What is Magnetic Particle Testing?

Magnetic Particle Testing (MPT) is a non-destructive testing technique used to detect surface and 

near-surface defects in ferromagnetic materials. It applies a magnetic field to the surface of the 

workpiece and uses magnetic particles to show the potential defect locations, helping engineers and 

technicians identify discontinuities such as cracks, inclusions, holes, etc. that may exist in the material 

or workpiece.

Principle

The basic principle of magnetic particle testing is based on the fact that ferromagnetic materials 

generate a magnetic field when they are magnetized. If there are cracks or other discontinuities in 

the material, these defects will distort the magnetic lines of force and form a leakage magnetic field. 

Sprinkle fine magnetic powder (usually iron powder or magnetic iron oxide powder) on the surface 

of the magnetized workpiece. The magnetic powder will be attracted by the leakage magnetic field and

 gather along the shape of the defect, forming magnetic marks visible to the naked eye on the surface

 of the workpiece. These magnetic marks reflect the location and shape of the defect.

Steps of Magnetic Particle Testing

1. Magnetization: Placing the workpiece in a magnetic field can be achieved by passing an electric 

current directly through the workpiece (direct magnetization) or by generating a magnetic field through

 a surrounding coil (indirect magnetization).

2. Applying magnetic powder: Sprinkle magnetic powder on the surface of the workpiece, usually

 immediately after magnetization. These magnetic powders can be dry powders or wet powders 

suspended in a liquid.

3. Inspection: Observe the surface of the workpiece through visual inspection or using tools such as 

magnifying glasses to find places where magnetic powders are concentrated. These concentrated areas 

often represent defects in the material.

4. Demagnetization and cleaning: After the inspection is completed, the workpiece may need to be 

demagnetized to remove the residual magnetic field. The workpiece is then cleaned to remove the 

magnetic powder attached to the surface.

Application Areas

Magnetic particle testing is widely used in various industries that require high reliability and safety, 

especially in situations where materials and components need to be guaranteed to be crack-free. 

Common applications include:

1. Aerospace: Detect cracks or fatigue damage in aircraft structural parts.

2. Automobile manufacturing: Used to detect defects in key components such as engine parts, 

drive shafts, gears, etc.

3. Petrochemical: Detect surface defects in pipelines, pressure vessels and welds.

4. Metallurgical industry: Used to test the quality of steel products to ensure that there are no cracks or 

inclusions.

Advantages and limitations

Advantages:

- Surface and near-surface defects of materials can be detected.

- The operation is relatively simple and the results are intuitive.

- Applicable to ferromagnetic material workpieces of various shapes.

Limitations:

- It is limited to the detection of ferromagnetic materials, and non-ferromagnetic materials such as

 aluminum and copper cannot be detected.

- Deep or very small defects may not be effectively detected.

- The surface of the workpiece needs to be well cleaned, and surface contaminants will affect the

 detection results.

Conclusion

Magnetic particle testing is a reliable and widely used non-destructive testing method for surface and 

near-surface defect detection of ferromagnetic materials. Through this technology, potential defects in

 materials can be effectively identified and located, ensuring product quality and safety. However, 

its application range and detection depth are limited, so in practical applications, it is often used in

 combination with other non-destructive testing technologies to ensure the comprehensiveness and 

accuracy of the detection.