Effects of environmental degradation on flexural failure strength of fiber reinforced composites

Fiber-reinforced composite laminates are often used in harsh environments that may affect their long-term durability as well as residual strength. In general, the environmental degradation is observed as the matrix cracking and erosion that can lead to deterioration of matrix-dominated properties. In this work, cross-ply carbon fiber-reinforced epoxy matrix laminates were subjected to various environmental conditions with controlled ultra-violet (UV) and moisture condensation exposures and their remaining performances were measured through elastic modulus and failure strength changes. Furthermore, the composites were also subjected to cyclic load to evaluate the synergistic effects of environmental degradation and mechanical fatigue. The experimental results show limited degradation effects under uniaxial load but far greater effects under bending or flexural load. Based on these results, we conclude the principal mechanism controlling these detrimental effects is the weakened interlaminar shear strength. It appears, during environmental degradation, the bonding strength between differently oriented plies is reduced. Such an effect is also enhanced by mechanical fatigue load. In order to quantify the influences of different degradation conditions, empirical formula is also established. The analysis shows the synergistic effects of various degraded conditions.