This work presents a novel test fixture for mode III fracture characterization of delaminations in monolithic laminates and face-core debonds in foam core composite sandwich specimens. The test fixture is configured as an extension of the already existing shear-torsion-bending (STB) test designed for monolithic laminates. The specimen sizing, lay-up configuration and the manufacturing process are presented. Accordingly, an overview of the test fixture is provided along with the data reduction method employed to compute the energy release rate. The results from representable fracture characterization tests are presented as force vs. displacement curves, where different definitions of the critical load for crack propagation can be defined. Thus, the critical value of the energy release rate is computed using analytically based equations for the different definitions given for the critical loads. The results show a stable crack growth for monolithic laminate specimens. However, a highly non-linear response of the sandwich specimens, before the onset of crack propagation, is observed. A non-linear numerical analysis and destructive specimen inspections are carried out in order to identify the source of the non-linear behaviour observed in the experimental results.