How to Choose the Right NDT Method

Start with the defect, not the method

The single most common selection error is starting from a familiar instrument instead of the flaw. A surface-breaking fatigue crack, a buried slag inclusion, mid-wall hydrogen damage, and external corrosion-under-insulation each have a different physics signature — and no single method is strong against all four.

Surface-breaking, open-to-surface cracks are the domain of penetrant (PT) on any non-porous material and magnetic particle (MT) on ferromagnetic steel, where MT also reaches slightly sub-surface. Volumetric flaws — porosity, slag, lack of fusion, mid-wall cracking — need a through-thickness method: radiography (RT) or ultrasonics (UT / PAUT / TOFD). Wall loss and corrosion are a thickness problem, best served by UT thickness gauging, profile radiography, or screening methods like guided wave and MFL.

The four-question decision

1. Defect type and orientation. Planar flaws (cracks, lack of fusion) reflect ultrasound strongly when the beam hits them near-normal — UT and PAUT excel, and TOFD is especially good at sizing through-wall height. RT is strong on volumetric flaws but can miss tight, favourably-oriented planar cracks that lie parallel to the beam.

2. Material and surface. RT and UT work on most metals; MT only works on ferromagnetic material; ET needs an electrically conductive surface and is sensitive to lift-off; PT needs a non-porous surface clean enough to hold and reveal the penetrant. Coatings, weld cap profile and surface roughness all shift the choice.

3. Access and geometry. One-sided access rules out film RT and favours UT/PAUT or digital radiography. Thin wall, small diameter, or complex nozzles change couplant and probe selection. Radiation exclusion zones can make RT impractical in a live plant, pushing the job to PAUT.

4. Code and acceptance. The governing code often decides the method for you — ASME Section V defines the technique families and ASME VIII / B31.3 / AWS D1.1 set acceptance. Some clients mandate PAUT-in-lieu-of-RT to a written procedure; others require specific coverage that only RT can demonstrate.

Method-by-method, in one place

Each method has a dedicated guide covering principle, equipment, procedure, advantages and limitations. Use these to confirm the technique can actually see your flaw population before you write the procedure.

When two methods compete: the head-to-heads

Most real decisions come down to a pair: UT vs RT on a pressure weld, PAUT vs RT for a turnaround, MT vs PT on a casting, TOFD vs PAUT for sizing. The comparison pages put the two side by side on sensitivity, speed, cost, access and code coverage so you can defend the choice in a procedure review.

Then check cost and throughput

Two methods that both find the flaw can differ 3–5× in cost per weld once you count crew, consumables, radiation exclusion and report turnaround. PAUT carries a higher day rate but inspects faster with no exclusion zone; RT is cheaper per shot but stops other trades during shooting. Use the free calculators to put real numbers on the trade before you award the work.