Comparative in-plane shear behaviour of long-carbon-fibre composites with thermoset or thermoplastic matrix

Abstract

This paper presents a comparative study of the in-plane shear behaviour of the AS4/PEEK thermoplastic and the thermoset T300/914 material. Tensile tests with several unloadings on (±45) 2 S specimens have shown that the laminae orientation, θ, varies greatly during the test, particularly in APC2 laminates which exhibit substantial plastic behaviour. These irreversible deformations induce considerable residual stresses in the plies of the unloaded specimens. The in-plane shear stresses and strains therefore need to be calculated, at each step of the tensile test, with the actual θ value. With this precaution, and taking into account the residual stresses, the elastic, damaged and plastic behaviour in shear of the thermoset and thermoplastic composites have been characterized for strain values below 10%. It is shown that T300/914 and AS4/PEEK materials exhibit very similar shear behaviour at the beginning of loading, with small variations of laminae orientation, θ, very scarce and short matrix cracks, but a very significant loss in shear stiffness associated with the development of large plastic strains. As soon as matrix cracks appear, damage development differs in the two materials: the precise observation, on the specimen edges, of matrix cracks and interply delaminations has shown that the earlier failure of the T300/914 specimens is primarily due to the lower shear strength of the thermoset matrix. Because of a much delayed failure, the (±45) 2 S PEEK matrix laminates can develop more pronounced plastic behaviour.