高温分离式Hopkinson压杆技术及其应用

本文介绍了在分离式Hopkinson压杆装置上通过使用一种气动同步机构,实现对试样进行高温高应变率加载的技术。利用此技术仅对试样加高温度而保持入射杆和透射杆与试样脱离且处在较低温度,到预定温度时,借助气动同步机构使入射杆、透射杆与试样接触并同时实现对试样加载。利用波形抑制技术,仅对试样实现一次加载,入射杆中的后续二次加载波通过反作用质量块吸收。通过这些技术的结合,1)可以进行材料在高温高应变率下应力应变测试;2)可以测试材料在高应变率不同温度下的等温曲线;3)可以间接对材料的塑性功热转换系数进行测试;4)可以进行不同温度高应变率下的中断跳跃试验等。在文中给出了一些典型的试验曲线和结果,并对测试方法和结果进行了分析讨论。     

高温高应变率下材料拉伸性能测试的一种新方法

为了实现高温环境下材料高应变率动态拉伸实验技术,将分离式Hopkinson杆直接拉伸装置中试样与拉杆的螺纹连接形式变成楔形连接形式,并加装了气动同步装置系统。这样,在对试样加高温时,能使靠近试样的入射和透射杆端处于较低温度。当撞击管向传递法兰运动时,气动同步装置瞬间拖动透射杆和试样,使两者之间的间隙为零,此时沿入射杆传递的入射波同时对试样拉伸加载。经实验验证,此方法可以有效实现材料高温高应变率拉伸加载。     

基于SHPB波传播系数法试验的衰减弥散修正研究

针对SHPB试验中的波传播衰减弥散问题,基于杆自由端撞击的波传播系数法试验原理,选取Φ100mm PA66尼龙杆为典型Hopkinson压杆,依据SHPB试验中获取试样应力-应变曲线的二波法,对原位测试试验中从加载试样传来的反射波弥散进行修正研究,获得入射杆反射波传播系数;同理,对透射杆获取透射波传播系数。根据修正后的实际反射波和透射波历程,获得试样的应力-应变曲线。通过比较分析,此方法合理可靠。     

PVDF压电薄膜用于Hopkinson压杆测量泡沫金属的动态性能

首先对PVDF(polyvinylidene fluoride)压电薄膜在不同温度不同压力作用下的响应进行了系统的试验研究。然后在Hopkinson压杆系统的透射杆之间夹上PVDF压电薄膜,对其动态响应进行了检验。最后应用这个镶嵌PVDF压电薄膜的Hopkinson压杆系统,测试了泡沫铜材料在不同应变率下的应力应变关系。结果表明:(1)PVDF压电薄膜的压电常数D33是随温度和压力而变,实际应用时应对其进行标定;(2)PVDF压电薄膜可有效的用于Hopkinson压杆系统来测试低强度泡沫材料或低阻抗材料的动态响应;(3)当应变率小于0.1/s时,泡沫铜的塑性流动应力对应变率不敏感,在约400/s到5000/s应变率范围,应变小于40%下泡沫铜对应变率也不敏感。但当应变大于约20%,应变率高于400/s时,与低应变率下的值比较,塑性流动应力的应变率敏感性增加。     

应力平衡和接触状态在Hopkinson杆测试聚合物动态弹性模量中的影响研究

针对用Hopkinson杆试验能否准确测量聚合物动态弹性模量以及其中主要影响因素的问题,本文基于重构试样初始加载阶段的应力波反射透射过程,分别计算了6个特征时间内的前三次反射波和透射波,得到试样的应力平衡度和试样的应力应变曲线。对于所研究的聚合物材料,通过比较重构的应力应变曲线的弹性模量与输入的材料弹性模量,发现在4个特征时间后,误差仅在3%左右。因此试样变形过程中的应力平衡与否不是材料在Hopkinson杆试验中弹性模量测不准的原因。通过环氧树脂试样试验发现,根据Hopkinson杆理论计算的应变结果要大于试样上应变片实测的结果,误差在11%左右。相应的数值模拟研究发现:试样和杆子端面接触状态直接决定着试样弹性模量测量的精度。关于惯性效应和压痕效应的研究也证实它们的影响是可以忽略的。     

定向凝固多晶铁磁形状记忆合金力-温度耦合的实验研究

铁磁形状记忆合金兼具大输出应变与高响应频率等综合特性,是新一代驱动与传感材料。采用定向凝固技术制成的多晶铁磁形状记忆合金具有较多优越的力学性能。本文对温度和应力耦合作用下的定向凝固多晶铁磁形状记忆合金的力学特性进行了实验测试,分别获得了定向凝固多晶Ni-Mn-Ga试样在不同恒定温度时的应力-应变循环曲线,以及试样在压缩时两个互相垂直方向的数字散斑图。结果表明:同一恒定温度时,随着应力循环次数的增加,其应变值逐渐减小;同一压缩应力时,不同温度作用过程中定向凝固方向的应力,随着温度的升高逐渐减小。本文结果可为铁磁形状记忆合金在工程中的应用提供一定的指导作用。     

纯锆在压缩加载下微观变形断裂机制研究

利用材料试验机和SHPB装置研究了常态下纯锆的准静态和动态压缩力学性能。塑性变形阶段的流动应力及硬化率随应变率提高而增大,应变10％～20％段的应力-应变曲线表现出凹向上趋势。金相观测表明孪晶是纯锆的重要变形机制,孪晶密度随应变及应变率的增长而增大。大应变时,在动态压缩试样表面观测到与压缩轴呈45°的宏观裂纹。试样纵向剖开后,金相观测到发展的绝热剪切带。     

采用改进的SHPB方法对泡沫铝动态力学性能的研究

本文改进了传统的分离式霍布金森压杆(SHPB)技术，采用夹在透射杆中的PVDF压电计直接测量透射杆中的应力时程．同时，采用输入波形整形技术，通过调整加载波形，使试样加载过程中保证均匀变形及应力平衡．利用此改进了的SHPB技术对泡沫铝进行了高应变率下的动态压缩实验．实验结果表明：泡沫铝的动态应力应变曲线具有泡沫材料的应力应变曲线的“三阶段”特征(elastic region，collapse region and densification region)，并且应变率对其力学性能影响明显．     

应力引起的镍钛单晶形状记忆合金相变的实验研究

在单晶形状记忆合金试样中，由于没有晶粒之间的约束，它的马氏体相界面移动比多晶容易，用实验方法研究其相变的特点，对建立新的理论模型有意义，因而对它的实验分析显得重要。本文利用高分辨率的CCD系统监测到NiTi单晶形状记忆合金在拉伸时的相变伪弹性的过程；利用X射线衍射法得到了NiTi单晶试样在拉伸方向的晶向；运用高分辨率的云纹干涉技术，获得了应力引起的NiTi单晶形状记忆合金相变时的变形场；利用高分辨率、高灵敏度的红外相机记录了NiTi单晶在拉伸状态下的温度变化规律；对低温下NiTi单晶的拉伸性能做了初步的研究，得到一些有意义的现象。     

一种新的高应变率复合压剪实验技术

提出了一种新的基于Hopkinson杆实验技术的在102~103s-1高应变率下实现压剪复合加载的实验装置,并给出了相应的理论分析和数值模拟。为了获取材料在复杂应力下的本构关系,借助斜飞片冲击实验的思想,对Hopkinson杆进行改造,将入射杆的末端改进为截锥形,以便在试样中同时产生压缩和剪切应力。利用有限元分析软件LS-DYNA对试样中的应力波传播进行模拟计算,并利用改进装置进行了初步实验。计算和分析结果表明,利用所设计的装置可以实现对试样的动态压剪复合加载,获得材料在高应变率复杂应力加载下的本构响应,进而建立材料在复杂应力状态下本构行为的描述。     

A quartz-crystal-embedded split Hopkinson pressure bar for soft materials

A dynamic experimental technique that is three orders of magnitude as sensitive in stress measurement as a conventional split Hopkinson pressure bar (SHPB) has been developed. Experimental results show that this new method is effective and reliable for determining the dynamic compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths, such as elastomeric materials and foams at high strain rates. The technique is based on a conventional SHPB. Instead of a surface strain gage mounted on the transmission bar, a piezoelectric force transducer was embedded in the middle of the transmission bar of a high-strength aluminum alloy to directly measure the weakly transmitted force profile from a soft specimen. In addition, a pulse-shape technique was used for increasing the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance and low-strength specimen.     

Pulse shaping techniques for testing brittle materials with a split hopkinson pressure bar

We present pulse shaping techniques to obtain compressive stress-strain data for brittle materials with the split Hopkinson pressure bar apparatus. The conventional split Hopkinson pressure bar apparatus is modified by shaping the incident pulse such that the samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration. A thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse. After impact by the striker bar, the copper disk deforms plastically and spreads the pulse in the incident bar. We present an analytical model and data that show a wide variety of incident strain pulses can be produced by varying the geometry of the copper disks and the length and striking velocity of the striker bar. Model predictions are in good agreement with measurements. In addition, we present data for a machineable glass ceramic material, Macor, that shows pulse shaping is required to obtain dynamic stress equilibrium and a nearly constant strain rate over most of the test duration.     

A split Hopkinson pressure bar technique to determine compressive stress-strain data for rock materials

This paper presents a split Hopkinson pressure bar technique to obtain compressive stress-strain data for rock materials. This technique modifies the conventional split Hopkinson bar apparatus by placing a thin copper disk on the impact surface of the incident bar. When the striker bar impacts the copper disk, a nondispersive ramp pulse propagates in the incident bar and produces a nearly constant strain rate in a rock sample. Data from experiments with limestone show that the samples are in dynamic stress equilibrium and have constant strain rates over most of the test durations. In addition, the ramp pulse durations can be controlled such that samples are unloaded just prior to failure. Thus, intact samples that experience strains beyond the elastic region and postpeak stresses can be retrieved for microstructural evaluations. The paper also presents analytical models that predict the time durations for sample equilibrium and constant strain rate. Model predictions are in good agreement with measurements.     

一种用于材料高应变率剪切性能测试的新型加载技术

高应变率下的冲击剪切实验技术是材料动态力学行为及其微观机理研究的重要基础.采用分离式霍普金森压杆(split Hopkinson pressure bar)装置一般可以获得材料在104s-1以内应变率的动态力学性能.在超过104s-1的应变率下对材料进行冲击剪切测试时,通常需要采用高速压剪飞片技术或由气炮发射子弹对试样进行直接加载.本文提出一种可用于传统霍普金森压杆技术的新型双剪切试样,可以在103~105s-1剪应变率范围实现对材料剪切性能的精确测量;同时,可以对材料的变形及失效过程进行直接观测.试样与压杆之间避免了复杂的界面或连接装置,通过转接头可以保证试样与压杆直接接触,提高测试精度,同时可以防止因试样的横向位移而导致的非均匀变形.获得了紫铜在1400~75000s-1应变率下的剪应力-剪应变曲线,并采用计算软件"ABAQUS/Explicit"对双剪切试样的动态加载过程进行了数值模拟和结果验证.分析表明,剪切区的主要区域内剪切成分占主导地位,其应力应变场沿厚度及宽度方向基本呈均匀分布.实验得到的剪应力-剪应变曲线与模拟结果吻合较好,说明所提出的基于分离式霍普金森压杆系统的双剪切试样可以为材料的高应变率力学性能测试提供一种方便有效的加载技术.      

基于PVDF薄膜传感器的猪肝动态压缩力学性能测试

瞬态冲击载荷作用下肝脏的力学响应是损伤生物力学的重要研究内容。本文提出了一种可用于软组织动态压缩力学特性测试的改进SHPB(分离式霍普金森压杆,Split Hopkinson Pressure Bar)方法。该方法采用PVDF(聚偏氟乙烯,Polyvinylidene Fluor)压电薄膜传感器测量实验过程中试件两端面的受力,以此来计算试件的应力,从而无需测量透射杆上的微弱透射信号。猪肝试样前后端面的PVDF压电信号对比表明,加载过程中试样达到了动态应力平衡状态。试样动态压缩中的惯性效应主要在加载的初始阶段对透射应力信号造成较大影响,在大变形阶段惯性效应引起的轴向应力较小。利用此方法对猪肝组织进行三种高应变率(1800s^-1,2500s^-1,3500s^-1)的动态压缩实验,并采用基于真实应变的惯性效应公式对实验数据进行修正计算。结果表明:猪肝组织在准静态与高应变率时的应力应变曲线都呈现出凹向上的非线性特征,即曲线初始阶段应力增长较缓慢,当应变达到15%后应力值则迅速增大;猪肝组织也具有明显的应变率效应,即随着应变率的增加,应力应变曲线的整体应力值也随之增大。最后,采用黏超弹性本构模型描述了猪肝组织的动态应力应变曲线。     

Determination of dynamic fracture-initiation toughness using three-point bending tests in a modified Hopkinson pressure bar

We present a procedure for measuring the dynamic fracture-initiation toughness of materials. The method is based on three-point bending tests at high loading rates, performed in an experimental device which is a modification of the classical split Hopkinson pressure bar. Coupled with the loading device, a high-speed photography system was used to measure the crack mouth opening displacement (CMOD) directly on the specimen. The stress intensity factor was calculated by three different simplified methods and the time to fracture was obtained from an appropriate specimen instrumentation. To evaluate the results derived from the simplified methods, a two-dimensional full-numerical analysis of the dynamic bending fracture test was made. The model includes the specimen, the input bar, the impacting projectile and the supporting device and takes into account the possible loss of contact during the experiment between the input bar and the specimen and between the specimen and its supports. From the tests and numerical results, it can be concluded that the CMOD procedure, together with the knowledge of the time to fracture determined using crack gages, seems to be the best method for measuring dynamic fracture-initiation toughness.     

Dynamic compressive responses of intact and damaged ceramics from a single split Hopkinson pressure bar experiment

A novel dynamic compressive experimental technique has been developed based on a split Hopkinson pressure bar. This new method dynamically loads the ceramic specimen by two consecutive stress pulses. The first pulse determines the dynamic response of the intact ceramic materiaal and then crushes the specimen, and the second pulse determines the dynamic compressive constitutive behavior of the ceramic rubble. Precise pulse shaping ensures that the specimen deforms at nearly constant strain rates under dynamic stress equilibrium during the loading by both stress pulses. Pulse shaping also controls the amplitudes of loading pulses, the values of strain rates, the maximum strains in the rubble specimens, and the proper separation time between the two loading pulses. The feasibility of the new technique is demonstrated by the experimental results obtained on an AD995 alumina.     

大尺寸Hopkinson压杆及其应用

本文介绍了国内最大尺寸的SHPB装置;讨论了在大尺寸SHPB装置上测量混凝土类材料动态力学性能将会出现的几个问题;采取了在入射杆的打击端加设波形整形器,在试件与杆件之间加设万向头及在试件上直接测量应变等新的实验技术及采用新的数据处理方法,提高了试验结果的精确度和可信度;简要介绍了利用ф100 SHPB装置对四种体积含量(0,2%,4%和6%)钢纤维高强混凝土进行三种应变率(10~20/s,35~45/s和75~85/s)的冲击压缩实验。实验结果表明,钢纤维高强混凝土具有较强的应变率效应,其破坏应力、峰值应变随应变率增加而显著增加,弹性模量也随应变率增加而增加。另外,钢纤维含量对混凝土具有增韧效应,随着钢纤维含量的增加,其韧性增大,脆性降低。