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Magnetic particle testing (MPT), also known as magnetic particle inspection (MPI), is a non-destructive testing method used to detect surface and near-surface flaws in ferromagnetic materials. It is commonly employed in industries such as manufacturing, automotive, aerospace, and construction to assess the integrity of components.

Here’s how magnetic particle testing generally works:

1. Magnetization: The first step involves magnetizing the test specimen. This is typically done by creating a magnetic field in the material using a magnetic yoke, electromagnetic coil, or permanent magnets. The magnetic field aligns the magnetic particles in the material, making them sensitive to surface defects.

2. Application of magnetic particles: Once the specimen is magnetized, ferromagnetic particles are applied to the surface of the material. These particles can be either dry powders or suspended in a liquid (wet method). The particles are attracted to and accumulate at areas of magnetic flux leakage caused by surface discontinuities.

3. Indication of defects: As the magnetic particles accumulate at the surface defects, they create visible indications that can be observed by the inspector. These indications appear as clusters or lines of particles that are drawn to the areas of magnetic flux leakage, highlighting the presence of surface cracks, discontinuities, or other flaws.

4. Inspection and interpretation: The inspector examines the magnetized specimen under appropriate lighting conditions, which may include using black lights or other methods to enhance the visibility of the indications. The inspector evaluates the size, shape, and distribution of the indications to determine the nature and severity of the detected flaws.

Magnetic particle testing is particularly effective for detecting surface cracks, weld defects, fatigue cracks, and other superficial flaws in ferromagnetic materials like steel, iron, nickel, and cobalt alloys. It is relatively quick and cost-effective, making it a widely used method for quality control and inspection purposes. However, it is limited to detecting surface and near-surface defects and may not be suitable for assessing the internal integrity of components.