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In the present study, the effects of both the microstructure and voids on the high-cycle fatigue behaviour of the 316L austenitic stainless steel are investigated by using finite element simulations of polycrystalline aggregates. The numerical analysis relies on a metallurgical and mechanical characterization. In particular, fatigue tests are carried out to estimate the fatigue limits at 2.106 cycles under uniaxial and multiaxial loading conditions (combined tension and torsion and biaxial tension) using both smooth specimens and specimens containing an artificial hemispherical defect. The simulations are carried out with several configurations of crystalline orientations in order to take into account the variability of the microstructure in the predictions of the macroscopic fatigue limits. These predictions are obtained, thanks to a probabilistic fatigue criterion using the finite element results. The capability of this criterion to predict the influence of voids on the average and the scatter of macroscopic fatigue limits is evaluated.

The work proposed in this paper is a possible way of modelling some local observations at the surface of mild steel specimens submitted to uniaxial and multiaxial loads. It is clearly seen that local plasticity, controlled by local microstructural heterogeneities, plays a fundamental role in microcrack nucleation and damage orientation is closely related to the applied loading mode. The framework of irreversible thermodynamics with internal variables for time-independent, isothermal and small deformations has been used to build a critical plane damage model by assuming the existence of a link between mesoplasticity and mesodamage. Non-associated plasticity and damage rules allow the evolution of some plastic slip before any damage nucleation, as seen during the observations. A key feature of this proposal is the capacity to reflect nonlinear damage accumulation under variable amplitude loading.

The stress gradient and the size of a component are known to influence the fatigue strength of metallic components. Indeed, in high-cycle fatigue, experiments prove that the stress distribution as well as the size of the loaded specimen can be responsible for changes in the fatigue limit (for instance, the fatigue limits in tension and bending are different, and decrease with the size of the specimen). When dealing with multiaxial load conditions, those effects still act but a relevant criterion must be used to account for the complex state of stress. The weakest-link concept together with a multiaxial endurance criterion based on a microplasticity analysis are then combined to describe the fatigue limit distribution of different metallic materials. Several load conditions are analysed: tension-compression, torsion, rotating bending and plane bending. By means of the proposed model, all the known effects on fatigue strength can be reflected. First, the endurance probability can be adequately predicted for any complex load conditions knowing some reference data from uniaxial fatigue tests. It can be linked to the probability of finding a defect with a critical size. The weakest-link theory also accounts for the decrease of multiaxial fatigue limit with the stressed volume. For the same load condition (i.e. for the same stress distribution in the volume), the probability of finding a critical defect increases with the component size and then according to the weakest-link theory the fatigue strength drops. A second model, based only on the damage developed at the surface, is also proposed. While the original Weibull theory makes no distinction between potential initiation sites at the free surface and in the volume and can lead to unsatisfactory predictions when applied to materials containing defects such as nodular cast iron, the new surface approach distinguishes between surface and volume effects.

The stress gradient effect on the fatigue limit is an important factor which has to be taken into account for an efficient transfer of fatigue data from laboratory tests to design of industrial components. A short review of some multiaxial high cycle fatigue criteria considering this effect is presented. On the basis of the two local mesoscopic approaches of Papadopoulos, two new non-local high cycle multiaxial fatigue criteria are developed. These proposals are based on the concept of volume influencing fatigue crack initiation. Their predictions are compared with experimental multiaxial fatigue data on four materials (a mild steel, two high strength steels and a spheroidal graphite cast iron). The accuracy of the two local Papadopoulos criteria and of the non-local proposals are compared and discussed, together with the physical interpretation of the threshold defining the volume influencing fatigue crack initiation.

This paper presents an easy to handle but efficient fatigue life prediction method based on a plasticity analysis carried out at the scale of the grains, i.e. the mesoscopic scale. The different steps of the model are described in the case of in-phase as well as out-of-phase bending-torsion load conditions. One striking feature of this method is that the definition of the critical material plane and the estimation of the number of cycles to initiation are made by means of analytical expressions that are functions of the loading parameters such as amplitudes, means and phase angles. The identification of the model parameters is readily carried out by means of one SN curve and two fatigue limits. This model is able to reflect the influence of the phase shift on damage accumulation with the introduction of no adjustable parameters. Furthermore, the comparison between the predictions and in-phase and out-of-phase fatigue data found in the literature leads to very good correlation.