Mechanical Properties And Strengthening Mechanism Of Novel Hourglass Metal Lattice Structure

Periodic lattice structures with lightweight,high srengths and stiffness,and mutilfunctional development have attracted significant interest as the lightweight cores for sandwich constructions.While pyramidal lattice topology systems usually offer significantly superior structural performance,improvements appear feasible,based on the following two limitations: the lattice cores are susceptible to buckling,and thick core leads to large inter-node spacing which weakens resistance to local face sheet buckling under compression and face tearing under blast loads.The purpose of this paper is to improve the mechanical properties of lattice structures by designing novel lattice topologies.Based on the design disadvantages of pyramidal lattice sandwich structures,a novel Hourglass truss lattice sandwich structure is developed and manufactured via a snap-fit and vacuum brazing approach.Compared with the pyramidal lattice,the hourglass lattice provide higher width to length ratio of strut and smaller internode spacing,and then provide higher resistance to buckling of both struts and facesheets.The out-of-plane compressive and in-plane shear mechanical properties of Hourglass lattice sandwich structures have been investigated theoretically and experimentally,and compared with those of pyramidal lattice structure.In theory,the compressive model,proving to be more accurate than the tensile model,is proposed to predict the compressive strengths.Under out-of-plane compressive loads,the measured collapse loads are in reasonable agreement with the predictions.The increase of resistance to buckling of struts for the hourglass lattice increase its out-of-compressive strength.The measured out-of-plane compressive strengths of the Hourglass lattices(relative densities from 1.15% to 2.71%)improve by 26%-47% compared with the snap-fit pyramidal lattices.The out-of-plane compressive strengths of the Hourglass lattices also outperform some other existing topologies.Under in-plane shear loads,the use of the hourglass lattices as the cores of sandwich structures increases their peak shear collapse strength due to the improvement of the inelastic buckling resistance of the lattice core members.The measured shear strengths of the hourglass lattices with relative densities ranging from 1.1% to 2.7% were about 40% to 60% higher than those of the pyramidal lattice structures with similar relative densities.In all,this novel lattice topology design increase the core strength of lattice structures.The in-plane compressive and three-point bending performances of hourglass lattice sandwich structures are investigated using various analytical methods and the obtained results were in good agreement with the experimental data.The use of the hourglass lattices as the cores of sandwich structures increases the elastic buckling resistance of the attached face sheets due to decreasing the inter-node spacing,which improve the in-plane compressive and the three-point bending peak loads.Under inplane compressive loads,At least four failure modes exist for Hourglass sandwich structures and pyramidal sandwich structures under in-plane compressive loading: macro elastic buckling,macro inelastic buckling,local elastic face sheet buckling,and local inelastic face sheet buckling.An in-plane compressive failure mechanism map is shown to guide the engineering design.The measured in-plane compressive collapse loads of Hourglass lattice sandwich structures are up to 3.5 times(the maximum time)that of pyramidal lattice sandwich structures with the same structure sizes.In addition,three-point bending tests were conducted for the two lattice structures to investigate their mechanical properties and failure mechanisms.The failure mechanism maps constructed in this study were validated by studying the mechanical behavior of the sandwich beams under three-point bending.The elastic buckling and yielding of the face sheets as well as core member yielding were observed for the hourglass lattice beam under three-point bending loading,while only the elastic buckling of the face sheets was detected for the pyramidal lattice beam.The obtained results indicate that the hourglass lattice design is superior to the pyramidal lattice design under three-point bending loading.A multi-layer hourglass truss lattice design is introduced to further reduce the facesheet fracture susceptibility by decreasing the internode spacing of lattice sandwich panels.The multi-layer hourglass lattice can be also fabricated by a snapfit and vacuum brazing approach.The in-plane and out-of-plane compressive properties of three different layers of hourglass lattices are investigated using analytical and experimental methods.Results indicate that the increasing of the layer number increases the in-plane compressive load of lattice truss sandwich structures and their energy adsorbtion,while changes the out-of-plane compressive strengths little.In addition,the layer number of the multi-layer hourglass lattice structures is optimized based on both the in-plane compressive collapse loads and the produced cost.A simulated water shock tube is used to test the underwater blast behavior of stainless steel single-layer hourglass lattice sandwich panels,which is compared with solid plates,the pyramidal lattice panels and the two-layer hourglass lattice panels of the identical areal mass.The performance of the different structures is compared in terms of the dynamic back-face and front-face deflections and the tearing susceptibility of the faces.Results indicate that the hourglass lattice sandwich panels provide higher underwater blast impact performance compared with not only the solid plate but also the pyramidal lattice panels.While compared with the singlelayer hourglass lattice structure,the two-layer hourglass lattice structure provide higher resistance to underwater blast impact.