Nonlinear assessment of “breathing” fatigue crack using steady-state vibration responses

Abstract

Fatigue cracks can occur in structural components under long-term cyclic loads. Such fatigue cracks can open and close like "breathing" under harmonic excitations. Fatigue cracks can develop and jeopardize safety of structures. Therefore, there is a great need to assess fatigue cracks especially in their early stages. Higher harmonics have been widely used as crack indicators, however, slight opening-closing motion can lead to weak higher harmonics with much less amplitudes compared to the first harmonics. In these situations, higher harmonics can be barely identified for crack detection. Addressing this problem, this study develops the damage assessment approach based on singular spectrum analysis to distinguish crack-induced nonlinearities in steady-state vibration responses, whereby nonlinear assessment of the crack can be achieved. The capability of the approach is experimentally validated on a steel beam with a “breathing” fatigue crack, steady-state velocity responses of which are acquired through non- contact laser measurement using a Doppler laser vibrometer.