On Friday, November 30, 2018, Amélie Cuynet, a doctoral student in engineering sciences at the Systems and Materials for Mechatronics (SYMME) department, will defend her thesis titled “Study of the mechanical behavior of plant-fiber composite materials during and after impact.”
The defense will take place at 2:00 p.m. in Room B120 at Polytech Annecy-Chambéry, 5 Chemin de Bellevue, on the Annecy campus.
Thesis summary
This thesis examines the low-speed impact and post-impact behavior of glass-fiber/epoxy composites. The study focuses on specimens produced using a vacuum infusion process involving a 2/2 twill woven reinforcement assembly consisting of 4, 6, or 8 plies. Various impact tests were conducted using a drop tower equipped with sensors (force and accelerometer) and two high-speed digital cameras. The cameras filmed the side of the specimen opposite the impact, and a digital image stereo-correlation technique was employed to analyze the composite’s behavior during impact in detail. The acquired image sequences were also post-processed for crack detection and tracking, particularly at maximum deflection. Additionally, flexural and fatigue tests were conducted on the impacted samples to characterize their post-impact residual properties and compare them to the results obtained on undamaged samples. A batch of samples was subjected to pre-impact stresses such as fatigue and hydro-aging or natural aging in order to analyze the material’s post-impact behavior at different stages of its life cycle. The study’s results highlight that (i) the 6-ply material can offer a good compromise between weight reduction and favorable impact and post-impact properties. For example, its normalized residual properties after a 10 J impact are of the same order as those of the 8-ply material. (ii) Depending on the impact energy, a three-stage material behavior leading to perforation was observed. Even at low energy levels with no visible macroscopic damage on the surface, the materials experience a loss of properties: the stress at failure and the flexural modulus decrease by 21% and 10%, respectively, for the 8-ply material at 5 J. The properties then stabilize before decreasing again as the energy level approaches the perforation energy. (iii) The implementation of scenarios representative of the material’s life cycle showed that hydroaging leads to a decline in the mechanical properties of the materials, particularly significant for dynamic stiffness and flexural modulus. This aggressive aging also alters their impact behavior. Natural aging, on the other hand, showed no notable influence.