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P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 25 (2013) 153-160 ; DOI: 10.3221/IGF-ESIS.25.22 153 Special Issue: Characterization of Crack Tip Stress Field Study of overload effects in bainitic steel by synchrotron X-ray diffraction P. Lopez-Crespo Department of Civil and Materials Engineering, University de Malaga, C/Dr Ortiz Ramos, s/n, 29071 Malaga, Spain P.J. Withers, J. R. Yates School of Materials, University of Manchester, Grosvenor St, Manchester, M13 9PL, UK A. Steuwer MAX IV Laboratory, Lund University, Box 118 SE-221 00 Lund, Sweden T. Buslaps ESRF, 6 rue J Horowitz, 38000 Grenoble, France Y. H. Tai Rolls-Royce plc, PO Box 31, Derby, DE24 8BJ, UK A BSTRACT . This work presents an in-situ characterisation of crack-tip strain fields following an overload by means of synchrotron X-ray diffraction. The study is made on very fine grained bainitic steel, thus allowing a very high resolution so that small changes occurring around the crack-tip were captured along the crack plane at the mid-thickness of the specimen. We have followed the crack as it grew through the overload location. Once the crack-tip has progressed past the overload event there is strong evidence that the crack faces contact in the region of the overload event (though not in the immediate vicinity of the current locations of the crack tip) at K min even when the crack has travelled 1mm beyond the overload location. It was also found that at K max the peak tensile strain ahead of the crack-tip decreases soon after the overload is applied and then gradually recovers as the crack grows past the compressive region created by the overload. K EYWORDS . Crack-tip strain field, overload effect, X-ray diffraction, Bainitic steel. I NTRODUCTION he concept of crack closure has been used to explain many crack retardation effects in fatigue of materials. Closure is associated with effects that cause the crack faces to close early during unloading so that the crack-tip does not experience the full crack-opening fatigue cycle. Plasticity induced crack closure is one of the most important mechanisms of crack closure, but is still a hotly debated subject. Some different researchers have suggested that closure does not occur at all [1] while others believe that it can only occur under plane stress [2]. To date, experimental measurements of crack closure have been inconclusive and have relied on either (i) measuring some secondary property of T