SHPB实验中试件内早期应力平衡分析

分析了影响SHPB实验试件中早期应力平衡的外部因素,如光滑入射波及其上升时间、压杆波阻抗及压杆与试件横截面积之比等。研究结果表明,当用传统的SHPB装置产生的入射波加载试件时,宜采用与试件波阻抗接近的压杆;在相同的试验条件下,直径较大的试件,其内部应力达到均匀分分布的状态快于小直径的试件,最后给出了改善试件早期应力均匀性的方法。     

岩石SHPB实验加载过程中应力平衡问题分析

运用一维应力波理论,分析了弹性应力波在分离式Hopkinson压杆(SHPB)实验中的传播过程,推 导出试件和压杆中应力分布相关计算公式。探讨了有关因素对试件应力平衡时间的影响规律,发现试件应力 平衡时间受试件/压杆广义波阻抗比和入射加载升时的影响显著,而不受试件/压杆截面积比和入射加载应力 幅值的影响。结合岩石SHPB实验,计算分析了不同入射加载应力幅值在不同入射加载升时情况下,试件达 到应力平衡时的应变变化特征,并提出了降低试件在应力平衡时的应变控制方法,使试件在未达到断裂应变 之前达到应力平衡,以保证实验的有效性。得出的结论对岩石类脆性材料SHPB实验方案设计具有一定的 参考意义。     

弹性SHPB试件内部纵向应力波传播及所产生的横向约束应力

采用考虑横向惯性效应的Rayleigh-Love杆理论分析了一个弹性试件在分离式霍普金森压杆(SHPB)加载过程中的内部弹性波传播过程，运用Laplace变换和反变换方法，得到了试件内部各点的变形、速度、应变和应力解析解.通过数值计算，得到梯形入射波加载情况下，纵向应力在试件内部的连续变化过程，以及波传播所伴随的横向附加应力.计算表明：在试件/入射杆界面附近，初次加载所产生的横向附加应力最大，可达入射波平台的12%;在大部分试件区域，纵向应力波传播将造成入射波平台4%～6%的横向附加应力；材料的泊松比越大，或者杆/试件声阻抗比越小，所伴随的横向附加应力越大；梯形波的上升时间和试件长径比对横向附加应力影响不大.     

SHPB实验中粘弹性试件内部应力波的传播

建立描述SHPB实验中线性粘弹性试件内部应力波传播的控制方程组,根据试件两端与入射杆及透射杆接触的应力波特征关系给出耦合边界条件.对方程组和定解条件进行Laplace变换,求得试件内部应力在变换域像函数的表达式.采用数值反变换技术进行反Laplace变换,获得试件两端的应力时程曲线.对现有的固定Tal-bot反变换算法进行改进:将入射波像函数分解为基本部分和延迟部分,利用固定Talbot算法对基本部分入射波作用下的波动问题求解,其他部分的解通过延迟定理得到,最终解为两部分的叠加.采用这种改进算法得到的不同入射波下粘弹性试件的内部应力解与传统的基于特征线数值模拟方法的结果吻合.在此基础上探讨了粘弹性试件的几何参数和材料本构参数对透射波波形的影响.     

浇铸类炸药应力应变曲线的SHPB测量

浇铸类炸药由于质地软、波阻抗及波速都很低,通过传统SHPB实验方法无法得到准确的应力应变数据。透射杆信号幅值过低、试样应力平衡均匀性不高以及大应变加载引起的入射波反射波重叠失效,是进行浇铸类炸药SHPB实验的难点所在。本文中对传统SHPB实验方法进行改进,在试样两端面加装石英晶体应力计,引入石英计所获得的应力数据与应变片测得数据共同对试样应力应变状态进行计算。该方法可以提高透射信号幅值,提供试样大应变加载,避免了入射波反射波重叠导致的信号失效问题,修正了SHPB实验过程中的应力时空不均匀性的影响,提高了实验结果的可靠性。利用改进后的实验方法对典型浇铸类炸药进行了实验研究,得到了较准确的应力应变曲线。     

基于数字图像的分离式霍普金森压杆实验中试件应变及两端应力的同步测量法

本文提出一种基于高速摄像和数字图像相关方法(DIC)的分离式Hopkinson压杆(SHPB)测量技术,从而实现试件应变和两端应力的同步测量。即在与试件接触的输入输出杆两端制作散斑,通过高速摄像获取SHPB实验过程中的散斑变形图像,由DIC测得各时刻试件的应变、输入输出杆端的应变(可直接换算为试件两端的应力)。由于试件和杆端的应变都是从同一张高速摄影的图像上分析得到的,因此它们是同步的。应用该方法对钢纤维混凝土试件的SHPB试验进行了测量,测量结果与传统应变片测量结果吻合,验证了该方法的可行性。该技术不仅实现了SHPB实验中试件应变和应力的同步测量,还将有助于直接检验各材料在SHPB实验中试件两端的力在实验过程中是否平衡。     

混凝土HJC本构模型参数的研究

运用非线性有限元动力分析软件LS-DYNA模拟了混凝土SHPB试验.将试验采集的入射波数据作为压杆的端面载荷,模拟条件尽量符合试验条件,确保模拟精度.将数值模拟与试验结果相结合,研究了混凝土HJC模型参数的确定方法,得到了C60混凝土的相关计算参数,模拟结果得到的应力-应变曲线与不同应变率下混凝土SHPB试验应力-应变曲线吻合较好.分析了HJC模型的本构特性,比较了与金属JC模型的差异与共性,讨论了HJC模型存在的问题.研究结果表明,未考虑混凝土弹性响应的应变率效应,横向效应的影响与采用的失效方式密切相关.     

SHPB试验中粘弹性材料的应力均匀性分析

采用特征线解法，对满足ZWT方程的粘弹性材料在高应变率SHPB试验中的应力均匀性进行了数值研究。着重分析了不同的材料本构粘性(松弛时间)、瞬态波阻抗比和入射波升时对于试样中应力均匀性、应变均匀性和平均应变率等的影响。为今后动态试验的试样设计提供了一定的理论依据。     

大直径SHPB弥散效应的二维数值分析

采用轴对称动态有限元HONDO程序对大直径SHPB装置中压杆横向泊松效应引起的应力波弥散进行二维数值分析,并从以下三个方面讨论波形弥散的影响：（1）SHPB装置中压杆直径和杆长对弥散结果（主要是升时）的影响;（2）压杆中的波形弥散对试件应力-应变曲线的影响;（3）弥散对试件应变率的影响。分析表明,在直径SHPB弥散效应对实验结果的影响很大,必须考虑。     

SHPB系统圆柱形试件的惯性效应分析

在对分离式Hopkinson压杆(SHPB)实验系统试件惯性效应分析的基础上,增加了界面摩擦做功对试件惯性效应影响的分析．在理论分析的基础上,就如何减小摩擦效应、惯性效应对实验结果的影响,对SHPB试件尺寸比例的设计以及摩擦系数的选取作了进一步的分析,另外还对SHPB实验过程中,试件两端应力平衡情况做了初步的讨论．     

Specimen-Bar Impedance Mismatch Effects on Equilibrium and Rate Constancy for Kolsky Bar Experiments

During a Kolsky bar, also known as a split Hopkinson pressure bar (SHPB), experiment, stress equilibrium and strain rate constancy conditions directly contribute to the measurement quality for rate-sensitive materials. A Kolsky bar specimen is initially at rest, and then gradually accelerated to a desired rate. Stress equilibrium is incrementally achieved by multiple stress pulse reflections inside the specimen to reach the desired mean stress. The critical time to achieve constant strain rate and equilibrium stress depends on the impedance mismatch between the bars and the specimen. This paper examined this critical time based on using linear elastic specimens under uniaxial compression. In the first part, the critical time is experimentally measured for PMMA specimens loaded by aluminum, titanium, and steel bars using linear ramp incident pulses. The results show that increasing impedance mismatch increases the time to reach a constant rate, while the time to satisfy equilibrium remains nearly the same. In the second part, optimal bilinear-shaped incident loadings were evaluated and shown to achieve both conditions faster than linear loadings. The time to satisfy both conditions was mapped via simulation using various bilinear pulses over a wide range of impedance mismatches. The analysis shows bilinear loadings with initial rise time between 1.75 and 2.15 transits in the sample require minimum time to equilibrium. There exists an optimum region of bilinear loadings that can reduce the time to reach constant rate. Within such region, the bilinear slope ratio can be approximated to be a reciprocal function of initial rise time.     

Effects of imperfect experimental conditions on stress waves in SHPB experiments

Experimental and numerical simulations were undertaken to estimate the effects of imperfect conditions on stress waves in split Hopkinson pressure bar(SHPB)experiments. The photonic Doppler velocimetry(PDV) measurement results show that the rise and fall times of an incident wave increases with an increasing inclination angle;also, the fluctuations of the incident wave disappear gradually with the increase of inclination angle. The following characteristics for various defects in the SHPB were obtained by numerical simulation:(1) the influence of a curved bar was negligible;(2) misalignment modestly affects the fluctuation characteristics, and bending waves were generated at this condition;(3) inclination and indentation of the impact endsurface had a great impact on the incident waves, and both of them increase the rise time of the incident wave by increasing the degree of defects. In view of the results, misalignment,inclination, and indentation in SHPB experiments should be minimized.     

肌肉类粘弹性超软材料SHPB实验的应力应变均匀性分析

试样的轴向应力和应变均匀问题是霍普金森杆实验行之有效的必要条件,而这对肌肉类粘弹性超软材料尤为重要.本文用量纲分析法,分析出了影响粘弹性材料SHPB实验应力和应变均匀性的无量纲量,结合LS-DYNA数值模拟方法,考察了各无量纲量对粘弹性材料均匀性的影响规律.结果表明,为使肌肉类材料SHPB实验更早达到均匀条件,理想的入...     

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.     

SHPB实验数据处理的规范化问题讨论

从SHPB实验的一维应力假设出发,确定了入射、反射和透射波的起点位置的判读办法,提高了SHPB实验结果的准确性,并规范了SHPB实验的数据处理。另外,从动量守恒的角度,提出了标准入射波平台高度的判读办法。该方法可以避免弥散振荡对判读入射波平台高度的干扰,并排除人为因素的影响,提高了压杆应变片灵敏系数的标定准确性和标定过程的规范化程度。通过对比理论结果和数值模拟研究的结果,这两种数据处理方法的准确性得到了验证。     

Propagation of a stress wave through a virtual functionally graded foam

Stress wave propagation through a Functionally Graded Foam Material (FGFM) is analysed in this paper using the finite element method. A finite element model of the Split Hopkinson Pressure Bar (SHPB) is developed to apply realistic boundary conditions to a uniform density foam and is validated against laboratory SHPB tests. Wave propagation through virtual FGFMs with various gradient functions is then considered. The amplitude of the stress wave is found to be shaped by the gradient functions, i.e., the stress can be amplified or diminished following propagation through the FGFMs. The plastic dissipation energy in the specimens is also shaped by the gradient functions. This property of FGFMs provides significant potential for such materials to be used for cushioning structures.     

单轴双向加载分离式霍普金森压杆的数据处理方法

本文中提出单轴双向加载分离式霍普金森压杆（bidirectional-load split Hopkinson compression bar,BSHCB）,即在传统的分离式霍普金森压杆（split Hopkinson pressure bar,SHPB）的基础上增加另一个对称的入射波,两边的入射波同时且对称地对试样进行动态加载。根据一维应力波传播理论,推导出单轴双向加载分离式霍普金森杆的数据处理公式。通过数值模拟分析发现,所推导的数据处理公式可以用于计算单轴双向加载实验中试样的工程应力、工程应变和工程应变率。此外,单轴双向对称加载不仅可缩短试样内部应力均匀化的过程,而且可以提高试样应变率。     

Modal Analysis of a Servo-Hydraulic High Speed Machine and its Application to Dynamic Tensile Testing at an Intermediate Strain Rate

This paper describes the experimental procedure to identify the predominant frequencies of the high speed testing machine by conducting modal analysis. The effects due to the predominant frequencies of the system and loading rate on the magnitude of system ringing and the flow stress were analyzed by using a single degree-of-freedom (SDOF) spring-mass-damper model. The system was then used to study the dynamic tensile behavior of two engineering materials, i.e., polyethylene (PE) fabric-cement composite and Alkaline Resistant (AR) glass fabrics at an intermediate strain rate. The stress oscillations in the response of these materials due to system ringing were addressed. The failure behavior of each material was studied by examining high speed digital camera images of specimens during the test. The validity of the dynamic tensile tests was investigated by examining the condition of dynamic stress equilibrium—a criterion used in split Hopkinson pressure bar (SHPB) tests. The results show that the quantitative criterion for a valid SHPB test is also applicable to dynamic tensile tests of these materials at the intermediate strain rate.