Modèles d'écrouissage pour flexion pure de tôles d'acier : de l'identification à l'estimation des contraintes résiduelles

National audienceLes composants pour la liaison au sol sont classiquement dimensionnés à la fatigue polycyclique. Le comportement du matériau est donc très simple à prédire car élastique. Néanmoins, la connaissance du comportement plastique ne peut être écartée pour deux raisons principales : tout d'abord, les pièces de liaison au sol sont mises en forme par déformation plastique (emboutissage), opération générant des états mécaniques locaux complexes qu'il est nécessaire de prendre en compte dans les étapes de dimensionnement ultérieures ; ensuite car le dimensionnement doit permettre d'assurer l'intégrité du composant lorsque celui-ci est soumis à des coups forts, pouvant générer localement de la plasticité. Dans ce contexte, les travaux exposés ici portent sur deux points. Dans un premier temps, nous nous sommes intéressés à l'identification de modèles élastoplastiques (isotrope, cinématique et mixte [3]) réalisée sur une base expérimentale constituée d'essais de pliage/dépliage sur tôle. Une attention toute particulière est portée à la compétition entre écrouissage cinématique et isotrope. Les essais sont réalisés sur un dispositif de flexion pure développé au CEMEF. L'identification est permise par un algorithme spécifique d'identification présenté en figure 2-b. Dans un second temps, les modèles identifiés sont testés sur une base expérimentale enrichie combinant des essais de pliage/dépliage cyclique à des mesures de contraintes résiduelles après relâchement. Un indicateur d'erreur est mis en place afin de hiérarchiser les capacités de prédiction des modèles. Celui-ci nous permet de conclure à la supériorité du modèle mixte

Load spectra and fatigue damage: applications to the automotive industry

High cycle fatigue behaviour of materials is historically assessed with constant amplitude and variable amplitude loads, respectively. Thus, a long-lasting debate is extremely active in the academic community, trying to link experimental results coming from these different approaches. Overcoming all this, since the 1970s several industrial fields have been choosing to consider representative customer load spectra (in terms of amplitude, not frequency) as the best way to test both materials and structures. In particular, the automotive industry makes use of specific car loading spectra, regularly fed by the customer knowledge and practised on proving grounds. This paper presents a highlight on such spectra, neglecting any sequence effect of the load time history, thus accepting the Palmgren-Miner’s rule as an assumption. Whereas a recent communication on this very topic focused on the basic occurrence spectra, which is absolutely independent from the material properties of the car parts, here we deal with the final damage assessment. Obviously, it is worth knowing which part of the spectrum is mainly responsible for the most relevant fatigue damage

Un modèle phénoménologique des vibrations induites par détachement tourbillonnaire

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Automotive stamped part fatigue design

Fatigue design of automotive axle parts is of prior concern because of these are high safety parts and they are expected to drive the overall vehicle mass reduction. In this framework, the stamping process is widely used to form axle parts, before assembling them by welding. Consequently, the mechanical and physical characteristics of the blank sheet are modified, having a strong influence on the fatigue behavior. In this paper, we address the consequences of the stamped process on the fatigue design and how they may be effectively taken into account in the automotive industry. Actually, the coupling between the stamping process and the fatigue design is under development at PSA Peugeot Citroën Company since several years. Such an analysis deals with some major topics: thickness variation, plastic hardening and deformation, and residual stress. All of these ingredients result in fatigue criteria evolutions: it is important to stress that the stamping process can have advantageous or disadvantageous consequences on the fatigue design. This is here highlighted on some examples, dealing with front and rear axles

Automotive stamped part fatigue design

Fatigue design of automotive axle parts is of prior concern because of these are high safety parts and they are expected to drive the overall vehicle mass reduction. In this framework, the stamping process is widely used to form axle parts, before assembling them by welding. Consequently, the mechanical and physical characteristics of the blank sheet are modified, having a strong influence on the fatigue behavior. In this paper, we address the consequences of the stamped process on the fatigue design and how they may be effectively taken into account in the automotive industry. Actually, the coupling between the stamping process and the fatigue design is under development at PSA Peugeot Citroën Company since several years. Such an analysis deals with some major topics: thickness variation, plastic hardening and deformation, and residual stress. All of these ingredients result in fatigue criteria evolutions: it is important to stress that the stamping process can have advantageous or disadvantageous consequences on the fatigue design. This is here highlighted on some examples, dealing with front and rear axles

Vortex-induced travelling waves along a cable

International audienceWe investigate the vortex-induced vibrations (VIV) of a very slender cable subjected to a stationary uniform cross-flow, using a travelling wave approach. A phenomenological model of the near wake based on van der Pol oscillators is developed and tested in comparison with numerical simulations and experimental data. A selection criterion for vortex-induced waves (VIW) is established: the fluid selects the frequency, according to Strouhal's law, and the structure fixes the wavenumber, as dictated by its dispersion relation

Fast fatigue properties identification by "self-heating" method : application to automotive welded joints in Fatigue design 2013 : 5th international conference on fatigue design, Senlis (France), 27-28 Nov. 2013 / edited By Mansour Afzali and Fabien Lefebvre

International audienceThe welded assemblies' high cycle fatigue properties identification for the automotive industry is a long and expensive process which has to be very efficient to perform both safety and lean design. Fatigue properties determination is a time and cost consuming process, it requires several days and specimens to be performed. The self-heating method offers the possibility to dramatically shorten the test duration. It consists in measuring the temperature of the structure under cyclic loading, and then linking it to fatigue properties. Only one specimen and few hours are required for fatigue properties identification providing important cost and time reduction. Thus, an experimental protocol is proposed to measure the rise of temperature of a lap joint welded specimen, with the use of infra-red thermography. Assuming that only self-heating and thermoelastic phenomena are responsible of the temperature evolution, self-heating temperature is extracted from experimental data. A 1D thermal model is proposed to describe the evolution of the temperature of the specimen due to self-heating. The welded joint concentrates dissipative phenomena, which corresponds to a concentrated heat source. This heat source field is then identified by fitting the model to experimental results, and its evolution with the applied loading is used to determine the welded assembly endurance limit. Self-heating method results are proven to be consistent to those obtained by classical fatigue tests

Loading classification proposal for fatigue design of automotive chassis-parts: A relevant process for variable amplitude and multi-input load cases

International audienceIt is essential for car manufacturers to ensure their vehicle robustness under a daily usage. As no detailed method nor standard prevails, each manufacturer is expected to setup its own methodology for safety parts life-assessment. The early stages of a renewed methodology are proposed in this study. This is based on proving-grounds measurements analysis to quantify the impact of various loads. The method, developed from the load measured at the vehicle wheels, considers the multi-input load case and its variable amplitude content. Measured time-series are analyzed and partitioned into two main categories depending on the time-correlation between the load axes. The driver-induced loads, Driven-Road, (DR) loads, are separated from the effects of random road conditions, Random-Road, (RR) loads. Then, for partition validation, a suitable life quantification method is applied in parallel to the two types of loading. On one hand, an assumption based on strong physical meaning eases the Rainflow Counting method application on the DR load. On the other hand, the use of spectral methods is explored to handle the random loads. This paper details the partition process of the time-series and each of the subsequent validation steps of the method to apply such a partition. Such a validation is performed via a comparison of the pseudo-damage between a referenced signal and the one issued from the partition