The effect of bulk stresses on magnetic flux leakage signals

3D finite element analysis has been used to examine the effect of simulated bulk tensile stress on magnetic flux leakage (MFL) signals. Non-linear anisotropic materials are used and the effects of applied bulk tensile stress are simulated by increasing permeability in the tensile stress direction. These simulated bulk stress effects on the radial MFL signal depend on the excitation flux density. The stress effects are a maximum at background flux density levels of 1.2-1.6 T, causing decreasing MFL signals. As flux densities are increased above 1.6 T the effect of the bulk stress on the MFL signal decreases, with no effect around 1.8 T. Interestingly, above 1.8 T stress begins to increase the MFL signal. The trends indicated by the model are similar to those obtained experimentally.