Ndt Process

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Destructive testing

In destructive testing, or (Destructive Physical Analysis DPA) tests are carried out to the specimen's failure, in

order to understand a specimen's structural performance or material behaviour under different loads. These tests

are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive

testing. Destructive testing is most suitable, and economic, for objects which will be mass-produced, as the costof destroying a small number of specimens is negligible. It is usually not economical to do destructive testing

where only one or very few items are to be produced (for example, in the case of a building). Analyzing and

documenting the destructive failure mode is often accomplished using a high-speed camera recording

continuously (movie-loop) until the failure is detected. Detecting the failure can be accomplish using a sound

detector or stress gauge which produces a signal to trigger the high-speed camera. These high-speed cameras

have advanced recording modes to capture almost any type of destructive failure. After the failure the high-speed

camera will stop recording. The capture images can be played back in slow motion showing precisely what happen

before, during and after the destructive event, image by image.

Some types of destructive testing:

Stress tests

Crash tests

Hardness tests

Metallographic tests

Benefits of Destructive Testing (DT)

Verifies properties of a material

Determines quality of welds

Helps you to reduce failures, accidents and costs

Ensures compliance with regulations

Hardness

Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when

a force is applied. Macroscopic hardness is generally characterized by strong intermolecular bonds, but

the behavior of solid materials under force is complex; therefore, there are different measurements of

hardness: scratch hardness, indentation hardness, and rebound hardness.

Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness,

viscoelasticity, and viscosity.

1. Scratch hardness

Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic

deformation due to friction from a sharp object. The principle is that an object made of a harder material

will scratch an object made of a softer material. When testing coatings, scratch hardness refers to the

force necessary to cut through the film to the substrate. The most common test is Mohs scale, which is

used in mineralogy. One tool to make this measurement is the sclerometer.

Another tool used to make these tests is the pocket hardness tester. This tool consists of a scale arm

with graduated markings attached to a four wheeled carriage. A scratch tool with a sharp rim is mounted

at a predetermined angle to the testing surface. In order to use it a weight of known mass is added to

the scale arm at one of the graduated markings, the tool is then drawn across the test surface. The use


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of the weight and markings allows a known pressure to be applied without the need for complicated

machinery.

2. Indentation hardness

Indentation hardness measures the resistance of a sample to material deformation due to a constant

compression load from a sharp object; they are primarily used in engineering and metallurgy fields. Thetests work on the basic premise of measuring the critical dimensions of an indentation left by a

specifically dimensioned and loaded indenter.

Common indentation hardness scales are Rockwell, Vickers, Shore, and Brinell.

1. Vickers hardness test

The Vickers hardness test was developed as an alternative to the Brinell method to measure the

hardness of materials. The Vickers test is often easier to use than other hardness tests since the required

calculations are independent of the size of the indenter, and the indenter can be used for all materials

irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe

the questioned material's ability to resist plastic deformation from a standard source. The Vickers test

can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness

given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH).

The hardness number can be converted into units of pascals, but should not be confused with a pressure,

which also has units of pascals. The hardness number is determined by the load over the surface area

of the indentation and not the area normal to the force, and is therefore not a pressure.

Vickers test scheme


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2. Brinell Hardness Test

The oldest of the hardness test methods in common use today, the Brinell test is frequently used to

determine the hardness of forgings and castings that have a grain structure too course for Rockwell or

Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and

ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force

independent as long as the ball size/test force relationship is the same.

All Brinell tests use a carbide ball indenter. The test procedure is as follows:

The indenter is pressed into the sample by an accurately controlled test force.

The force is maintained for a specific dwell time, normally 10 - 15 seconds.

After the dwell time is complete, the indenter is removed leaving a round indent in the sample.

The size of the indent is determined optically by measuring two diagonals of the round indent

using either a portable microscope or one that is integrated with the load application device.

The Brinell hardness number is a function of the test force divided by the curved surface area of

the indent. The indentation is considered to be spherical with a radius equal to half the diameterof the ball. The average of the two diagonals is used in the following formula to calculate the

Brinell hardness.

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the

sample gets harder. Tables are available to make the calculation simple. A typical Brinell hardness is

specified as follows:

356HBW

Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Note- Previous

standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.


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Nondestructive testing

Nondestructive testing or Non-destructive testing (NDT) is a wide group of analysis techniques used in

science and industry to evaluate the properties of a material, component or system without causing

damage. The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and

Nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT

does not permanently alter the article being inspected, it is a highly valuable technique that can save

both money and time in product evaluation, troubleshooting, and research. Common NDT methods

include ultrasonic, magnetic-particle, liquid penetrant, radiographic, remote visual inspection (RVI),

eddy-current testing, and low coherence interferometry.

NDT methods may rely upon use of electromagnetic radiation, sound, and inherent properties of

materials to examine samples. This includes some kinds of microscopy to examine external surfaces in

detail, although sample preparation techniques for metallography, optical microscopy and electron

microscopy are generally destructive as the surfaces must be made smooth through polishing or the

sample must be electron transparent in thickness. The inside of a sample can be examined with

penetrating electromagnetic radiation, such as X-rays or 3D X-rays for volumetric inspection. Sound

waves are utilized in the case of ultrasonic testing. Contrast between a defect and the bulk of the sample

may be enhanced for visual examination by the unaided eye by using liquids to penetrate fatigue cracks.

One method (liquid penetrant testing) involves using dyes, fluorescent or non-fluorescent, in fluids for

non-magnetic materials, usually metals. Another commonly used method for magnetic materials

involves using a liquid suspension of fine iron particles applied to a part while it is in an externally applied

magnetic field (magnetic-particle testing). Thermoelectric effect (or use of the Seebeck effect) uses

thermal properties of an alloy to quickly and easily characterize many alloys. The chemical test, or

chemical spot test method, utilizes application of sensitive chemicals that can indicate the presence ofindividual alloying elements. Electrochemical methods, such as electrochemical fatigue crack sensors,

utilize the tendency of metal structural material to oxidize readily in order to detect progressive damage.

1. Dye penetrant inspection

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing (PT), is

a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-

porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous

materials and ferrous materials, although for ferrous components magnetic-particle inspection is often

used instead for its subsurface detection capability. LPI is used to detect casting, forging and weldingsurface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-

service components.

1. Section of material with a surface-breaking crack that is not visible to the naked eye.

2. Penetrant is applied to the surface.

3. Excess penetrant is removed.

4. Developer is applied, rendering the crack visible


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Inspection steps

1. Pre-cleaning:

The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep

penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may includesolvents, alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean

surface where any defects present are open to the surface, dry, and free of contamination. Note that if

media blasting is used, it may "work over" small discontinuities in the part, and an etching bath is

recommended as a post-blasting treatment.

Application of the penetrant to a part in a ventilated test area.

2. Application of Penetrant:

The penetrant is then applied to the surface of the item being tested. The penetrant is allowed "dwell

time" to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon thepenetrant being used, material being tested and the size of flaws sought. As expected, smaller flaws

require a longer penetration time. Due to their incompatible nature one must be careful not to apply

solvent-based penetrant to a surface which is to be inspected with a water-washable penetrant.

3. Excess Penetrant Removal:

The excess penetrant is then removed from the surface. The removal method is controlled by the type

of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic

post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and

chemically interact with the oily penetrant to make it removable with a water spray. When using solventremover and lint-free cloth it is important to not spray the solvent on the test surface directly, because

this can remove the penetrant from the flaws. If excess penetrant is not properly removed, once the

developer is applied, it may leave a background in the developed area that can mask indications or


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defects. In addition, this may also produce false indications severely hindering your ability to do a proper

inspection.

4. Application of Developer:

After excess penetrant has been removed a white developer is applied to the sample. Several developer

types are available, including: non-aqueous wet developer, dry powder, water suspendable, and watersoluble. Choice of developer is governed by penetrant compatibility (one can't use water-soluble or

suspendable developer with water-washable penetrant), and by inspection conditions. When using non-

aqueous wet developer (NAWD) or dry powder, the sample must be dried prior to application, while

soluble and suspendable developers are applied with the part still wet from the previous step. NAWD is

commercially available in aerosol spray cans, and may employ acetone, isopropyl alcohol, or a propellant

that is a combination of the two. Developer should form a semi-transparent, even coating on the

surface.

The developer draws penetrant from defects out onto the surface to form a visible indication, commonly

known as bleed-out. Any areas that bleed-out can indicate the location, orientation and possible typesof defects on the surface. Interpreting the results and characterizing defects from the indications found

may require some training and/or experience [the indication size is not the actual size of the defect]

5. Inspection:

The inspector will use visible light with adequate intensity (100 foot-candles or 1100 lux is typical) for

visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per

centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for

fluorescent penetrant examinations. Inspection of the test surface should take place after 10 to 30

minute development time, depends of product kind. This time delay allows the blotting action to occur.

The inspector may observe the sample for indication formation when using visible dye. It is also good

practice to observe indications as they form because the characteristics of the bleed out are a significant

part of interpretation characterization of flaws.

6. Post Cleaning:

The test surface is often cleaned after inspection and recording of defects, especially if post-inspection

coating processes are scheduled.

Advantages and disadvantages

The main advantages of DPI are the speed of the test and the low cost. Disadvantages include thedetection of only surface flaws, skin irritation, and the inspection should be on a smooth clean surface

where excessive penetrant can be removed prior to being developed. Conducting the test on rough

surfaces, such-as "as-welded" welds, will make it difficult to remove any excessive penetrant and could

result in false indications. Water-washable penetrant should be considered here if no other option is

available. Also, on certain surfaces a great enough color contrast cannot be achieved or the dye will stain

the workpiece.

Limited training is required for the operator although experience is quite valuable. Proper cleaning is

necessary to assure that surface contaminants have been removed and any defects present are clean

and dry. Some cleaning methods have been shown to be detrimental to test sensitivity, so acid etching

to remove metal smearing and re-open the defect may be necessary.


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2. Radiographic testing

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of

inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation

(high energy photons) to penetrate various materials.

Either an X-ray machine or a radioactive source are used in a X-ray computed tomography machine as asource of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses

neutrons instead of photons to penetrate materials. This can see very different things from X-rays,

because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils.

Since the amount of radiation emerging from the opposite side of the material can be detected and

measured, variations in this amount (or intensity) of radiation are used to determine thickness or

composition of material.

The beam of radiation must be directed to the middle of the section under examination and must be

normal to the material surface at that point, except in special techniques where known defects are best

revealed by a different alignment of the beam. The length of weld under examination for each exposure

shall be such that the thickness of the material at the diagnostic extremities, measured in the direction

of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen

to be inspected is placed between the source of radiation and the detecting device, usually the film in a

light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length

of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying

densities according to the amount of radiation reaching each area. It is known as a radio graph, as

distinct from a photograph produced by light. Because film is cumulative in its response (the exposure

increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the

exposure until the film can record an image that will be visible after development. The radiograph is

examined as a negative, without printing as a positive as in photography. This is because, in printing,

some of the detail is always lost and no useful purpose is served.


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Before commencing a radio graphic examination, it is always advisable to examine the component with

one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it

may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in

which the surface irregularities (which will be visible on the radio graph) may make detecting internal

defects difficult.

3.

Ultrasonic testing

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-

15 MHz and occasionally up to 50 MHz are transmitted into materials to detect internal flaws or to

characterize materials. A common example is ultrasonic thickness measurement, which tests the

thickness of the test object, for example, to monitor pipework corrosion.

Principle of ultrasonic testing. LEFT: A probe sends a sound wave into a test material. There are two

indications, one from the initial pulse of the probe, and the second due to the back wall echo. RIGHT: A

defect creates a third indication and simultaneously reduces the amplitude of the back wall indication.

The depth of the defect is determined by the ratio D/Ep

Working: In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed

over the object being inspected. The transducer is typically separated from the test object by a couplant

(such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted with

an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.

There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection

(or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves

as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as

the back wall of the object or from an imperfection within the object. The diagnostic machine displays

these results in the form of a signal with an amplitude representing the intensity of the reflection and

the distance, representing the arrival time of the reflection. In attenuation (or through-transmission)

mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount

that has reached it on another surface after traveling through the medium. Imperfections or other

conditions in the space between the transmitter and receiver reduce the amount of sound transmitted,


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thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the

losses in the ultrasonic wave energy due to separation between the surfaces.

Advantages

1. High penetrating power, which allows the detection of flaws deep in the part.

2. High sensitivity, permitting the detection of extremely small flaws.

3.

Only one surface needs to be accessible.4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws

and the thickness of parts with parallel surfaces.

5. Some capability of estimating the size, orientation, shape and nature of defects.

6. Non hazardous to operations or to nearby personnel and has no effect on equipment and

materials in the vicinity.

7. Capable of portable or highly automated operation.

Disadvantages

1.

Manual operation requires careful attention by experienced technicians. The transducers alertto both normal structure of some materials, tolerable anomalies of other specimens (both

termed noise) and to faults therein severe enough to compromise specimen integrity. These

signals must be distinguished by a skilled technician, possibly, after follow up with other

nondestructive testing methods.[1]

2. Extensive technical knowledge is required for the development of inspection procedures.

3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to

inspect.

4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that

is properly bonded to a surface need not be removed.

5.

Couplants are needed to provide effective transfer of ultrasonic wave energy betweentransducers and parts being inspected unless a non-contact technique is used. Non-contact

techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).

6. Inspected items must be water resistant, when using water based couplants that do not contain

rust inhibitors.

Electromagnetic testing

Electromagnetic Testing (ET), as a form of nondestructive testing, is the process of inducing electric

currents or magnetic fields or both inside a test object and observing the electromagnetic response. If

the test is set up properly, a defect inside the test object creates a measurable response.

4. Eddy-current testing

Eddy-current testing uses electromagnetic induction to detect flaws in conductive materials. There are

several limitations, among them: only conductive materials can be tested, the surface of the material

must be accessible, the finish of the material may cause bad readings, the depth of penetration into the

material is limited by the materials' conductivity, and flaws that lie parallel to the probe may be

undetectable.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the test

specimen (which must be electrically conductive).The alternating current in the coil generates changing

magnetic field which interacts with test specimen and generates eddy current. Variations in the phase

and magnitude of these eddy currents can be monitored using a second 'receiver' coil, or by measuring
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changes to the current flowing in the primary 'excitation' coil. Variations in the electrical conductivity or

magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy

current and a corresponding change in the phase and amplitude of the measured current. This is the

basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.

However, eddy-current testing can detect very small cracks in or near the surface of the material, the

surfaces need minimal preparation, and physically complex geometries can be investigated. It is alsouseful for making electrical conductivity and coating thickness measurements.

The testing devices are portable, provide immediate feedback, and do not need to contact the item in

question. Recently tomographic notion of ECT has been explored see for example:

Another eddy-current testing technique is pulsed eddy-current testing. A major advantage of this type

of testing is that there is no need for direct contact with the tested object. The measurement can be

performed through coatings, weather sheetings, corrosion products and insulation materials.[1] This

way even high temperature inspections are possible. Compared to the conventional eddy-current

testing, pulsed eddy-current testing allows multi-frequency operation.

5. Magnetic particle inspection

Magnetic Particle Inspection (MPI) also sometimes called as Magnetic Test (MT) is a non-destructive test

method for the detection of surface and sub-surface discontinuities in ferrous materials. fatigue-crack-in-a-shaft-magpartThe test method involves application of magnetic field externally or applying electric

current through the material which in turn produces magnetic flux in the material. Simultaneously,

visible ferrous particles on sprinkled or sprayed on the test surface. The presence of a surface or near

surface discontinuities in the material causes distortion in the magnetic flux which in turn causes leakage

of the magnetic fields at the discontinuity. The magnetic particles are attracted by the surface field in

the area of the discontinuity and adhere to the edges of the discontinuity appearing the shape of the

discontinuity.

There are several methods of magnetising the test parts.


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A current flow method through contact heads, encircling coil magnetising, threaded bard magnetising

are the examples of magnetising methods on a magnetic particle bench. The most common method

utilised in general industries is the magnetic flow method using electromagnetic yoke. The particles are

often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet

(UV) light (black light). The test method using fluorescent coated particles is called as Fluorescent

Magnetic Particle Inspection or test (FMPI) and the usage of other coloured particles is termed as colourcontrast Magnetic Particle Inspection or test (MPI).

Magnetic Particle Inspection (MPI) is the economical and comparative faster non-destructive test

method used widely in Aerospace, Locomotive, automotive, power generation, nuclear, petrochemical

industries. The most common examples are testing of crank shafts, cam shafts, connecting rods, engine

gears, landing gear, bearing caps, engine blocks, motor shafts, engine bolts, nuts, washers, threaded

bars, studs, piping joints ( fabricated joints, welds) in power generation and petrochemical industries,

etc.

Magnetic particle test (MT) is very sensitive test method. It can detect tight in-service fatigue cracks in

rotating parts or creep cracks on steam piping. Magnetic Particle Inspection cannot be used for non-

ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels.

In theory, magnetic particle inspection (MPI) 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 the case of 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.

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


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reenters at the south pole. The magnetic field

spreads out when it encounters 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 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 inspection.

The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. Ifany defects on or near the surface are present, the defects will create a leakage field. After the

component has been magnetized, iron particles, 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.

Source: www.wikipedia.org

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