Magnetic Particle Inspection Level 2 Practice Exam

What is the preferred MT method when testing for fine shallow surface cracks?

1. Dry AC

2. Dry DC

3. Wet AC

4. Wet DC

The correct answer is: Wet AC

The preferred method for detecting fine shallow surface cracks in Magnetic Particle Inspection (MT) is Wet AC. This method utilizes a suspension of magnetic particles in a liquid carrier that enhances the visibility of small surface discontinuities. The wet method allows the magnetic particles to spread more evenly and conform to the surface of the test piece, which provides better coverage and access to tiny cracks that may be present. The alternating current (AC) used in this method generates a magnetic field that is primarily effective at detecting surface flaws. AC induces a magnetic field that aligns the particles parallel to the surface, making it easier for the particles to cluster at the site of a discontinuity like a crack, thereby highlighting it significantly. In contrast, other methods like dry techniques may not provide the same level of sensitivity to superficial cracks due to the limitations of particle adherence and visibility. Using direct current (DC), while useful for detecting subsurface defects, is less effective for shallow surface cracks since it can penetrate deeper into the material, missing the very cracks that the wet AC method is particularly suited to find.