Investigation of internal friction and fracture fatigue entropy of CFRP laminates with various stacking sequences subjected to fatigue loading

The heat dissipation caused by internal friction plays an important role during fatigue property prediction based on self-heating data but it is still unclear for Carbon Fiber Reinforced Polymer (CFRP). In this paper, fatigue tests and thermal data analysis are carried out in order to investigate internal friction and Fracture Fatigue Entropy (FFE) of CFRP laminates with various stacking sequences. The fatigue limit is firstly estimated based on an improved graphic method developed by our previous study and then used to mark the beginning of fatigue damage occurs. It is found that the relationship between internal friction and applied stress amplitude is non-linear which is different from metals. Quadratic function is employed to characterize the internal friction region below fatigue limit and the trend line is used to separate heat dissipation related to damage and internal friction beyond fatigue limit. Results show that the internal friction effect of cross-ply laminates is significantly greater than that of unidirectional laminates. FFEs estimated by the heat dissipation related to damage are found to be independent of applied stress amplitude, which can be used to predict S-N curve of CFRP laminates efficiently and precisely.