The mechanical properties together with the tensile testing method of orthogonal and quasi-isotropic CFRP laminates made of finite number of unidirectional and cloth prepreg sheets were discussed.
(1) Two different test specimens used in ISO and ASTM standards were checked. The dumbell-type specimen was found to be undersirable even for quasi-isotropic laminates, since the fracture usually takes place near the intersection of parallel and curved portions. The strip-type specimen with tabs at both ends recommended for oriented composites frequently yielded unexpected tensile fracture because of fracture near the tab end or in the grip. The specimen having a circular notch with large radius of curvature thicknesswise is recommended for the unidirectional composites.
(2) The Young's modulus of orthotropic and quasi-isotropic laminates can be predicted by the lamination theory in good agreement with the experimental values independently of the type of prepreg sheets. However, the tensile strength of laminates made cloth prepreg is lower than that made of unidirectional prepreg because of the local high stress at the cross-over points.
(3) It should be noted in the quasi-isotropic laminates that the in-plane elastic modulus is isotropic, but the tensile strength is different depending on the interval of lamination angle and on the tensile direction. These experimental results can be reasonably explained by considering that the fracture results from the tensile fracture transverse to fibers in a lamina.
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