Why is direct current used instead of alternating current for circular magnetization to detect subsurface discontinuities?

Using direct current for circular magnetization when conducting Magnetic Particle Testing is crucial for optimal detection of subsurface discontinuities. Direct current generates a steady magnetic field, which is essential because alternating current changes direction, thereby creating a varying magnetic field. This variation can reduce the effective depth to which magnetic fields can penetrate the material being tested.

The presence of subsurface discontinuities can often be challenging to detect with alternating current due to the rapid changes in polarity causing a reduction in the effective magnetic flux and possibly masking undetected flaws. As a result, the ability to identify defects at greater depths is compromised when alternating current is employed.

In contrast, the steady nature of direct current ensures a strong and consistent magnetic field, which enhances the likelihood of detecting subsurface defects. This attribute is especially important in situations where the depth of potential flaws is significant, as it allows for a more reliable assessment of material integrity.

By choosing direct current, testers can ensure they maintain maximum sensitivity to discontinuities, thereby leading to safer and more effective inspections.