Magnetic Particle Inspection (MPI) is one of the most common non-destructive testing (NDT) techniques for detecting surface and near-surface discontinuities in ferromagnetic materials. MPI equipment finds its use in industries like aerospace, automotive, construction, and manufacturing, where material integrity becomes a serious concern. But how does the MPI machine operate? This blog will look into the working principle of MPI machines, parts of an MPI machine, and applications for such machines.
What is Magnetic Particle Inspection (MPI)?
This is a non-destructive testing technique that detects defects such as cracks, seams, laps, and inclusions in ferromagnetic materials like iron, nickel, and cobalt. The principle of using magnetic particles for inspection consists of magnetizing an object and then applying small magnetic particles to it so that defects may become evident. MPI is particularly effective for detecting surface and shallow subsurface defects and is, therefore, one of the most preferred methods for safety assurance and quality control.
How Does an MPI Machine Work?
The principle of working of MPI machine lies in the science of magnetism and magnetic flux leakage.Here is the step-wise process:
- Magnetization Of The Material
The first step in MPI is to magnetize the test material. The magnetization of the test materials is carried out in an MPI machine that provides an external magnetic field. The material can be magnetized by any of the following methods:
Direct Magnetization: Passing electric current directly through the material.
Indirect Magnetization: Using a magnetic yoke or coil in order to create a magnetic field around the material.
Any magnetization of the material creates inside it magnetic lines of force (flux lines). If there are no defects in the material, these flux lines would pass uniformly through the material.
Introduction of Magnetic Particles
Once the substance is magnetized, dry or liquid-suspended magnetic particles will coat the material surface. These magnetic particles, made of iron oxide or iron carbide, are extremely sensitive to any external magnetic fields.
- Detection of Defects
Any surface or near-surface discontinuity (i.e., a crack) would disturb the otherwise uniform magnetic flux lines. Consequently, the flow of magnetic flux lines is leaked off at the defect location. The magnetic particles will gather at the site of these flux leakages, which in turn gives a visible cue to the presence of a defect. The shape and size of the accumulation of the particles will assist in defining the characteristics and location of the flaw.
- Inspection and Interpretation
The inspector looks for material flaws using the right lighting conditions (the use of UV light for fluorescent particles being quite common). The indications are then interpreted to assess the severity and type of defect.
- Demagnetization (Optional)
In case of inspection, a material can be demagnetized to remove the residual magnetism. This process is important for components that will eventually be used in applications that demand sensitivity-examples being rotating machines, where the effect due to residual magnetism could cause serious troubles.
MPI Machine of the Key Components
An MPI machine has a lot of components that work together to ensure reliable and accurate testing:
Power Supply: Provides current for magnetization
Magnetizing Equipment: Yokes, coils, or prods; the equipment is used to create the magnetic field
Magnetic Particles: Dry or wet particles supplied for detecting defects.
Lighting System: UV or white light, which is used to look at the accumulations of particles.
Control Panel: Used by the operator to set current and magnetization time, among other things.
Types of the MPI Machines
These applications are applicable to only some types of MPI machines:
Portable MPI Machines: Convenient to carry, preferably used with portable applications.
Bench-Top MPI Machines: Used for testing in laboratories or workshops, delivering much higher precision.
Wet MPI Systems: Magnetic particles suspended in a liquid carrier provide much greater sensitivity than dry systems.
Dry MPI Systems: Consists of dry magnetic particles, mostly used for rough surfaces or outdoors.
Advantages of MPI Machines
High sensitivity: Detects very small surface or near-surface defects.
Fast results: Instant visual indications of defects.
Versatility: Applicable to various ferromagnetic materials.
Cost-effective: Fairly inexpensive compared to the other methods of NDT.
Limitations of MPI Machines
Material restriction: It only works on ferromagnetic materials.
Surface defects: It detects primarily surface and near-surface defects and not deep internal defects.
Skill requirement: Accurate interpretation requires trained personnel.
Applications of MPI Machines
MPI machines are used in industries for ensuring safety and reliability of components:
Aerospace: Inspection of aircraft components-a crack and defect analysis.
Automotive: Testing of engine parts, axles, and others critical components.
Construction: Checking of welds and structural steel.
Manufacturing: Quality assurance of machines and equipment.
Oil and gas: Inspection of pipelines and drilling equipment.
Safe Considerations
MPI is a really safe and reliable method, and some precautions need to take while it is being performed in practice.
Electrical Safety: Ensure proper insulation and grounding to prevent electric shocks.
Magnetics: Use protective equipment to avoid breathing in or swallowing magnetic particles.
Eye Protection: Use eye protection when exposing yourself to UV light.
Machines for Magnetic Particle Inspection are true workhorses in probing surface and near-surface discontinuities found in ferromagnetic materials. Their operation, which is based on magnetism and magnetic flux leakage, stands them in good stead for quality control and safety assurance across various industries. However, it is imperative that companies should take measures and are aware of the operation of these machines, their components, as well as their application so they can implement this important method of NDT where necessary.They are used in a variety of industries, including aerospace, automotive, manufacturing, and others. MPI machines are built to ensure the integrity of your materials by providing reliable, cost-effective applications. Thinking about purchasing an MPI machine? Check with the experts on the type appropriate for your use and ensure proper training provision for your team.