tables in the general

resources section of this site.

5.6 ACOUSTIC IMPEDANCE

Sound travels through materials under the influence of sound pressure. Because

molecules or atoms of a solid are bound elastically to one another, the excess

pressure results in a wave propagating through the solid.

The acoustic impedance (Z) of a material is defined as the product of density (ρ)

and acoustic velocity (V) of that material.

Z = ρV

Acoustic impedance is important in

1. The determination of acoustic transmission and reflection at the boundary of

two materials having different acoustic impedance

2. The design of ultrasonic transducers.

3. Assessing absorption of sound in a medium.

The following figure will help you calculate the acoustic impedance for any material,

so long as you know its density (ρ) and acoustic velocity (V). We can also compare

two materials and "see" how they reflect and transmit sound energy. The red arrow

represents energy of the reflected sound, while the blue arrow represents energy of

the transmitted sound. The reflected energy is the square of the difference divided by

the sum of the acoustic impedances of the two materials.

Note that Transmitted Sound Energy + Reflected Sound Energy = 1

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5.7 ULTRASONIC WAVE GENERATION

5.7.1 PIEZOELECTRIC TRANSDUCERS

The conversion of electrical pulses to mechanical vibrations and the conversion of

returned mechanical vibrations back into electrical energy is the basis for ultrasonic

testing. The active element is the heart of the transducer as it converts the electrical

energy to acoustic energy, and vice versa. The active element is basically a piece

polarized material (i.e. some parts of the molecule are positively charged, while other

parts of the molecule are negatively charged) with electrodes attached to two of its

opposite faces. When an electric field is applied across the material, the polarized

molecules will align themselves with the electric field, resulting in induced dipoles

within the molecular or crystal structure of the material. This alignment of molecules

will cause the material to change dimensions. This phenomenon is known as

electrostriction. In addition, a permanently-polarized material such as quartz (SiO2)

or barium titanate (BaTiO3) will produce an electric field when the material changes

dimensions as a result of an imposed mechanical force. This phenomenon is known

as the piezoelectric effect.

The active element of most acoustic transducers used today is a piezoelectric

ceramics, which can be cut in various ways to produce different wave modes. A

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large piezoelectric ceramic element can be seen in the image of a sectioned low

frequency transducer. In the early 1950's, piezoelectric crystals made from quartz

crystals and magnetostrictive materials were primarily used. When piezoelectric

ceramics were introduced they soon became the dominant material for transducers

due to their good piezoelectric properties and their ease of manufacture into a variety

of shapes and sizes. They also operate at low voltage and are usable up to about

300oC. The first piezoceramic in general use was barium titanate, and that was

followed during the 1960's by lead zirconate titanate compositions, which are now

the most commonly employed ceramic for making transducers. New materials such

as piezo polymers and composites are also being used in some applications.

The thickness of the active element is determined by the desired frequency of the

transducer. A thin wafer element vibrates with a wavelength that is twice its

thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the

desired radiated wavelength. The higher the frequency of the transducer, the

thinner will be active element. The primary reason that high frequency contact

transducers are not produced in because the element is very thin and too fragile.

5.8 REFRACTION AND SNELL'S LAW

When an ultrasonic wave passes through an interface between two materials at an

oblique angle, and the materials have different indices of refraction, it produces both

reflected and refracted waves. This also occurs with light and this makes objects you

see across an interface appear to be shifted relative to where they really are.

Refraction takes place at an interface due to the different velocities of the acoustic

waves within the two materials. The velocity of sound in each material is determined

by the material properties (elastic modules and density) for that material. In the

animation below, a series of plane waves are shown traveling in one material and

entering a second material that has a higher acoustic velocity. Therefore, when the

wave encounters the interface between these two materials, the portion of the wave

in the second material is moving faster than the portion of the wave in the first

material. It can be seen that this causes the wave to bend.

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Snell's Law describes the relationship between the angles and the velocities of the

waves. Snell's law equates the ratio of material velocities v1 and v2 to the ratio of the

sine's of incident (θ) and refraction (θ2) angles, as shown in the following equation.

Where:

VL1 is the longitudinal wave velocity in material 1.

VL2 is the longitudinal wave velocity in material 2.

Note that in the diagram, there is a reflected longitudinal wave (VL1) shown. This

wave is reflected at the same angle as the incident wave because the two waves are

traveling in the same material and, therefore, have the same velocities. This

reflected wave is unimportant in our explanation of Snell's Law, but it should be

remembered that some of the wave energy is reflected at the interface.

When a longitudinal wave moves from a slower to a faster material, there is an

incident angle that makes the angle of refraction for the wave 90°. This is known as

the first critical angle. The first critical angle can be found from Snell's law by putting

in an angle of 90° for the angle of the refracted ray. At the critical angle of incidence,

much of the acoustic energy is in the form of an inhomogeneous compression wave,

which travels along the interface and decays exponentially with depth from the

interface. This wave is sometimes referred to as a "creep wave." Because of their

inhomogeneous nature and the fact that they decay rapidly, creep waves are not

used as extensively as Rayleigh surface waves in NDT. However, creep waves are

sometimes useful because they suffer less from surface irregularities and coarse

material microstructure, due to their longer wavelengths, than Rayleigh waves.

5.9 CALIBRATION METHODS

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Calibration refers to the act of evaluating and adjusting the precision and accuracy of

measurement equipment. Different type of standard calibration blocks used for

various applications described below. In ultrasonic testing, several forms of

calibration must be done. First, the electronics of the equipment must be calibrated

to assure that they are performing as designed. This operation is usually performed

by the equipment manufacturer and will not be discussed further in this material. It is

also usually necessary for the operator to perform a "user calibration" of the

equipment. This user calibration is necessary because most ultrasonic equipment

can be reconfigured for use in a large variety of applications. The user must

"calibrate" the system, which includes the equipment settings, the transducer, and

the test setup, to validate that the desired level of precision and accuracy are

achieved. The term calibration standard is usually only used when an absolute value

is measured and in many cases, the standards are traceable back to standards at

the National Institute for Standards and Technology.

In ultrasonic testing, there is also a need for reference standards. Reference

standards are used to establish a general level of consistency in measurements and

to help interpret and quantify the information contained in the received signal.

Reference standards are used to validate that the equipment and the setup provide

similar results from one day to the next and that similar results are produced by

different systems. Reference standards also help the inspector to estimate the size

of flaws. In a pulse-echo type setup, signal strength depends on both the size of the

flaw and the distance between the flaw and the transducer. The inspector can use a

reference standard with an artificially induced flaw of known size and at

approximately the same distance away for the transducer to produce a signal. By

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comparing the signal from the reference standard to that received from the actual

flaw, the inspector can estimate the flaw size.

This section will discuss some of the more common calibration and reference

specimen that are used in ultrasonic inspection. Some of these specimens are

shown in the figure above. Be aware that are other standards available and that

specially designed standards may be required for many applications. The information

provided here is intended to serve a general introduction to the standards and not to

be instruction on the proper use of the standards.

5.10 INTRODUCTION TO THE COMMON STANDARDS

Calibration and reference standards for ultrasonic testing come in many shapes and

sizes. The type of standard used is dependent on the NDE application and the form

and shape of the object being evaluated. The material of the reference standard

should be the same as the material being inspected and the artificially induced flaw

should closely resemble that of the actual flaw. This second requirement is a major

limitation of most standard reference samples. Most use drilled holes and notches

that do not closely represent real flaws. In most cases the artificially induced defects

in reference standards are better reflectors of sound energy (due to their flatter and

smoother surfaces) and produce indications that are larger than those that a similar

sized flaw would produce. Producing more "realistic" defects is cost prohibitive in

most cases and, therefore, the inspector can only make an estimate of the flaw size.

Computer programs that allow the inspector to create computer simulated models of

the part and flaw may one day lessen this limitation.

5.11 THE IIW TYPE CALIBRATION BLOCK

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The standard shown in the above figure is commonly known in the US as an IIW

type reference block. IIW is an acronym for the International Institute of Welding. It is

referred to as an IIW "type" reference block because it was patterned after the "true"

IIW block but does not conform to IIW requirements in IIS/IIW-23-59. "True" IIW

blocks are only made out of steel (to be precise, killed, open hearth or electric

furnace, low-carbon steel in the normalized condition with a grain size of McQuaid-

Ehn #8) where IIW "type" blocks can be commercially obtained in a selection of

materials. The dimensions of "true" IIW blocks are in metric units while IIW "type"

blocks usually have English units. IIW "type" blocks may also include additional

calibration and references features such as notches, circular groves, and scales that

are not specified by IIW. There are two full-sized and a mini version of the IIW type

blocks. The Mini version is about one-half the size of the full-sized block and weighs

only about one-fourth as much. The IIW type US-1 block was derived the basic "true"

IIW block and is shown below in the figure on the left.

5.11.1 IIW TYPE US-1

5.11.2 IIW TYPE US-2

5.11.3 IIW TYPE MINI

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IIW type blocks are used to calibrate instruments for both angle beam and normal

incident inspections. Some of their uses include setting metal-distance and

sensitivity settings, determining the sound exit point and refracted angle of angle

beam transducers, and evaluating depth resolution of normal beam inspection

setups. Instructions on using the IIW type blocks can be found in the annex of

American Society for Testing and Materials Standard E164, Standard Practice for

Ultrasonic Contact Examination of Weldments.

5.11.4 THE MINIATURE ANGLE-BEAM CALIBRATION BLOCK

The miniature angle-beam is a calibration block that was designed for use in the field

for instrument calibration. The block is much smaller and lighter than the IIW block

but performs many of the same functions. The miniature angle-beam block can be

used to check the beam angle and exit point of the transducer. The block can also

be used to make metal-distance and sensitivity calibrations for both angle and

normal-beam inspection setups.

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5.11.5 AWS SHEAR WAVE DISTANCE/SENSITIVITY CALIBRATION

(DSC) BLOCK

A block that closely resembles the miniature angle-beam block and is used in a

similar way is the DSC AWS Block. This block is used to determine the beam exit

point and refracted angle of angle-beam transducers and to calibrate distance and

set the sensitivity for both normal and angle beam inspection setups. Instructions on

using the DSC block can be found in the annex of American Society for Testing and

Materials Standard E164, Standard Practice for Ultrasonic Contact Examination of

Weldments.

5.11.6 AWS SHEAR WAVE DISTANCE CALIBRATION (DC) BLOCK

The DC AWS Block is a metal path distance and beam exit point calibration standard

that conforms to the requirements of the American Welding Society (AWS) and the

American Association of State Highway and Transportation Officials (AASHTO).

Instructions on using the DC block can be found in the annex of American Society for

Testing and Materials Standard E164, Standard Practice for Ultrasonic Contact

Examination of Weldments.

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5.11.7 AWS RESOLUTION CALIBRATION (RC) BLOCK

The RC Block is used to determine the resolution of angle beam transducers per the

requirements of AWS and AASHTO. Engraved Index markers are provided for 45,

60, and 70 degree refracted angle beams.

5.11.8 30 FBH RESOLUTION REFERENCE BLOCK

The 30 FBH resolution reference block is used to evaluate the near-surface

resolution and flaw size/depth sensitivity of a normal-beam setup. The block contains

number 3 (3/64"), 5 (5/64"), and 8 (8/64") ASTM flat bottom holes at ten metal-

distances ranging from 0.050 inch (1.27 mm) to 1.250 inch (31.75 mm).

5.11.9 MINIATURE RESOLUTION BLOCK

The miniature resolution block is used to evaluate the near-surface resolution and

sensitivity of a normal-beam setup It can be used to calibrate high-resolution

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thickness gages over the range of 0.015 inches (0.381 mm) to 0.125 inches (3.175

mm).

5.11.10 STEP AND TAPERED CALIBRATION WEDGES

Step and tapered calibration wedges come in a large variety of sizes and

configurations. Step wedges are typically manufactured with four or five steps but

custom wedge can be obtained with any number of steps. Tapered wedges have a

constant taper over the desired thickness range.

5.11.11 DISTANCE/SENSITIVITY (DS) BLOCK

The DS test block is a calibration standard used to check the horizontal linearity and

the dB accuracy per requirements of AWS.

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5.12 COUPLANT

A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic

energy from the transducer into the test specimen. Couplant is generally necessary

because the acoustic impedance mismatch between air and solids, such as the test

specimen, is large and, therefore, nearly all of the energy is reflected and very little is

transmitted into the test material. The couplant displaces the air and makes it

possible to get more sound energy into the test specimen so that a usable ultrasonic

signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water

is generally used between the transducer and the test surface.

When scanning over the part or making precise measurements, an immersion

technique is often used. In immersion ultrasonic testing both the transducer and the

part are immersed in the couplant, which is typically water. This method of coupling

makes it easier to maintain consistent coupling while moving and manipulating the

transducer and/or the part

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5.13 NORMAL BEAM INSPECTION

Pulse-echo ultrasonic measurements can determine the location of a discontinuity in

a part or structure by accurately measuring the time required for a short ultrasonic

pulse generated by a transducer to travel through a thickness of material, reflect

from the back or the surface of a discontinuity, and be returned to the transducer. In

most applications, this time interval is a few microseconds or less. The two-way

transit time measured is divided by two to account for the down-and-back travel path

and multiplied by the velocity of sound in the test material. The result is expressed in

the well-known relationship

d = vt/2 or v = 2d/t

where d is the distance from the surface to the discontinuity in the test piece, v is the

velocity of sound waves in the material, and t is the measured round-trip transit time.

The diagram below allows you to move a transducer over the surface of a stainless

steel test block and see return echoes, as they would appear on an oscilloscope.

The transducer employed is a 5 MHz broadband transducer 0.25 inches in diameter.

Precision ultrasonic thickness gages usually operate at frequencies between 500

kHz and 100 MHz, by means of piezoelectric transducers that generate bursts of

sound waves when excited by electrical pulses. A wide variety of transducers with

various acoustic characteristics have been developed to meet the needs of industrial

applications. Typically, lower frequencies are used to optimize penetration when

measuring thick, highly attenuating or highly scattering materials, while higher

frequencies will be recommended to optimize resolution in thinner, non-attenuating,

non-scattering materials.

In thickness gauging, ultrasonic techniques permit quick and reliable measurement

of thickness without requiring access to both sides of a part. Accuracy's as high as

±1 micron or ±0.0001 inch can be achieved in some applications. It is possible to

measure most engineering materials ultrasonically, including metals, plastic,

ceramics, composites, epoxies, and glass as well as liquid levels and the thickness

of certain biological specimens. On-line or in-process measurement of extruded

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plastics or rolled metal often is possible, as is measurements of single layers or

coatings in multilayer materials. Modern handheld gages are simple to use and very

reliable.

5.14 ANGLE BEAM INSPECTION

Angle Beam Transducers and wedges are typically used to introduce a refracted

shear wave into the test material. An angled sound path allows the sound beam to

come in from the side, thereby improving detectability of flaws in and around welded

areas.

5.15 WELDMENTS (WELDED JOINTS)

The most commonly occurring defects in welded joints are porosity, slag inclusions,

lack of side-wall fusion, lack of inter-run fusion, lack of root penetration, undercutting

and longitudinal or transverse cracks.

With the exception of single gas pores all the defects listed are usually well

detectable by ultrasonic. Most applications are on low-alloy construction quality

steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has

long been the preferred method for nondestructive testing in welding applications.

This safe, accurate, and simple technique has pushed ultrasonics to the forefront of

inspection technology.

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Ultrasonic weld inspections are typically performed using a straight beam transducer

in conjunction with an angle beam transducer and wedge. A straight beam

transducer, producing a longitudinal wave at normal incidence into the test piece, is

first used to locate any laminations in or near the heat-affected zone. This is

important because an angle beam transducer may not be able to provide a return

signal from a laminar flaw.

The second step in the inspection involves using an angle beam transducer to

inspect the actual weld. Angle beam transducers use the principles of refraction and

mode conversion to produce refracted shear or longitudinal waves in the test

material. This inspection may include the root, sidewall, crown, and heat-affected

zones of a weld. The process involves scanning the surface of the material around

the weldment with the transducer. This refracted sound wave will bounce off a

reflector (discontinuity) in the path of the sound beam. With proper angle beam

techniques, echoes returned from the weld zone may allow the operator to determine

the location and type of discontinuity.

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To determine the proper scanning area for the weld, the inspector must first calculate

the location of the sound beam in the test material. Using the refracted angle, beam

index point and material thickness, the V-path and skip distance of the sound beam

is found. Once they have been calculated, the inspector can identify the transducer

locations on the surface of the material corresponding to the crown, sidewall, and

root of the weld.

5.16 DISTANCE AMPLITUDE CORRECTION (DAC)

Acoustic signals from the same reflecting surface will have different amplitudes at

different distance in the same material. A distance amplitude correction curve

(DACC) can be constructed from the peak amplitude responses from reflectors of

equal area at different distances in the same material.

Such curves are plotted specifically for a flat-bottom hole target and engraved on a

transparent plastic sheet for attachment to the CRT screen. Disk-shaped reflectors,

side drilled holes and hemispherical bottom holes are used as equivalent reflectors

when using contact probes. A small steel ball helps to measure a distance-amplitude

curve for immersion probes. These techniques are important because the amplitude

of ultrasonic pulses varies with the distance from the probe, which causes the echo

from a constant reflector to vary with distance. Therefore, to evaluate echoes of

reflectors for all kind of probes, distance-amplitude curves are needed.

The following figure shows a test block with a side drilled hole. The transducer was

chosen so that the signal in the shortest pulse-echo path is in the far-field

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(Sec.3.1.2). The transducer may be moved finding signals at depth ratios of 1, 3, 5,

and 7. Red points are "drawn" at the peaks of the signals and are used to form the

distance amplitude correction curve drawn in blue. Start by pressing the green "Test

now!" button. After determining the amplitudes for various path lengths (4), press

"Draw DACC" and then press the green "Test now!" button.

5.17 WAVELENGTH AND DEFECT DETECTION

In ultrasonic testing the inspector must make a decision about the frequency of the

transducer that will be used. As mentioned in the earlier page, changing the

frequency when the sound velocity is fixed will result in a change in the wavelength

of the sound. The wavelength of the ultrasound used has significant affect on the

probability of detecting a discontinuity. A rule of thumb in industrial inspections is that

discontinuities that are larger than one-half the size of wavelength can be usually be

detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to

describe a technique's ability to locate flaws. Sensitivity is the ability to locate small

discontinuities. Sensitivity generally increases with higher frequency (shorter

wavelengths). Resolution is the ability of the system to locate discontinuities that are

close together within the material or located near the part surface. Resolution also

generally increases as the frequency increases.

The wave frequency can also affect the capability of an inspection in adverse ways.

Therefore, selecting the optimal inspection frequency often involves maintaining a

balance between favorable and unfavorable results of the selection. Before selecting

an inspection frequency, the grain structure, material thickness, size, type, and

probable location of the discontinuity should be considered. As frequency increases,

sound tends to scatter from large or course grain structure and from small

imperfections within a material. Cast materials often have coarse grains and other

sound scatters that require lower frequencies to be used for evaluations of these

products. Wrought and forged products with directional and refined grain structure,

can usually be inspected with higher frequency transducers.

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Since more things in a material are likely to scatter a portion of the sound energy at

higher frequencies, the penetrating power (or the maximum depth in a material that

flaws can be located) is also reduced. Frequency also has an effect on the shape of

the ultrasonic beam. Beam spread, or the divergence of the beam from the center

axis of the transducer, and how it is affected by frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables

would also affect the ability of ultrasound to locate defects. These include pulse

length, type and voltage applied to the crystal, properties of the crystal, backing

material, transducer diameter, and the receiver circuitry of the instrument. These are

discussed in more detail in the material on signal-to-noise ratio.

Advantages of ultrasonic testing is very fast and surface defects as well as internal

defects also detected. Only disadvantage is needs expert personnel and linear

defects such as cracks which are not perpendicular to beam will not be detected.

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CHAPTER - 7

BIBLIOGRAPHY 1. American Society for Metals Hand Book for Non Destructive Testing, Volume

14.

2. ASME Hand Book for Non Destructive Testing.

3. ASME Section V – Non Destructive Testing.

4. Internet sites