Energy Absorption And Low Velocity Impact Damage Resistance Of Composite Lattice Structures

Sandwich structures are commonly composed of two thin but stiff skin layers separated by the soft lightweight core material. In engineering applications, the tensile and compressive stresses are usually balanced by the top and bottom facesheets while the shear loads are mainly supported by the core. At present, sandwich structures are widely used in aerospace and astronautic industries due to the designable characteristic. Compared with the conventional sandwich structures, such as the foam and honeycomb sandwich panels, the lattice core sandwich structures have higher specific strength and stiffness. Except that, the unique open cell architecture allows for lattice core sandwich structures have the potential in multi-functional applications. In this dissertation, the pyramidal lattice core sandwich structures consisting of carbon fiber reinforced polymer (CFRP) facesheets and aluminum alloy cores are manufactured. Then, the basic mechanical properties are also investigated by the experimental and numerical methods. The main contents are as follows:Firstly, based on the slot-fitting method, the pyramidal lattice core sandwich structures are manufactured. Quasi-static compression tests are conducted to get the stress-strain curves and to evaluate the energy absorption mechanism. While, the shear properties are investigated by the shear tests. Furthermore, the finite element method is employed to simulate the compressive behavior.Secondly, a combined experimental and numerical method is conducted to assess the damage resistance of such structures subjected to low velocity impact. The effects of impact energy, impact site and core density are considered in the low velocity impact tests. In view of the complicated composite failure and contact involved in the tests, in general, the numerical results give a good agreement when compared with the experiments, which can be helpful in the structural design.Then, the compression-after-impact (CAI) strength of pyramidal lattice core sandwich structures is explored by the experimental and numerical methods. The parameters affecting the CAI strength, such as impact energy, impact site and core density, are considered in this research. It is observed that the small energy impact has negligible effect on the CAI strength. In addition, the failure modes of such structures are distinct during the CAI tests when the impact site is different and that the specimens with higher density cores have slightly lower normalized CAI strength reduction. In the finite element analysis, the impact damage of impact-damaged sandwich structures are initially obtained in the impact analytical step and then transferred to the compression step to forecast the residual compressive strength. Generally speaking, the numerical results are satisfied.Finally, the polyurethane foam filled pyramidal lattice core sandwich panel is fabricated in order to improve the energy absorption and low velocity impact resistance. Based on the compression tests, a synergistic effect that the foam filled sandwich panels have a greater load carrying capacity compared with the sum of the unfilled specimens and the filled polyurethane block is found. During the impact tests, it is found that the filled polyurethane foam does not demonstrate significant influence on the impact resistance.