EM-100 - Magnetic particle inspection yoke

Magnetic Particle Inspection (MPI) is a widely used nondestructive testing technique designed to detect surface and near-surface discontinuities in ferromagnetic materials like iron, nickel, cobalt, and their alloys. This method works by introducing a magnetic field into the object being tested. If there are any cracks or flaws, they disrupt the magnetic field, causing magnetic flux leakage. To visualize these imperfections, small ferrous particles (either dry powder or suspended in a liquid) are applied to the object's surface. These particles gather at the areas of flux leakage, forming visible indications that mark the location of the defects.

MPI can use two different magnetization methods: direct and indirect. In direct magnetization, an electric current passes directly through the test object, creating a magnetic field within the material. Indirect magnetization uses an external magnetic source to magnetize the object without passing current through it. The type of electrical current used for magnetization also plays a crucial role. Alternating current (AC) is often used for detecting surface defects due to its shallow penetration. For subsurface flaws, full-wave direct current (FWDC) or half-wave direct current (HWDC) is preferred, as these currents penetrate deeper into the material, providing more comprehensive inspection.

Types of Electrical Currents Used in MPI

The selection of the right electrical current for MPI depends on the geometry of the part, the type of material, the depth of the discontinuity, and the required magnetic field penetration. Each type of current has specific characteristics that make it suitable for certain applications:

• Alternating Current (AC): Best for detecting surface-level defects, AC creates a magnetic field that remains mostly on the surface of the object due to a phenomenon called the "skin effect." The current alternates polarity rapidly (50-60 cycles per second), meaning it does not penetrate deep into the material. This limits its use for subsurface detection but makes it ideal for identifying surface discontinuities.

• Full-Wave Direct Current (FWDC): FWDC is used when deeper penetration is required to find subsurface discontinuities. Unlike AC, FWDC can magnetize thicker cross-sections of material, making it more suitable for larger or thicker parts where surface inspection is insufficient.

• Half-Wave Direct Current (HWDC): HWDC provides a balance between surface and subsurface detection. Its pulsating current helps move the magnetic particles during inspection, increasing the likelihood of detecting surface cracks. This mobility of particles, combined with deeper penetration than AC, makes HWDC effective for both surface and shallow subsurface inspection.

An AC electromagnet is most effective for detecting surface-breaking indications, while some form of DC, such as FWDC or HWDC, is better suited for identifying subsurface defects. MPI is a versatile inspection method used in industries such as aerospace, automotive, and construction, where the integrity of metal components is critical.