Using Barkhausen Noise and Digital Image Correlation to Investigate the Influence of Local Residual Stresses on Fatigue-Crack Propagation

Abstract

Abstract Fatigue-crack propagation is, by far, the most crucial failure mechanism of technical systems. The key to understanding microscopic processes that lead to failure lies in the knowledge of local stresses, the driving force of cracks. We present the mapping of stress and strain fields induced by a single overload on a fatigue crack and their influence on transient crack-growth retardation. In contrast to former work, in which synchrotron x-ray diffraction (XRD) was used, this investigation was performed by using a calibrated magnetic Barkhausen noise microscope in combination with digital image correlation based on in situ scanning electron microscope imaging. The underlying mechanisms, residual stress effects in front of the crack tip and plasticity-induced crack closure caused by the plastic wake, have been studied. Specifically, a crack in S960Q steel has been followed through the overload (OL) region while examining measurements at distinctive overload points: before OL, after OL, at maximum retardation, and at recovery. We observe a strong correlation of the local fatigue-crack-growth rate with the local residual stress distribution obtained from magnetic Barkhausen noise, which remains nearly unchanged after the crack has passed by. Digital image correlation results reveal the influence of these residual stresses on crack-tip opening reactions and strain fields under external loads. Although strain fields show a strong decrease because of the OL event, differences in crack opening stresses remain rather low at first, but prevail in the second part of the OL region. The applicability of new measurement methods and their results regarding the dominating retardation mechanism are discussed. These indicate that the residual stress effect on the strain fields can be associated to be more significant than plasticity-induced crack closure at maximum retardation with a change of mechanisms on reacceleration.