Time to failure prediction in rubber components subjected to thermal ageing: A combined approach based upon the intrinsic defect concept and the fracture mechanics

In this contribution, we attempt to derive a tool allowing the prediction of the stretch ratioat failure in rubber components subjected to thermal ageing. To achieve this goal, the mainidea is to combine the fracture mechanics approach and the intrinsic defect concept. Using an accelerated ageing procedure for an Ethylene–Propylene–Diene Monomer (EPDM), it is first shown that the average molar mass of the elastically active chains (i.e. between crosslinks) can be used as the main indicator of the macromolecular network degradation. Byintroducing the time–temperature equivalence principle, a shift factor obeying to an Arrhenius law is derived, and master curves are built as well for the average molar mass as for the ultimate mechanical properties. Fracture mechanics tests are also achieved and the square root dependence of the fracture energy with the average molar mass is pointed out. Moreover, it is shown that the mechanical response could be approximated by the phantom network theory, which allows to relate the strain energy density function to the average molar mass. Assuming that the fracture of a smooth specimen is the consequence of a virtual intrinsic defect whose the size can be easily estimated, the stretch ratio at break can be therefore computed for any thermal ageing condition. The estimated values are found in a very nice agreement with EPDM experimental data, making this approach a useful tool when designing rubber components for moderate to high temperature environments

Crystallization and mechanical behavior of semi-crystalline polyethylene

International audienceAbstract Molecular dynamics simulations are employed to study the crystallinity and mechanical properties of multi-chain polyethylene systems. Results show that structural composition (length and number of chains) and temperature lead to different crystallinity, which are obtained by using two general measurement methods, namely chain orientation and global order. The semi-crystalline polyethylene systems are deformed under various mechanical loading modes and at different temperatures representing different polymer states. The stretching temperature and structural composition have a strong influence on the mechanical properties, including elastic modulus, yield stress and inelastic mechanisms. The orientation crystallization caused by the heat treatment stage induces a significant directional effect on the different parts of the large-strain stress-strain response. Besides, the competition of the two main inelastic deformation mechanisms, namely shear yielding and cavitation damage, are revealed during the course of stretching

J integral as a fracture criterion of rubber-like materials using the intrinsic defect concept

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Modelling of the heat build-up temperature and damage fields in bulk filled-rubber samples during fatigue

Proceedings of the 11th European Conference on Constitutive Models for Rubber (ECCMR 2019), June 25-27, 2019, Nantes, FranceInternational audienceIn the present work, a finite strain thermo-viscoelastic-damage model is used to predict the cyclic thermo-mechanical response of bulk rubber samples during fatigue. The model parameters were identified and verified using experimental observations on stress-softening, hysteresis and dissipative heating obtained in thin rubber samples containing different amounts of carbon-black and cyclically loaded under different minimum stretch levels. Predicted evolutions of the heat build-up temperature and damage fields in bulk rubber samples during fatigue are discussed

Experimental and finite element investigation of void nucleation in rubber-like materials

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Damage interaction and angle effects on the erosion behavior of soda-lime-silica glass

International audienceThe present contribution aims to examine the erosion behavior of soda-lime-silica glass in connection with damage interaction and angle effects. Experimental observations are reported on glass plates subjected to sandblasting process using alumina abrasive particles for different sandblasting durations and impact angles. The damage and erosion mechanisms are computed through a numerical model of the sandblasted glass plate. The glass internal stiffness degradation due to impact process is accounted for by an anisotropic stress-based continuum damage mechanics model. The glass erosion is simulated by means of a vanishing element technique using the critical values of damage components as failure criterion. A parametric numerical study is carried out to bring insights into damage interaction and angle effects on the material loss

A two-phase hyperelastic-viscoplastic constitutive model for semi-crystalline polymers: Application to polyethylene materials with a variable range of crystal fractions

International audiencePolyethylene-based polymers as biomedical materials can contribute to a wide range of biomechanical applications. Therefore, it is important to identify, analyse, and predict with precision their mechanical behaviour. Polyethylene materials are semi-crystalline systems consisting of both amorphous and crystalline phases interacting in a rather complex manner. When the amorphous phase is in the rubbery state, the mechanical behaviour is strongly dependent on the crystal fraction, therefore leading to essentially thermoplastic or elastomeric responses. In this study, the finite deformation stress-strain response of polyethylene materials is modelled by considering these semi-crystalline polymers as two-phase heterogeneous media in order to provide insight into the role of crystalline and amorphous phases on the macro-behaviour and on the material deformation resistances, i.e. intermolecular and network resistances. A hyperelastic-viscoplastic model is developed in contemplation of representing the overall mechanical response of polyethylene materials under large deformation. An evolutionary optimization procedure based on a genetic algorithm is developed to identify the model parameters at different strain rates. The identification results show good agreement with experimental data, demonstrating the usefulness of the proposed approach: the constitutive model, with only one set of identified parameters, allows reproducing the stress-strain behaviour of polyethylene materials exhibiting a wide range of crystallinities, the crystal content becoming the only variable of the model

Mullins effect in polyethylene and its dependency on crystal content: A network alteration model

International audienceThis contribution is focused on the Mullins effect in polyethylene. An ultra-low-density polyethylene with 0.15 crystal content, a low-density polyethylene with 0.3 crystal content and a high-density polyethylene with 0.72 crystal content are subjected to cyclic stretching over a large strain range. Experimental observations are first reported to examine how the crystal content influences the Mullins effect in polyethylene. It is found that the cyclic stretching is characterized by a stress-softening, a hysteresis and a residual strain, whose amounts depends on the crystal content and the applied strain. A unified viscohyperelastic-viscoelastic-viscoplastic constitutive model is proposed to capture the polyethylene response over a large strain range and its crystal-dependency. The macro-scale polyethylene response is decomposed into two physically distinct sources, a viscoelastic-viscoplastic intermolecular part and a viscohyperelastic network part. The local inelastic deformations of the rubbery amorphous and crystalline phases are considered by means of a micromechanical treatment using the volume fraction concept. Experimentally-based material kinetics are designed by considering the Mullins effect crystal-dependency and are introduced into the constitutive equations to capture the experimental observations. It is shown that the model is able to accurately reproduce the Mullins effect in polyethylene over a large strain range. The inherent deformation mechanisms are finally presented guided by the proposed constitutive model

Surgery in vertebral fracture: Epidemiology and functional and radiological results in a prospective series of 518 patients at 1year's follow-up

International audienceINTRODUCTION:Recent epidemiological data for spinal trauma in France are sparse. However, increased knowledge of sagittal balance and the development of minimally invasive techniques have greatly improved surgical management.OBJECTIVES:To describe the epidemiology and management of traumatic vertebral fracture, and to analyze evolution and risk factors for poor functional outcome at 1 year's follow-up.MATERIALS AND METHODS:A prospective multicenter French cohort study was performed over a 6-month period in 2011, including all cases of vertebral fracture surgery. Data were collected by online questionnaire over the Internet. Demographic characteristics, lesion type and surgical procedures were collected. Clinical, functional and radiological assessment was carried out at 1 year.RESULTS:Five hundred and eighteen patients, with a mean age of 47 years, were included. Sixty-seven percent of fractures involved the thoracic or lumbar segment. Thirty percent of patients had multiple fractures and 28% neurological impairment. A minimally invasive technique was performed in 20% of cases and neurological decompression in 25%. Dural tear was observed in 42 patients (8%). Seventy percent of patients were followed up at 1 year. Functionally, SF-36 scores decreased on all dimensions, significantly associated with age, persistent neurological deficit and previous spine imbalance. Thirty-eight percent of working patients had returned to work. Radiologically, sagittal balance was good in 74% of cases, with fracture consolidation in 70%.DISCUSSION:Despite progress in management, spinal trauma was still a source of significant morbidity in 2011, with pronounced decrease in quality of life. Conserved sagittal balance appeared to be associated with better functional outcome