Hopkinson拉杆的入射波波形整形实验研究

目前,分离式Hopkinson杆实验技术已经被广泛用于测试材料在10~2~10~4s~(-1)应变率范围内的动态力学特性。为了抑制入射波的高频振荡,实现恒定应变率加载,本文利用分离式Hopkinson拉杆(SHTB)实验装置,研究了加载金属短杆(2A12T4铝合金)及整形垫片(纸板、PVC软塑料及带磁性胶皮)对入射波波形的影响。实验结果表明,整形垫片降低了入射应力脉冲的高频振荡,获得了比较平滑的入射应力脉冲,延长了上升时间。同时,利用所得的波形整形结果,对2A12T4铝合金进行了拉伸应力波脉冲加载的拉伸和断裂实验测试。     

采用磁致伸缩逆效应原理测定Hopkinson杆的应力变化率

利用Villari效应,以非接触方式测得不同撞击速度下Hopkinson杆的应力变化率波形,并介绍了传感器的标定方法。     

Hopkinson压杆技术的推广应用

简要介绍了Hopkinson杆的几种工程应用,包括:(1)用同步组装系统进行高温、高应变率耦合作用下材料动态力学性能的测试;(2)在Hopkinson压杆技术中实现单脉冲加载及其在动态损伤力学中的应用;(3)用Hopkinson压杆加载三点弯曲试样测定材料的动态起裂韧性;(4)用Hopkinson压杆技术对加速度传感器在3 000~200 000g范围内进行g值校准。还介绍了这些应用的基本原理、测试方法及所取得的成果。     

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.      

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。     

单脉冲加载的Hopkinson扭杆装置

分析了反复加载现象及其对变形微结构的影响，介绍了对现有标准Ｈｏｐｋｉｎｓｏｎ扭杆的改进。改进后的新型Ｈｏｐｋｉｎｓｏｎ扭杆是具有特殊设计的加载头和传动器的四杆系统，它完全消除了应力波的反复加载效应，实现了过程的单脉冲加载。改进前后的实验波形和变形局部化微结构形态的比较证实了改进的有效性。     

Limiting conditions for compression testing of flat specimens in the split hopkinson pressure bar

The split Hopkinson pressure bar (SHPB) technique is analyzed during the initial stages of loading by means of axisymmetric finite element simulations of dynamic compression tests. Limiting strains as functions of the test parameters such as the specimen diameterd and heighth were found to ensure a one-dimensional stress state and axial stress homogeneity in specimens of elastic-perfectly plastic material. The one-dimensional stress state is necessary and sufficient for accurate test results for flat specimens (h/d≤0.5) and nonflat specimens, respectively, with diameters up to half of the bar diameter. Only very small values of the Coulomb friction constraint (μ≈0.01) seem to be acceptable. The significance of the determined limiting conditions to the more practical case of a rate dependent material is investigated using an elastic-viscoplastic material for the specimen. The stress and strain rate reconstructed from the calculated bar signals (according to the SHPB analysis) are compared with stresses and strain rates averaged over the cross section of the specimen. Well-known inertia corrections improve the results of the SHPB procedure, but errors remain for small strains and highly time dependent strain rates.     

动态拉伸试验中试样应变测试的有效性分析

为了评估将试样通过胶粘连接到加载杆的Hopkinson杆装置所获得试样应变的有效性,对四种强度刚度差异较大的纤维增强复合材料进行了动态拉伸试验。试验时,试样通过环氧胶和杆夹层粘接,试样的应变分别按照Hopkinson杆一维应力波理论计算和试样上应变计直接准确测量得到。结果证明：对小变形碳纤维复合材料,按一维应力波理论计算的应变与试样上直接所测应变值偏差超过100%;对较大变形的GFRP和KFRP层合板,两者偏差小于40%。说明采用Hopkinson杆一维应力波理论计算的试样应变不准确。为修正不准确性,一是通过大量数据分析建立按一维应力波理论计算值与直接测量应变之间的关系式,用此式可使此试验装置获得有效的试样应变;二是借助ABAQUS有限元模拟分析得出粘胶层以及试样过渡弧段的变形,用一维应力波理论计算的应变减去此变形,也可获得有效的试样应变。     

Development of testing systems for dynamic assessment of sheet metal specimens

The work presented herein regards the analysis of an experimental technique for the execution of dynamic tensile tests on structural material sheet specimens. Dynamic tensile testing of sheet is becoming more important due to the need for more optimized vehicle crashworthiness analysis in the automotive industry. Positive strain-rate sensitivity, i.e. the strength increases with strain-rate, offers a potential for improved energy absorption during a crash event. Tests have been carried out in the Reliability and Safety Laboratory of the 2^nd Engineering Faculty of the Politecnico di Torino. Different types of testing techniques have been used to generate data under dynamic conditions. However, no guidelines are available for the testing method, specimen dimensions, measurement devices, and other important issues which are critical for the quality of the results. Accurate signal processing and curve smoothing are often necessary to make the testing data usable.     

Biaxial Testing of Sheet Materials at High Strain Rates Using Viscoelastic Bars

A dynamic bulge testing technique is developed to perform biaxial tests on metals at high strain rates. The main component of the dynamic testing device is a movable bulge cell which is directly mounted on the measuring end of the input bar of a conventional split Hopkinson pressure bar system. The input bar is used to apply and measure the bulging pressure. The experimental system is analyzed in detail and the measurement accuracy is discussed. It is found that bars made of low impedance materials must be used to achieve a satisfactory pressure measurement accuracy. A series of dynamic experiments is performed on aluminum 6111-T4 sheets using viscoelastic nylon bars to demonstrate the capabilities of the proposed experimental technique. The parameters of the rate-dependent Hollomon–Cowper–Symonds J2 plasticity model of the aluminum are determined using an inverse analysis method in conjunction with finite element simulations.