12.3 Method Selection & Comparison Matrix

Choosing the Right Method

At Level III, method selection is one of the most heavily tested skills because roughly 40 percent of the Basic outline is application of NDT methods. The task is a matching problem: identify the likely discontinuity type, orientation, and location, then pick the method whose physics detects it, and finally filter by practical constraints. The physics of VT, PT, MT, ET, UT, and RT is taught in Chapters 8 through 11, and LT, AE, and IRT in this chapter; here we compare rather than re-teach.

The selection variables

1. Discontinuity type and location. Is it surface-breaking, near-surface, subsurface, or fully internal? Is it planar (crack, lack of fusion, lamination) or volumetric (porosity, slag, void)? Planar flaws favor UT and, at the surface, PT/MT; volumetric flaws favor RT.
2. Material. MT needs a ferromagnetic material; ET needs an electrical conductor; PT needs a nonporous surface (it fails on porous or very rough parts because background bleeds); IRT/shearography/microwave shine on composites and dielectrics.
3. Geometry and access. RT generally needs two-side access (source one side, detector the other); UT needs one-side access plus couplant; large areas favor IRT, MFL, or AE for coverage.
4. Portability and setting. VT, PT, MT, and portable UT go to the field easily; RT needs radiation control and immersion UT needs a tank, pushing them toward fixed facilities.
5. Sensitivity, cost, throughput, record, and safety. Tighter flaw sizes and permanent records raise cost and slow throughput; RT produces a permanent image but demands the strictest radiation safety; PT raises chemical-handling concerns.

Capabilities and Limitations Matrix

Reading the matrix

Surface reach: VT/PT/MT/ET find surface and near-surface flaws, while UT/RT go deep and LT/AE/IRT answer specialized questions (tightness, activity, temperature). Volumetric versus planar: RT is preferred for volumetric flaws and gives a permanent image but can miss a tight, unfavorably oriented planar crack; UT is preferred for planar flaws and thickness but needs skill and couplant. Only MT/ET are constrained by material class; only RT carries a heavy radiation-safety burden.

Selection Logic and Common Traps

A reliable decision path: (1) perform VT first on essentially every job, because it is fast, cheap, non-contact, and screens obvious conditions; (2) for a surface crack on any metal, choose PT, or MT if the part is ferromagnetic (MT is faster and finds slightly subsurface flaws); (3) for conductive tubing or high-speed sorting without couplant, choose ET; (4) for internal volumetric flaws such as porosity or slag, choose RT; (5) for internal planar flaws, laminations, or thickness/wall loss, choose UT; (6) for a through-leak in a sealed system, choose LT, escalating to helium mass spectrometry for very low rates; (7) to monitor a whole vessel for growing flaws under load, choose AE; (8) for large-area subsurface disbond in composites, choose IRT; and for steel tank-floor thinning, choose MFL (Chapter 9).

Common traps the exam exploits: choosing RT for tight planar cracks (UT is better); choosing PT on a porous or very rough surface (background masks indications); choosing MT on aluminum or stainless (not ferromagnetic, so PT or ET applies); forgetting that process history such as grinding or hardening can create the very flaw you are hunting; and skipping VT before a costly specialized method. The best answer usually names a primary method matched to the flaw and, where sensible, a complementary confirming method.

A worked selection example

Consider a thin aluminum panel suspected of fatigue cracks around fastener holes. Walk the variables: the flaw is surface-breaking, the material is non-ferromagnetic but conductive, and access is one-sided. That immediately rules out MT (needs ferromagnetism) and makes RT awkward (crack orientation and access). Two strong candidates remain: PT, a low-cost first choice that reveals surface-breaking cracks on nonporous aluminum, and ET, which finds surface and near-surface cracks in a conductor at high speed without couplant and can inspect around and beneath fastener heads. A practical program often screens with PT and confirms or sorts with ET. The lesson is that the same defect can have more than one defensible method; the exam rewards the choice that best fits the stated material, access, and productivity constraints, not a memorized single answer.

Putting it together

Every selection question can be answered by moving down the same checklist: flaw type and depth, material class, geometry and access, portability, sensitivity, cost and speed, record, and safety. Read what the stem emphasizes, because that emphasis is the deciding filter. If it stresses a permanent image, lean RT; if it stresses one-side access on thick steel, lean UT; if it stresses a sealed system, lean LT; if it stresses monitoring under load, lean AE; and if it stresses a large composite area, lean IRT.