Comportement thermo-mécanique et rupture de polypropylènes. Etude expérimentale et modélisation.

Engineering plastics are used in a variety of applications such as automotive systems. Among the materials used today for these applications, polypropylene is one of the most attractive. Knowledge of the behaviour of such material is then of prime importance when designing polymer-components. One important mechanism, which can lead to plastic deformation, is shear banding often associated with simplistic assumption of isochoric deformation. However several damage processes such as voiding or crazing can play a significant role in the deformation of polymers. Recently experimental evidence has proved the important role of non-cohesive mechanisms in the deformation of semicrystalline polymers and blends, making it necessary to revisit usual experimental protocols. In this study a commercial polypropylene compounds were analysed over a wide range of strain rate in tension. Hourglass shaped specimens were machined out from 3 mm-thick injection-moulded plates. The strain and stress were deduced from local 3D-measurement in the smallest section. This geometry included a small rectilinear zone aiming at localising the deformation in the zone where all the parameters were determined while the triaxial effects, which could result from a small radius of curvature, were limited. A high-speed video camera combined with image analysis and image correlation systems made it possible measuring the 3D-strain field as a function of time. Experiments were run at room temperature but the temperature of the sample was measured using a high sampling rate infrared pyrometer. Resulting from the measurement of the three components of strain, the volume change during tensile test can be calculated. It could be concluded that dilatation phenomenon was significant even for low strains and that volume change mechanism was not sensitive to strain rate. As a consequence isochoric assumption appeared to be totally invalid.In order to reveal the mechanisms of plastic damage, series of observations using a scanning electron microscope were performed on transversal surfaces of samples at different strain levels. Nucleation of voids took place in the early stage of deformation. As the strain increased the number and size of voids increased significantly. In parallel, tension with partial unloadings allowed to quantify damaging through the decrease of elastic modulus. Compression and shearing were also characterised.Finally a constitutive model including damaging, volume change and strain rate effects can be prososed and was first validated using ABAQUS Code.Les exigences sécuritaires dans le domaine automobile incitent à une connaissance toujours plus approfondie du comportement et de la rupture des polymères utilisés en tant que pièces de structure de l'habitacle. Nous étudions deux polypropylènes couramment utilisés dans ce domaine dans le but de dégager les processus physiques responsables de leur tenue mécanique en vue de la construction d'une loi de comportement pertinente. La caractérisation mécanique de la réponse de ces polymères sous divers types de sollicitations simples et complexes s'appuie sur une méthodologie expérimentale originale et performante alliant des techniques de mesure de champs de déformations et de température.Le comportement des deux polypropylènes en grandes déformations implique plusieurs processus dissipatifs. Un phénomène d'autoéchauffement a ainsi été mis en évidence. Ses conséquences sur les propriétés mécaniques des matériaux sont significatives. Nous avons également observé le développement d'un endommagement se manifestant par la nucléation et la croissance de cavités. Une variable scalaire lui a été associé et mesuré au cours d'un chargement cyclique. L'analyse de ces résultats permet de souligner que la plasticité nos polymères implique une dépendance à la vitesse des propriétés mécaniques ainsi qu'une sensibilité à la pression hydrostatique.L'ensemble de ces observations permet de poser les bases d'une modélisation thermodynamique du comportement et de l'endommagement de cette classe de polymères. L'apport de cette approche a été démontré par la confrontation des résultats expérimentaux à la simulation numérique de la réponse de nos matériaux sous différents modes de chargement