On the evaluation of damage-entropy model in cross-ply laminated composites

•Viscoelasticity, irreversible deformations and damage cause hysteresis energy.•Loading conditions and fiber orientation affect the rate of temperature changes.•The released damage-entropy reaches to nearly constant value before final failure.•The damage-entropy grows nearly linear until final failure.•Stacking sequence of lay-ups affects temperature patterns during fatigue loading. Entropy production during cyclic loading can serve as a measurement of degradation for composite materials. The results of a series of experiments on 0° plies, 90° plies and cross-ply laminates of carbon/epoxy are presented to characterize the fatigue behavior in terms of temperature evolution, hysteresis energy and the entropy associated with the damage and irreversible deformations. A theoretical approach based on the first and second laws of thermodynamics is presented which takes into account the damage energy, the work of irreversible deformations and the dissipated heat due to the viscoelastic nature of the polymeric matrix. The experimental and analytical results show that the concept of tallying up entropy owing to damage and irreversible deformations until final failure is useful to predict the fatigue life of composite laminates. The fatigue fracture entropy values of fiber and matrix, so-called FFE-F and FFE-M in this study, are considered as final failure criteria for 0° and 90° plies, respectively. The results of entropy accumulation indicate that interpretation of fatigue behavior of laminated composites requires to consider both the entropy associated with damage and irreversible deformations.