Choosing the Right NDT Method: UT, MT, PT, ET, and VT Compared

Nondestructive Testing isn't a single technique — it's a family of methods, and picking the wrong one for the job either misses the defect you're looking for or costs more than it should. The right choice comes down to a few questions: what material are you inspecting, what kind of defect are you looking for, is the flaw likely to be on the surface or buried inside, and what does your governing specification require?

Internal flaws — Ultrasonic Testing (UT). When a defect could be below the surface — a void in a casting, a crack under a fastener head, delamination in a bonded structure — Ultrasonic Testing is usually the first method to reach for. UT sends high-frequency sound waves into the material and reads back the echoes, which also makes it the standard method for precision thickness measurement on parts prone to corrosion or erosion.

Surface cracks on ferromagnetic metal — Magnetic Particle Testing (MT). For steel, iron, and other ferromagnetic alloys, Magnetic Particle Testing is fast, inexpensive, and extremely sensitive to fatigue cracks and other surface or near-surface discontinuities. It doesn't work on aluminum, titanium, or other non-magnetic materials — that's where Liquid Penetrant Testing takes over.

Surface defects on non-magnetic materials — Liquid Penetrant Testing (PT). PT relies on capillary action rather than magnetism, which makes it the go-to method for aluminum aircraft skins, stainless steel, and other non-porous, non-ferrous materials. It's highly sensitive to fine, surface-breaking cracks, but unlike UT and ET, it can't detect anything below the surface.

Fast, portable inspection of conductive metals — Eddy Current Testing (ET). ET uses electromagnetic induction instead of sound or dye, so there's no couplant or wet process involved. That makes it well suited to quick, repeatable field inspection of fastener holes, aircraft skins, and tubing — often the preferred method when a part needs to be checked in place rather than pulled for lab testing.

The baseline — Visual Testing (VT). Every inspection program starts here. VT catches obvious damage, corrosion, and surface condition issues before more specialized methods are even considered, and it's often what determines whether a deeper inspection is warranted at all.

In practice, most inspection programs use more than one method — a borescope check that flags an area for eddy current follow-up, or an ultrasonic thickness survey paired with penetrant testing on a suspect weld. Lion NDT supports all five core methods, so method selection is based on what the part and the specification actually require.