13th International Conference on Fracture June 16–21, 2013, Beijing, China -2with a length of 5~8mm. The unsaturated polyester resin (UPR) was provided by Ashland Chemicals (AROPOL 6585) with density of 1.12 g/cm3. The curing agent was obtained from Acros Orgaincs. The sandwich core was Divinycell H-grade H35 PVC foam (DIAB AB Group). Kenaf fibers reinforced unsaturated polyester resin (UPR) composites were fabricated first as the face sheet of sandwich structure. Then polyesters, fibers, and curing agent were initially weighed to ensure the total fiber loading was 55 wt.%. Each mixture of sandwich face sheet was put into a steel mold and pressed at 5MPa for 2 min to form a mat. Then, the polyester resin with curing agent was poured into the mold and the mat was hot-pressed under a pressure of 5MPa and at a temperature of 150oC for 2 hours. When the fabrication was accomplished, all the panel samples were put into a drier at room temperature for 48 hours for further investigation. The compliant PVC foam core were joined with the upper and lower kenaf fiber reinforced UPR composite panels using epoxy resin. 3. Mechanical properties properties of sandwich sandwich composites composites 3.1. Tensile test Tensile specimens were cut from the composite panels. The tensile strength and modulus of the kenaf fiber composite panel were tested on the basis of three replicates with a MTS 810 universal testing system (USA) in accordance with the procedure described in ASTM D 638-10 [8]. The specimen dimensions were 100×20×1.60 mm3 (Length × Width × Thickness). A tensile fixture was used, with a constant gauge of 60 mm and a crosshead speed of 0.50mm/min. An FEI Quanta Environmental Scanning Electron Microscope (ESEM; FEI Company, Oregon, USA) were used to examine the fracture morphology of the specimens. Figure 1. Tensile stress versus strain of kenaf fiber reinforced UPR composite panels Figure 2. SEM micrographs of composites after tensile failure
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