A new shear test method for sandwich core materials is proposed and evaluated. Sandwich beams are loaded in four-point bending, and the shear deformation is measured with two rotary sensors. Conditions of idealized sandwich theory are assumed to prevail, and the accuracy of the proposed methodology is thus dependent on a few mechanical and geometric relations between the sandwich constituents. The stress-strain responses for two polymer foam core materials, one relatively brittle and one relatively ductile, are extracted and compared with results from single-block shear tests of the same material batch. The new method provides several benefits with respect to the block shear test. It does not suffer from extreme stress concentrations and the specimens are tested under in-service conditions. Problems arise, however, for the ductile material, predominantly related to large deformations during the test eventually resulting in bending failure of the face sheet instead of shear failure of the core.

International audienc

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Material candidates for energy absorption in head impact countermeasures for automotive applications are evaluated using both quasi-static and dynamic test methods. Ranking of different materials turns out to be difficult since the mechanical response of a material could vary considerably with temperature, especially for polymers. Twenty-eight selected materials, including foams, honeycombs and balsa wood are tested and evaluated. The materials are subjected to a sequence of tests in order to thin out the array systematically. Quasi-static uni-axial compression is used for initial mapping of the selected materials, followed by quasi-static shear and dynamic uni-axial compression. The quasi-static test results show that balsa wood has by far the highest energy absorption capacity per unit weight but the yield strength is too high to make it suitable for the current application. The subsequent dynamic compression tests are performed for strain rates between 56 s−1 and 120 s−1 (impact velocities between 1.4 and 3 m/s) and temperatures in the range −20 - 60 °C. The test results emphasize the necessity of including both strain rate and temperature dependency to acquire reliable results from computer simulations of the selected materials.