Non Destructive tests (NDT) are tests carried out without destroying the weld joints. Non-Destructive Test plays an important role in reducing the chances of weld failure, both through their application during fabrication and through their service.
Non-destructive tests are applied to welded components to determine their suitability for the service conditions to which they will be subjected. These tests neither break nor alter the structure or appearance of the welded component. Non-destructive tests have the ability to detect invisible sub-surface defects.
The standard Non Destructive tests (NDT) methods are:
- Dye/Liquid Penetrant Test (PT)
- Magnetic Particle Test (MP)
- Ultrasonic Test (UT)
- Radiographic Test (RT)
- Eddy Current Test
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This method is used to detect discontinuities open to surfaces in non-porous materials. This test is based on the capillary action of the liquid. Capillarity or Capillary action is the action by which the surface of the liquid, where it is in contact with a solid, is elevated or depressed.
Magnetic Particle Testing
Magnetic particle testing is a method used to detect surface and subsurface discontinuities in ferromagnetic materials. Magnetic particle testing is ideal for identifying surface or near-surface discontinuities in parts and assemblies whose materials have a certain degree of magnetism, such as billets, bars, castings, forgings, fasteners, weldments, brazes, and extrusions.
Radiography is a non-destructive testing tool widely used in industry. Its unique advantage over other NDT methods is that it presents a permanent record, simple to read out. Its greatest handicap is its hazardous nature of harmful radiation and the recurring cost involved.
Radiography employs the penetrating capacity of ionizing radiation like X or Gamma rays to produce a shadow of the internal condition of a job on a recording medium
Mechanical vibrations which have a frequency higher than the audible range of the human ear viz. 20 to 20 kHz are called ultrasonic waves. Unlike electromagnetic radiation, these waves require a medium, and the velocity of propagation depends on the elasticity of the medium.
Eddy Current Testing
Eddy current tests can be carried out on all materials which conduct electricity. They have been used for crack detection, thickness measurements of metallic plates, foils, tubes, and cylinders and of nonmetallic coatings on metals, for the detection of corrosion, measurements of conductivity, the investigation of chemical compositions of metals, and so on. A great advantage of the method is that direct contact with the test piece is not necessary.