Non-destructive testing (NDT) is a process that is used to assess the properties of a material without causing any damage. There are many different types of NDT, but one of the most common methods is magnetic particle inspection (MPI). In this blog post, we will discuss what MPI is and how it is used to test materials. Stay tuned for future posts in our MPI Basics series!
MPI is a non-destructive testing technique that uses magnetic fields to detect flaws or defects in ferrous materials. This method is often used to test welds, but it can also be used on castings, forgings, and other types of metal components. MPI is generally considered to be a relatively low-cost and quick testing method.
How does MPI work?
When a magnetic field is applied to a material, thefield lines will be perpendicular to the surface of the material. If there are any flaws or defects in the material, thefield lines will be disturbed. This disturbance can be used to detect the presence of flaws or defects. There are three methods:
Dry Powder technique – The Hi-Temperature and gross cracking inspection technique is not very popular or sensitive, but it uses electromagnets or permanent magnets.
Contrast Technique – This inspection technique uses a background paint to magnify crack indications. It is most commonly used for the inspection of welds in structural steel and plant equipment, and can be performed with electromagnets or permanent magnets.
Wet Fluorescent Technique – MPI technique that involves using UV light in a dark room. This is the most sensitive form of MPI and can find tiny indications, saving time while maintaining consistent results.This method can detect damage to assets without stopping production, making it ideal for in-service inspections.
Dry powder MPI is the least sensitive of the three methods, but it is also the most common. In this method, a magnetic field is applied to the surface of the material using an electromagnet or permanent magnet. A dry powder is then applied to the surface of the material. The powder will be attracted to any areas where the field lines are disturbed, which will indicate the presence of a flaw or defect.
Contrast MPI is a more sensitive method than dry powder MPI. In this method, a magnetic field is applied to the surface of the material using an electromagnet or permanent magnet. A magnetic particle suspension is then used to coat the surface of the material. The suspension will be attracted to any areas where the field lines are disturbed, which will indicate the presence of a flaw or defect.
Wet fluorescent MPI is the most sensitive method of the three.
Magnetic Particle Inspection Benefits:
- The ability to detect surface and near-surface indications
- Inspection of irregularly-shaped parts
- less essential pre-cleaning of the asset being inspected , as existing contaminants should not affect flaw detection
- time efficient and lower cost than other NDT methods with real=time results, as indications are visible directly on the material’s surface. Additionally, it is a portable inspection method which makes it ideal for field inspection services
Magnetic Particle Inspection limitations:
- DETECTS Ferromagnetic materials ONLY
- Cannot be used if a thick coating is present
- May require de-magnetization in some situations
- Surface and indications very near the surface may only be seen
MISTRAS Group, Inc. is our expert at Corrosion Alliance to help you with questions related to NDT inspections.