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NDE Methods Compared
The five nondestructive examination methods — VT, PT, MT, RT, and UT — side by side: what each detects, the materials it works on, cost, and whether it leaves a record. Know these trade-offs cold for the CWI exam.
VT — Visual Testing
Key point: Most widely used and lowest-cost method; every CWI must master it. Requires good lighting.
Main limitation: Surface only — cannot see internal flaws.
| Method | Detects | Best for | Materials | Permanent record | Relative cost | Main limitation |
|---|---|---|---|---|---|---|
| VT | Surface | Surface flaws: profile, undercut, overlap, surface cracks, weld size. | Any | Limited | Low | Surface only — cannot see internal flaws. |
| PT | Surface | Surface-breaking flaws only, including tight cracks. | Any non-porous (works on non-magnetic: stainless, aluminum) | No | Low | Detects only flaws OPEN to the surface; cannot find subsurface flaws. |
| MT | Surface & slightly subsurface | Surface and near-surface cracks in ferromagnetic parts. | Ferromagnetic only (carbon steel) — NOT aluminum or austenitic stainless | Limited | Low | Only works on ferromagnetic materials; flaw must be at/near the surface. |
| RT | Volumetric (internal) | Volumetric flaws: porosity, slag inclusions. | Most | Yes | High | Radiation hazard; poor at tight planar cracks aligned with the beam. |
| UT | Volumetric & planar (internal) | Planar flaws: cracks, incomplete fusion; also thickness. | Most (needs couplant) | Limited | High | Requires a skilled operator and careful interpretation; needs a couplant. |
Rule of thumb: RT is best for volumetric flaws (porosity, slag); UT is best for planar flaws (cracks, incomplete fusion). VT, PT, and MT find surface or near-surface flaws only.
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Frequently Asked Questions
What is the difference between RT and UT?
Radiographic testing (RT) passes X-rays or gamma rays through the weld and is best for volumetric flaws such as porosity and slag, and it produces a permanent record. Ultrasonic testing (UT) reflects high-frequency sound off internal flaws and is best for planar flaws such as cracks and incomplete fusion. The rule of thumb: RT for volumetric, UT for planar.
Which NDE methods find only surface flaws?
Visual testing (VT) and liquid penetrant testing (PT) find surface flaws only. Magnetic particle testing (MT) finds surface and slightly subsurface flaws, but only in ferromagnetic materials. To find deeper internal flaws you need a volumetric method — RT or UT.
Why can't magnetic particle testing be used on aluminum?
MT relies on magnetizing the part so that flaws disrupt the magnetic field and attract iron particles. Aluminum and austenitic stainless steel are not ferromagnetic, so they cannot be magnetized this way. For those materials, use liquid penetrant (PT) for surface flaws or a volumetric method for internal flaws.
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