Fatigue behavior and cyclic damage of peek short fiber reinforced composites

Fatigue strength and failure mechanisms of short fiber reinforced (SFR) PEEK have been investigated in the past by several research groups. However some relevant aspects of the fatigue behavior of these materials, like cyclic creep and fatigue damage accumulation and modeling, have not been studied yet, in particular in presence of both fillers and short fibers as reinforcement. In the present research these aspects were considered by carrying out uni-axial fatigue tests in load control (cycle ratio R = 0) on neat PEEK and PEEK based composites reinforced either with short carbon fibers only or with addition of fillers (graphite and PTFE). For each material stress-life curves were obtained and compared. Fatigue fracture surfaces were analyzed to identify failure mechanisms in presence of different reinforcement types. The evolution of cyclic creep strain was also monitored as a function of the number of cycles, thus allowing investigation on the correlation between cyclic creep parameters and fatigue life. The evolution of cyclic damage with loading cycles was then compared by defining a damage parameter related to the specimen stiffness reduction observed during the tests. Progressive cyclic damage evolution of short fiber reinforced PEEK composites presented significantly different patterns depending on applied stress level and on the presence of different reinforcement typologies. In order to reproduce the different fatigue damage kinetics and stages of progressive damage accumulation observed experimentally, a cyclic damage model was finally developed and implemented into a finite element code by which a satisfactory agreement between numerical prediction and experimental data at different stress levels for each examined material.