Gradient TPMS Structure Design And Mechanical Behavior Study Of 316L Stainless Steel And NiTi Shape Memory Alloy

Porous structures,with their low relative density,high specific strength,large specific surface area and good energy absorption,are a hot research topic in the field of mechanics and have led the way in exploratory research into new types of crashworthy engineering structures.Triply Periodic Minimal Surface(TPMS)is significantly better than other porous structures in terms of strength,stiffness,surface area,platform stress and energy absorption,and avoids the problem of stress concentration with its zero mean curvature.The deformation process of metallic TPMS structures is divided into a linear-elastic phase,an elasto-plastic plateau phase and a densification phase.The magnitude of energy absorption depends on the combination of high plateau stresses and large densification strains,however it is difficult to enhance both at the same time with a uniform thickness design.Many scholars have conducted studies on the optimal design and mechanical behaviour of TPMS structures,but they are still at a preliminary stage and lack systematic research.The shear bands that occur during compression in uniform-thickness structures weaken their overall energy absorption capacity,while gradient design offers a new direction of optimisation for existing structures,and the study of gradient structural design is of great importance for the application of structural energy absorption.In this paper,multiple gradient designs are proposed,in addition to the design for the direction parallel to the loading,multiple directional gradient designs are also proposed for the macroscopic adjustment of the double shear band,thus effectively improving the energy absorption characteristics of the structure.In this paper,the TPMS gradient structures are printed using selective laser melting technique using 316 L stainless steel powder and Ni Ti powder as raw materials respectively,and the effects of different gradient designs on the mechanical behaviour,damage modes and failure mechanisms of plastic and brittle TPMS structural alloys are investigated.It was found that the 316 L stainless steel TPMS structure in the [100] direction(loading direction)gradient design of the specimen,the gradient thickness of 0.6-0.3-0.6 for the largest performance improvement,compared with the uniform thickness design,energy absorption performance increased by 18.3%.In the shear band position gradient design,the [110] and[111] direction gradient design scheme is set to the thinnest wall thickness in the vertical direction of the shear band unit in order to avoid shear band generation.However,as the thinnest wall thickness passes through the main compression unit,the yield strength decreases compared to the even-thickness sample,but the wall thickness of the other adjacent units gradually thickens.The stress/strain curves of these two design solutions show multiple decreases and increases,and the overall load capacity is at an average value without any significant increase.The bearing capacity is the key factor to determine whether the energy absorption performance is excellent or not.Due to the low strength in the early stage,although the strength is improved in the later stage,the average bearing capacity is slightly lower than that of the uniform thickness structure,so the energy absorption performance is slightly worse than that of the uniform thickness structure.The shear zone thickening scheme thickens the main bearing units and weakens their peripheral units,so that the load carrying capacity of the main bearing units is increased and the structure rapidly enters the densification phase when the main bearing units yield.The strength of the structure depends on the strength of the thicker units,and the greater the difference in the magnitude of the sample wall thickness,the higher the load carrying capacity and the faster it enters the densification phase after yielding.This solution results in a 26.8% improvement in energy absorption compared to uniform thickness,which is better than a solution designed for loading in a parallel direction.Cyclic compression tests on brittle Ni Ti TPMS structures have shown that the mechanical behaviour of brittle TPMS structures is quite sensitive to changes in wall thickness and that the strength of the specimen depends on the strength at the thinnest part of the wall thickness,with the sample breaking down after cracking at the thinner part of the wall thickness.The specimens still show shear damage when the difference in wall thickness of the specimen unit is not large,but when the specimen unit variation is more than twice as large and above,the specimen eventually breaks at the weak point of the wall thickness.The plasticity performance is best when the specimen unit wall thickness variation is 2 times.Among the five gradient designs,the best plastic performance is the [111] directional gradient design,which increases plasticity by 41.5% compared with the uniform thickness design,but decreases strength by 18.9%,and the thickened design at the shear zone position can increase structural strength and plasticity simultaneously,increasing strength by 7.6% and plasticity by 38.5%.