Non-destructive testing (NDT) is the examination of an object or material with technology that does not affect its future usefulness. It is an approach to test the weld that involves in evaluating the weld without causing physical damage.
A variety of non-destructive testing methods have been developed to evaluate materials, components and welded joints. The following five basic methods are commonly used to examine finished welds.
● Visual Inspection (VT)
● Dye-penetrant inspection or Liquid penetration inspection (LPI).
● Magnetic particle inspection (MPI)
● Radiographic inspection (RT)
● Ultrasonic inspection (UT).
1. Visual Inspection
Visual inspection is the oldest and most common form of NDT. It is most widely used welding inspection method. This inspection procedure may be greatly enhanced by the use of appropriate combinations of magnifying instruments, borescopes, light sources, video scanners and other similar devices.
The use of optical aids for visual inspection is beneficial and recommended. Optical aids magnify defects that cannot be seen by the unaided eye and also permit visual inspection in inaccessible areas.
Common defects such as spatter, excessive built-up, incomplete slag removal, lack of root penetration, distortion, undercut and surface cracks can be detected by a visual inspection of the welds. Cracks may also be detected by hitting the casting with a mallet and listening to the quality of the tone.
2. Magnetic Particle Inspection
Magnetic particle inspection is a method of detecting invisible cracks and other defects in ferromagnetic materials such as iron and steel. It is not applicable to non-magnetic materials.
The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected. If a defect is present, the magnetic lines of force will be disturbed and opposite poles will exist on either side of the defect. The magnetized particles form a pattern, ås shown in Figure 2.46, in the magnetic field between opposite poles. This pattern known as “indication” assumes the approximate shape of the surface projection of the defect.
In this experiment, commercially available magnetic powder manufactured for NDT inspection will be used. A strong U shape magnet will be used to provide the necessary magnetic field at the inspected area.
This test is used to detect cracks, porosity and inclusions in the welding. It is mainly used for testing ferromagnetic materials (those that can be magnetized). Magnetic particle inspection can detect the surface and near surface defects.
Advantages:
1. Large surface areas of complex parts can be inspected rapidly.
2. The test can detect surface and subsurface flaws.
3. Surface preparation is less critical than it is in penetrant inspection.
4. Equipment costs are relatively low.
Limitations:
1. Only ferromagnetic materials can be inspected.
2. A proper alignment of magnetic field and defect is critical.
3. It requires relatively smooth surface.
4. Paint or other nonmagnetic coverings adversely affect sensitivity.
3. Dye-Penetrant Inspection or Liquid Penetrant Inspection
Penetrant inspection is used on non-porous metal and non-metallic components to find material discontinuities that are open to the surface and may not be evident to normal visual inspection. The dye-penetrant method is frequently used for the detection of surface breaking flaws in non-ferromagnetic materials. The basic purpose of penetrant inspection is to increase the visible contrast between discontinuity and its background.
Penetrant solution is applied to the surface of a pre-cleaned component. The liquid is pulled into surface-breaking defects by capillary action. Excess penetrant material is carefully cleaned from the surface. A developer is applied to pull the trapped penetrant back to the surface where it is spread out and it forms an indication. The indication is much easier to see. than actual defect.
Penetrant inspection detects defects such as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat treat cracks. This inspection also indicates a lack of bond between joined metals.
Advantages:
1. It is highly sensitivity (small discontinuities can be detected).
2. It ensures rapid inspection of large areas and volumes.
3. It is portable (materials are available in aerosol spray cans)
4. Cost is low (materials and associated equipment are relatively inexpensive).
Limitations:
1. The defect must be open to the surface in order to let the penetrant get into the defect.
2. Only materials with a relatively nonporous surface can be inspected.
3. Pre-cleaning is critical since contaminants can mask the defects.
4. The inspector must have direct access to the surface being inspected. 5. Surface finish and roughness can affect the inspection sensitivity.
4. Radiographic Inspection
Radiography (X-ray) is an NDT method used to inspect material and components using the concept of differential adsorption of penetrating radiation. This technique is suitable for the detection of internal defects in ferrous and non-ferrous metals, and other materials. X-rays and gamma rays are used in the radiographic test.
If a void present in the object is being radiographed, more X-rays will pass in that area and film under the part in turn will have more exposure than non-void area. Hence, the voids show as darkened area on a clear background as shown in Figure 2.47.
In this test, X-ray film or a photographic plate is placed behind and in contact with weld surface. The portion is exposed to a beam of X-rays. X-rays are produced in an X-ray tube. During exposure, X-rays penetrate through the weldment and then affect the X-ray film. After developing the film, a radiograph is obtained. This radiograph shows the nature of defect.
This test is used to detect the internal defects such as cracks, porosity, blow holes, inclusions. Blow holes, cracks, cavities and porosity appear darker than the surrounding area. This is a quicker method but the cost of test is high. The radiograph is used as a permanent record. The radiation may affect human beings.
Advantages:
1. It can be used to inspect virtually all materials.
2. Detects both surface and subsurface defects can be identified.
3. Minimum part preparation is required.
4. Information is presented pictorially and permanent record obtained.
5. It can be used for inspecting hidden areas (direct access to surface is not required).
Limitations:
1. Extensive operator training and skill are required.
2. Access to both sides of the structure is usually required.
3. Field inspection of thick section can be time-consuming.
4. Relatively expensive equipment investment is required.
5. Possible radiation hazard for personnel occurs.
5. Ultrasonic Inspection
This method is used to find internal defects by using ultrasonic sound waves. Very minute defects such as cracks, porosity, blowholes etc. can be accurately detected in castings. Sound waves can pass through solids without any absorption. It can also be reflected from a surface. Hence, ultrasonic waves are used in this test. These ultrasonic waves are produced by a transducer. The transducer can change the high frequency electrical energy into ultrasonic sound waves. It is called transmitter which can also change the ultrasonic sound waves into electrical energy.
High frequency sound waves are sent into a material by the use of a transducer. The sound waves travel through the material and they are received by the same transducer or a second transducer. The amount of energy transmitted or received and time to receive the energy is analyzed to determine the presence of flaws. Changes in material thickness and properties can also be measured.
If the work is defect free, the wave will strike the bottom of the work and return to the receiver. The striking of waves at the bottom surface, defect and top surface are indicated in the form of pip (echo) in CRT as shown in Figure 2.48.
Ultrasonic inspection techniques:
Two basic ultrasonic inspection techniques are employed such as pulse-echo and through-transmission.
(a) Pulse-Echo inspection:
This process uses a transducer to both transmit and receive the ultrasonic pulse as shown in Figure 2.48(a). The received ultrasonic pulses are separated by the time.
(b) Through-transmission inspection:
This inspection employs two transducers. Among them, one is to generate and the second one is to receive the ultrasound as shown in Figure 2.48(b). A defect in the sound path between two transducers will interrupt the sound transmission. The magnitude (the change in the sound pulse amplitude) of the interruption is used to evaluate test results.
Ultrasonic inspection is used to detect surface and subsurface discontinuities such as cracks, shrinkage cavities, bursts, flakes, pores, delaminations and porosity.
Advantages:
1. It is a fast and reliable process.
2. It is sensitive to both surface and subsurface discontinuities.
3. The depth of penetration for flaw detection or measurement is superior.
4. It is highly accurate in determining reflector position and estimating size and shape.
5. Its equipment can be highly portable or highly automated.
Limitations:
1. Surface must be accessible to transducer and couplant so that ultrasound can be transmitted.
2. Surface finish and roughness can interfere with inspection.
3. Thin parts may be difficult to inspect.
4. It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
5. Linear defects oriented parallel to the sound beam may go undetected.
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