13th International Conference on Fracture June 16–21, 2013, Beijing, China -4was randomly distributed in the unsaturated polyester resin matrix. Bending strength was relative high at rich levels of fiber loading. Otherwise, at a relative low level of fiber loading, due to the limitation of resin strength, during the whole stress transfer process, fracture tended to occur in the voids between fiber and resin. Meanwhile those could be used to explain the low stress fracture in no.1 specimen of experimental results, shown in Fig. 4. Table 3. Three-point bending test results of sandwich composites no. Maximum load (N) Max Deflection (mm) f σ (MPa) τ (MPa) lc σ (MPa) Failure modes 1 365.30 18.82 7.79 0.24 1.37 Upper skin fracture, local indentation 2 381.13 18.17 8.13 0.25 1.43 local indentation 3 443.14 13.65 9.45 0.29 1.67 local indentation 4 378.92 14.19 8.08 0.25 1.42 local indentation, crack in the core 5 391.17 17.01 8.34 0.26 1.47 local indentation *where,bending stress in the face sheet f σ , shear stress in the core τ and local indentation stress lc σ were as follows [10]: hb t M f f max = σ (1) hb Vmax =τ (2) A P lc = σ (3) Mmax is the maximum moment, tf is the thickness of face sheet, h is the distance between centroids of the face sheets, and b is the specimen width, Vmax is the maximum shear, Pis the applied load and Ais local crushing area. Figure 4. Three-point bending experiment set-up and main failure modes 3.3. Double cantilever beam test The objective of this part experimental investigation was to understand the fracture behavior and
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