13th International Conference on Fracture June 16–21, 2013, Beijing, China -3In the present study, the tensile properties of the kenaf fiber reinforced UPR composites are displayed in Fig. 1 and summarized in Table 1 showing the peak load, tensile strength, strain at break and young’s modulus. Among all tested specimens, kenaf fiber reinforced UPR composites exhibited relative high strength and stiffness characteristics. The tensile stress of sandwich specimens was about 18.21~19.32MPa. On the other hand, the failure strains was 0.81~0.92%. The young’s modulus of sandwich panel was 2.05~2.39GPa. Fig. 2 shows the fracture micrograph of kenaf fiber reinforced UPR composites. Good interfacial adhesion of kenaf fiber with polyester resin can be found even though there are some local micro voids. Fiber breakage is the major failure mode. Table 1. Tensile test results of kenaf fiber reinforced UPR composites Specimen Peak load (N) Max stress (MPa) Strain at break (%) Elastic modulus (GPa) 1 582.72 18.21 0.82 2.22 2 621.39 19.32 0.81 2.39 3 594.28 18.86 0.92 2.05 3.2. Bending test To investigate the characteristics of bending behavior of this novel sandwich panels and also to analyze the shear effects of PVC foam core, three point bending tests were carried out. Sandwich specimens for three-point bending test were cut from the whole sandwich panels. The bending load and deflection of the sandwich composites were tested on the basis of five replicates with a MTS 810 universal testing system (USA) in accordance with the procedure described in ASTM D 7264 [9]. The specimen dimensions were 140×28×28.80 mm3 (Length × Width × Thickness). Three-point bending fixture was used, with a constant span of 110 mm and a crosshead speed of 2.0mm/min. Figure 3. Flexural load versus mid-span deflection of sandwich composite specimens Fig. 3 shows the flexural load and mid-span deflection of sandwich composite specimens. It could be seen that the whole three-point bending deformation tests included three similar stages for all of specimens. They were linear elastic stage, depressed region and final fracture. The bending properties of sandwich composites were also summarized in Table 2 showing the maximum load, deflection at break, bending stress of face sheet, shear stress in the core, local indentation stress and main failure modes. According to the analysis of [10] , bending stress in face sheet of all tested specimens were 7.79~9.45MPa. The maximum shear stress in the core was 0.29MPa. It was denoted that no shear failure initiated in the sandwich composites. Due to local concentration of loading at mid-span, the major failure mode was local indentation. In the present study, kenaf fiber
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