Non-Destructive Testing and Defect Evaluation
Domain 6: Non-destructive testing and defect evaluation
This domain tests method selection and the interpretation of indications against acceptance criteria. You need to know what each method can and cannot find, and how defect type, orientation and location influence detection.
Method capabilities
Visual testing finds surface discontinuities, profile and alignment problems. Penetrant testing finds surface-breaking defects in non-porous materials. Magnetic particle testing finds surface and near-surface defects in ferromagnetic materials; it is unsuitable for austenitic stainless steel. Radiography is volumetric and sensitive to porosity, slag and lack of penetration, but less reliable for planar defects aligned with the beam. Ultrasonic testing is volumetric and strong for planar defects such as lack of fusion and cracks when correctly oriented.
Acceptance criteria
ISO 5817 defines weld quality levels B, C and D for steel, with B the most stringent. Acceptance levels for NDT are set in standards such as ISO 11666 for ultrasonic testing and ISO 10675 for radiography. Evaluation compares indication type, size and location with the specified quality level, not with a generic 'good weld' idea.
Worked example
A 25 mm carbon-steel butt weld must be examined for lack of sidewall fusion. Radiography may miss a tight planar defect lying in the beam direction. Ultrasonic testing with a suitable angle probe is more appropriate; if the weld is austenitic, a dedicated technique and probe pair are needed. The reasoning is defect geometry plus material, not 'UT is always better'.
Traps
Using MT on austenitic stainless steel. Assuming PT finds buried defects. Treating a radiograph as a photograph of all defects. Applying the wrong quality level or acceptance standard.