13th International Conference on Fracture June 16–21, 2013, Beijing, China -6displacement curve. In order to calculate the work required to propagate the crack, for all of the specimens, load vs. open displacement of stage II were selected. Meanwhile, the DCB results were summarized in table 4 showing the peak load, open displacement and strain energy release rate Gc. Also compared with some published works, one was for honeycomb sandwich and another was for copper-epoxy interface, it was denoted that this novel sandwich structure exhibited better interlaminar fracture behaviors. Figure 7. Fracture behavior and propagation of interlaminar delamination of sandwich composite Table 4. Double cantilever beam test results of sandwich composites Specimen Peak load (N) Open displacement (mm) Energy release rate (kJ/m2) Main failure mode 1 15.37 0.41 0.476 Interface delamination 2 18.27 0.89 0.277 upper skin fracture 3 12.48 1.50 0.423 Interface delamination 4. Conclusions The following results are obtained, (1) A novel natural fiber reinforced UPR composite sandwich structure is developed and fabricated. (2) Mechanical properties results show the elastic modulus of kenaf composite material is about 2.05~2.39GPa. Main failure mode in three-point bending tests is local indentation, while the equivalent flexural stiffness is about 1476~1592N/mm. Meanwhile failure mechanism analysis is accomplished by digital camera and SEM. (3) Due to lightweight, relative high strength, high elastic modulus and absorb energy, this kind of sandwich structures can be potentially used in civil engineering constructions. Acknowledgements This research work was financially supported by the central university scientific research basic project (HEUCFZ1128) of China, National Science Foundation of USA (NSF-PFI 1114389 and NSF-CMMI 1031828). The authors thank Kengro, Inc. and DIAB group, for supplying the raw materials. Also show thanks to Dr. Wen Hu, Mr. Changlei Xia, Mr. Mangesh Nar and Dr. Kaiwen Liang for their help in experiments. References [1] D Zenkert. An introduction to sandwich construction. Solihull UK: EMAS, 1995. [2] SD Clark, RA Shenoi, HG Allen. Modelling the fatigue behaviour of sandwich beams under monotonic, 2-step and block-loading regimes. Compos Sci Technol, 4(1999) 471–486.
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