Вторая Школа молодых ученых «Мониторинг природных и техногенных систем» 16-18 ноября 2020 г.
Выступление Thierry PALIN-LUC
In service, most of the mechanical components or structures are submitted to cylic loading. To avoid failure of such systems, engineers have to design them against fatigue crack initiation (and then propagation). For short lifetimes (in low cycle fatigue, LCF) regime, up to ~105 cycles) macroscopic plasticity is responsible of crack initiation. For longer lifes (i.e. in high cycle fatigue, HCF) up to approximatively 107 cycles, the key role of microplasticity at the grain scale is now well established. Very promissing results have been obtained in literature with polycristal plasticity and critical plane based or energy based approaches. In the two previous regimes (LCF and HCF) cracks initiate at stress concentrators and at the surface of metals with a scenario where persistent slip bands are the precusors of crack. But for very long life, named gigacycle regime (that is beyond 108 cycles) when very low cyclic load level is applied (the stress amplitude could be ~0.3 times the material yield stress) fatigue crack initiation occurs in the core of the material whereas it is at the surface for higher load levels (in LCF and / or HCF). In gigacycle fatigue, non-metallic inclusions are often crack initiation triggers, but internal crack initiation occurs also without any inclusion (in titanium alloy for example). Consequently the mechanism of crack initiation is not so clear. There is no model able to explain why crack initiation shifts from the surface to the core of the material when the cyclic load level is decreased. Some ways for future researches are proposed to progress in the understanding of this open question.