What Does MPI Mean?
MPI stands for Magnetic Particle Testing and is a non-destructive testing (NDT) method that detects surface-breaking defects and subsurface defects close to the surface. It is non-destructive due to no damage or harm caused to the metal during the process.
How Does MPI Testing Work?
MPI (Magnetic Particle Inspection) works by magnetising the material. Magnetising the metal results in the metal producing lines of magnetic flux or force. As the magnetic flux travels through the metal, it meets any discontinuities (if they are present), and as a result, the magnetic flux will leak at the discontinuity generating secondary magnetic poles.
Magnetic flux leak occurs due air does not support as much magnetic field per unit volume as metals. If a discontinuity/defect is present, air will also be present due to the void in the metal structure. The size of the defect controls the amount of magnetic flux leak.
To locate the defects, a suspension containing magnetic particles is applied to the metal. The magnetic ferrous particles are attracted to the magnetic flux leak and gather around it, giving a clear visual and size of the defect.
Using the ‘black and white’ method helps straightforwardly identify the defect. The ‘black and white’ method consists of initially spraying the area of the metal you wish to inspect with a white contrast paint. Following this, black ink containing ferrous particles is sprayed over the top of the white paint. Following magnetisation, commonly using an A.C yoke, the black ferrous particles are attracted to any defect via a magnetic flux leak.
What is The Principle of MPI testing?
The principle of Magnetic Particle Inspection testing is to detect surface-breaking defects and sub-surface discontinuities close to the surface by inducing a magnetic field on the tested component.
Methods of Magnetisation
The component to be tested can either be directly or non-directly magnetised. Direct magnetisation is the application of an electric current through the test component, which results in a magnetic field forming in the material.
Indirect magnetisation is when a magnetic field is applied from an external source – no electrical current is passed through the material.
The most common application of MPI testing is by using an AC electromagnet yoke. The magnet is connected to a power source and then applied to the test material allowing a current to flow over the surface.
The second most common method of magnetisation is by placing the specimen inside of a current-carrying coil. If the test specimen is hollow, a current-carrying bar can be inserted through the specimen to induce magnetism.
In all scenarios, the current used can be either AC (alternating current) or DC (direct current), but dependent on the level of magnetism required and the penetration into the material, one will be more effective than the other.
Types of Electrical Currents
For this type of non-destructive testing, both AC (alternating current) and DC (direct current) are used. Influential factors which help determine the correct type of current are part geometry, penetration of magnetic field required and material.
In most cases, AC is used, which is the preferred method to detect surface-breaking defects. Unfortunately, using AC is not suitable to detect sub-surface discontinuities due to a phenomenon known as the skin effect. As the name suggests, the AC runs along the surface and does not penetrate further into the metal. Altering the frequency of the alternating current will help increase or decrease its penetration into the metal.
The other type of current, DC (direct current), is also used in MPI testing and comes in two forms. It is either used as full-wave DC or half-wave DC.
Full-wave DC is used to detect subsurface defects due to its larger penetration, and the level of penetration is determined by the amount of current applied.
Half-wave DC is a middle ground as it can provide magnetisation deeper into the metal than an AC, but it can also detect surface-breaking defects, unlike full-wave DC.
Using AC is the preferred method for detecting surface-breaking defects, whereas some form of DC is better for subsurface discontinuities.
What material can be tested by MPI
MPI can only be undertaken on ferromagnetic materials such as iron and steel. Fortunately, steel is the most used metal in the world, which means MPI can be applied across a variety of industries and applications. Only ferromagnetic materials can be tested because they can be magnetised. Unfortunately, Austenitic Steel cannot undergo MPI testing due to its lack of magnetic properties.
MPI Testing of Welds
MPI testing is most applied to check the structural integrity of welds. Welds are critically important in dynamically loaded structures and determine the life due to them tending to fail before any other part of the structure.
Due to their small size, the best method of inspecting welds via MPI is by using an AC electromagnet yoke.
Equipment
A horizontal MPI inspection bench is one of the most common pieces of equipment for mass testing of similar parts. Commonly the bench is made for specific applications as it contains a coil used to magnetise the parts. A ‘bath’ collects any residual inspection fluid and is recirculated for reuse. Reusing MPI fluid is particularly common when using fluorescent magnetic ink as it is not mixed with any other fluid.
When using the bench for fluorescent and UV (ultraviolet) applications, a large tent is pulled over the bench to reduce light inside, allowing the fluorescent ink to be detected easier using UV.
For mobile testing and testing of smaller components, a handheld electromagnet yoke is the preferred option. The two contacting points create opposing magnetic fields. As a result, a handheld electromagnet is only suitable for the testing of smaller components and welds. To help discover all types of discontinuities, the yoke should be rotated 90 degrees to capture discontinuities in the horizontal and vertical plane.
After undertaking MPI, it is important the component is demagnetised, and there is a range of demagnetising equipment such as pull through coils for either AC or DC.
A Gauss meter is used to measure the residual magnetism of the component after testing. If the maximum amount of residual magnetism is exceeded, the component will require additional demagnetisation.
Standards for MPI
There are many standards for undertaking MPI on products and even more for NDT procedures! Magnetic particle inspection is an activity that can find potential catastrophic defects and hence is regulated heavily.
Testing is required for each batch of magnetic ink used to measure and ensure the correct volume of ferromagnetic particles are within the suspension. Fortunately, a batch of paint covers a large number of canisters, so the testing isn’t overregulated. Additionally, a light reading is required and must meet the minimum standard of 1000 lux. After all, MPI is heavily visual so ensuring sufficient light is present is a good idea. ISO 9934 – ‘Non-destructive testing – Magnetic particle testing’ is the central standard for MPI testing and the standard that the majority of operators adhere to. ISO 9934 (International Organization for Standardization) has 3 parts to cover general principles, equipment and detection media:
- ISO 9934-1 – Non-destructive testing – Magnetic particle testing – Part 1 – General Principles
- ISO 9934-2 – Non-destructive testing – Magnetic particle testing – Part 2 – Detection Media
- ISO 9934-3 – Non-destructive testing – Magnetic particle testing – Part 3 – Equipment
Summary
MPI is the most popular non-destructive testing method used in many industries and plays a pivotal role in crack detection. We now understand how MPI works, how cracks are identified, the different types of magnetisation methods and the currents used. It is a process you are likely to encounter in your career, therefore, it will be a benefit to understand the process and how it works.
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