Listening to understand and characterize the plasticity and damage of semi-crystalline polymers – POLYCOUSTIC

The present proposal is based on the idea that the technique of acoustic signals analyses could be extremely useful in the domain of polymers and especially concerning plasticity and rupture. Indeed, this technique is classically used to characterise fracture mechanisms of composites, metals or ceramics. In return, it has rarely been tested on polymers since the latter are considered too attenuating. Yet, the understanding of the initiation of plasticity, of damage (cavitation) and of the fracture of semi-crystalline polymers remains a great challenge even for the most known polymer: the polyethylene. Therefore, the use of a new experimental technique is an approach that could enable overcoming the actual difficulties. In particular, a better understanding could come from the discrimination of the different phenomena occurring at the same time: shearing of crystallites, cavitation, fracture of fibrils and also martensitic transformation.
A preliminary study has shown the relevance of this approach. Several tensile samples have been instrumented in order to record the acoustic activity simultaneously and acoustic signals have been recorded before the yield stress denoted ?y. These first encouraging results already justify the project in part since now the idea is to launch a much vaster and more systematic study, allowing to link the microstructure of the material to the acoustic signals and this for the initiation of plasticity as well as for the fracture through crack propagation. In order to reach this objective, we will benefit, on one side, from the expertise of the laboratory concerning the obtainment of well-controlled-microstructure model polyethylenes and on the other side, from the expertise in the acoustics domain (acoustic emission (AE) and ultrasonic (US) techniques) and in statistical analyses of the AE signals recorded in the material. It will first consist in realising a complete study on tensile specimens both in AE and US that would allow discriminating the elasto-plastic part from the damage part. Finally, the response of a specimen dedicated to EWF testing will be analysed. Indeed, this test is particularly appropriate for the fracture analysis since the energies useful to propagate a crack and the energies useful in the plasticity may be separated. The information obtained thanks to acoustic techniques should allow us, thanks to a quantitative and statistical study of the signals, to propose one (or several) local mechanism of damage and plasticity as well as a chronology of these events. Otherwise, it has to be mentioned that the order of appearance of the events may be modified by the temperature since the latter may favour crystallite shearing and may reinforce the interphase between amorphous and crystalline phase. By modifying the coupling and the mobility, the scenario of fracture can be strongly affected by the temperature. Therefore, we will perform experiments at various temperatures. Finally, thanks to the discrimination of the different phenomena, we could propose a global scenario leading to the final fracture.
Beyond the clear fundamental interest of this study, we need to put the proposal in context. Nowadays, several scientists are working on polymers of the future i.e biosourced polymers or at least recyclable. These new polymers as PLA or PHB are often unsatisfactory when considering fracture properties. Yet, they all are semi-crystalline polymers, therefore, we understand the interest to, first, better understand the damage mechanisms of semi-crystalline polymers, from plasticity to fracture, in order to consider afterwards a careful scientific explanation for the causes of an early damage of less known polymers. The end of the project will be dedicated to the extension of the concepts proposed on model materials to the semi-crystalline polymers whose vocation is to replace the polymers obtained from oil.