Issue 35

C. Gandiolle et alii, Frattura ed Integrità Strutturale, 35 (2016) 232-241; DOI: 10.3221/IGF-ESIS.35.27 232 Focussed on Crack Paths Fretting fatigue crack propagation rate under variable loading conditions C. Gandiolle, S. Fouvry LTDS, Ecole Centrale de Lyon, 36 avenue Guy de Collonges, 69134 Ecully Cedex, France camille.gandiolle@ec-lyon.fr , siegfried.fouvry@ec-lyon.fr A BSTRACT . Fretting fatigue experiments aim to represent industrial problems and most of them endure variable loading. Being able to assess lifetime of assemblies, especially for low propagation rate conditions, is essential as experimental validation is often too expensive. Both experimental and numerical approaches are proposed to follow the crack propagation rate of steel on steel cylinder/plane fretting fatigue contact submitted to variable loading conditions. An original experimental monitoring has been implemented on the fretting-fatigue test device to observe crack propagation using a potential drop technique. A calibration curve relating crack length and electrical potential was established for the studied contact. It allows direct knowledge of the crack length and crack propagation rate. It was applied to mixed load test showing crack arrest for the last loading condition. To explain this behavior, a 2-dimensional FE modeling was implemented to simulate the complexes multi-axial contact stressing. The crack propagation rate was formalized using an effective stress intensity factor amplitude ΔK eff coupled with Paris law of the material. The crack arrest condition for a given loading was related to ΔK eff along the expected crack path crossing the material crack arrest threshold ΔK 0 . The failure was related to ΔK eff reaching the critical stress intensity factor K IC . A good correlation with experiments was observed allowing to predict the crack arrest condition although the model tends to overestimate the final crack length extension. K EYWORDS . Fretting fatigue; FEM; Cracking; Variable loading. I NTRODUCTION retting is defined as a small oscillatory movement between two bodies in contact which induce relative displacement between the two surfaces. Combined with cyclic bulk fatigue loading, the so called fretting-fatigue loading can induce catastrophic damages such as wear or cracking, which critically reduce the endurance of assemblies. In addition, fretting fatigue experiments aim to represent industrial problems and most of them endure variable loadings. Being able to assess lifetime of assemblies, especially for variable loading applied for very high number of cycles, is essential as experimental validation is often too expensive. This study concentrates on cracking damage and is restricted to the partial slip domain. Crack nucleation risk is usually investigated by applying multi-axial fatigue criterion. Predictions were improved by considering the severe stress gradients imposed by the contact loading, using non-local process volume stress averaging strategy [1] or equivalent critical distance [2]. F