An improved analysis of a STB specimen for fracture characterization of laminates and foam-cored sandwich composites under mode III loads

This work presents the development of an improved data reduction method for the shear-torsion-bending (STB) test designed for monolithic laminates. The analytical derivation extends the applicability of the STB rig to composite sandwich specimens subjected to an out-of-plane shear loading. The data reduction method consists of an analytical model that expresses the global energy release rate in terms of applied loads, specimen geometry and material properties. The mathematical derivation of the energy release rate relies on first order shear deformation theory, Vlasov theory for non-uniform torsion of beams and near tip effects are also taken into consideration by the analytical model. Face sheets and core are modelled as homogeneous, linear elastic and orthotropic materials. The analytical expression is verified using the energy release rate extracted from a high-fidelity 3D FE based fracture mechanics model of the specimen. A compliance based method is used to generate global predictions, while local predictions are extracted using a displacement-based mode separation method. Local predictions are used to discuss accuracy and limitations of the approximate analytical model.