An instantaneous-baseline multi-indicial nonlinear ultrasonic resonance spectral correlation technique for fatigue crack detection and quantification

This article presents an instantaneous-baseline multi-indicial nonlinear ultrasonic resonance spectral correlation technique for fatigue crack detection and quantification. A reduced-order nonlinear oscillator model is tailored to illuminate the contact acoustic nonlinearity (CAN) and the nonlinear resonance phenomena. The analytical formulation considers the rough surface condition of the fatigue cracks, with a crack open–close transitional range for the effective modeling of the variable-stiffness nonlinear mechanism. Multiple damage indices (DIs) associated with the degree of nonlinearity of the interrogated structures are then proposed by correlating the ultrasonic resonance spectra. Three perspectives of the nonlinear resonance phenomena are investigated to detect and monitor the fatigue crack growth: (1) time-history dependence, which evolves different resonance states depending on the loading history; (2) amplitude dependence, which renders significantly different nonlinear responses under various levels of excitation amplitudes; (3) breakage of superposition, which effectively distinguishes nonlinear resonant responses from the linear counterparts. These DIs are established using instantaneous baselines, facilitating the fatigue damage monitoring without the prior knowledge of a pristine structure. Fatigue tests on a thin aluminum plate with a rivet hole are conducted to induce fatigue cracks in the specimen. The experimental results demonstrate that the proposed technique shows remarkable sensitivity to the nucleation and growth of the fatigue cracks. This paper differs from the existing literature on nonlinear resonance-based techniques in that it focuses on the resonance phenomenon aroused by the contact acoustic nonlinearity from localized fatigue cracks, rather than the diffused material nonlinearity. The novelty of the paper resides in the establishment of an instantaneous baseline technique utilizing the nonlinear resonance features without the need of referring to a pristine baseline situation. The paper finishes with discussion, concluding remarks, and suggestions for future work.