Magnetic Field Frequency Optimisation for MFL Imaging using QWHE Sensors

Magnetic particle and other magnetic flux leakage-based methods for the detection and evaluation of surface-breaking flaws in ferromagnetic materials typically use high-strength (~ 0.5 T rms), low frequency (≤ 50 Hz) magnetic fields. The rationale behind this is the ready availability of strong permanent magnets and mains power to create high strength electromagnets. This high field strength is needed to saturate the sample and compensate for the insensitivity of magnetic particles, silicon Hall sensors, coils and other magnetic transducers. As such, the frequency of applied magnetic field is typically limited to ≤ 50 Hz and does not take into account the frequency response of the material under test (some MFL applications use this low frequency to detect sub-surface or flaws on the back wall). In this study, a probe consisting of a Quantum Well Hall Effect (QWHE) sensor, illuminating electromagnet and sensor circuitry was controlled using an automated XYZ scanner with an x y measurement step size (i.e. magnetic image pixel size) of 100 microns. This probe was used to apply a magnetic field of various frequencies (DC to 1 kHz) and field strengths (5 mT to 100 mT) to ascertain a frequency and field range best suited to detecting 10 and 11 mm length longitudinal surface-breaking toe cracks in ground mild steel welds. A lift-off distance of < 1 mm was controlled using a proximity laser and z direction motor module to autonomously control the probe lift-off and conform to sample geometry. This study found that an applied magnetic field of frequency 800 Hz and strength 10 mT rms was optimal, based on the ratio of MFL responses from the two flaws and the weld. The power dissipation of the coil was taken into account in this determination, where other frequency-field combinations had comparable or higher detection but were discounted as they had substantially higher power consumption.