Magnetic Particle Inspection (MPI), commonly abbreviated as MT or MPI test, is a non-destructive testing technique used to identify linear flaws near the surface of ferromagnetic materials, mainly for surface examination. It is particularly effective in detecting surface-breaking and minor sub-surface defects, including cracks, pores, cold laps, and lack of sidewall fusion in magnetic materials.
A range of techniques can be found, and the most versatile option involves a handheld electromagnetic yoke magnet operating on 110v AC, which can be obtained through reputable test equipment distributors. A white removable paint acts as a contrasting background, accompanied by a magnetic “ink” that contains iron powder particles suspended in a liquid carrier base.
Advantages of MPI Test (Magnetic Particle Inspection):
Non-invasive: MPI test constitutes a type of non-invasive testing, ensuring your component remains unharmed after the procedure. This method is well-suited for components essential for reference during development or for continuous use without requiring replacement.
Fast: Maintaining operational timelines as a priority is crucial. MPI test is a speedy procedure capable of yielding outcomes within a brief timeframe.
Portable: Utilizing portable magnetization tools and materials, inspectors can execute MPI test in various locations. This benefit is particularly valuable for oversized components, eliminating the need for shipping and optimizing inspection efficiency.
Simple: The procedure is user-friendly, allowing inspectors of various expertise levels to adeptly learn and execute it. Additionally, it entails minimal pre- and post-treatment demands, such as cleaning, streamlining your operation’s tasks.
Highly perceptive: MPI test is capable of detecting surface-breaking and near-surface indications, including delicate, shallow cracks.
Adaptable: Magnetic particle inspection effectively accommodates irregular forms, encompassing features like external splines, connecting rods, and crankshafts.
MPI test involves four stages:
- Generating a magnetic field within the sample
- Applying magnetic particles onto the sample’s surface
- Observing the surface for particle clusters resulting from defects
- Demagnetizing and cleansing the sample.
There are two main types of mpi test
- Dry Magnetic Particle Testing (DMPT): In the process of dry magnetic particle testing (DMPT), inspectors utilize magnetic powder to coat a material’s surface. This powder is evenly spread by dusting, creating a thin layer. After powder application, inspectors gently remove excess powder. This step requires precision to eliminate excess while retaining powder settled in the flux leakage field.
DMPT is suitable for surfaces like as-cast and unground welds. It’s effective for detecting shallow subsurface flaws. An electromagnetic yoke is commonly used to apply the magnetic field, utilizing AC or half-wave DC current pulses to enhance powder mobility.
- Wet Magnetic Particle Testing (WMPT): Wet magnetic particle testing (WMPT) employs particles within a liquid carrier. Inspectors can apply this via spraying or free-flowing liquid, ensuring swift and uniform coverage. The liquid sustains particle mobility, aiding their collection at flux leakage fields for enhanced defect visibility.
WMPT is suitable for smooth surfaces with small defects. However, on rough surfaces, particles in the liquid carrier may settle in surface textures, diminishing mobility.
How is the mpi test (magnetic particle inspection) done?
- Material under inspection is magnetized
- As the magnetic field travels through the material, encountering a defect disrupts the field, causing it to disperse from the defect’s location, known as a ‘flux leakage field.’
- After identifying a defect, particles are applied to the material’s surface. These particles, in either powder form or suspended in liquid, are attracted to the leakage field. Typically black or fluorescent, these particles enhance defect visibility for inspectors.
Magnetic Particle Inspection (MPI test) is a valuable non-destructive testing technique used to identify surface and near-surface flaws in ferromagnetic materials. This method, whether through Dry Magnetic Particle Testing (DMPT) or Wet Magnetic Particle Testing (WMPT), offers advantages like quick results, portability, and simplicity. MPI’s test ability to detect defects, its adaptability to irregular shapes, and its sensitivity make it an essential tool for flaw identification.