Abstract

This thesis presents an experimental analysis of the tensile behavior of unidirectional carbon/epoxy towpreg, focusing on the nonlinearity observed at the beginning of the stress-strain curve. Due to the matrix's high viscosity, securely holding specimens during testing was challenging, prompting modifications in the gripping method to ensure reliable data. By using a longer gauge length, the slippage impact on elastic modulus measurement was minimized, resulting in good repeatability among the test samples. Experimental findings highlighted the significant interaction between fiber waviness and the viscous matrix, leading to nonlinear tensile behavior in the beginning. The linear stiffness of the samples closely matched that of the fibers and remained unaffected by temperature variations. However, at higher temperatures, the epoxy matrix's decreased viscosity caused an upward shift in the stiffness within the non-linear region. To support the experimental findings, a micromechanical model of prepreg tow with fiber waviness was proposed. An RVE model of periodically distributed unidirectional wavy cylindrical fibers embedded within the matrix was developed to predict effective material stiffness parameters. The simulation outcomes aligned well with the result of the uniaxial tensile test of the towpreg, demonstrating the proposed RVE model's capability to predict elastic properties, considering factors like fiber arrangement, waviness, and temperature.