Which NDT method should I use to find a crack?

It depends on where the crack is and what the material is. Surface-breaking cracks in ferromagnetic steel are found fastest and most reliably with magnetic particle testing (MT). Surface cracks in any non-magnetic material (stainless steel, aluminum, titanium) call for liquid penetrant testing (PT). Subsurface or internal cracks require a volumetric method — ultrasonic testing (UT) for depth-located flaws, or eddy current testing (ET) for shallow near-surface cracks on conductive material where speed matters.

Can radiographic testing find tight cracks?

Not reliably. Radiography detects defects by the change in radiation absorbed along the beam path, so it is excellent at revealing volumetric flaws like porosity and inclusions, which have real thickness in every direction. A tight crack has almost no thickness in the beam direction unless the beam happens to align closely with the crack plane, so RT frequently misses cracks that UT, MT, or PT would catch easily. This is why weld codes increasingly favor ultrasonic phased-array inspection over film radiography for crack-sensitive applications.

Why can't magnetic particle testing be used on stainless steel?

Magnetic particle testing works by magnetizing the part and detecting the magnetic flux leakage at a discontinuity — a phenomenon that only occurs in ferromagnetic materials. Austenitic stainless steels (the 300 series), aluminum, titanium, and most other nonferrous alloys are not ferromagnetic, so they cannot be magnetized and MT simply produces no usable signal. Liquid penetrant testing is the standard substitute for surface-breaking flaws on these materials.

What is the difference between ASNT Level II and Level III certification?

A Level II inspector is qualified to set up and calibrate equipment, perform and supervise tests, interpret results against a written procedure or code, and issue inspection reports — this is the working inspector qualification held on most job sites. A Level III is the technical authority: qualified to write and approve procedures, interpret codes and specifications, and train and examine Level I and II personnel. Level III typically requires broader examinations and more years of documented experience.

What is the main limitation of eddy current testing?

Eddy currents are strongest right at the surface and decay rapidly with depth — the skin effect — so eddy current testing has shallow effective penetration, especially at the higher test frequencies used for fine surface cracks. It is also sensitive to probe lift-off and to conductivity changes from alloy composition, heat treatment, or part geometry, which can mask real flaws or create false indications unless the equipment is carefully calibrated against known reference standards.

Engineering·13 min read·July 12, 2026

🔍 Nondestructive Testing (NDT) Methods for Materials Inspection

A practical guide to nondestructive testing — ultrasonic, radiographic, magnetic particle, dye penetrant, and eddy current testing, how each method physically works, which defects it finds best, and ASNT certification levels.

📖Materials Science & Metallurgy Handbook (Full Access)

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Why Nondestructive Testing Matters

A pressure vessel, a pipeline girth weld, an aircraft wing spar, or a crane hook can look flawless on the outside and still hide a crack, void, or inclusion that will grow under service loads until the part fails. Nondestructive testing (NDT) is the family of inspection techniques that find these hidden defects without cutting, sectioning, or otherwise damaging the part being examined. Because the component remains fully usable afterward, NDT can be applied to every unit in production rather than a sacrificial sample, and it can be repeated throughout a component's service life to track whether a known flaw is growing. Five methods dominate industrial practice: ultrasonic testing (UT), radiographic testing (RT), magnetic particle testing (MT), liquid (dye) penetrant testing (PT), and eddy current testing (ET). Each exploits a different physical principle, and each has a distinct sweet spot of defect type, material, and geometry — choosing the wrong one means missing the very defect you were looking for.

Topics covered

nondestructive testingNDTultrasonic testingradiographic testingmagnetic particle testingdye penetrant testingliquid penetrant testingeddy current testingASNT certificationweld inspectionflaw detectionmaterials inspectionASNT Level IINDT methods