Experimental Characterization of Interlaminar Shear Strengths of Graphite/Epoxy Laminated Composites

Abstract

An investigation is conducted to study the effects of fiber orientations on the interlaminar shear (ILS) strengths of CYTEC® G40-800/5276-1 graphite/epoxy laminated composites. The results of this investigation are going to be reported in two separate parts; experimental characterization and stochastic simulation. The present paper demonstrates the experimental program and the experimental results. The ILS strengths at the θ-interfaces (θ=0, 10, 20, 30, 40, 50, 60, 70, 80 and 90°), around which the two neighboring plies have a fiber orientation difference angle, are experimentally measured using the modified double notch shear specimens with an elaborately designed lay-up. Because of the potentially considerable scatter in the matrix-dominated ILS strengths, each data set at a specified interface is obtained through a larger sample with 35 specimens. A 3D finite element analysis is also performed to aid in the design of the multi-directional laminated composite specimens, so as to minimize the edge effects and obtain a relatively uniform stress state in the specimen gage sections. The experimental results show that not only the mean values but also the probability distribution forms of ILS strengths depend on the fiber orientations of the neighboring plies around the interfaces. Five failure modes are also found by observing the fracture surfaces; i.e., interface crack, in-ply crack, crack jumping, fiber bridging, and fiber breakage. The inherent relationships between the failure modes and the probability distributions of ILS strengths are studied on the basis of rank-sum test. The general forms of the individual probability density functions corresponding to each experimental data set are determined by means of the maximum entropy technique and verified by Kolmogorov–Smirnov good-of-fit test.