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The high cycle fatigue (HCF) is a major element for a great design of automotive parts. A wide part of the steel sheets for the automotive industry are stamped, sometimes deeply. During this operation, the steel is plastically strained in different directions, so that a good prediction of the fatigue behavior requires the determination of the fatigue properties of the pre-strained material. Nowadays, the evolution of HCF properties is often neglected, because of prohibitive time dedicated to traditional fatigue campaigns. To reduce the characterization time, self-heating measurements are used. This approach permits to identify the influence of homogeneous pre-strain on fatigue properties. The aim of this paper is to develop an original experimental test to identify this influence for a wide range of pre-strain with only one specimen. The study of a particular case of specimen with a constant gradient of pre-strain is presented. Digital image correlation is a way to determine the heterogeneity of the plastic pre-strain on the specimen and infrared measurements with a ”1D” approach allows the determination of the influence of a plastic pre-strain on the fatigue properties of the studied steel.

Abstract: As elastomeric materials are heterogeneous by nature, their fatigue behavior is strongly driven by the initiation and the growth of cavities. The explanations on the microscopic mechanisms are usually supported by SEM observations. SEM is a 2D destructive technique that can induce artifacts and is not suitable to analyze 3D phenomena. Moreover, the time needed to observe a single slice prevents from scanning a whole massive sample. In this study X-ray micro-tomography is used. This non destructive method has already been widely applied to elastomeric materials to control the fillers size and dispersion or to analyze the cavitation induced under high hydrostatic pressure fatigue loading, for example. Here, this technique is used with a good resolution to analyze the evolution of the defects population during a fatigue campaign on hourglass-shaped specimens. The initiation and propagation mechanisms are clearly illustrated on 3D observations, and the influences of the maximum principal strain and of the number of cycles on several damage parameters (size repartition, porosity, defect volumic density) are investigated. A scenario for the fatigue damage evolution is proposed and some fatigue initiation criteria are finally discussed with the help of the micro-structural measurements. [Copyright &y& Elsevier]

Abstract: The temperature of rubber-like materials increases under cyclic loadings due to their dissipative behavior and low thermal conductivity. This well-known phenomenon, called “heat build-up” in the rubber community, has attracted the attention of researchers for a long time. But, to our knowledge, no published studies have tried to link the temperature rise to a fatigue behavior, as already done on some metallics. Two main points are discussed in this paper. The first one is dedicated to heat buildup measurements: a dedicated experimental protocol is developed to capture the instantaneous heat build-up (taking into account the thermomechanical couplings) and, based on this protocol, a heat build-up experiment is defined in order to link the steadystate temperature to the maximum principal strain. A discussion on the correlation between these results and the fatigue behavior is opened and illustrated for several industrial materials. The second point investigates the ability to couple X-ray -tomography measurements presented in another paper  to the former heat build-up results in order to predict the initiation lifetime. An approach based on a critical energy criterion is proposed and the comparison to a classic Wöhler curve approach gives good results. [Copyright &y& Elsevier]

It is proposed to develop and identify a probabilistic two-scale model for HCF that accounts for the failure of samples, but also for the thermal effects during cyclic loadings in a unified framework. The probabilistic model is based on a Poisson point process. Within the weakest link theory, the model corresponds to a Weibull law for the fatigue limits. The thermal effects can be described if one considers the same hypotheses apart from the weakest link assumption. A method of identification is proposed and uses temperature measurements to identify the scatter in anS/ Ncurve. The validation of the model is obtained by predictingS/ Ncurves for different effective volumes of a dual-phase steel. [ABSTRACT FROM AUTHOR]