PSI - Issue 57

Kimmo Kärkkäinen et al. / Procedia Structural Integrity 57 (2024) 271–279 K. Ka¨rkka¨inen et al. / Structural Integrity Procedia 00 (2023) 000–000

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conservation of volume (c.f., Poisson’s e ff ect). This results in increased tensile plasticity in front of the crack tip in the following tensile loading, leading to temporarily higher closure levels. If the crack tip is within the underload compressive plastic zone of the initial defect, the total plastic strain in the crack tip is reduced, leading to decreased closure levels until the crack propagates out of the defect compressive plastic zone. Thus, the underload e ff ect is dependent on the ratio of the underload / tensile plasticity, as well as the distance from the initial defect, i.e., crack length. In Fig. 5 the crack tip at the time of the underload is still within the defect compressive plastic zone, but far enough that the compressive plastic strains are small. The surplus of plastic flow from behind the crack tip, caused by the underload volume conservation e ff ect, strongly outweighs the small compressive plastic strain in this case. The discrepancy to literature results, that an underload temporarily accelerates propagation, may stem from that the mechanism described above is unique to the simulated case, or that the removal of far-field closure due to compression is significant in terms of crack driving force. Extended acceleration of crack propagation due to a single underload might not be attributable to plasticity-induced crack closure under plane strain conditions. In this paper, the link between empirically observed overload and underload phenomena, and plasticity-induced crack closure, was investigated in a short, interior defect-initiated crack under near-threshold loading conditions. The main findings are as follows: • A significant reduction of crack driving force is observed following an overload. Overload behavior predicted by the present model is qualitatively very similar to what is commonly reported from a wide array of analysis configurations in literature. • A single underload produces a similar but weaker e ff ect as the overload in this case, which is explained primarily by volume conservation e ff ects in plane strain plasticity, but depends, for example, on the distance to initial defect. This could have major implications especially in the very high cycle fatigue regime. • Crack growth acceleration is not predicted apart from the initial spike and could suggest that the removal of far-field closure due to compression is significant in terms of crack driving force. • Numerical simulation allows for observing crack closure e ff ects for the entire crack surface. Total closure influ ence should be a weighted sum of entire crack surface contact. 4. Conclusions

Acknowledgements

Co-funded by the European Union Grant Agreement No. 101058179 ENGINE project. The authors wish to ac knowledge CSC – IT Center for Science, Finland, for computational resources.

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