What type of current is used to detect subsurface defects when using a magnetic particle test?

• A. AC
• B. DC
• C. Pulsating current
• D. None

Answer: (B)

In magnetic particle testing, how deep a defect can be detected depends on how the material is magnetized. A steady direct current creates a constant magnetic field that penetrates deeper into the part, so any subsurface discontinuity disturbs the flux and produces leakage at the surface where the particles can gather. Because the field remains present, the leakage path associated with a subsurface flaw becomes visible to the inspector. Alternating current, in contrast, changes direction rapidly and tends to concentrate leakage effects at surface defects, making subsurface flaws harder to see. Pulsating current is a form of time-varying magnetization, but the stable field from direct current is generally more effective for revealing defects below the surface. Therefore, direct current is the best choice for detecting subsurface defects in magnetic particle testing.