Creep and fatigue lifetime prediction of polymer matrix composites based on simple cumulative damage laws

Presently, due to the energy saving concerns, there is an increase demand for lightweight structures for transportation industry, electric power wind generators among others. In this context design and maintenance of these civil engineering structures has put a stress on durability issues. The time scale for aeronautical applications is different for the rest of civil engineering structural applications therefore the durability experience accumulated on aeronautics is of little use. Usually civil engineering structures must remain in service for 50 years or more. Therefore the needs for real long-term mechanical performance prediction become quite important. Presently, for instance, the standards for buried GRP (Glass Fiber Reinforced Polymer Composites) pipes demand more than 10,000 h of creep tests in order to extrapolate data for two decades (in the time log scale) with a high level of confidence. The accelerated methodology proposed by Miyano et al. [Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of FRP laminates for marine use. J Composite Mater 2005;39:5–20] on Carbon and Glass Fiber Reinforced Polymers (CFRP and GFRP) laminates, rests on the fact that the time-superposition principle is the same for static, creep and fatigue strengths. Although not universally verified, it has been used successfully to predict fatigue lifetimes of many typical composites. In this context time-dependent failure criteria for viscoelastic materials is reviewed. Comparison between creep failure and static failure under constant stress/strain rate tests are analyzed. Cumulative damage laws, including the classical Linear Cumulative Damage law (LCD), are introduced to enlighten these comparisons. Finally some applications to lifetime prediction under variable amplitude fatigue loading are presented. The empirical LCD law performance is compared against the other physical based cumulative damage laws using fatigue loading sequences invented to expose the incoherencies of LCD lifetime predictions.