Dependence of magnetic properties on crack size in steels

A crack induces a local inhomogeneity in the magnetic flux path in a material and any change in crack area results in change in the distribution of magnetic flux. From the results of measurements on A533B steel and Cr-Mo steels, not only were the magnetic properties found to change with crack area, but also a linear relationship between measured remanence and crack area was observed. A model equation has been developed which shows agreement with the observed relationship. On the other hand, the coercivity and its relationship to crack area were found to depend upon the sensor position relative to the crack. 3D finite element analysis of the leakage field above a crack also showed the position dependence. Therefore, using the remanence measurement method described here fatigue damage and crack area in steel components can be evaluated nondestructively