Issue 33

F. Morel et alii, Frattura ed Integrità Strutturale, 33 (2015) 404-414; DOI: 10.3221/IGF-ESIS.33.45 413 C ONCLUSIONS he purpose of this study has been to investigate the multiaxial fatigue performance of metals in the presence of defects through statistical modeling of the microstructure. More exactly, the goal has been to analyze the competition existing between the stress concentration induced, on one hand, by a small defect and, on the other hand, by the most highly stressed regions of the microstructure caused by the anisotropic behavior of the grains. In a first phase, the cyclic mechanical responses of polycrystalline aggregates, obtained from finite element simulations, have been analysed for various loading modes and in the presence of geometrical defects. It has been shown the local stress field is highly scattered and differs significantly from the macroscopic response of the polycrystalline aggregate. In a second phase, an experimental campaign conducted on an austenitic steel 316L has shown the influence of hemispherical defect on the fatigue strength. The role of the loading modes for different defect sizes has been clearly established. It has been observed an evolution of the experimental ratio between the fatigue limits under fully reversed torsion and fully reversed tension with respect to the defect size. The predictions reached from a probabilistic criterion (originally proposed by Morel) using the FE computations appear to be in good agreement with most of the experimental results. The role of the shape factor m of the probabilistic criterion is discussed and it is shown to play a role similar as a material characteristic length for the crack initiation mechanism. The Kitagawa effect is perfectly reflected by means of the model proposed and the related loading mode influence is also adequately accounted for. The simulations discussed in this paper are of 2D types but some extra 3D EF simulations have been conducted for the smallest defect size (50  m) and the same trends have been observed. In particular, both 2D and 3D simulations together with the probabilistic criterion show an increase of the ratio between the fatigue limits under fully reversed torsion and fully reversed tension with respect to the defect size (Fig. 6). R EFERENCES [1] Lukàš, P., Kunz, L., Weiss, B., Stickler, R., Notch size effect in fatigue, Fatigue Fract Eng Mater Struct, 12(3) (1989) 175-186. [2] Susmel, L., Taylor, D., A simplified approach to apply the theory of critical distances to notched components under torsional fatigue loading, Int J Fatigue, 28(4) (2006) 417-30. [3] Endo, M., Ishimoto, I., The fatigue strength of steels containing small holes under out-of-phase combined loading. Int J Fatigue, 28(56) (2006) 592-597. [4] Dang Van, K., Macro-micro approach in high-cycle multiaxial fatigue, ASTM Special Technical Publication; (1993) 120-130. [5] Papadopoulos, I., A new criterion of fatigue strength for out-of-phase bending and torsion of hard metals, Int J Fatigue, 16(6) (1994) 377-384. [6] Monchiet, V., Charkaluk, E., Kondo, D., Plasticity-damage based micromechanical modelling in high cycle fatigue, Comp Rendus Mécan, 334(2) (2006) 129-136. [7] Morel, F., Huyen, N., Plasticity and damage heterogeneity in fatigue, Theor Appl Fract Mech, 49(1) (2008) 98-127. [8] Bennett, V., McDowell, D., Polycrystal orientation distribution effects on microslip in high cycle fatigue, Int J Fatigue, 25(1) (2003) 27-39. [9] Guilhem, Y., Basseville, S., Curtit, F., Stéphan, J.-M., Cailletaud, G., Investigation of the effect of grain clusters on fatigue crack initiation in polycrystals, Int J Fatigue, 32(11) (2010) 1748-1763. [10] Robert, C., Saintier, N., Palin-Luc, T., Morel, F., Micro-mechanical modelling of high cycle fatigue behaviour of metals under multiaxial loads, Mech Mater, 55 (2012) 112-129. [11] Fang, X., Yan, W., Gao, H., Yue, Z., Liu, J., Wang, F., Finite element simulation of surface deformation of polycrystal with a rough surface under repeated load, Finite Elements Anal Des, 60 (2012) 64-71. [12] Le Pécheur, A., Curtit, F., Clavel, M., Stephan, J.M., Rey, C., Bompard, Ph., Polycrystal modelling of fatigue: pre- hardening and surface roughness effects on damage initiation for 304L stainless steel, Int J Fatigue, 45 (2012) 48-60. [13] Koutiri, I., Bellett, D., Morel, F., Augustins, L., Adrien, J., High cycle fatigue damage mechanisms in cast aluminium subject to complex loads, Int J Fatigue, 47 (2013) 44-57. [14] Pessard, E., Morel, F., Verdu, C., Flaceliere, L., Baudry, G., Microstructural heterogeneities and fatigue anisotropy of forged steels, Mat Sc Engng A, 529 (2011) 259-299. T