Time and temperature dependence of carbon/epoxy interface strength

Characterization of fiber/matrix interface is essential for the understanding of long-term properties of fiber reinforced composite materials. In this research, time and temperature dependence of carbon/epoxy interface strength were investigated. Unidirectional specimens were tested under tensile load up to failure, at various temperatures and testing speeds. The failure modes were identified as matrix dominant failure or interface dominant failure. A unit-cell model was considered to evaluate the stresses at the microscopic level and identify the critical points of highest stresses. Time and temperature dependent stress-concentration factor and thermal residual stress at the critical points were calculated using viscoelastic FEA. The micro stresses at the critical points were found to be properly represented by a bilinear curve with the interface dominant failure mode associated with the horizontal portion of the curve, suggesting that the interface strength is independent of time and temperature.