Comportement en fatigue à la traction des composites tissés carbone/époxy sous l'effet de la température, l’humidité et la fréquence de chargement : Essais et modélisation

Abstract

Due to their low weight and superior mechanical properties, carbon fiber-reinforced polymer (CFRP) composites are becoming more and more used in engineering applications where low weight and high stiffness and strength are essential, specifically in the aerospace industry. However, during service these materials are often subjected to a wide range of manufacturing defects and incidental damages. One of these damages that mostly occur is delamination. Furthermore, these materials are exposed to environmental conditions such as temperature and moisture combined to different load frequencies. Consequently, it is necessary for aerospace applications to know the fatigue behavior of flawed CFRP laminates in such operational conditions. The main goal of this thesis is first to study the effect of temperature, humidity and load frequency on fatigue behavior of quasi-isotropic plain weave carbon/epoxy aerospace laminates containing an artificial flaw under axial cyclic loading in tension-tension, secondly to propose a phenomenological model of the residual stiffness, and finally to carry out finite element modeling on mesoscopic and macroscopic scales by using the homogenization technique. Dry and wet specimens were tested at tensile load-controlled cyclic loading with a stress ratio R = 0.1 and two load frequencies (7 Hz and 15 Hz) at room temperature (RT) and at 82°C under different stress levels. Allowable stiffness reduction as a failure onset criterion was used to determine the delamination propagation threshold under cyclic tensile loading at each frequency and environmental condition. The propagation of delamination was verified using the ultrasonic imaging (C-Scan) technique. The experimental results show that fatigue life of CFRP specimens was more individually affected by moisture than by temperature and combined moisture and temperature cause a drastic decrease in fatigue life. The general behavior is kept with the same overall trend in each of the two loading frequencies (7 Hz and 15 Hz). However, under RT/Wet and 82°C/Dry conditions, the loading frequency increase played a positive effect by increasing the fatigue life of specimens. While in both conditions RT/Dry and 82°C/Wet, this effect depends on the cyclic loading stress level. In addition, an investigation of the effect of hygrothermal conditions on stiffness degradation and damage of composite laminates subjected to tensile fatigue loading has been carried out. Based on the residual stiffness degradation, a phenomenological model of composite laminates damage under different hygrothermal conditions has been proposed. Finally, finite element modeling at the mesoscopic and macroscopic scales was performed using the homogenization method for a woven composite laminate to investigate the impact of the presence of fatigue damage mechanisms on the macroscopic mechanical properties of woven composite laminates.

Date	18 Sept 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Anh Dung Ngô (Supervisor) & Rajamohan Ganesan (Co-supervisor)