General NDT Knowledge, Codes, and Other Methods Interface
Method selection and interfaces
Penetrant testing detects surface-breaking discontinuities in nonporous materials. It cannot find subsurface defects; that is the province of radiography, ultrasonics, or magnetic particle (for surface and near-surface in ferromagnetic materials). Candidates must know when penetrant is inappropriate: porous materials, rough surfaces that trap penetrant, and applications where residue is unacceptable.
Codes and standards landscape
Know the roles of consensus standards (ASTM, ISO, ASME) and how they are invoked. ASME Section V covers method requirements; Section VIII and B31 codes carry acceptance criteria. Customer specifications may add requirements.
Comparison traps
- Penetrant versus magnetic particle: MP can detect near-surface defects in ferromagnetic materials; PT is limited to surface-breaking and works on nonmagnetic materials.
- Penetrant versus visual: PT amplifies visibility of tight surface cracks that visual inspection misses.
- Penetrant versus eddy current: ET detects surface and near-surface in conductive materials without consumables.
Worked example
A stainless steel weld is suspected of having subsurface porosity and surface cracks. Penetrant finds surface-breaking cracks only; subsurface porosity requires radiography or ultrasonics. The correct answer combines PT for surface and RT or UT for volumetric.
Rule of thumb
Match the method to the defect location, material, and geometry. Surface-breaking and nonporous: PT. Surface and near-surface in ferromagnetic: MT. Volumetric: RT or UT. Surface in conductive: ET.