Teaching durability in automotive applications using a reliability approach

Fatigue phenomena, which appear generally below the yield stress, is the cause of more than 80 % of in-service mechanical failures. However, the optimization of the weight and cost when designing mechanical components or structures, linked to improved performance, leads to increasingly stressed components. Therefore a fatigue design approach must be done by the engineer. This paper shows the experience gained over five academic years of teaching fatigue the assessment of automotive components using a reliability approach to predict probability of failure, in the engineering school, Arts et Métiers ParisTech, in France. The choice was made to present a comprehensive fatigue assessment approach using a method, initially developed in the automotive industry and since extended to the aeronautical and mechanical industries. This method is known as the “Stress-Strength interference analysis”. The “Stress” represents the distribution of the driver severity, and the “Strength” represents the distribution of the fatigue strength of all the components. A suspension arm is used to illustrate the approach. The Dang Van multiaxial fatigue criterion is implemented in a Finite Elements Code and a danger coefficient is visualized on the meshed structure. The fatigue analysis is interpreted with respect to the target reliability sought by the car- manufacturer

A Biaxial Fatigue Specimen for Uniaxial Loading

The aim of this paper is to present a novel un-notched fatigue test specimen in which a biaxial stress state is achieved using a uniaxial loading condition. This allows the problem of multi-axial fatigue to be studied using relatively common one-axis servo-hydraulic testing machines. In addition the specimen presented here is very compact and can be made using a small volume of material (100x40x4.5mm). For this specimen, the degree of biaxiality, defined by the parameter is equal to approximately 0.45. The specimen geometry was optimised using the Dang Van multi-axial fatigue criterion. In addition to use as a fatigue specimen, it has been demonstrated that the biaxial specimen presented here is also suitable for biaxial tensile tests, to determine the rupture strength of a material in a biaxial stress state. Two different materials have been investigated: The first was wrought aluminium 2024-O in the form of 5mm sheets. The second was a cast aluminium-silicon alloy AlSi7Cu0.5Mg0.3, commonly used in automotive and aeronautical applications. The fatigue strengths were determined at 2x106 cycles and at various R-ratios using a staircase procedure. For the aluminium 2024, it is shown that the biaxial stress state increases the maximum permissible first principal stress when compared to the uniaxial condition. However, in terms of the cast aluminium alloy, it has been demonstrated that this type of fatigue specimen is not suitable for materials containing casting defects, in particular micro-shrinkage pores, because the volume of material, in which the stress state is biaxial, is not large enough.The authors gratefully acknowledge the financial support of PSA – Peugeot Citroën and also that of the Conseil Général du Département de Maine-et-Loire, France

Characterising the impact of surface integrity on the fatigue behaviour of forged components

The present study focusses on analysing and modelling the influence on fatigue behaviour of the surface of a hot-forged C70 connecting rod which undergoes a shot-blasting treatment. The shot-blasting heavily affects the surface and thus the fatigue properties. In addition, the forging process introduces large defects which also have an effect on the fatigue strength. So as to be able to determine which aspects of the surface integrity are the most influential in fatigue, various surface states were thoroughly characterised and then tested in high cycle fatigue in bending. The various aspects studied are the surface roughness and large defects, residual stresses, microstructure and hardness

A probabilistic high cycle fatigue model applied to cast Al-Si alloys

In this work, the high cycle fatigue behavior of cast hypo-eutectic Al-Si alloys is investigated. It is observed that two different coexisting fatigue initiation mechanisms can occur in these materials depending on the presence of different microstructural heterogeneities (i.e. micro-shrinkage pores, Si particles, Fe-rich inter-metallics, DAS of the Al-matrix, etc).Firstly, an experimental study is presented, highlighting the coexistence of these two fatigue damage mechanisms and their dependence on the presence of different micro-structural heterogeneities. A probabilistic high cycle fatigue model, which has the capacity to describe these two mechanisms, is then presented. The model contains the same principal ingredients as the one developed by [1]. It uses a probabilistic framework to link the two different fatigue damage mechanisms, and can take into account the mean stress and the effect of a biaxial stress state.This work was undertaken in partnership with PSA Peugeot Citrooën and was financially supported by the French region, Pays de la Loire

Micromechanical investigation of the influence of defects in high cycle fatigue

This study aims to analyse the influence of geometrical defects (notches and holes) on the high cycle fatigue behaviour of an electrolytic copper based on finite element simulations of 2D polycrystalline aggregates. In order to investigate the role of each source of anisotropy on the mechanical response at the grain scale, three different material constitutive models are assigned successively to the grains: isotropic elasticity, cubic elasticity and crystal plasticity in addition to the cubic elasticity. The significant influence of the elastic anisotropy on the mechanical response of the grains is highlighted. When considering smooth microstructures, the crystal plasticity have has a slight effect in comparison with the cubic elasticity influence. However, in the case of notched microstructures, it has been shown that the influence of the plasticity is no more negligible. Finally, the predictions of three fatigue criteria are analysed. Their ability to predict the defect size effect on the fatigue strength is evaluated thanks to a comparison with experimental data from the literature

Characterising the impact of surface integrity on the fatigue behaviour of a shot-peened connecting rod

The present study focuses on analysing and modelling the influence on fatigue behaviour of the surface of a hot-forged C70 connecting rod which undergoes a shot-blasting treatment. The shot-blasting heavily affects the surface and thus the fatigue properties. In addition, the forging process introduces large defects which also have an effect on the fatigue strength. So as to be able to determine which aspects of the surface integrity are the most influential in fatigue, additional surface states were generated by shot-peening the as-forged surface. The various surface states were thoroughly characterised and then tested in high cycle fatigue in bending. The various aspects studied are the surface roughness and large defects, residual stresses, and microstructure

Evolution du bilan énergétique dans les matériaux métalliques sous sollicitation cyclique.

In the case of cyclic plasticity, the validity of a constitutive model is usually assessed using stress-strain curves. However, this description can be enriched by adopting an energetic point of view. Thus, in the present work, a multiscale model is developed to estimate the amount of energy which is either dissipated into heat or stored in the material in a medium carbon steel under cyclic loading. The results emphasize the heterogeneous aspect of the stored and dissipated energy fields at a microscopic scale

Modelling, identification and application of phenomenological constitutive laws over a large strain rate and temperature range

A review of the different phenomenological thermo-viscoplastic constitutive models often applied to forging and machining processes is presented. Several of the most common models have been identified using a large experimental database (Hor et al., 2013). The latter consists of the tests were done in compression on cylindrical shaped specimens and in shear using hat-shaped specimens. The comparison between these different models is shown that the group of decoupled empirical constitutive models (e.g. the Johnson and Cook (1983) model), despite their simple identification procedures, are relatively limited, especially over a large range of strain rates and temperatures. Recent studies have led to the proposal of coupled empirical models. Three models in this class have also been studied. The Lurdos (2008) model shows the best accuracy but requires a large experimental database to identify its high number of parameters. After this comparison, a constitutive equation is proposed by modifying the TANH model (Calamaz et al., 2010). Coupling between the effects of strain rate and temperature is introduced. This model is easier to identify and does not require knowledge of the saturation stress. Compared to other models, it better reproduces the experimental results especially in the semi-hot and hot domains. In order to study real machining conditions, an orthogonal cutting tests is considered. The comparison between experimental test results and numerical simulations conducted using the previously identified constitutive models shows that the decoupled empirical models are not capable of reproducing the experimental observations. However, the coupled constitutive models, that take into account softening, improve the accuracy of these simulations

The effect of quenching and defects size on the HCF behaviour of Boron steel

This work investigates the effect of natural and artificial surface defects and quenching on the fatigue strength of a Boron steel (22MnB5). A vast experimental campaign has been undertaken to study the high cycle fatigue behaviour and more specifically the fatigue damage mechanisms observed in quenched and untreated materials, under different loading conditions and with differents artificial defects sizes (from 25 μm to 370 μm radius). In order to test the sheet metal in shear an original test apparatus is used. The critical defect size is determined to be 100 ± 50 μm. This critical size does not appear to depend on the loading type or the microstructure of the material (i.e. ferritic–perlitic or martensitic). However, for large defects, the quenched material is more sensitive to the defect size than the untreated material. For a defect size range of 100–300 μm the slope of the Kitagawa–Takahashi diagram is approximately −1/3 and −1/6 for the quenched and untreated materials respectively. A probabilistic approach that leads naturally to a probabilistic Kitagawa type diagram is developed. This methodology can be used to explain the relationship between the influence of the heat treatment and the defect size on the fatigue behaviour of this steel

Experimental and numerical study of the evolution of stored energy in metallic materials under cyclic loading

In-service loading conditions usually generate complex cyclic stress states. As such, the choice of an appropriate multiaxial fatigue criterion plays a crucial role in obtaining correct fatigue predictions. In the case of high cycle fatigue, the observation of the stabilized behavior is generally required to build either stress-based criteria or energy-based criteria. ...