Derivation of fatigue damage accumulation curves based on changes in composite material properties

The life of composite materials (CM) subjected to fatigue conditions is associated with the gradual damage of their structure. Since the damage accumulation is reflected by changes in CM properties, the derivation of damage accumulation curves may be based on test results providing the reduction of the residual strength. Examined in this work is a non-linear fatigue damage accumulation function for cross-ply CF/PEEK laminates. This damage function is specified with respect to damage parameters derived by the strength decrease during fatigue for the CF/PEEK. Taking into account this concept, an improvement of the theory of isodamage lines is proposed. Since the nonlinearity of the derived damage function depends on an exponent that is a function of the fatigue stress amplitude, the effect of the loading order and the previous damage history on the total fatigue life is investigated. Numerical examples of fatigue life prediction under two-stage loading conditions and a damage accumulation map for the CP/PEEK are presented and discussed.