What role does the magnetic field play in MPT?

The magnetic field plays a critical role in Magnetic Particle Testing (MPT) by causing magnetic particles to cluster around discontinuities or defects in the test material. When a ferromagnetic material is magnetized, any surface or near-surface flaw that disrupts the magnetic field will create leakage fields. The magnetic particles, which are often coated with a fluorescent dye for visibility, are attracted to these leakage fields. As the particles accumulate at the site of the defect, they form an indication that can be visually inspected, revealing information about the integrity of the material.

In contrast, other options are not representative of the central function of the magnetic field in MPT. Heating the material, cooling the inspection surface, or aligning molecular structures do not directly contribute to the detection of flaws using magnetic particles. The primary diagnostic capability of MPT hinges on the attraction and clustering of particles around defects, making option B the most accurate representation of the magnetic field's role in this testing method.