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A B S T R A C TThe surface crack opening displacements are characterised by digital image correlation for a (thin) plane stress 316 stainless steel compact tension sample subjected to an overload event. This supports a traditional plasticity-induced closure interpretation showing a knee in the closure response 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 shows no evidence of significant crack face contact stresses behind the crack tip on approaching minimum loading. Rather the changes during loading and overloading can 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 shows very significant compressive reverse plastic strains ahead of the crack that start to form early during unloading. At the moment it is not clear whether this difference is because of the increasing stress intensity applied as the crack grows, or for some other reason, such as prevention of the crack faces closing mid-thickness due to the reverse plastic zone.Keywords overload effect; synchrotron X-ray diffraction. N O M E N C L A T U R Ea = crack length COD = crack opening displacements da/dN = crack growth rate K I = opening mode stress intensity factor K max = maximum stress intensity factor K min = minimum stress intensity factor N = number of fatigue cycle applied OL = overload P max = maximum load P min = minimum load R = load ratio (P min /P max ) r p = size of the monotonic plastic zone r c = size of the cyclic plastic zone t = thickness UTS = ultimate tensile strength x = crack growing direction y = crack opening direction K = range of stress intensity factor (K max -K min ) P = load range (P max -P min )

Digital images of fatigue crack behaviour have been acquired in real time at high cycle fatigue rates (77 Hz) using a high-speed camera at 1000 frames/s. Digital image correlation has then been used to determine the crack-tip position and stress intensity variations (K I and K II ) within selected cycles. This has been achieved for a pre-cracked aluminium compact tension (CT) specimen subjected to constant load amplitude fatigue crack cycling. The crack-tip displacement field has been determined at 14 points within each cycle. In this proof of concept study, despite noise in the inferred displacement fields, by least squares fitting the displacement field rather than the strain field to the Muskhelishvili's form crack-tip stress field, the crack-tip stress intensity inferred from the measured crack-tip displacement field was found to good accuracy (around 0.2 MPa m 1/2 ). Furthermore, the observed sinusoidal variation was in excellent agreement with the nominal DK obtained from the applied fatigue amplitude confirming the accuracy of the method; a drift of around 0.42 MPa m 1/2 was observed in K mean from cycle to cycle. No evidence of closure was observed at low K. In principle this method permits the identification of crack-tip closure and crack growth, as well as the application of constant DK eff cycling, through intermittent monitoring throughout the duration of a fatigue test. The current method is well suited to more complex cases where crack closure, residual stresses, or more complex geometries mean that it is difficult to predict DK eff a priori and a number of potential applications of the method are identified.