Non-Destructive Testing
Meaning ~ Inspection techniques that evaluate the internal structure of manufactured materials without causing damage represent a formal verification category.
Magnetic Particle Testing
Meaning
Detection of flaws in ferromagnetic materials relies on the use of magnetic fields and iron particles to reveal discontinuities. The process of magnetic particle testing involves magnetizing the part and applying fine metal powder to the surface. If a crack exists it creates a magnetic leakage field that attracts the powder.
This method is faster and more sensitive than liquid penetrant testing for steels and irons.
Physical Principle
Distortion of the magnetic lines of force indicates the presence of a structural flaw. During magnetic particle testing the orientation of the field must be perpendicular to the expected crack direction. Using a yoke or a prods setup allows the technician to control the direction of the flux.
Proper magnetization is verified using a pie gauge or a shim.
Material Compatibility
Application of the method is restricted to materials that can be strongly magnetized. Aluminum and most stainless steels cannot undergo magnetic particle testing because they do not support a magnetic field. Heat treatment and alloying elements change the magnetic properties of the base metal.
Testing Limit
Surface roughness and thick coatings interfere with the movement of the particles. While magnetic particle testing can find flaws just below the surface, it cannot detect deep internal voids. Demagnetizing the part after the test is often a mandatory step to prevent interference with later machining or welding.
Residual magnetism can attract metallic debris and cause wear in moving parts.