Final answer:
In magnetic particle testing, AC is most effective for surface defects due to strong magnetic fields at the surface, while DC is used for subsurface defects because of its deeper penetration into the material.
Step-by-step explanation:
In magnetic particle testing, the type of current used is significant in detecting different kinds of defects in materials. For detecting surface defects, alternating current (AC) is most effective because it does not penetrate deeply into the material, resulting in strong magnetic fields at and near the surface. On the other hand, to detect subsurface defects, direct current (DC) or rectified current is used because it provides deeper penetration into the material, facilitating the identification of flaws that are not immediately at the surface but are still close enough to cause disruption in the magnetic field.
It is essential to choose the correct type of current when conducting magnetic particle testing to ensure that all potential defects are identified, whether they are on the surface or subsurface. The efficiency of detection is maximized by aligning the type of current with the defect depth one is trying to identify.