Validation of finite element analysis of the magnetic fields around fine cracks

Reliable non-destructive testing of crack-like defects in welded structures, using magnetic particle inspection techniques, requires accurate information about the leakage flux resulting from applied current and fields. A range of cracks was prepared in carbon-manganese steel with maximum widths of 10 to 15μm and depths of 0.5 to 4.5mm. The magnetic disturbance caused by such defects was measured using a 0.12μm wide magneto-resistive thin film. Defects of similar dimensions were subsequently modelled using existing finite element software, tuned to meet the requirements imposed by the discretisation of regions having large aspect ratios. The predictions of the models, which were in close agreement with flux measurements, were used to determine the surface forces produced by magnetisation which act upon particles used in magnetic particle inspection.