Concerns in tension-tension fatigue testing of unidirectional composites: Specimen design and test setup

Abstract

Tension-tension fatigue of unidirectional (UD) composites is often used to represent the fatigue behavior of composites. The standard proposes to use end tabs for UD composites. However, obtaining a reliable S–N curve requires avoiding premature failure, which in turn requires minimizing stress concentrations near the grips and end tabs. This work examines the fatigue life of different specimen and end tab designs. Conventional end tabs and other well-known designs, along with novel arrow end tabs, are tested to identify the method that is least influenced by the stress concentrations and yields the highest and the most valid fatigue life. Rectangular specimens with rectangular and tapered end tabs, which is the standard configuration, yielded the highest fatigue life. This differs from the preferences in the quasi-static test, where arrow end tabs performed the best. Moreover, the effect of parameters in the manufacturing process and test setup on the fatigue life of two different carbon fiber/epoxy prepregs material systems are discussed. The results reveal the significant effect of the test setup on tension-tension fatigue of UD composites and will inform the community on how to perform more reliable fatigue tests.