Scaling effects in angle-ply laminates

The effect of specimen size upon the response and strength of +/- 45 degree angle-ply laminates was investigated for two graphite fiber reinforced plastic systems and several stacking sequences. The first material system was a brittle epoxy based system, AS4 fibers in 3502 epoxy, and the second was a tough thermoplastic based system, AS4 fibers in PEEK matrix. For the epoxy based system, two generic +/- 45 degree lay-ups were studied: (+45 degrees sub n/-45 degrees sub n) sub 2S (blocked plies), and (+45 degrees/-45 degrees) sub 2nS, for n=1 and 2. The in-plane dimensions of the specimens were varied such that the width/length relationship was 12.7 x n/127 x n mm, for m=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 lay-up used. 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 degree tensile test which is used to determine the in-plane shear response of unidirectional composites are discussed.