13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Table 1. Results of the finite element analyses Maximum von Mises equivalent stresses [MPa] Load case Type of analysis Sheets Core Adhesive Maximum deflection [mm] Buckling safety coefficient Fundamental eigenfrequency [Hz] 1 linear 94.3 4.6 7.17 2.589 5.97 365 1 nonlinear 96.6 4.58 7.12 2.581 5.97 365 2 linear 134 8.95 18.4 2.864 8.09 262 3 linear 133 8.93 18.3 2.779 8.24 265 In the two cases of cylindrical bending the stresses in the sheets, but especially in the core are unacceptably high. As the stresses in the foam have to be reduced to half, the loading has to be decreased from 0.05 MPa to 0.025 MPa. If the pressure has an imposed value (as 0.05 MPa) the thickness of the sheets may be increased and/or a foam with higher density and resistance has to be chosen. It is interesting to notice that the distribution of the reactions along the edges on which the panel is supported is quite uniform (Figs. 14 and 18), but there are peak values in the vicinity of the corners where extreme values of the stresses in all the components were obtained. Although it was expected to obtain differences between the two cases of cylindrical bending, the results are very similar; this represents an advantage given by the use of the meta-tetrachiral configuration. 5. Results of the FEA in the case of the panel under torsion The loading scheme is presented in Fig. 20, in each corner of the plate being applied a force of 500 N, which in the numerical model were distributed on squares of 20x20 mm, as to avoid undesired local effects. The obtained results are presented in Figs. 20-23. In the aluminum sheets the maximum equivalent stress obtained in the corners is 65 MPa (Fig. 22), and in the center is 25 MPa (Fig. 23). In Fig. 23 the middle of the panel is considered the origin of the horizontal axis. The equivalent stress in the adhesive was obtained as about 2 MPa along the border of the core, and a maximum value of 6 MPa resulted again in the corners. In the chiral core the equivalent stress was almost zero in the middle area, about 1 MPa in the border, and a maximum value of 2.9 MPa was again obtained in the corners. Figure 20. The loading scheme Figure 21. Displacement of the twisted panel
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