Method Selection, Limitations, and Comparison of Methods
Selection framework
Method selection is a decision problem with constraints: material, geometry, discontinuity type and orientation, location (surface vs. internal), access, surface condition, production rate, codes/contract requirements, and cost. A candidate should be able to walk through these in order and justify a choice.
Comparison points examiners test
- Sensitivity to tight cracks: MT > PT for ferromagnetic surface cracks; PT struggles with tight, shallow, or contaminated cracks.
- Volumetric coverage: RT gives a permanent image and good for porosity; UT gives better planar flaw detection and depth sizing but requires couplant and skilled interpretation.
- Throughput: PT and MT can be automated for high volume; RT film is slow; digital RT faster.
- Safety burden: RT involves radiation safety; UT and ET are comparatively benign.
- Material limits: RT works on most materials including nonmetals; UT needs acoustic coupling; ET only conductors.
Traps
- "Most sensitive method overall" - there is no universal answer; it depends on flaw and material.
- Confusing penetration with sensitivity.
- Ignoring surface condition: rough or painted surfaces degrade PT, MT, and ET.
- Forgetting that codes may mandate a method regardless of technical superiority.
Rule of thumb
Match the method to the discontinuity orientation and location first, then filter by material and access, then by code and cost. If two methods are both capable, the code governs.
Worked example
Detecting cracks on an aluminum aircraft skin: MT is out (nonferromagnetic); PT works for surface-breaking cracks; ET is preferred for surface and subsurface cracks in conductors and is fast over large areas. Best answer: ET.