Developing a new model to predict the fatigue life of cross-ply laminates using coupled CDM-entropy generation approach

In this paper, a new model based on thermodynamic entropy approach is proposed to predict the fatigue life of cross-ply laminates. The basis of the model is that, during fatigue loading, the accumulated entropy up to final failure that is generated because of damage in fiber and matrix is nearly constant and independent of the magnitude of loading and conditions. In the present study, for the first time, these values called the fracture fatigue entropy of fiber (FFE-F) and fracture fatigue entropy of matrix (FFE-M) which are considered as a final failure criterion in 0° and 90° layers, respectively. A CDM-based constitutive model is utilized to predict the damage energy in the fatigue degradation of composite laminates. FFE-F and FFE-M values are calculated using experimental results of available tension-tension fatigue tests. The validity of the proposed damage-entropy model in fatigue life prediction of T700/Cycom890 carbon/epoxy laminated composite is investigated and the obtained results are in good agreements with experimental ones. It is noted that the developed model can discriminate between different stacking sequences in fatigue loading and also is sensitive to the frequency of loading and different ambient temperature. In addition, the obtained results show that the proposed damage-entropy model is applicable in life prediction of specimens undergoing both high-cycle and low-cycle fatigue loading.