Which method is commonly used for applying magnetic fields in MPI?

The application of magnetic fields in Magnetic Particle Inspection (MPI) primarily utilizes electromagnetic coils or yokes. This method is effective in establishing a magnetic field in ferromagnetic materials. Electromagnetic coils can be used to either induce a magnetic field when an electric current is passed through them or to create a static field with a yoke, which is particularly useful in inspecting large components or when portability is essential.

This method provides flexibility and control in adjusting the magnetic field strength, which is crucial for detecting surface and near-surface discontinuities within a variety of materials. The ability to apply different polarities and adapt the configuration of the magnetic field enhances the sensitivity of the inspection.

Other methods mentioned, such as heat induction, are not standard means for applying magnetic fields in MPI and typically pertain to processes affecting material properties rather than directly creating magnetic fields for inspection purposes. Permanent magnets have limited scope compared to the versatility of electromagnetic coils and yokes, as they cannot be adjusted or varied once set. Ultrasonic testing devices operate on a completely different principle, using sound waves rather than magnetic fields for detection and evaluation of material integrity.