Search

International audience; This study focuses on the influence on fatigue behaviour of the surface integrity of a steel connecting rod. The component is hot-forged and shot-blasted, producing a complex surface state with large surface defects and high residual stresses. In a prior study, the surface was thoroughly characterized and fatigue tests were performed. Several different surface states were analysed in order to quantify the influence of the various surface aspects. These tests showed that the forging defects and the residual stresses are the most influential aspects of the surface. The goal of this paper is to develop an approach capable of taking into account the influence of both these aspects on fatigue behaviour. Two methods were developed. First, using surface scans of the fatigue specimens, the forging defects were fitted with ellipsoids so as to determine their size and shape. This allows to easily compare the numerous defects and test various criteria in order to identify the critical defect of each specimen. The second method used was the finite element simulation of the defects based on real topography scans. Using a non-local approach based on the theory of critical distances, the simulations were used to accurately predict the influence of the defects' geometry. The residual stress profiles were integrated in the simulations using Dang Van's criterion. The predictions are accurate and show the importance of taking into account the real defect geometry when estimating the fatigue strength. (C) 2017 Elsevier Ltd. All rights reserved.

Cold extrusion is a process commonly used to manufacture drive train components in the automotive industry. Large plastic strains can be applied during this operation (up to 1.5) and greatly changes the mechanical properties of the resulting material. This study focuses on the impact of cold extrusion process parameters on the multiaxial fatigue behaviour of steel components. A specific set of forward rod extrusion tools was developed to get original fatigue specimen able to characterise the effect of the manufacturing process on the fatigue behaviour. The specimens were extruded from two different initial diameters, giving two different reductions in cross-section of 18% and 75% respectively. To understand the influence of cold extrusion, the following analyses have been undertaken for each condition and on the initial material: monotonic tensile properties, microstructure, EBSD, residual stresses and hardness. Simulation of the forming process and microstructural observations show that the plastic strain is homogeneous in the specimen section. For both reduction factors, the forming process has a positive effect on the components properties: induced residual stresses in compression and improved hardness and roughness (Ra decreasing). Tension, plane bending and torsion fatigue tests show that the fatigue strength is about 30% higher for the batch with 75% reduced cross-section. All investigations show that strain hardening is the principal material parameter responsible for the increase in fatigue strength. A multiaxial fatigue criterion taking into account the effects of the forward rod extrusion process was also developed. This work has been performed within the ANR (National Research Agency) DEFISURF project, in a partnership including several industrial (Ascometal, Cetim, PSA, Transvalor, Atelier des Janves, Gévelot) and academic (INSA Lyon MATEIS, ENSMP-CEMEF, Arts et Métiers ParisTech LAMPA) institutions.

This paper is dedicated to the high cycle fatigue (HCF) behaviour of cast Al–Si alloys. In particular, three similar alloys with different microstructural characteristics are investigated. The result of an experimental campaign is presented, in order to characterise the fatigue behaviour, and more specifically the fatigue damage mechanisms related to the different microstructural heterogeneities (i.e. casting porosity, dendrite size, SDAS, non-metallic inclusions and silicon particles), observed under different multiaxial loading conditions: pure tension, plane bending, pure torsion and combined tension–torsion with a load ratio R=−1. It is shown that casting porosity has a very detrimental influence on the uniaxial and combined tension–torsion fatigue strengths. However, a much lower influence is observed for the torsional fatigue strength. For the porosity-free alloy, it is observed that the formation of persistent slip bands (PSB) in the aluminium matrix is the major fatigue crack initiation mechanism regardless of the loading modes, at a load ratio of R=−1. It is also shown that the aluminium matrix has a large role in the formation of PSB and that the Si particles facilitate the formation of PSB.

Lien vers la version éditeur: http://www.sciencedirect.com/science/article/pii/S0142112312002356; International audience; This article is dedicated to the high cycle fatigue behaviour of cast hypo-eutectic Al-Si alloys. In particular, the AlSi7Cu05Mg03 alloy is investigated. It presents the results of a vast experimental campaign undertaken to investigate the fatigue behaviour, and more specifically the fatigue damage mechanisms observed under complex loading conditions: plane bending with different load ratios, fully reversed torsion and equibiaxial bending with a load ratio of R = 0.1. A specific test set-up has been designed to create an equibiaxial stress state using disk shaped specimens. A tomographic analysis is also presented with the aim of characterising the micro-shrinkage pore population of the material. It is shown that two distinct and coexisting fatigue damage mechanisms occur in this material, depending on the presence of different microstructural heterogeneities (i.e. micro-shrinkage pores, Silicon particles in the eutectic zones, Fe-rich intermetallic phases, etc.). Furthermore, it is concluded that the effect of an equibiaxial tensile stress state is not detrimental in terms of high cycle fatigue. It is also shown that the Dang Van criterion is not able to simultaneously predict the multiaxial effect (i.e. torsion and equibiaxial tension) and the mean stress effect for this material.

An analysis of high cycle multiaxial fatigue behaviour is conducted through the numerical simulation of polycrystalline aggregates using the finite element method. The metallic material chosen for investigation is pure copper, which has a Face Centred Cubic (FCC) crystalline microstructure. The elementary volumes are modelled in 2D using an hypothesis of generalised plane strain and consist of 300 equi-probability, randomly oriented grains with equiaxed geometry. The aggregates are loaded at levels equivalent to the average macroscopic fatigue strength at 10 7 cycles. The goal is to compute the mechanical quantities at the mesoscopic scale (i.e., average within the grain) after stabilization of the local cyclic behaviour. The results show that the mesoscopic mechanical variables are characterised by high dispersion. A statistical analysis of the response of the aggregates is undertaken for different loading modes: fully reversed tensile loads, torsion and combined in-phase tension–torsion. Via the calculation of the local mechanical quantities for a sufficiently large number of different microstructures, a critical analysis of certain multiaxial endurance criteria (Crossland, Dang Van and Matake) is conducted. In terms of material behaviour models, it is shown that elastic anisotropy strongly affects the scatter of the mechanical parameters used in the different criteria and that its role is predominant compared to that of crystal plasticity. The analysis of multiaxial endurance criteria at both the macroscopic and mesoscopic scales clearly show that the critical plane type criteria (Dang Van and Matake) give an adequate estimation of the shear stress but badly reflect the scatter of the normal stress or the hydrostatic stress.

This paper deals with the presentation of a high cycle multiaxial fatigue life prediction method for metallic materials. By means of the mesoscopic approach introduced by Dang Van and developed by Papadopoulos, accumulated plastic strain due to external loading is estimated at a scale on the order of a grain or a few grains. Its evaluation requires the use of a critical plane type fatigue criterion. As soon as the accumulated plastic mesostrain, considered as the damage variable, reaches a critical value, a crack is considered to be initiated. The complex and combined cases of loading (multiaxial and variable amplitude) can be analysed with this new method. Particular attention is given to a description of the detrimental effect of out-of-phase loadings. A good agreement has been found between the predicted and experimental results for in-phase and out-of-phase sinusoidal constant amplitude loadings by examining a large amount of experimental data.