Abstract
The evolution of crack-tip strain fields in a thin (plane stress) compact tension sample following an overload (OL) event has been studied using two different experimental techniques. Surface behaviour has been characterised by Digital Image Correlation (DIC), while the bulk behaviour has been characterised by means of synchrotron X-ray diffraction (XRD). The combination of both surface and bulk information allowed us to visualise the through-thickness evolution of the strain fields before the OL event, during the overload event, just after OL and at various stages after it. Unlike previous work, complete 2D maps of strains around the crack-tip were acquired at 60?m spatial resolution by XRD. The DIC shows less crack opening after overload and the XRD a lower crack-tip peak stress after OL until the crack has grown past the compressive crack-tip residual stress introduced by the overload after which the behaviour returned to that for the baseline fatigue response. While the peak crack-tip stress is supressed by the compressive residual stress, the crack-tip stress field changes over each cycle are nevertheless the same for all Kmax cycles except at OL.
Highlights
M any crack retardation effects occurring during the fatigue of materials have been explained by the concept of crack closure [1]
We selected a thin specimen so that the variation of the strain field on the surface, where Digital Image Correlation (DIC) measures the total strain, and the elastic strain averaged through thickness as measured by X-ray diffraction (XRD), is minimised
Lopez-Crespo et al have already combined these complementary methods to examine the strain fields local to a crack-tip in a plane stress stainless steel compact tension sample prior and subsequent to an overload event [15]
Summary
M any crack retardation effects occurring during the fatigue of materials have been explained by the concept of crack closure [1]. Steuwer et al [6] were the first to map the stresses in thick (plane strain) samples in 2D at high (25 m) resolution, but in this case the plastic zone was small and the crack-tip stress fields dominated largely by elastic behaviour.
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