Fundamental Engineering Studies of Magnetic Particle Inspection and Impact on Standards and Industrial Practice

Abstract

<p>Magnetic particle inspection (MPI) has been widely utilized for decades, and sees considerable use in the aerospace industry with 90% of the steel parts being inspected with MPI at some point in the lifecycle. Typical aircraft locations inspected are landing gear, engine components, attachment hardware, and doors. In spite of its numerous applications the method remains poorly understood, and there are many aspects that would benefit from in-depth study. This shortcoming is due to the fact that MPI combines the complicated nature of electromagnetics, metallurgical material effects, fluid-particle motion dynamics, and physiological human factors into a single inspection. To promote understanding of the intricate method issues that affect sensitivity, or would assist with the revision of industry specifications and standards, research studies will be prioritized through the guidance of a panel of industry experts. This approach has worked successfully in past fluorescent penetrant inspection (FPI) research efforts[i].</p> <p>The magnetic particle inspection technique has been used for many years for aviation applications, but unfortunately very few aids exist that assist in proper test setup. There are many ‘rule-of-thumb’ equations available to calculate current settings for a given sample geometry, but very often this results in gross over-magnetization and reduced sensitivity. Further, magnetic particle test specifications prescribe current values, which are affected by the control waveforms used for regulating the current intensity. This introduces harmonics in the waveforms, which makes it difficult to establish a relationship between peak and rms values of a current waveform, which is important in the practical use of MPI. Each of the waveforms has its own characteristics and interactions between leakage fields at discontinuities and the particles can vary significantly. It is therefore possible to miss the detection of defects by choosing inappropriate current waveforms. In recent Air Transport Association NDT Forums, the airlines have identified the need for additional research to support fundamental understanding of the MPI technique and the factors which affect sensitivity. Of particular concern is the direction “complete 100% magnetic particle inspection” without specific guidance on setup parameters which is common in OEMprocedures. Another issuewith overwhelming support is the impact of coatings and platings on MPI sensitivity.</p>