Jason De Silveira discusses metal fatigue and flaw detection with magnetic particle inspection
Magnetic particle inspection (MPI) is a valuable method of NDT, with one proviso: users must be aware of aspects that affect results.
One of the greatest of these is metal hardening and fatigue. If this isn’t taken into account, the probability of detection (POD) can be impacted. This makes the understanding of such a problem vital for accurate analysis of the results.
To fully comprehend why this is so, we need to have a basic overview of what MPI is and how it’s carried out.
MPI: what do I need to know?
In basic terms, MPI uses metal particles and a magnetic field to show defects and minor imperfections in a ferrous metal surface. It’s quick to carry out, returning immediate results. There are various methods used to carry out the testing, with the most common being the Yoke technique.
After cleaning the surface to be inspected, the tool is used to create a magnetic field in the metal. Magnetic particles are then applied to the surface. These group together, showing areas at or near the surface that are flawed or have imperfections. The technique is inexpensive, easy to use and, in most cases, highly accurate.
Unless, that is, the metal has undergone physical deformation through metal fatigue and work hardening.
This is because they cause the magnetic properties of metal to alter. Fatigue is typically caused by welding, repeated stresses or plastic deformation.
Metal fatigue makes the metal far more difficult to magnetise, more resistant to such changes, and – once the force is removed – further decreases its ability to be magnetised.
In short, fatigued metal will return an increased POD because the tester no longer has full control over the magnetic current. This is especially relevant with minor flaws or those that are just below the surface.
Overcoming the issue
As long as the tester is aware of the problem, there are various techniques that can be employed to help overcome this. These consist of a variety of magnetisation capabilities, including longitudinal magnetisation, yoke magnetisation, circular magnetisation, and prods magnetisation. All of these provide more control over the permeability of the metal (AKA, how easy it is to create a magnetic force within the fatigued metal).
Although experience is vital to ensure the correct results from MPI, so too is using the right equipment. The ability to generate a strong magnetic field is crucial, as is using and advanced magnetisation tool that’s easy to operate and reduces the physical stresses on the user.
One such MPI tool is the AC Electromagnetic Yoke (Y-2). Not only is it ergonomically designed to ensure operator comfort and increase productivity, but it also uses the latest technology to help create an accurate magnetic force, even within fatigued metal.
Of course, even the best tool won’t provide the desired results with an inexperienced user. But the combination of machine quality and knowledge of how to overcome metal fatigue during NDT testing is the key to getting the best results. With the associated expenses of carrying out such necessary inspections, it’s crucial that the results can be relied on. Therefore, learning the techniques and putting them into action with advanced NDT inspection technology is the answer to gaining the very best probability of inspection.
Jason De Silveira is managing director of Nexxis
