Physically based non-linear stress–strain relations for the inter-fibre fracture analysis of FRP laminates

The high quality of a lamina-by-lamina failure analysis of FRP laminates depends on both realistic failure criteria and a correct stress–strain analysis. This paper presents some advances in the non-linear stress–strain analysis of UD laminae with non-flexibilized thermoset matrix as they are used in structural applications. In addition to the well-known non-linearity of the stress–strain curve in pure longitudinal shear τ21 and uniaxial transverse compression σ2 there is an influence under combined (σ2, τ21)-stress of the normal stress σ2 on the shear strain γ21 and an influence of the shear stress τ21 on the transverse strain ε2. Although this strain-interaction is already known for a rather long period none of the participants of the Worldwide Failure Exercise (WWFE) has included this effect in the stress–strain analysis. In this paper, a new computational model on a physical basis is presented by which these problems can be handled very easily. The variable of decisive importance for this procedure is the stress exposure ratio fE, which is a measure for the ‘risk of inter-fibre fracture’. In the numerical examples fE is calculated from the new realistic action plane fracture criteria for brittle inter-fibre fracture.