PSI - Issue 45

James Vidler et al. / Procedia Structural Integrity 45 (2023) 82–87 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. The ratio of = / as a function of the excitation loading a , material properties, and , and the ratio of the plastic stretch to the half crack length, / = . 4. Conclusion In conclusion, we would like to highlight the main features of fatigue crack behaviour under loading, which have been observed from the simple model presented in sections 2 and 3. Fatigue cracks in the absence of the applied loading or residual stresses are partially closed due to the plasticity induced closure mechanism. Therefore, if a plate structure is subjected to low intensity ultrasonic inspections then the length of the detected crack(s) may be underestimated. Under sufficiently small level of the applied stress the length of the open region changes linearly with the intensity of the applied loading. The latter does not support the popular bilinear stiffness modelling assumption, which assumes a sharp change in the plate stiffness during loading and unloading portions of the cyclic excitation (Broda et al. 2014). The behaviour of a crack subjected to cyclic loading therefore cannot be accurately described using the mechanical diode analogy or clapping mechanism, as the variation of the open region during harmonic excitation could be much smaller than predicted. It is important to note that the current model is concerned with relatively large cracks, (e.g. larger than 5 mm), propagating in Mode I (opening fracture mode) for which the plasticity-induced closure normally dominates over the roughness- and oxide-induced closure mechanisms. Despite many modelling assumptions, the developed simple model is much more realistic than all previous considerations (Broda et al. 2014, Xu et al. 2021); and it can serve as an initial point for the developments of more adequate mechanisms and accurate models for different crack geometries and load histories. The latter may help to develop effective nondestructive techniques for detection and identification of fatigue cracks and prevention of fatigue failures. Acknowledgements This research was partially supported by the Australian Government through the Australian Research Council's Discovery Projects funding scheme (projects DP200102300 and DP210103307). The authors are grateful for the support.