Mode-III fracture toughness measurements of foam-cored sandwich composites using a constrained Shear-Torsion-Bending specimen

This work presents a constrained version of the Shear-Torsion-Bending (STB) test fixture, specially developed for testing of the mode-III face/core fracture toughness of sandwich specimens. Compared to the traditional STB test fixture, the free end of the specimen is now constrained between two support rollers, restraining the twisting deformation of the specimen resulting from the mode-III shear forces. The constrained STB test fixture was applied for mode-III fracture characterization of sandwich specimens with longitudinal slits and consisting of E-glass/epoxy face sheets and Divinycell PVC foam cores. Three density classes of the foam material were considered and tested, that is, H45, H100, and H160. Furthermore, two core thicknesses were tested for the H160 cored specimens. The experimental results from the constrained STB test method show that stable crack growth is obtained without kinking into the core. However, post-mortem investigations showed that longitudinal 45 degrees tension cracks in the core just below the face/core interface occurred, before interface crack opening. Consequently, an apparent mode-III fracture toughness was computed using a three-dimensional Finite Element (FE) model in conjunction with the Crack Surface Displacement Extrapolation (CSDE) mode-mixity method and an optical Digital Image Correlation (DIC) based crack propagation method. Finally, based on the experimental observations presented in this paper, a general discussion of the existence of pure mode-III face/core interface propagation in foam-cored sandwich composites in relation to core and face/core bond strength is included.