What happens to magnetic particles when a defect is present during MPI testing?

When magnetic particles are used during Magnetic Particle Inspection (MPI) testing, their behavior in the presence of a defect is critical for identifying flaws in a material. When there is a discontinuity, such as a crack or void, the magnetic field at that site is disrupted. This disruption creates a localized area of concentration where the magnetic lines of force bend or leak.

As a result, the magnetic particles, which are typically suspended in a liquid medium, are attracted to the defect site due to the changes in the magnetic field. They accumulate and form a visible indication at the location of the defect, making it easier for inspectors to identify and analyze the flaw. This accumulation provides a clear visual representation of the defect, which is essential for both evaluating the integrity of the material and ensuring safety and reliability in engineering applications.

The other provided options do not accurately describe the behavior of magnetic particles during MPI in the presence of defects. Magnetic particles do not disperse evenly, dissolve, or lose their magnetic charge when a defect is present. Instead, their attraction to the defect is what makes MPI such an effective non-destructive testing method.