Piezomagnetism and fatigue

Piezomagnetism refers to a change in the intrinsic magnetization of a material subjected to mechanical actions such as tension or compression. In a ferromagnet such as a mild steel these effects are easily measured: typically a stress of 2 x 10(4) lbs in 2 or 140 MPa induces a magnetic moment of the order of 5 x 10(-3) emu or 5 x 10(-6) A m(2), resulting in flux densities in the range 10 mG or 1 mu T in the vicinity of the specimen, Since piezomagnetic effects are due to interactions between the mechanical and magnetic mesostructure of materials microplastic processes that alter the arrangement of the ferromagnetic domain structure affect the intensity of the associated magnetic fields. The progressive degradation of such materials under cyclic loading can therefore he tracked by following the evolution of the piezomagnetic field. Specifically, if the measurements are displayed as loci in a three-dimensional stress (sigma)-strain (epsilon)-field (B) space, the approach to fatigue failure is paralleled by a series of conspicuous geometric transformations of these curves. Complementary information also appears in continuous-time records of B(f): these magnetograms clearly show the abrupt incidence of 'infarcts' (microcracks) and the cumulation of phase shifts as the material degrades.