On Friday November 30, 2018, Amélie CUYNET, a PhD student in engineering sciences at Systèmes et Matériaux pour la Mécatronique (SYMME), will present her thesis "Étude du comportement mécanique à l'impact et en post impact de matériaux composites à fibres végétales".
The defense will take place at 2pm, in room B120, at Polytech Annecy-Chambéry, 5 chemin de Bellevue, on the Annecy campus.
Summary of the thesis
The work in this thesis concerns the low-speed impact and post-impact behavior of linen/epoxy composites. The study concerns samples obtained from a vacuum infusion process of a 2/2 twill woven reinforcement assembly composed of 4, 6 or 8 plies. Various impact tests were carried out using a drop tower equipped with sensors (force and accelerometer) and two high-speed digital cameras. The latter filmed the opposite side of the sample to impact, and a digital image stereo-correlation technique was used to analyze in detail the composite's behavior during impact. The image sequences acquired were also post-processed to detect and monitor cracks, particularly at maximum deflection. In addition, bending and fatigue tests were carried out on impacted samples to characterize their residual post-impact properties and compare them with results obtained on healthy samples. A batch of samples was subjected to pre-impact stresses such as fatigue and water or natural aging, in order to analyze the post-impact behavior of the material at different stages of its life cycle. The results of the study show that (i) the 6-ply material can offer a good compromise between mass gain and interesting impact and post-impact properties. Its normalized residual properties after a 10 J impact, for example, are of the same order as those of the 8-ply material. (ii) As a function of impact energy, a three-stage material behavior leading to perforation was demonstrated. Even at low energies, with no visible macroscopic surface damage, the materials suffer a loss of properties: stress at break and flexural modulus fall by 21% and 10% respectively for 8-ply at 5 J. Properties then stabilize, before falling again as the energy level approaches the perforation energy. (iii) Setting up scenarios representative of the material's life cycle showed that water aging leads to a drop in the material's mechanical properties, particularly significant for dynamic stiffness and flexural modulus. Aggressive aging also modifies impact behavior. Natural aging, on the other hand, showed no significant influence.