Stiffness Degradation and the Fatigue Behavior of Low-flow Carbon Fiber Sheet Molding Compounds

Abstract

Carbon Fiber Sheet Molding Compounds (CF-SMC) combine the properties of high-performance prepregs with the cheaper and easier manufacturing possibilities of SMCs. Understanding the fatigue behavior of these composite materials is essential for demanding engineering applications where components are exposed to cyclic loading conditions. This contribution investigates the fatigue behavior of CF-SMC based on discontinuous prepreg platelets with a random orientation distribution. CFSMC plates for specimen preparation were compression molded in a low-flow setup. Fatigue tests were performed under uniaxial cyclic tension-tension (R = 0.1), compression-compression (R = 10) and tension-compression (R = -1) loading conditions. The stiffness degradation was significant and fundamentally different in tension compared to compression fatigue. Stiffness reduction happened sooner in the early stages of fatigue life for compression-compression and tension-compression fatigue loading than for tension-tension. In-situ passive thermography measurements for the tension-tension fatigue tests revealed diffuse damage as well as localized major damage sites. The final failure site could be correlated with the first visible appearance of a localized temperature increase by passive thermography. These findings suggest that the weakest link (the least favorable local mesoscale morphology) defines the fatigue strength of CF-SMC. A damage accumulation time was introduced which could potentially be a useful tool in terms of fatigue life prediction and structural health monitoring of CF-SMC structures.