Magnetic Particle Testing (MPT) – Non-Destructive Testing (NDT) Services

Magnetic Particle Testing (MPT) – Complete Guide

1. What is MPT?

Magnetic Particle Testing (MPT), also called Magnetic Particle Inspection (MPI), is a non-destructive testing (NDT) method used to detect surface and near-surface defects in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys.

The method relies on the principle that when a ferromagnetic material is magnetized, magnetic flux leakage occurs at defects, and these leaks can attract fine magnetic particles, making defects visible.

2. Principle of MPT

• Magnetization: The material is magnetized using an electric current or permanent magnet.

• Flux Leakage: Discontinuities like cracks distort the magnetic field, causing flux leakage.

• Particle Attraction: Magnetic particles (dry or suspended in liquid) accumulate at the flux leakage, forming a visible indication of the defect.

Key Formula:
Magnetization can be circular (around a conductor) or longitudinal (along the length of the part), depending on defect orientation.

3. Equipment Used in MPT

1. Magnetizing Devices:

   • AC/DC yokes

   • Coil or prods

   • Permanent magnets

2. Magnetic Particles:

   • Dry Powder: For field inspections

   • Wet Suspension: Fluorescent particles in a liquid carrier

3. Auxiliary Equipment:

   • Ultraviolet (UV) lamp for fluorescent particles

   • Demagnetizer (to remove residual magnetism)

   • Cleaning and drying stations

4. Procedure

1. Surface Preparation: Clean the part of oil, grease, or dirt.

2. Magnetization: Apply AC or DC current to magnetize the part.

3. Application of Particles:

   • Sprinkle dry magnetic powder or apply wet suspension.

4. Inspection: Observe areas where particles accumulate; these indicate defects.

5. Demagnetization: Remove residual magnetism if necessary.

6. Cleaning & Reporting: Clean the part and document results with photos and reports.

5. Types of Magnetization

• Longitudinal: Magnetic field along the length of the part; detects transverse cracks.

• Circular: Magnetic field around a circular object; detects longitudinal cracks.

• Residual Magnetism: Uses remaining magnetism in the material.

6. Applications of MPT

• Detecting surface cracks in welded joints, shafts, gears, rails.

• Aerospace: Inspecting turbine blades, landing gear.

• Automotive: Engine components, transmission parts.

• Construction & Manufacturing: Pressure vessels, pipelines, structural steel.

• Quality assurance and preventive maintenance.

7. Advantages

• Highly sensitive to surface and near-surface defects.

• Fast and cost-effective.

• Can inspect complex shapes and large components.

• Portable equipment allows field inspection.

8. Limitations

• Only works on ferromagnetic materials.

• Cannot detect very deep internal defects.

• Requires clean and accessible surfaces.

• Some defects may be missed if orientation is not aligned with the magnetic field.

9. Safety Considerations

• Avoid exposure to strong magnetic fields if you have implants.

• Wear gloves and eye protection when using magnetic powders.

• Handle fluorescent particles carefully under UV light.

10. Standards & Codes

• ASTM E1444 / E709 – Standard for magnetic particle inspection.

• ASME Section V – NDT procedures for pressure vessels.

• ISO 9934 – Non-destructive testing standards for magnetic particle inspection.

11. Summary

Magnetic Particle Testing is a reliable, fast, and widely used NDT method for ensuring the structural integrity of ferromagnetic components. It helps prevent failures, ensures safety, and supports quality compliance across industries.