Description
A Master of Science thesis in Mechanical Engineering by Mostafa Elyoussef entitled, “Notch Sensitivity of Fiber Reinforced Composite”, submitted in April 2019. Thesis advisor is Dr. Maen Alkhader and thesis co-advisor Dr. Wael Abuzaid. Soft and hard copy available.
Abstract
In most applications, structures made of composite materials involve features such as drilled assembly holes, which induce stress concentrations in their vicinities resulting in a reduction in the load carrying capacity of the structure. The nature of the damage resulting from such geometric features in orthotropic CFRP composites has been the subject of extensive research. Nevertheless, few works have investigated the behavior of notched CFRP composites exposed to elevated temperatures. Accordingly, the aim of this research is to investigate the effect of elevated temperatures on the notch sensitivity of CFRP composites. To achieve the goal of this study, both the nominal and local responses of woven CFRP samples were experimentally investigated, with the aid of Digital Image Correlation technique (DIC). Tensile tests were conducted on notched (i.e. with circular hole) and un-notched samples at 25°C, 50°C, 75°C, and 100°C. The experimental results obtained from the global stress-strain response of un-notched samples showed a decreasing trend in the mechanical properties with increasing temperatures. However, the global response of notched samples at 50°C surprisingly deviated from the expected trend and exhibited higher tensile strength than that at 25°C. Moreover, the notch sensitivity, assessed through un-notched to notched strength ratio, was found to decrease with increasing temperatures. Fractured surface examination showed two different damage mechanisms: Transverse cracks and axial splitting. It was noticed that transverse cracks was evident at the four temperature levels, while axial splitting was absent at room temperature. Measuring the local axial strains at the transverse crack initiation site showed a clear deviation from the linear response at the onset of transverse cracking. Moreover, investigating the local response at the axial splitting initiation site revealed a sudden change in the transverse and shear strain evolution. The blunting effect of axial splitting was found to become more significant at higher temperatures. Furthermore, residual shear strains were measured at the end of loading-unloading cycle for different temperature levels. It was found that residual strains are negligible at room temperatures and become more significant at higher loading temperatures.