The effects of shear and near tip deformations on interface fracture of symmetric sandwich beams

The effects of shear on energy release rate and mode mixity in a symmetric sandwich beam with isotropic layers and a debond crack at the face-sheet/core interface are investigated through a semi-analytic approach based on two-dimensional elasticity and linear elastic fracture mechanics. The semi-analytic expressions for the shear components of energy release rate and mode mixity phase angle which have been derived in Li et al. (2004) for bi-material beams are extended to sandwich beams and the necessary numerical coefficients derived through accurate finite element analyses. The expressions are combined with earlier results for sandwich beams subjected to bending moments and axial forces in order to obtain solutions for general loading conditions and for an extensive range of geometrical and material properties. The physical and mechanical significance of the terms of the energy release rate which depend on the shear forces are explained using structural mechanics concepts and introducing crack tip root rotations to account for the main effects of the near tip deformations. The results are applicable to laboratory specimens used for the characterization of the fracture properties of sandwich composites for civil, marine, energy and aeronautical applications, provided the lengths of the crack and the ligament ahead of the crack tip are above minimum lengths which are defined in the paper.