The effect of specimen size upon the response and strength of ± 45° angle-ply laminates has been investigated for two graphite fiber-reinforced plastic systems and several stacking sequences. The first material system was an epoxy-based system, AS4 fibers in 3502 epoxy, and the second was a thermoplastic-based system, AS4 fibers in PEEK matrix. For the epoxy-based system, two generic ±45° layups were studied; ( + 45°n/-45°n)2s (blocked plies), and ( + 45°/-45°)2nS (distributed plies), where n = 1, 2, 3, and 4. In the case of the thermoplastic system, only the layup with distributed plies was investigated, ( + 45°/-45°)2nS, for n = 1 and 2. The in-plane dimensions of the specimens were varied such that the width/length relationship was 12.7 × n/127 × n mm, for n = 1, 2, 3, or 4.
It is shown that the stress/strain response and the ultimate strength of these angle-ply laminates depends on the laminate thickness and the type of generic layup used. The ultimate strength of the epoxy matrix material was found to be much more sensitive to specimen size when compared to the thermoplastic matrix system. Scaling effects defined with respect to the first ply failure, strain at ultimate failure, and ultimate strength are isolated and discussed. Furthermore, it is shown that first ply failure occurs in the surface plies as a result of normal rather than shear stresses. The implications of the experimental findings upon the validity of the ±45° tension test, which is used to determine the in-plane shear response of unidirectional composites, are discussed.