Issue 33

P.J. Whithers et alii, Frattura ed Integrità Strutturale, 33 (2015) 151-158; DOI: 10.3221/IGF-ESIS.33.19 152 K EYWORDS . Linear elastic fracture mechanics; Closure; Plastic strain; Bainitic steel; Effective stress intensity factor. I NTRODUCTION any crack retardation effects occurring during the fatigue of materials have been explained by the concept of crack closure [1]. Closure of the crack-faces either near to, or distant from, the crack-tip means that the crack- tip does not experience the full crack-opening fatigue cycle. Such effects have been held to be responsible for the immediate acceleration and then subsequent retardation of the crack growth rate observed following an overload during fatigue cycling. Diffraction peak shifts are predominantly sensitive to the elastic strain and a number of studies have mapped the strain ahead of, and behind, the crack-tip using neutron [2, 3] or synchrotron x-ray [4, 5] beams. The problem with linescans along the crack plane is that there is a danger that the location of the peak stress may be missed. 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. In this paper, we employ Digital Image Correlation (DIC), a non-contact full-field measurement technique, to measure the total (elastic plus plastic) strain on the surface of the specimen. The accuracy of DIC to measure elastic strain is debateable, but its capability to measure total strain (considering that plastic strains are orders of magnitude bigger than elastic strains) is unrivalled. The combination of the two techniques provides both elastic (from XRD) and total (DIC) strains if the measurements are carried out on the same gauge volume. Therefore, we selected a thin (i.e. plane stress) specimen so that the variation of the strain field on the surface, where DIC measures the total strain, and the elastic strain averaged through thickness as measured by XRD, is minimised. DIC has found increasing application for the study of crack-tip strain fields [7] and it has been possible to extract fracture mechanics information such as closure stresses [8, 9], plastic zone sizes [10], crack-tip opening displacements (CTOD) and effective stress intensity factors at the crack-tip, K eff [10-14]. Lopez-Crespo et al. have already combined these complementary methods to examine the strain fields local to a crack-tip in a plane stress (thin) stainless steel compact tension sample prior and subsequent to an overload event [15]. Even under plane stress where the surface and bulk states might be expected to be the same, important differences were observed between DIC and XRD. Surface DIC measurements suggested that under fatigue cycling the cracks faces appear to be in contact for around 50% of the cycle supporting a traditional plasticity-induced closure interpretation. Indeed they observed a knee in the closure response for baseline fatigue prior to overload, an absence of closure in the accelerated growth regime followed by accentuated closure in the retardation regime. By contrast, measurement of the mid-thickness elastic strain field behind and ahead of the crack made by synchrotron X-ray diffraction showed no evidence of significant crack-face contact stresses immediately behind the crack-tip on approaching minimum loading. Though the results were affected by point-to-point scatter due to an insufficiently small grain size, the changes during loading and overloading could mostly be explained by a simple elastic plastic analysis using a value of the yield stress intermediate between the initial yield stress and the UTS. This showed very significant compressive plastic strains ahead of the crack that start to form early during unloading. Here we revisit this topic by studying a bainitic rather than stainless steel. This has an inherently much finer grain size allowing us to achieve much higher spatial resolution in the X-ray diffraction data with much reduced point to point scatter [16]. Further the steel has sufficient toughness that a large plastic zone can be introduced in contrast to the Al-Li studied by Steuwer et al. [6]. This has enabled us to study the elastic strain field and the effect of plasticity on closure in unparalleled detail. M ATERIAL AND SPECIMEN compact tension (CT) fatigue specimen was machined from quenched and tempered bainitic steel similar to Q1N (HY80) [17]. Its chemical composition is summarised in Tab. 1. The tensile properties are as follows: Yield Stress (  y ) = 570 MPa and Ultimate Tensile Stress,  uts = 663 MPa. The CT specimen had a width ( W ) of 60 mm and thickness ( B ) of 3.3mm. M A