The effects of stitch orientation on the tensile and open hole tension properties of carbon/epoxy plain weave laminates

Abstract

Carbon/epoxy composites have been used in critical parts of aircraft such as wing box, fuselage and empennage. Apparently, some areas are prone to impact damage, and therefore, interlaminar strength is of prime concern. One method to improve interlaminar strength is to use textile yarns to bind fabric preforms prior to curing process. Albeit effective, such through-thickness reinforcement may alter the in-plane mechanical behavior of composites. During the design phase using stitched composites, certain parameters are considered, e.g. stitch orientation, to ensure that the in-plane response is still acceptable. In addition to that, presence of geometrical changes such as cut-outs may complicate the issue at hand. Therefore, evaluation of stitched composites containing geometrical changes is critical. Present paper reports the influence of stitch orientation on the tensile behavior of stitched plain weave with and without circular holes. Two stitch orientations are discussed: longitudinal direction (parallel to the loading direction) and transverse direction (perpendicular to the loading direction). Experiment shows that specimens with transverse stitches suffer 24% reduction of tensile strength and failure strain. The reduction is attributed to the stitch-induced misalignment of the load-bearing tows in transverse stitched specimen. Tensile modulus, however, does not show sensitivity to the stitch orientation. Failure mechanism for different stitch orientations is elucidated by microscopic observation. It is revealed that in the specimen with longitudinal stitches, the crack initiation may originate from the resin-rich region around the stitch hole, and the crack subsequently propagates rapidly throughout the width of specimen. While the crack initiation site is similar for the transverse stitched specimens, the crack progression would occur along the stitch lines. Microscopic observation also reveals that the plain-weave laminates eventually fail by brittle fracture regardless the stitch orientation. In open hole specimens, the role of stress concentration around the hole outperforms the fiber misalignment effect. Stress measurement confirms that normal stress distribution and KT are insensitive to the stitch orientation. Lekhnitskii theory (1968) is also employed to predict the stress distribution, and the theory is found in a good agreement with experiment.