形状记忆合金点阵材料的变形、失稳与优化设计

Exhibiting recoverable strains that is nearly ten times greater than those of the base materials or conventional metal lattice materials, shape memory alloy (SMA) lattice materials are promising candidates to make reusable energy absorbers, actuators, and other smart devices. Therefore SMA lattice materials have attracted much attention in academia and industry. Design of the lattice microstructures with respect to any specific application requires systematic understanding of the mechanisms of deformation and instability of the lattice materials, which is still an open topic and a challenge. In this project, a constitutive model accounting for the characteristics of SMAs that affect the performance of SMA lattice materials, i.e., thermomechanical coupling, tension/compression asymmetry, material softening under tension and plastic yielding at high stress levels will be developed for the accurate description of the behaviors of SMAs. Specimens of SMA lattice materials with six commonly used lattice configurations will be fabricated and tested in order to obtain experimental data on the deformation and instability patterns, as well as the mechanical performance of the specimens. Combined with the experimental observations and data, finite element modeling will be performed to reveal the effects of material characteristics and lattice microstructure on the mechanical behavior of SMA lattice materials, based on which the rule of design and optimization will be established. Finally, the design and optimization for an energy absorber based on SMA lattice materials will be conducted for demonstration. This project tends to deepen the understanding of the mechanics of SMA lattice materials and provide significantly larger room for design and optimization for such materials.

形状记忆合金点阵材料可获得近十倍于基材或传统金属点阵材料的可恢复应变，有望用于制造可重复使用的吸能装置、致动器等，在学术界和工业界备受瞩目。针对具体应用对形状记忆合金点阵材料性能提出的要求进行微结构设计，需要对其变形和失稳机理进行系统的研究，而目前这方面的工作还不成熟。本项目将针对影响形状记忆合金点阵材料吸能能力、可恢复应变等力学性能的材料特性，建立一个同时考虑热力耦合、拉压非对称性、拉伸下的相变软化、高应力下塑性屈服的本构关系；制备六种典型点阵构型的形状记忆合金点阵材料并对其开展力学测试，获得其变形与失稳机理以及性能数据；结合有限元模拟，揭示材料特性、点阵微结构等因素对形状记忆合金点阵材料性能指标的影响规律，从而建立点阵微结构设计和优化准则，并以应用于航天器的点阵材料吸能板为例进行优化设计。本项目的实施将深化对形状记忆合金点阵材料力学行为的认识，极大地拓展轻质智能点阵材料的设计空间。

形状记忆合金点阵材料有望用于制造可重复使用的吸能装置、致动器等，且在弹热制冷等新兴领域具有广阔的应用前景，因此受到了学术界和工业界的瞩目。基于点阵中基材的受载特点，本项目对超弹性形状记忆合金的热力响应进行实验测试、本构建模和有限元仿真，并对基本构件的力学行为进行系统的研究，取得的成果包括：(1)针对该材料在循环拉伸作用下的功能性疲劳开展实验研究，基于不可逆热力学框架进行本构建模，引入超弹性参数随相变累积的演化，成功模拟了恒温条件下随着循环进行而出现的相变应力减小、相变应变减小、残余塑性应变累积等功能性疲劳现象。(2)在前述模型的基础上，加入与温度有关的热力学自由能，引入位错密度重新建立残余应变的演化规则，将超弹性参数随着循环次数增加而发生的演化与位错累积相关联，从而描述超弹性形状记忆合金的热力耦合功能性疲劳现象，通过有限元仿真验证了模型的有效性。(3)晶粒细化是调控材料性质的一种有效手段，在本构模型中采用Voigt假设对细观结构进行建模，将其视为可变相与不可变相的复合，建立可变相的体积分数与晶粒尺寸间的关系，从而准确描述热力性质的晶粒尺寸相关性。(4)对薄壁管的单轴拉伸进行了有限元模拟，揭示了热力耦合是引起应力应变曲线、局部化应变场、温度场等热力响应率相关性的原因，并探讨了管的几何对局部化相变模式的影响。(5)对42组不同径厚比和长径比组合的超弹性形状记忆合金管的单轴压缩响应进行了有限元仿真与理论分析，获得了稳定压缩、柱型屈曲-退屈曲、柱型屈曲、壳型屈曲四种变形模式在径厚比-长径比参数空间中的分布相图。(6)对测量段宽高比为0.25、1、2和4的四种哑铃型试件进行了单轴拉伸实验和有限元仿真，揭示了几何因素即宽高比对应力应变曲线和局部化相变带演化的影响，并指出应力双轴度随宽高比增大而上升是造成上述影响的原因。对上述问题的研究验证了本构模型的有效性，为进一步设计和分析形状记忆合金点阵材料奠定了基础。