【作者】 宋卿争；

【导师】 唐志平；

【作者基本信息】 中国科学技术大学 ， 工程力学， 2014， 博士

【摘要】 NiTi形状记忆合金是一种典型的相变材料,相变可引起材料强烈的非线性,对其力学响应和应力波形造成重大影响。本文主要对复合加载下NiTi合金的准静态和动态相变特性进行了实验研究,并对NiTi合金薄壁管中的复合应力相变波的传播规律进行了理论分析和实验研究。利用MTS实验装置,对NiTi合金在不同加载路径(拉、压、扭以及复合加载)和不同温度(28-150℃)下的准静态相变行为进行了较系统的实验研究,结果表明材料呈现明显的伪弹性效应和拉压不对称性。随着温度的升高,相变起始应力逐渐增大,伪弹性效应和拉压不对称性逐渐减弱。当温度达到一定阂值时,拉压不对称性消失,材料基本呈现弹塑性状态。通过实验曲线确定了NiTi合金的相变临界参数,在σ-τ应力空间中对三种常用的宏观相变临界准则进行了讨论,比较了它们各自的适用性。最后对NiTi合金实心圆轴试样在纯扭转实验过程中的剪应力和剪应变的计算方法做了初步探讨,并对复合加载下的实验数据进行了修正。采用一种基于Hopkinson压杆的压剪复合加载实验装置,对NiTi合金在压剪复合加载下的准静态和动态力学行为进行了研究。对NiTi合金材料进行了不同倾斜角度的准静态压剪实验,验证了该实验方法的可行性。对NiTi合金进行了不同倾斜角度、不同速度下的压剪冲击加载实验,并给出了动态不同应变率下NiTi合金的相变临界点在σ-τ应力空间的分布。采用一个合适的动态相变临界准则,对实验得到的相变临界点进行了预测,发现NiTi合金材料存在应变率效应。在390-860s-1纵向应变率范围内,NiTi合金材料的相变临界点的应力随应变率增加明显。通过高分辨率相机和高速摄影分别跟踪拍摄了准静态和动态实验过程,根据不同时刻实验照片的对比,发现实验中试样和斜端面垫块之间不存在相对滑动,说明实验结果是的可靠的。采用考虑静水压力与偏应力共同作用的相变临界准则,构建了增量形式的相变本构关系,建立了薄壁圆管中复合应力相变波的控制方程。通过特征线理论求解出了相变快波和相变慢波,并在σ-τ应力平面上给出了相应的复合应力路径图。由于静水压力的影响,相变区可分为三个区域,其中τ轴和初始及后继相变临界椭圆顶点所围成的区域是传统弹塑性材料所没有的,加载最终状态在该区域或通过该区域的应力路径的相变复合波形可能有新的变化。以NiTi合金薄壁圆管为例,对两种典型的应力路径进行了分析,给出了相应的复合应力相变波信号。若只考虑相变临界准则中的应力项,则与Drucker-Prager屈服准则形式一致,因此可将得到的相变材料中的结果可以推广应用于符合该模型的其他材料,比如岩土材料。在分离式Hopkinson压杆的基础上,设计并建立了分离式压扭Hopkinson杆实验装置,实现了薄壁长管的压扭复合加载。对不锈钢薄壁管进行了压扭复合加载实验,得到了典型的塑性快波和塑性慢波信号,并采用粘塑性模型对信号做了定量分析,验证了该实验方法的可行性。采用NiTi合金薄壁管的材料参数,对预扭纵向冲击加载下NiTi管中的复合加载路径和相变复合波信号进行了分析和预测,为设计NiTi管的压扭实验提供了支持。对NiTi管分别进行了相变临界面内和相变临界面外的预扭压缩复合加载实验,并在应力空间中对复合加载路径进行了分析和计算,计算结果和实验结果吻合较好,但管中纵向信号的应变平台和相变慢波的区分不够清楚。由于实验所用NiTi管存在相变临界应力较高,破坏应变较低等问题,同时还具有拉压不对称性,若要获得理想的复合应力相变波信号,还需改进实验装置为拉扭复合加载或通过热处理降低NiTi合金管的相变临界应力。更多还原

【Abstract】 NiTi shape memory alloy is a typical phase transition material, phase transition can greatly affect the mechanical response and wave propagation properties of material and structure. In this paper, the quasi-static and dynamic phase transition behaviors of pseudoelastic NiTi alloy under combined stress are investigated, and the theoretical and experimental research of combined phase transition waves in thin-walled NiTi tubes are proposed.Phase transition behavior of NiTi shape memory alloy was studied systematically through quasi-static experiments under different loading paths (uniaxial compression and tension, pure torsion and proportional tension/compression-torsion) and different temperatures range from28℃to150℃. Experimental results show apparent pseudoelastic effect and tension-compression asymmetry. The initial phase transition stress increased with the rise of temperature; meanwhile the pseudoelastic effect and tension-compression asymmetry were gradually weakened. NiTi alloy show elastic-plastic behavior when the temperature reached a certain threshold, and the tension-compression asymmetry disappeared. Based on the data of critical phase transition points measured from the experiments, three phase transition criterions were discussed on σ-τ stress plane, and their applicability were compared respectively. Finally preliminarily discussed the calculated method of shear stress and shear strain during the torsion of NiTi circular bar, and revised the experiment data of combined loading.The quasi-static and dynamic phase transition behaviors of pseudoelastic NiTi alloy under compression-shear loading were studied through a new combined compression-shear technique based on split Hopkinson pressure bar. The data processing methods were studied and verified by quasi-static experiment of NiTi alloy. A series of experiments of NiTi alloy were performed at different impact velocities and different loading angles, and the dynamic equivalent pressure curves were given. The initial phase transition points and the phase transition surfaces predicted by a dynamic phase transition criterion were given. The results show that the material was sensitive to the strain rate, and the stress of initial phase transition increased with the strain rate during390-860s-1. By a high resolution camera and high-speed photography following the quasi-static and dynamic experimental process respectively, there was no relative slip between the specimen and the beveled ends from the comparison of the photographs of the experiment, which confirmed the validity of the experiment method.The incremental constitutive relation and governing equations with combined stresses for phase transition wave propagation in a thin-walled tube were established based on the phase transition criterion considering both the hydrostatic pressure and the deviatoric stress. It was found that the centers of the initial and subsequent phase transition ellipses were shifted along the σ-axis in the σ-τ plane due to the tension-compression asymmetry induced by the hydrostatic pressure. The wave solution offered the "fast" and "slow" phase transition waves under combined longitudinal and torsional stresses in the phase transition region. The results show some new stress paths and wave structures in a thin-walled tube with phase transition, differing from those of conventional elastic-plastic materials.The split combined compression torsion Hopkinson bar was designed and built based on SHPB, and experiment technique of pre-torqued tubes under longitudinal impact was achieved. The combined stress wave of strainless steel tube was measured, and the quick wave and slow wave of the strain signal was quantitative analysed by elastic-viscoplastic models, which show the experiment technique was feasible. The combined stress and strain signals of pre-torqued tube under impact were calculated by the material parameters of NiTi. The final stress rate in and out the initial phase transition surface were achieved during the combined loading of NiTi tube, and the property of combined phase transition was analysed. The separate of quick phase transition waves and slow phase transition waves was not clear at present experiment, but has a good agreement with the theoretical model. As the NiTi tube have several problems such as:the initial phase transition stress is too high, the crack strain is low, and the tension-compression asymmetry. To obtain desired signals of combined phase transition waves, it must be improved the compression-torsion loading into tension-torsion loading or decreased the initial phase transition stress through heat treatment.更多还原