PIEZOMAGNETISM AND FATIGUE

Piezomagnetism refers to a change in the intrinsic magnetization of a material subjected to mechanical actions such as tension or compressio n. In a ferromagnet such as a mild steel these effects are easily meas ured: typically a stress of 2 x 10(4) lbs in 2 or 140 MPa induces a ma gnetic moment of the order of 5 x 10(-3) emu or 5 x 10(-6) A m(2), res ulting 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 b etween the mechanical and magnetic mesostructure of materials micropla stic 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 the refore he tracked by following the evolution of the piezomagnetic fiel d. Specifically, if the measurements are displayed as loci in a three- dimensional stress (sigma)-strain (epsilon)-field (B) space, the appro ach to fatigue failure is paralleled by a series of conspicuous geomet ric transformations of these curves. Complementary information also ap pears in continuous-time records of B(f): these magnetograms clearly s how the abrupt incidence of 'infarcts' (microcracks) and the cumulatio n of phase shifts as the material degrades.