In MPI, what type of defects can be most effectively detected?

In Magnetic Particle Inspection (MPI), the method is particularly effective for detecting defects that are at or near the surface of a material. This technique relies on the use of magnetic fields and ferromagnetic particles to identify surface flaws such as cracks, porosity, and laps. When a magnetic field is applied to a ferromagnetic material, any surface discontinuities will disrupt the magnetic field lines, causing the magnetic particles to accumulate at these faults.

This ability to reveal surface-breaking defects is what makes MPI a popular choice in various fields including aerospace, automotive, and manufacturing where surface integrity is critical. While MPI can provide some level of indication for subsurface defects, it is primarily designed to highlight issues that are near the surface, making it crucial for inspectors to evaluate the areas of interest accordingly.

In contrast, techniques such as ultrasonic testing or radiographic testing are better suited for locating internal or deep-seated defects. Therefore, MPI's strength lies in its capability to effectively detect surface-level defects, making the focus on surface-breaking flaws the correct answer.