Thermomechanical effects in PVTα measurements

Knowledge of the different properties of thermoset composite materials is of great importance for the manufacturing of high quality composite parts. The resin bulk modulus is one of them and is essential to define the composite parts compressive behaviour under uniform compression. The evolution of this property with temperature and conversion degree of reaction is a challenging task and has been tentatively measured with a home-made apparatus, named PVT alpha, on which temperature, volume change and degree of cure are simultaneously recorded. But as the sample is contained in a non-reactive and deformable capsule, which mechanical behaviour may interfere with the measurement, a validation is required. The aim of this work is to develop a finite element model of the problem in order to simulate the thermal-mechanical behaviour of the sample and the capsule, and so to validate the measurement process. The multiphysical numerical model accounts for phase change kinetics and non-linear thermal properties as well as thermo-dependent elastic properties, all problems being solved through a strong iterative coupling scheme. Mechanical contact problems between the capsule and the resin sample are handled through a penalty method contact algorithm which enables to capture the effects of chemical and thermal shrinkage in the sample and the capsule. The heterogeneous stress state generated by the material transformation is assumed to induce heterogeneous strain states which may lead to misinterpretations of macroscopic measurements. This model is a first approach and will be improved using a more sophisticated rheological model. Nevertheless, results show that the usual experimental analysis method can be used as long as the gel point is not reached. After a certain conversion degree, the measured bulk modulus is different from the effective one so corrections have to be brought.