Nondestructive Examination Methods
Method selection and capability
Nondestructive examination (NDE) must detect the expected discontinuity without damaging the component. Visual testing (VT) is the first line and requires good access, lighting, and surface condition. Penetrant testing (PT) finds surface-breaking flaws in nonporous materials. Magnetic particle testing (MT) finds surface and near-surface flaws in ferromagnetic steel but is unsuitable for austenitic stainless steel. Radiographic testing (RT) detects volumetric discontinuities such as porosity and slag; it is less reliable for tight planar cracks unless orientation is favorable. Ultrasonic testing (UT) is strong for internal planar flaws and thickness measurement; phased array and time-of-flight diffraction improve imaging and sizing. Eddy current testing is sensitive to surface and near-surface flaws in conductive materials.
Personnel qualification, equipment calibration, procedure qualification, surface preparation, and acceptance criteria are essential. Coverage and orientation are recurring themes: a discontinuity must be open to the surface for PT, magnetizable for MT, and aligned with the sound beam for UT.
Traps
- Using PT to find subsurface lack of fusion.
- Applying MT to austenitic stainless steel or aluminum.
- Assuming RT will reliably detect a tight crack parallel to the radiation beam.
- Forgetting that UT needs couplant and access.
- Comparing indications without the correct acceptance criteria or calibration.
Worked rule
Start with the discontinuity and location: surface-breaking crack in carbon steel - VT, MT, or PT; internal volumetric porosity - RT or UT; internal planar crack - UT. If the method cannot physically interact with the flaw, do not expect a valid result.