Fracture surface evolution and apparent delamination toughness in split composite beam specimens subjected to mixed mode I–III loading

A novel test fixture for mode III fracture characterization of monolithic laminates and composite sandwich specimens

“…The vertical main actuator in the test machine, is an axialtorsional actuator with an axial capacity of 25 kN and torsional capacity of 250 Nm. The vertical actuator allows to sufficiently open the crack flanks in order to avoid contact friction between the crack flanks during mode III testing due to the torsional moments described in Sabbadin et al 25 Two load cells are present in the text fixture: one is mounted on the horizontal actuator and has a capacity of 5 kN (Figure 4) and a second one is installed on the vertical actuator and has an axial capacity of 10 kN and torsional capacity of 200 Nm (Figure 3). The shearing load P is applied through compressive contact between the horizontal actuator and the load-block which is allowed to slide on a rail with linear bearings.…”

“…The nominal geometric dimensions of the monolithic laminate and the composite sandwich specimens are reported in Table 1. The initial crack length is different in the sandwich specimen compared to the monolitic specimen for the following reason: longer delaminated arms are needed in the sandwich specimen in order to satisfy the assumptions originating from modelling the lower cracked arm as a composite beam (see Sabbadin et al 25 ) The specific geometrical parameters of the individual specimens varied only slightly (þ/À 0.5 mm) with respect to the nominal values in Table 1.…”

“…The longitudinal slits were created by using a specific pattern for the Teflon insert as shown in Figure 6. The presence of the side slits is needed in order to mitigate the free-edge influence on the mode-III energy release rate distribution along the crack front as it is discussed in Sabbadin et al 25 The longitudinal side slits have already been used in the literature and their influence on the energy release rate distribution is accurately studied in Davidson and Sediles 20 for monolithic laminates. The mode-III energy release rate component trend is studied along the crack front for different widths of the longitudinal slits for monolithic laminates in Davidson and Sediles.…”

“…Therefore, the novel test fixture presented in this work, is inspired by the STB test fixture in its pure mode III configuration only. The main difference between the STB fixture (Davidson and Sediles 20 ) and the novel test fixture is related to testing of sandwich specimens (in addition to monolithic laminates) and to the fact that a new data reduction method (Sabbadin et al 25 ) was used to reduce the energy release rate at the crack front from measured data.…”

“…It is carried out on a limited amount of monolithic laminates and foam core sandwich specimens loaded under pure mode III loading, to investigate the experimental performance of the novel dedicated mode III test fixture and modified STB specimen geometry presented in this study. The analytical equations derived in Sabbadin et al 25 are used to extract the energy release rate from measured forces and moments.…”