13th International Conference on Fracture June 16–21, 2013, Beijing, China -5performance of this novel sandwich composite. The double cantilever beam of the sandwich composites were tested on the basis of three replicates with a Shimadzu universal testing system (model SCG-5KNA, Japan) in accordance with the procedure described in ASTM D 5528 [11]. The specimen dimensions were 160×30×28.8 mm3 (Length × Width × Thickness) with a 25 mm pre-crack. A tensile fixture was used, with a piano hinge amounted on the upper and lower sandwich face sheet and a crosshead speed of 2.0mm/min, shown in Fig. 5. During the process of test, the digital camera was used to real-time record the fracture behavior of the pre-crack and also used to take pictures of the typical fracture behavior. Figure 5. The dimension of double cantilever beam specimens and experiment set-up A typical load against displacement curve of sandwich specimens during the DCB tests is shown in Fig. 6. The process of fracture can be approximately categorized into four stages based on the observation results and the characteristic load against displacement behavior at each stage, named as I, II, III and IV. In stage I, the load increased approximately linearly to the point A along with the increasing of the open displacement. From the point A to the point B (the stage II), the load against open displacement curve became non-linear. In this stage, the interlaminar delamination initiated between upper sandwich face sheet and PVC foam core and pre-crack developed stably. When stage II fracture completed, the load decreases rapidly from the point B to the point C (the stage III). In stage III, the interlaminar delamination propagated unstably. Beyond the point C, the interlaminar delamination propagation slowly went on. Due to the relative lower randomly distributed fibers in the upper face sheet, skin brittle fracture was found in no.2 specimen. Figure 6. Load against open displacement curves for sandwich composite According to [12], the strain energy release rate during crack propagation was calculated by, ba U Gc ∆ = (4) Where, Gc is strain energy release rate, bis the width of the specimen and ais the incremental crack length during the process of test, the energy or work U∆ , is equal to the area under load vs.
RkJQdWJsaXNoZXIy MjM0NDE=