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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6 ; DOI: 10.3221/IGF-ESIS.25.01 5 Hence, a plot of u y against √r should give a straight line with a gradient of 8K I /(E√(2  )) and an intercept of  , and values of K and  may be obtained from the experimental results in a similar manner to the purely elastic case. Fig. 6 shows the variation of K with  for the cycle soon after the overload (i.e. corresponding to the elastic analysis shown in Fig. 5b). This shows evidence of the hysteresis in of K /  space, which Pommier and Hamam suggest is associated with fatigue crack propagation. According to their hypothesis, absence of a cyclic variation in  corresponds to loading beneath the threshold value of  K. Figure 5 : Variation of measured stress intensity factor with load for specimen CTF6, 23,500 cycles after the overload. Figure 6 : Variation of K with  when the elastic/plastic approach is used to analyse a cycle after the overload. C ONCLUSIONS he results presented here have demonstrated that displacement information collected in the crack wake can be analysed using digital image correlation in order to give reliable values of the stress intensity factor. For the case of constant amplitude loading, the results are broadly as expected and show a linear relationship of K with load, suggesting that the behaviour is predominantly elastic. There is, however, an offset to the experimental curve, compared with the linear elastic model, which can be attributed to the existence of crack closure. The situation when a non-uniform load history is examined appears more complex. Here, a simple case of a single overload in an otherwise constant amplitude loading has been examined. Even this relatively simple load history gives rise to some surprising results. For some time after the overload, negative stress intensity factors are measured when using a purely elastic model. Normally it is only possible to obtain positive values, as the crack closes and allows transmission of compressive forces across the T