Thermodynamic analysis of fatigue failure in a composite laminate

We put forward a thermodynamic approach for analyzing fatigue failure in a composite laminate. We show that fatigue is an irreversible progression of increasing entropy that accumulates until it reaches a critical value called the fracture fatigue entropy (FFE) at the onset of failure. Extensive series of fatigue tests are carried out that involve load-controlled tension-tension, and displacement controlled fully-reversed bending fatigue with three different stress ratios as well as constant- and variable-loading. The role of hysteresis energy in the entropy generation is investigated. FFE values are calculated based the experimental data obtained for temperature and hysteresis energy of a woven Glass/Epoxy (G10/FR4) laminate. The concept of tallying entropy accumulation and the use of FFE are useful for determining the fatigue life of composite laminates undergoing cyclic loading.