Experiment and equivalent sine wave non-linear numerical analysis method using AC magnetization curve for alternating magnetic flux leakage testing

摘要

Alternating magnetic flux leakage testing is one type of inspection of the surface of steel in the steel industry. This method detects leakage flux from cracks in ferromagnetic material magnetized by alternating magnetic flux. The exciting frequency for this method is becoming higher, because precision detection of very small surface cracks in steel materials is being demanded. But no technique to evaluate this test has been established, because ferromagnetic material under a high power magnetic field has a non-linear alternating magnetic property. This phenomenon can be addressed by development of a non-linear alternating magnetic field numerical analysis. Generally speaking, such a numerical analysis requires a series analysis (a step-by-step analysis method), but this involves numerous time steps in a cycle and a long calculation time. Therefore, we propose an approximate numerical analysis method using the equivalent sine wave for use in alternating magnetic flux leakage testing. We also propose an effective AC magnetization curve using this analysis technique. Model tubes with inner and outer tubes are constructed of two exciting coils and three search coils (outer part, central part and inner part). The AC magnetization curve is obtained from both output voltages of the three search coils and numerical analysis. And used to evaluate alternating magnetic flux leakage testing. The result of the numerical analysis using the AC magnetization curve better fits the experimental voltages than does the DC magnetization curve.