Which NDT method is typically used for detecting internal flaws?

• A. Magnetic Particle Testing (MT)
• B. Liquid Penetrant Testing (PT)
• C. Ultrasonic Testing (UT)
• D. Visual Inspection (VT)

Answer: (C)

Ultrasonic testing is the method that probes the interior of a material. It sends high-frequency sound waves into the specimen; when these waves reach a flaw such as a crack, void, or inclusion, part of the wave is reflected back to the detector. By measuring the travel time and strength of those echoes, you can locate and size internal flaws and even assess thickness. This interior-detection capability is what makes ultrasonics the go-to choice for identifying flaws that aren’t visible on the surface.

The other methods are primarily surface-focused. Magnetic particle testing detects surface or near-surface discontinuities in ferromagnetic materials by revealing flux leakage around defects. Liquid penetrant testing requires the defect to be open to the surface so penetrant can enter and later bleed out for visualization, which means it can only show surface-breaking defects. Visual inspection looks at the surface and cannot reveal internal inconsistencies hidden beneath.