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P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 25 (2013) 153-160; DOI: 10.3221/IGF-ESIS.25.22 154 the cracked body such as compliance or electrical resistance or (ii) measurement of crack-opening displacements on the surface of the cracked body. Third generation synchrotron X-ray facilities allow experimental measurement of the strain at the interior of the specimen. Recent works have shown that it is possible to map in 2D the strain fields around the crack- tip [3-5]. However, our highest spatial resolution measurements (25  m) were made on a very fine grained Al-Li alloy, with low fracture toughness such that the plastic zone was very small. Nevertheless it was possible to extract accurate measures of the crack tip stress intensity factor at minimum, maximum and overload stresses, K min , K max and K OL . More recent experiments allowed the measurement of crack-tip strains under plane stress conditions with characteristically larger plastic zones [6, 7]. Overload events were studied, both at the surface via digital image correlation and in the interior, via synchrotron X-ray diffraction. However, the large grain size (~50  m) did not allow sufficiently high resolution strain mapping to resolve the changes occurring immediately local to the crack-tip. The current work aims to examine the effect of overload in a bainitic material having large plastic-zone and very fine grain size, thus achieving excellent resolution in mapping the strain fields around the plastic one of a crack-tip. M ATERIAL AND SPECIMEN compact tension (CT) fatigue specimen was machined from quenched and tempered steel similar to Q1N (HY80) [8]. Its chemical composition is summarised in Tab. 1. The tensile properties are as follows: Yield Stress = 570 MPa and Ultimate Tensile Stress = 663 MPa. The CT specimen had a width (W) of 60 mm and thickness (B) of 12 mm. The steel has an acicular microstructure as shown in Fig. 1 with an approximate grain size of 5 µm. Alloy C Si Mn P S Cr Ni Mo Cu Q1N 0.16 0.25 0.31 0.010 0.008 1.42 2.71 0.41 0.10 Table 1 : Chemical composition in weight % of Q1N steel. The balance is Fe. Figure 1 : Optical micrograph of the bainitic steel used in the current work. The micrograph was obtained at 1000X magnification. X- RAY DIFFRACTION EXPERIMENTAL SETUP he crack-tip elastic strain fields were measured on the ID15 beamline at the European Synchrotron Radiation Facility (ESRF), using the same arrangement as that described in [4] shown schematically in Fig. 2. The incident beam slits were opened to 60 × 60 µm giving a lateral resolution (x,y) of 60 m and a nominal gauge length A T