Topology and Shape Optimization of 2-D and 3-D Micro-ArchitecturedThermoelastic Metamaterials Using a Parametric Level Set Method

2-D and 3-D micro-architectured multiphase thermoelastic metamaterials are designed and analyzed using a parametric level set method for topology optimization and the finite element method.An asymptotic homogenization approach is employed to obtain the effective thermoelastic properties of the multiphase metamaterials.Theε-constraint multi-objective optimization method is adopted in the formulation.The coefficient of thermal expansion(CTE)and Poisson’s ratio(PR)are chosen as two objective functions,with the CTE optimized and the PR treated as a constraint.The optimization problems are solved by using the method of moving asymptotes.Effective isotropic and anisotropic CTEs and stiffness constants are obtained for the topologically optimized metamaterials with prescribed values of PR under the constraints of specified effective bulk modulus,volume fractions and material symmetry.Two solid materials along with one additional void phase are involved in each of the 2-D and 3-D optimal design examples.The numerical results reveal that the newly proposed approach can integrate shape and topology optimizations and lead to optimal microstructures with distinct topological boundaries.The current method can topologically optimize metamaterials with a positive,negative or zero CTE and a positive,negative or zero Poisson’s ratio.

Mechanical Response of Honeycomb Structures Under Impact

A honeycomb structure with a negative Poisson’s ratio(NPR)was designed,fabricated,and analyzed for utilization in personal protective clothing(PPC).The mechanical properties were investigated using a quasi-static mechanical testing and the Hopkinson pressure bar experimental system,and results were compared with similar samples containing regular hexagonal and regular quadrilateral honeycomb structures.The experimental results showed that under quasi-static loadings,the concave honeycomb structure had the highest compressive modulus and yield strength,which produced the highest strain absorption energy,anti-deformation performance and energy absorption.When exposed to a dynamic load at a high strain rate,the concave honeycomb also exhibited the highest dynamic compression modulus,the best impact resistance and best energy absorption among the three structures.In summary,the concave honeycomb structure was more resistant to deformation and had higher impact resistance than the regular hexagonal and regular quadrilateral honeycombs,and exhibited better energy absorption,which makes it a good candidate for application as a personal safety protection material.

负泊松比效应锚索的力学特性及其在冲击地压防治中的应用研究

冲击地压发生强度、危害程度及频次呈急剧增加趋势,现有支护材料无法满足冲击力作用下巷道防护的要求,基于负泊松比材料的特殊力学特性,结合井下巷道冲击大变形控制的需求,研发了具有负泊松比效应新型高恒阻大变形锚索。采用室内力学实验和现场爆破模拟冲击试验相结合的方法,对新型锚索的防冲力学特性进行了研究,结果表明恒阻锚索能够在静力拉伸作用下产生滑移拉伸变形的同时保持350 kN左右的恒定阻力,多次落锤冲击动力作用下,能够通过保持恒定阻力并产生拉伸变形来吸收冲击能量。以沈阳红阳三矿1213回风联络巷为工程背景,提出了现场采用爆破形式模拟冲击地压的现场防冲方案,试验表明高恒阻大变形锚索在爆炸冲击力作用下可以产生瞬间滑移变形,从而吸收爆炸产生的冲击能量,并具有保持恒定阻力的特殊力学性能;通过现场对比试验可知,在相同当量爆破冲击能量作用下,普通锚索试验段完全崩垮,恒阻锚索试验段整体稳定,验证了恒阻大变形锚索比普通锚索具有更好的抗冲击力学性能。

抗拔桩的承载性能研究

针对抗拔桩的侧摩阻力很难准确计算的问题,采用弹性力学M ind lin解并结合非线性接触面单元建立了一个能考虑桩体泊松效应的桩土体系力学模型.通过对原型抗拔试验桩进行模拟验证了该模型的适用性.分析表明,抗拔桩属于“突进型破坏”,而抗压桩属于“缓进型破坏”.且在极限荷载作用下抗拔桩的变形较小,破坏前兆不易察觉.抗拔桩的桩侧阻力随桩长的增加近似按线性关系减少,随桩径的增加按非线性关系减少.

不同冲击速率下恒阻大变形锚杆动力学特性分析

为了研究恒阻大变形锚杆的动态力学特性,基于恒阻大变形锚杆冲击拉伸试验系统对锚杆进行动力学试验,分析了锚杆在不同速率冲击时的受力和变形等力学特性,采用CAD/CAE多软件集成有限元分析方法,对锚杆在经受冲击时的工作状态进行了数值模拟,并简要介绍了唐口矿工程监测试验的相关结果。室内外试验结果表明:恒阻大变形锚杆承受的冲击力与外界瞬时冲击力呈明显的正比关系,且锚杆变形过程是弹性—塑性,验证了恒阻大变形锚杆具有高恒阻值、大变形量和负泊松比效应的特点。数值模拟与室内试验结果吻合,这不仅体现了有限元分析方法的准确性,也表明运用有限元分析来佐证或替代部分室内试验的可行性。

泡沫金属的微惯性效应和动态塑性泊松比

采用三维Voronoi技术和显式有限元方法来研究闭孔和开孔两种泡沫金属的动态塑性泊松比问题和微惯性效应。细观数值模拟的结果表明:塑性泊松比随着轴向应变的增加而下降,塑性泊松比的峰值随着冲击速度的增加而下降;相对密度增加时,泡沫金属塑性泊松比增加;微惯性对平台应力的影响不大。该数值模拟结果能够解释侧向约束情况下闭孔泡沫金属的压溃应力随着加载速率的提高而下降的实验现象。

内凹工字形蜂窝结构的面内冲击性能

提出一种具有负泊松比效应的内凹工字形蜂窝结构,采用数值模拟方法研究了不同冲击速度下内凹工字形蜂窝的变形模式和能量吸收等动力学响应特性,并将其与正六边形蜂窝和内凹六边形蜂窝在面内冲击载荷作用下的抗冲击性能进行对比。结果表明,在中低速冲击载荷下,能够明显观察到内凹工字形蜂窝结构受到轴向压缩时的“颈缩”现象,其主要与工字形节点梁的弯曲旋转变形有关。相比于内凹六边形蜂窝结构,内凹工字形蜂窝结构峰值应力更低,平台应力阶段时间更长,并且随着冲击速度的提高,表现出更好的冲击载荷一致性和能量吸收特性。

钢筋混凝土的负泊松比设计及其常规力学性能研究

负泊松比材料,拉伸时在弹性范围内横向发生膨胀,压缩时横向则发生收缩,具有优异的抗断裂性能与回弹韧性,可有效抵抗剪切应力,大幅度吸收爆炸荷载能量.将负泊松比引入混凝土结构,设计并系统研究了具有负泊松比效应的功能钢筋混凝土体系及其制备工艺,利用电子万能试验机测试分析了在拉、压、弯、剪等静载作用下3种钢筋混凝土结构的抗拉性能、抗压性能、弹性模量和剪切模量等常规力学性能,应用数字散斑相关方法(DSCM)表征了不同类型荷载状态下各类钢筋混凝土在形变过程中应变场及其泊松比动态分布,并提出了功能负泊松比钢筋混凝土的优化设计方案,为建筑材料的结构设计及应用提供了实验基础与理论依据.

负泊松比圆弧曲线蜂窝芯结构的力学分析

负泊松比机械超材料独特的力学性能,使其在航空航天、医疗设备、国防工程等领域具有广泛应用前景.文章提出了一种具有负泊松比特性的弧角可变的圆弧曲线蜂窝芯结构,改进了传统内凹六边形蜂窝芯结构的胞元设计.运用能量法给出了不同弧角下圆弧曲线蜂窝芯结构在小形变范围内等效弹性模量和泊松比的理论公式,与有限元结果有较好的吻合度,验证了理论公式的正确性.同时,给出了圆弧曲线弧角,胞元长宽比和杆厚等结构参数对本结构等效弹性模量和泊松比的影响规律.研究结果可为新型负泊松比机械超材料的优化设计及应用提供新的思路.

一种正、负和零泊松比相互转换的新策略

针对正、负和零泊松比值难以调节的问题,本文基于曲杆/板组成的内凹式十边形多孔结构,提出了一种转换策略。采用数值模拟和实验相结合的方法验证了在不同胞壁曲率下,结构的泊松比不同(范围包括正、负和零)。研究发现:结构的泊松比绝对值随着有效真实应变的增大而逐渐减小,随后出现了相反的泊松比值。对这种现象进行了分析,并提出了曲率效应影响机制。

周期性矩形管浮筏结构的振动分析

为减小舰船动力设备机械振动的传播,提高舰船减振能力,本文设计了一种在压缩载荷下具备负泊松比特性的周期性矩形管浮筏结构。通过数值模拟分析了矩形管层数、梯度排列方式和矩形管单胞壁厚对结构振动传递特性的影响。结果表明:矩形管层数和矩形管单胞厚度对结构的固有频率影响较大。在相同面密度下,梯度排列的矩形管可改变结构的共振频率,同时对减振效果有一定程度的提升。此外随着矩形管壁厚的增加,结构的固有频率逐渐增高,同时结构整体的减振效果呈现先增强后减弱的趋势。

Computational Studies of Porous Head Protection Structures for Human Cranium under Impact Loading

This paper looks into the effects of various porous structures used in the construction of the shell of a protective helmet on the energy absorption capacity and their efficacy in protecting the head/skull against impact force.It is well known that porous structures are very effective for energy absorption;hence,they have been widely used to reduce the negative effects of impact and explosion loads on the human skull.Porous shell structures,made from titanium alloy(Ti–6Al–4V)and,comprised of several periodic topological configurations,namely the more common rectangle and hexagonal honeycomb,as well as those having auxetic properties,namely the concave honeycomb and double-arrow,are studied by means of numerical modeling.The reliability of the numerical model is validated with the published experimental results.For the double-arrow configurations,the study involves three different densities,and the structural energy absorption capacity of the double-arrow shells increases with density.For the same density,the energy absorption capacity of the rectangular shell is the best,and that of the honeycomb is the worst.The superior performance of the rectangular configuration is partly derived from the fact that the orientation of the struts in this structure is aligned along the direction of the impact force.Further comparison of energy absorption capacity is made between the porous shell and a shell having a traditional titanium monolayer.The severe plastic deformation in the solid titanium shell(traditional monolayer shell)is detrimental to the overall effectiveness of head protection gear.Apart from this,compared with the Kevlar composite laminated shell of the same mass,both the solid and porous titanium shells provide considerable protection to the human head.The comprehensive comparisons show that the porous design on the titanium shell is beneficial for mitigating the risks of traumatic brain injuries(TBIs).

恒阻大变形锚杆冲击拉伸实验及其有限元分析

为了研究在动态冲击作用下恒阻大变形锚杆的力学特性,采用自主研发的恒阻大变形锚杆霍普金森拉杆动力冲击拉伸实验系统,对某批次恒阻大变形锚杆进行动态冲击拉伸实验,从支护阻力、锚杆伸长量和膨胀量等方面对锚杆的动态力学性能进行分析,并验证该锚杆的负泊松比效应。并通过有限元分析的方法与实验结果进行比较,验证有限元结果的准确性。实验结果表明,该锚杆通过产生弹塑性结构变形来吸收冲击能量并具有独一无二的负泊松比效应,因而能良好地适应动态冲击。有限元分析结果与实验结果吻合度较高,不仅体现了有限元分析方法的准确性,也表明今后可以运用有限元分析来佐证或替代部分室内实验的方法。