Evaluation of the fill yarns effect on the out-of-plane compressive fatigue behavior for an unidirectional glass fiber reinforced epoxy composite

Understanding the out-of-plane mechanical behavior of composite materials is necessary to optimally design thick structures. In unidirectional laminates, the out-of-plane mechanical behavior is generally characterized by the in-plane data assuming transverse isotropy; nevertheless unidirectional woven contain a fraction of fill yarns that might change this property. To evaluate the fill yarns effect over the transverse isotropy, this work compares the in-plane mechanical performance to the out-of-plane behavior under a cyclic load for an epoxy glass fiber reinforced by a unidirectional weave.First, the quasi-static tensile and compression strengths were determined for both planes. Fatigue tests were then performed at 95%, 90%, 80% and 70% of the compressive strength using stress ratios of R= 2, R= 10 (in compression-compression fatigue) and the critical stress ratio defined by the quotient between the compressive and tensile strengths. The results show that the quasi-static tensile strength depends on the plane, mainly due to the influence of the weave fill fibers. These fibers also influence the observed differences in compressive strength; however, the specimen geometry and boundary conditions also have an effect. Despite these differences, the fatigue strength detriment responds to the same causes and their magnitude in terms of the number of cycles is similar. An in-plane S-N curve of the compression-compression fatigue can be transformed into an out-of-plane curve using the compressive strength in both planes and vice versa.