Stiffness Degradation of Glass-Fiber Reinforced Epoxy Composites Due to Matrix Cracking Under Quasi-Static Loading

A micromechanical, empirical quasi-static degradation model for glass-fiber reinforced epoxy laminates is presented, assuming that stiffness degradation begins with the first microcrack and is driven by the matrix damage behavior. A degradation factor is introduced, directly applying to the matrix' Young's modulus upon damage initiation. As input, the model requires fiber and matrix mechanical properties only. Based on micromechanical rules of mixture, ply stresses are transformed into stresses on the matrix level. The matrix stress exposure factor is calculated with a classical strain energy approach and is implemented in a damage onset criterion. Stiffness degradation curves are derived experimentally. As a result, two generic degradation functions for tensile/shear and for compressive loading as a function of the matrix stress exposure factor are proposed, that can be implemented in a layer-wise strength analysis.