A nonlinear acoustic measurement is studied for fatigue-damage monitoring. An electromagnetic acoustic transducer (EMAT) magnetostrictively couples to a surface-shear wave resonance along the circumference of a rod specimen during rotating bending fatigue of carbon steels. Excitation of the EMAT at the half of the resonance frequency caused the standing wave containing only the second-harmonic component, which was received by the same EMAT to determine the second-harmonic amplitude. Thus measured surface-wave nonlinearity always showed two distinct peaks at 60 % and 85 % of the total life. We attribute the earlier peak to the crack nucleation and growth, and the later peak to an increase of free dislocations associated with crack extension in the final stage.
