Nonlinear ultrasonic to assess localized plastic deformation during high cycle fatigue

Nonlinear ultrasonic is the new approach for the effective evaluation of material degradation in the area of non-destructive evaluation (NDE). Fatigue damage of a material produces a substantial distortion of ultrasonic waves propagating through the degraded material. The wave distortion is quantified by means of a material nonlinearity parameter beta that is defined as the ratio of the amplitude of the 2(nd) harmonic to the square of the amplitude of the fundamental. This nonlinear parameter changes when, for instance, a distribution of microcracks appears inside the material. This feature has recently been found as a new potential application in the characterization of fatigued and degraded materials. In this present work, an attempt has been made to assess the localized plastic deformation during high cycle fatigue to locate the position of crack initiation much before the failure along the gage length of hourglass type specimen in polycrystalline copper by nonlinear ultrasonic technique.