This paper deals with the development of constitutive equations to model the mechanical behaviour of compressible elastomers. These materials are naturally incompressible but can be made compressible by the addition of hollow microspheres, for example. Such a material is referred to as syntactic foam. The CEA (French Commission for Atomic and Alternative Energies) employs such compressible materials as seals in complex structures to reduce the internal stresses in vulnerable components and prevent their failure. The behaviour of these structures is predicted by finite element simulations. It is important to know and model the mechanical behaviour of the seals. Like elastomers, they can undergo large deformations. The microspheres enable the material to undergo large volume change unlike pure elastomer that is nearly incompressible. This compressibility also intensifies dissipative phenomena encountered in elastomers such as viscosity or plasticity. Furthermore, the Mullins stress softening effect is also intensified even for loadings that only bring about volumetric changes. To model these behaviours, a phenomenological approach was developed based on the isochoric/volumetric decomposition of the deformation gradient. The method of intermediate dissipative configurations was employed to introduce multiple phenomena, including viscosity (with several characteristic times) and viscoplasticity, for these two parts of the deformation. The constitutive equations and their flow rules were implemented in Abaqus through a UMAT subroutine and using a numerical approach to define the tangent operator. The parameters of the behaviour law were identified using a model reduction technique known as shape manifold approach. The resulting model can be compared with experimental data.

Les élastomères peuvent être sujets à de grands nombre cycles de sollicitation mécanique conduisant à un amoindrissement de leurs propriétés qui peut compromettre leur performance, on parle alors de dégradation en fatigue. Le présent chapitre propose de s’intéresser à la caractérisation du comportement en fatigue par approches en initiation au travers de la présentation d’un cas pratique mené sur le polychloroprène (CR) pour lesquels différents critères de fatigue (expérimentalement ou numériquement déterminés) et/ou représentations classiques sont explicités et utilisés.

Currently, rubber automotive valves are appropriate for passenger vehicles that operate at speeds lower than 210 km/h. However, beyond this threshold, the mechanical stress imposed on the elastomer is far more intense, increasing the risk of cracks caused by the cyclic accelerations and decelerations of the vehicle. This work delves into valve damage at high speed to gain insights into the factors contributing to failures. Fractographic analysis on valves has facilitated a thorough comprehension of valve damage by precisely pinpointing the location of crack initiation and its propagation within the volume of the elastomer. Nevertheless, the correlation between failures and valve durability is not straightforward, primarily due to variations in bench test equipment. Therefore, in order to eliminate the influence of bench test equipment-related variations, a fatigue campaign was conducted on laboratory specimens. This aimed to exclusively characterize the variability in rubber fatigue. Additionally, to achieve a higher level of representativeness of valve application, Hencky's invariants were employed to establish an equivalent kinematic mechanical valve state on these specimens. Experimental results attest an intrinsic variability of the rubber material. Subsequently, a fractography study on these specimens has provided a clearer insight into the primary material weaknesses, specifically focusing on the agglomeration of black carbon. A microstructural analysis using scanning electron microscopy (SEM) was performed to assess the batch dispersion state in correlation with specimen durability.

The network structure developed during the curing process will greatly define the mechanical response of a rubber. Nonetheless, crosslinking mechanisms commonly admitted for non-fluorinated rubbers may not be applicable for fluoroelastomers due to the steric hindrance of fluorine. The aim of this experimental study is to apprehend the crosslinking processes of a fluoroelastomer cured by peroxide in presence of triallyl isocyanurate (TAIC). TAIC is commonly used as coagent to increase the crosslinking efficiency of numerous rubbers by reducing undesirable reactions. Several formulations based on FKM/peroxide/TAIC were prepared, varying the amount of TAIC. The cure characteristics and physico-mechanical properties were determined using Moving Die Rheometer (MDR), differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), swelling tests and tensile tests.

Automotive legislations request a monitoring system on wheel's pressure for safety reasons. Those systems are called TPMS (Tire Pressure Monitoring System) and are attached to the rear of the valve inside the tire. For high speed application, the centrifugal force generated by the wheel rotation causes high stress located in the rubber element and may lead to cracks. A dedicated bench test replicates the mechanical profile of vehicle accelerations and decelerations. The results obtained on the latter show a large dispersion when valve lifespan characterization results are analyzed.The aim of this work is first to understand cracks initiation on rubber valves and then, to characterize the standard deviation. Fracture study has been carried out using field emission gun combining with CT-scans analysis. Results attest that initiation occurs on surface to the contact with the rim thanks to striations observed on valves features. CT-scan experiment also exhibits the presence of decohesion near carbon black agglomerates while analyses indicate high thermo-oxidative phenomenon.Finally, rubber fatigue variability has been investigated on laboratory specimen to counter variability related to test bench equipment. Test parameters and amplitudes have been set by numerical calculation using Hencky invariants calculated via a FE model of the valve. The main interest was to determine which mechanical loading type (shearing, tension or compression) predominates among rubber valves and then to re-apply an equivalent one on specific specimens during low cycles fatigue tests campaign. Experimental fatigue results on specimens are compared with experimental tests ran on valves, mechanically representative of the industrial conditions.

The thermal weak point of a peroxide-cured fluoroelastomer (FKM) in presence of triallylisocyanurate (TAIC) has been identified. To that purpose, the thermal stability was assessed by TGA measurements. In parallel, thermo-oxidative degradation was characterized after the aging of several samples at 250 degrees C for various durations. Specific attention has been directed to the structural evolution, evaluated by both FTIR and equilibrium swelling experiments. Experimental results highlight that TAIC, the system's crosslink node, is the thermal weak point of a peroxide-cured fluoroelastomer (FKM). Nonetheless, crosslinks are not entirely broken since the crosslinking density decreases with aging time before stabilizing for one week at 250 degrees C. This allows the material to retain some of its mechanical properties such as good elongation at break. For longest exposure times at 250 degrees C (beyond one week), chain scission mechanism becomes predominant leading to a loss of the mechanical properties. Regarding the degradation mechanism, TGA-FTIR results are rather in favor of a dehydrofluorination mechanism as hydrogen fluoride (HF) is detected upon the first times of thermo-oxidative aging. In a second step, chain scission starts to occur. Recombination reactions between double bonds provided by dehydrofluorination and macroradicals formed during chain scission could explain that FKM is still crosslinked after one week at 250 degrees C under air.

This article deals with the modelling of RTV rubber seals ("Room Temperature Vulcanized") containing compressible thermoplastic microspheres. The materials were tested for different types of loadings such as uniaxial tension, hydrostatic compression and simple shear. In addition to being highly compressible, these materials exhibit dissipative behaviour and damage phenomena like Mullins effect. Thus, the modelling approach in this work is based on the finite strain theory. Compressibility is introduced thanks to a split of the deformation gradient into isochoric and volumetric parts. This lead to a model able to represent dissipative phenomena for both deviatoric and spherical loadings. The parameters of this model are identified through a model reduction method involving Proper Othogonal Decomposition (POD) and an interpolation technique.

ABSTRACT The evolution of mechanical properties of NR with carbon black fillers was examined after a thermal aging step through both experimentation and non-deterministic numerical simulations. A quantification of mechanical properties and associated variability is first proposed for a set of specimens exposed at different temperatures and exposure times. Second, a family of stretch–stress laws is numerically built with a James' hyperelastic model. Next, the whole of the behavior evolution is modeled with a Kriging model to quantify the effects of properties on a macroscopic stiffness, useful in dynamic simulations, and the least-favorable scenario is so determined. Finally, Arrhenius method is performed to numerically draw the evolution bounds of macroscopic stiffness as a function of aging exposure, followed by a comparison with a naturally aged suspension component. To our knowledge, the methodology developed has not already been proposed in this area.

The aim of the paper is to provide a numerical model of nanoindentation tests carried out on a synthetic elastomer. Some works deal with such numerical model but on classical elastomer like silicon rubber. Our study focuses on a filled fluoro-elastomer (FKM). At first, a 2D numerical model equivalent to a Berkovich test is built. The law of behaviour used in the simulation is obtained from the results of a single traction test. Then a numerical nanoindentation test can be carried out and compared with experimental nanoindentation curves of a previous study. The relevance of the most suitable laws of behaviour is deduced.

The Laser Metal Deposition (LMD) is an additive manufacturing process which is gaining good competence in manufacturing and repairing complex functional parts. However, the produced parts require conventional machining operations in order to enhance the surface quality and the material properties. Due to the highly localized heat input experienced by the sample during the building process, significant variation of the local material properties can appear within the produced components. This could affect the machinability of the parts produced by the LMD process. This study aims to investigate the milling process and its effect on the resulted surface integrity of Ti-6Al-4V components produced by the LMD process. The heat treatment was performed in order to homogenize the microstructure of the material. The conventional Ti-6Al-4V was taken as a reference material sample. Depending on the cutting process parameters, the cutting forces and the surface roughness of the machined LMD parts were 10-40% and 18-65% respectively higher than the conventional samples. The compressive residual stress in the machined LMD samples were 11-30 % higher than the conventional specimen. These differences are related to microstructure and grain size differences between the tested parts.

This paper presents the establishment of an equation fitting the evolution of the self-heating of a polychloroprene rubber (CR) and a natural rubber (NR) during uniaxial fatigue tests. For the CR, dumbbell shaped samples underwent a wide range of loadings was investigated in order to cover low and high cycle fatigue behavior for several load ratios taken between −0.3 and 0.5. The study consists in firstly measuring the evolution of the self-heating during a fatigue test. Then, after computing the derivative, the resulting curve is fit with an equation composed of three components: two of the form of a negative exponential and a constant. It is then integrated to obtain an equation that would fit the evolution of the self-heating for all the loadings investigated. It is observed that the parameters are correlated with the intensity of the stress and the load ratio applied during the test. The same observations were made for the NR on slim samples. Consequently, it is proposed that one of the parameter is linked to the viscosity, the parameter linked to the first negative exponential is related with the viscosity, the parameter linked to the second negative exponential is related to the damage induced by the low cycle fatigue loading and the parameter integrated from the constant is related to the influence of the presence of a sizable crack on the temperature field.