Compound model of residual stiffness degradation for FRP composites

In order to establish the relationship between residual stiffness, material damage quantity and residual life, failure modes of fiber-reinforced polymer (FRP) laminates under tension-tension fatigue load effect are divided into three types: inter fiber fracture, fiber random fracture and delamination. Based on the analysis of quantitative relationship between different failure modes and residual stiffness degradation quantity, a compound model of residual stiffness degradation, integrated each failure mode's influence, was put forward. This model is applicable to stage Ⅰ and Ⅱ, which occupy most of the life, avoids the influence of stiffness reduction uncertainty in stage Ⅲ. Normalization of residual stiffness degradation curve by time scale, eliminates individual specimens' dispersion effect, significantly reduces the dispersion. Statistical analysis was conducted for fatigue test results of four kinds of E-glass/Epoxy glass fiber composite laminates and three kinds of AS-4/polyether-ether-ketone (PEEK) carbon fiber composite laminates, which shows that the proposed model is suitable for an accurate description of the residual stiffness decline law of composites.