Prediction of the Scatter of Crack Initiation under High Cycle Fatigue

Abstract

Under fatigue loading, the number of cycles to failure and its associated scatter increase when the loading level decreases. The High-Cycle Fatigue (HCF) regime is thus characterized by a large scatter in the number of cycles to failure [1]. Cracks initiation represents an important part of the lifetime of the structures. A stochastic method is used to study the fatigue crack initiation prediction in the 316L austenitic stainless steel. The present work proposes to show that this scatter can be attributed to the random orientation of individual grains, which influences the crack initiation localization. The stresses in grains are determined by finite element computations (FEM [2]), using a configuration representative of a polycrystalline aggregate. This approach takes into account the crystallographic orientations of the grains in the aggregate as well as the deformation incompatibilities between neighbouring grains due to crystalline anisotropic elasticity and elasticplasticity [3]. Then, the scatter of the number of cycles to crack initiation is derived from the FEM stress fields using two fatigue crack initiation criteria: an usual one, Mura’s criterion [4] and a more recent one [5], based on Discrete Dislocation Dynamics (DDD) simulations and taking into account plastic slips, cross slip and stress tensor components.

Identification and Probabilistic Modeling of Mesocrack Initiations in 304l Stainless Steel

“…Several stochastic models have been proposed to simulate mesocrack densities. They are often based on Monte-Carlo simulations (Suh et al, 1992;Bataille and Magnin, 1994;Argence, 1996;Osterstock et al, 2007) and identified by using few experimental data. Since the initiation phenomenon in fatigue occurs generally on the surface and at a very small scale, classical macroscopic measurements do not allow the experimentalist to get quantitative data of this process.…”

Stage I surface crack formation in thermal fatigue: A predictive multi-scale approach

“…Equi-probable coalescence along X and Y and homogeneous partition of the crack habit planes and twist angles can be explained in the following way. Applying an equi-biaxial loading in the (X, Y) plane is equivalent to applying uni-axial tension along Z, superimposed to a hydrostatic pressure [49,50,56]. The mechanical stress field is therefore exactly the same in every direction parallel to the (X, Y) plane, yielding no preferential crack initiation and linking directions.…”

“…Namely, twist angles comprised between ±20 • and ±70 • are equi-probable, whereas the probability to findˇi = 0 ± 20 • (crack habit planes ∼parallel to the free surface) andˇi = 90 ± 20 • (crack habit planes ∼normal to the free surface) is very low. Using the same biaxial load equivalence as before [49,50,56], the average Schmidt factor f in a specific grain can be evaluated from angles and :…”

“…When anisotropic elasticity is accounted for, expression (3) yields realistic prediction of the crack initiation scattering, in function of applied cyclic strain amplitude (see Fig. 7a and [49,50]). A sharp increase of N i /2N i with decreasing cyclic loading is indeed consistent with well-known experimental trends [51].…”

Modelling of the effect of dislocation channel on intergranular microcrack nucleation in pre-irradiated austenitic stainless steels during low strain rate tensile loading

“…This is due to the channel non-linear behaviour. Nevertheless, elastic strain measurements [74] and computations carried out for higher plastic strains [74,75], corresponding to generalized plasticity in the polycrystal show that the scatter of the mean grain plastic strain distribution decreases dramatically with the macroscopic strain increasing. And the computations taking into account the variable orientation of the next grain lead to negligible scatter of the GB stress field close to the channel-GB intersection.…”