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Nonlinear resonant ultrasonic spectroscopy (NRUS) is a resonance-based acoustic testing technique that yields the hysteretic elastic nonlinearity parameter by measuring the resonance frequency shift with increasing driving amplitude. NRUS offers great potential for nondestructive evaluation since it is relatively simple to implement and can detect incipient damage thanks to the high sensitivity of hysteretic nonlinearity parameter to micro-damage. Previous research has shown that NRUS can monitor distributed damage in a wide variety of materials, but the application of NRUS for detection of local defects in metals is less explored. In this study, the feasibility of using NRUS to assess local progressive damage in aluminum is investigated. We use three-point bending fatigue test to initiate a single fatigue crack in a large aluminum specimen. The cyclic loading is interrupted at several stages in order to image the crack using Scanning Electron Microscopy (SEM) and to perform NRUS and other tests. As the crack grows in length, NRUS records a gradual increase in the resonance frequency shift. However, the trend for the amplitude dependency of resonance frequency shift is different from what has been previously reported in materials with distributed damage; the resonance frequency changes are larger at low strain than at high strains. In addition, the utility of multi-modal NRUS for locating the fatigue crack is demonstrated. Finally, we compare the results of NRUS and those from an impact-based NRUS (INRUS) that uses an automated impact hammer as the excitation source in differentiating the fatigue-damaged from the intact specimen. Our findings suggest the potential of NRUS and INRUS in detecting local damage in metals.

Nonlinear Resonant Ultrasonic Spectroscopy (NRUS) for Monitoring Fatigue Crack Growth in Aluminum