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
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
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: 2 of 83
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 /
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
Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095
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Rev: 01 Date: 09-11-2004 Pages: 70 of 83
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
Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095
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Rev: 01 Date: 09-11-2004 Pages: 71 of 83
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
Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095
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Rev: 01 Date: 09-11-2004 Pages: 72 of 83
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.
Category: IPCL Module No MECHANICAL NC: Training Module IPCLDSMEC095