13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Figure 22. Equivalent stresses in the sheets Figure 23. Variation of stresses σeq, σ1, σ2, along a half of the diagonal, on the surface of the panel 6. Experimental evaluation of the sandwich panel under torsion In Fig. 24 (left) is shown the core of the sandwich panel tested in torsion with the loading scheme from Fig. 24 (right). A strain gage with two measuring grids (SG 1 and SG 2) of type XY93 10/120 (HBM, Germany) was positioned in the middle of the panel along its diagonals in order to measure the principal strains. With these strains were calculated the principal stresses σ1, σ2, and then the von Mises equivalent stress, σeq. For an applied force of 500 N (as in the FEA model) the experimentally established equivalent stress resulted as 23.55 MPa, being a little bit smaller than the 25 MPa value obtained numerically. This result validates the modeling procedure established in the FEA. Figure 24. The meta-tetrachiral core manufactured from rigid foam strips and the loading scheme 7. Conclusions The meta-tetrachiral core analyzed hereby is a new solution which has auxetic properties, and the resulting sandwich panel is stiff and light. Although it has only one plane of symmetry its behavior in bending (Table 1) is almost the same on the two in-plane directions. This type of geometry, characterized by robust cells having thick walls, confers an important advantage concerning the local stability of the core. We intend to use this meta-tetrachiral core for curved sandwich panels. By changing the geometry of the basic block (Fig. 5) one can obtain similar chiral core geometries with properties which worth to be investigated. Due to the simple manufacturing technology such panels can be obtained with significant economical advantages.
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