NDE methods on Part A: which method finds which discontinuity

Part A is closed book, so the limits of each method have to be in your head. Visual examination (VT) is covered in the Part A exam guide. This page covers the methods that back it up.

Surface methods: PT and MT

Both find discontinuities at or near the surface. Neither sees deep inside a weld.

• Penetrant testing (PT). A liquid is drawn into surface-open flaws, then pulled back out by a developer so you can see it. It works on almost any nonporous material, including stainless steel and aluminum. It only finds flaws that are open to the surface, and poor precleaning hides them.
• Magnetic particle testing (MT). The part is magnetized, and iron particles gather where a flaw leaks the field. It finds flaws at the surface and just below it, but only on ferromagnetic material. It is most sensitive to flaws lying across the field, which is why the part is magnetized in two directions.

The exam trap is material. MT does not work on austenitic stainless steel or aluminum, so a surface crack there points to PT.

Volumetric methods: RT and UT

Both look inside the weld.

• Radiographic testing (RT). X-rays or gamma rays pass through the part onto film or a digital detector, and an image quality indicator shows whether the image is sensitive enough. It is good at volumetric flaws such as porosity and slag and leaves a permanent record. It is weaker on planar flaws unless they line up with the beam, it usually needs access to both sides, and it brings radiation controls.
• Ultrasonic testing (UT). Sound goes into the part through a couplant and reflects from interfaces such as a crack face. Angle beams suit welds, because the beam can strike the fusion faces. It is strong on planar flaws, needs one side only and gives depth information. It needs a skilled operator, and coarse-grained material such as austenitic stainless welds is difficult.

Eddy current (ET) is the method you meet least often. Know its place, but spend your time on the four above.

Choose a method in three questions

1. Where is the flaw likely to be? At the surface, near it, or deep inside.
2. What shape is it? Planar or volumetric.
3. What is the material and joint? Magnetic or not, thickness, access and geometry.

Then rule out what cannot work before you pick what can. MT on stainless and PT for a subsurface flaw are the two eliminations that settle most scenario questions.

This is a study guide, not a rule. The contract documents and the governing code set the method, the extent and the acceptance criteria, and the inspector confirms they were followed. Most codes also expect visual examination first.

Practice it

• Run the NDE methods quiz: 18 original questions on which method finds which flaw, material limits and principles, each answer explained.
• Take the free 10-question Part A check and look at how you do on the NDE fundamentals domain.
• Work the public Part A sample. It covers reading welding and examination symbols, which is the other half of NDE on Part A.
• For each miss, write which method, which material or joint, and whether you mixed up surface and volumetric or planar and volumetric. Then redo the three questions above without the choices in front of you.

To see what each discontinuity looks like before you pick a method, use the weld discontinuities guide: one page and one drawing per type.

For the whole exam, use the CWI practice overview. To learn the symbol side, read how to read welding symbols.

Related exam prep

• NDT methods compared on Angle Beam: how UT, MT, PT and the other methods differ, for Level II candidates.

Sources

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Checked 23 October 2026.

• AWS Certified Welding Inspector, AWS: the certification program.

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