Influence of formulation on the fatigue properties of elastomeric materials
Influence de la formulation sur les propriétés en fatigue d'élastomères industriels
Résumé
Because of the heterogeneous nature of these filled materials, the fatigue properties of elastomers are strongly connected to their microstructure,. The description and understanding of the basic mechanisms involved are therefore of primary importance to optimize the fatigue design of the industrial parts.
At first, in the present study, a complete analysis of the initiation stage of fatigue crack was performed during interrupted fatigue tests and after failure to investigate the mechanisms occurring at the microstructural scale for different natural rubbers (NR) vulcanized with sulphur and filled with various amounts of carbon black.
Fatigue Wöhler curves were first established in fully relaxing conditions for the different NR compounds. The scattering of the fatigue life results is relatively low (compared to synthetic rubbers) and is related to a multi-initiation phenomenon and relatively slow crack propagation. The analysis was then performed on fatigued samples and on fracture surfaces with scanning electron microscope (SEM) and energy dispersive spectrometry of x-rays (EDS) to characterize the morphology and the chemical composition of the microstructural defects leading to the initiation of fatigue cracks. An important database listing the nature and the number of these defects was build for various strain levels and various NR compounds. This procedure revealed in particular that inclusions such as carbon black agglomerates or zinc oxides are generally responsible for the initiation. However, those two types of inclusions correspond to different crack initiation mechanisms and only the initiations on carbon black agglomerates are followed by a propagation stage that leads to fatigue failure. The carbon black agglomerates appear to have a stronger cohesion than zinc oxides and a stronger adhesion to the matrix. Consequently two kinds of initiation mechanisms are proposed considering the nature of the inclusions, their cohesion and their interface properties with the matrix.
Another objective of this study was to take advantage of experimental observations at the scale of the inclusions to characterize the early stages of the fatigue damage scenario. The first range of observations is based on SEM techniques applied to samples after interrupted fatigue tests. These measurements enable describing the early stages of initiation. Nevertheless, this description is geometric and the access to local behavior or criteria remains limited. In order to unlock these limitations, thermal measurements at the microstructure scale were achieved and a specific protocol is applied to infer the dissipation fields from the temperature measurements. Firstly developed on a classical structural sample, this approach was applied to samples with well mastered typical flaws at the microscopic scale and then samples exhibiting isolated carbon blacks agglomerates. The experimental results obtained from SEM observations and thermal measurements were finally correlated and are clearly building a precious database which comparison to finite element simulations should allow for a quantitative evaluation of the mechanisms at stake during the initiation of the fatigue cracks induced by inclusions.
In former papers a fast characterization of the fatigue properties based on thermal measurements has been proposed. The temperature measurements are performed with an infrared camera. An experimental protocol was developed to obtain dissipation mappings directly from temperature fields. Applied on hourglass fatigue samples, the data were used to predict the fatigue lifetime throughout an energy based criterion. The comparison to classical fatigue tests showed that this approach provides a very efficient prediction of the deterministic Wöhler curve with only one sample, within less than one day. In the present study, this method is challenged on many industrial compounds and a very good agreement is found for most of the material considered, which illustrates that this considered approach is highly useful to speed up the fatigue characterization of elastomeric materials. Moreover, the evolution of the defects population was investigated using a wide campaign of interrupted fatigue tests on one NR compound. These measurements enable to describe the evolution of the defects population along the fatigue lifetime and its sensitivity to the local strain. Another energetic criterion, using a single parameter is then suggested, that couples the micro-structural measurements to the dissipation curves previously obtained. In this last case, the evaluation is not fast anymore as the interrupted test and their analysis requires some time, but the very good agreement between the predicted fatigue curve and the mean experimental curve illustrates the strong physical meaning of an approach based on dissipated energy.
Cette étude concerne l’influence de la formulation sur les propriétés en fatigue des élastomères. Pour cela, douze matériaux ont été préalablement choisis de façon à ce que leur formulation soit simplifiée mais représentative de celles de matériaux industriels. La première partie de l’étude vise la description des mécanismes et des scénarios d’endommagement par fatigue, pour différents matériaux, niveaux de sollicitation et pourcentages de durée de vie. Une vaste campagne d’essais de fatigue interrompus a été menée permettant une analyse statistique de la population de défauts. Ces données ont permis d’une part de proposer des scénarios de ruine pour les différents matériaux étudiés, et d’autre part d’identifier deux types de mécanismes d’amorçage autour d’inclusions de différentes natures. La deuxième partie de l’étude vise à comprendre les mécanismes d’amorçage de fissure de fatigue. Un protocole expérimental permettant d’obtenir les champs d’énergie dissipée directement à partir des champs de température mesurés a été mis en place. Ce protocole a d’abord été développé à une échelle macroscopique et validé grâce à des simulations par éléments finis. Il a ensuite été appliqué avec succès à l’échelle des inclusions pour des cas 2D. Enfin, la troisième partie de cette étude propose un critère énergétique basé sur un protocole d’auto-échauffement permettant une détermination rapide des propriétés en fatigue. L’approche proposée utilise un critère à deux paramètres et permet de prédire la courbe de Wöhler déterministe avec une seule éprouvette, en une demi-journée d’essai et uniquement à partir de mesures thermiques. Ce critère a été validé sur une large gamme d’élastomères, et s’avère capable de rendre compte de l’influence de la gomme, du taux et du type de charges.
Loading...