多孔硬脆性材料的SHPB实验技术

以SHPB实验技术中一维假定和应力均匀假定为基础, 针对多孔硬脆性材料 的力学特性, 分析了利用传统SHPB装置在实验研究中存在的问题, 并从入射杆和透射杆的选 取、传感器的选取、试件参数的确定、入射波形的确定和数据处理方法的选取等方面总结了 克服这些问题的技术和方法.     

A split Hopkinson bar technique for low-impedance materials

An experimental technique that modifies the conventional split Hopkinson pressure bar has been developed for measuring the compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths such as elastomers at high strain rates. A high-strength aluminum alloy was used for the bar materials instead of steel, and the transmission bar was hollow. The lower Young's modulus of the aluminum alloy and the smaller cross-sectional area of the hollow bar increased the amplitude of the transmitted strain signal by an order of magnitude as compared to a conventional steel bar. In addition, a pulse shaper lengthened the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance specimen. Experimental results show that the high strain rate, compressive stress-strain behavior of an elastomeric material can be determined accurately and reliably using this technique.     

用于高强度材料的SHPB实验添加垫块法

提出了用于高强度材料的改进的SHPB实验方法添加垫块法,运用数值模拟方法,利用有限元程序LS-DYNA3D分析了添加垫块实验方法的合理性和可行性。根据一维应力波理论,给出了数据处理的修正方法。作为应用实例,采用改进的实验方法对高强度的Al2O3陶瓷材料的动态力学性能进行了研究,得到了比常规方法较高的应变率及应力应变范围的动态应力应变曲线,表明Al2O3陶瓷为应变率相关的非线性弹脆性材料。结果表明,添加垫块实验方法可有效地防止实验中压杆端面的变形,提高试件的应力应变及应变率水平。添加垫块实验方法为在SHPB装置上实现高强度材料的动态实验提供了一种方便实用的途径。     

混凝土类材料动态压缩强度在多维应力状态下的应变率效应

对混凝土类材料动态压缩应变率效应研究的发展及问题进行了概述,对比不同应力状态下混凝土类材料动态压缩应变率效应的表现特征,揭示了不同加载路径下实测动态强度提高系数的显著差异。研究表明,在高应变率下,基于初始一维应力加载路径的试件将因横向惯性效应导致的侧向围压而演化至多维应力状态,传统霍普金森杆技术无法获得高应变率下基于真实一维应力路径的动态强度提高系数,在强度模型中直接应用实测数据将过高估计材料的动态强度。鉴于应变率效应的加载路径依赖性,将仅包含应变率的强度提高系数模型扩展至同时计及应变率和应力状态的多维应力状态模型,并结合Drucker-Prager准则在强度模型中给予了实现。针对具有自由和约束边界试件开展的数值霍普金森杆实验表明,多维应力状态下的应变率效应模型可以考虑应变率效应随应力状态改变的特点,从而准确预测该类材料的动态压缩强度。研究结果可为正确应用霍普金森杆技术确定脆性材料的动态压缩强度提供参考。     

Thickness Effect of Pulse Shaper on Dynamic Stress Equilibrium and Dynamic Deformation Behavior in the Polycarbonate Using SHPB Technique

0Introduction Thehighstrainratestress strainresponsesofpolymersandpolymericcompositematerialshave receivedincreasedscientificandindustrialattentioninrecentyears.Polymericmaterialsaresubjected todynamicloadingandhighstrainratedeformationinavarietyofimporta…     

一种用于软材料测试的改进SHPB装置

本文提出了一种新的、用于测试橡胶、高弹体及高聚物软泡沫材料动态力学性能的SHPB改进装置。该装置取消了常见的入射杆而采用长杆弹直接撞击试件从而实现了持续的长时间加载，使得在相当大的应变率范围内试件的最大应变在一个加载过程中即可达到。配合该装置采用了瞬态响应优良、分辨率良好的光电式位移测试系统来测量试件的变形；为记录微弱的应变信号，在透射杆中使用了半导体应变片。本方案克服了传统SHPB在测试软材料时由于子弹长度限制带来的加载幅度不足及由于阻抗失配导致的应变信号微弱的困难；与采用高聚物杆的SHPB改进方案相比，本方案的测试结果也更为可靠。在试验装置中还运用了加载整形技术以改善试件中的应力均匀性。从测试结果看，该装置能有效地实现大变形范围、近似恒应变率持续加载以及相应的微弱应变信号的测量。     

直撞式霍普金森压杆二次加载技术

利用传统分离式霍普金森压杆（split Hopkinson pressure bar, SHPB）实验技术来实现试件在较低应变率下的大变形时,需要使用超长的压杆系统,杆件的加工和实验空间限制了该技术的推广应用。鉴于此,提出一种直撞式霍普金森压杆二次加载实验技术,利用透射杆中的应力波在其末端的准刚性壁反射实现对试件的二次加载,并分析了准刚性质量块尺寸对二次加载的影响规律;采用二点波分离方法对叠加的应力波进行了有效分离和计算,在总长4 m的压杆系统中实现了1.2 ms的长历时加载,并可以准确获得试件的加载应变率曲线和应力应变关系。建立了直撞式霍普金森压杆二次加载有限元模型,数值仿真结果表明,该实验技术能有效地实现试件的二次加载,与超长SHPB系统获得的仿真结果相比较,两者的试件应力应变关系完全一致。利用该技术对1100铝合金材料进行动态压缩实验,实现了其在102 s?1量级应变率下的大变形动态力学性能测试。     

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。     

Dynamic stress equilibration in split Hopkinson pressure bar tests on soft materials

The condition of dynamic stress equilibrium is not satisfied automatically when a split Hopkinson pressure bar (SHPB) is employed to determine the dynamic properties of soft materials. In order to develop guidelines for the proper design of SHPB experiments under valid testing conditions, an integrated experimental/analytical study has been conducted to examine the process of dynamic stress equilibrium in a soft rubber specimen. Dynamic compressive experiments on a RTV 630 and an ethylene-propylene-diene monomer rubber with a SHPB modified for soft material testing were conducted to determine the effects of specimen thickness and loading rate on the stress equilibrating process. An analytical model was employed to analyze the equilibrating processes observed in experiments. It is found that the incident loading rate dominates the initial non-equilibrium stress state, and the specimen thickness mainly affects the dynamic stress equilibrium after the initial stage.     

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。     

Flattened Brazilian Disc Method for Determining the Dynamic Tensile Stress-Strain Curve of Low Strength Brittle Solids

An indirect tensile testing method is proposed to measure the full dynamic tensile stress-strain curve of low strength brittle solids. In this method, the flattened-Brazilian disc (FBD) sample is loaded by modified split Hopkinson pressure bars (SHPB) system. Low amplitude dynamic forces were measured with a pair of piezoelectric force transducers embedded in the incident bar and the transmitted bar. The evolution of tensile stress at the center of the disc sample was determined through finite element analyses using the measured stress in SHPB as inputs. In a traditional Brazilian test, a strain gauge is mounted at the center of the specimen to measure the tensile strain, which is difficult to apply for low strength brittle materials. Thus, two types of non-contact methods, the Digital Image Correlation (DIC) technique and the Laser Gap Gauge (LGG), were used to measure the strain. The DIC method was used to monitor the displacement and the strain map of the specimen during the test, from which the strain at the center of the specimen can be obtained. The accuracy of the DIC results was assessed, and the displacement and strain uncertainties of our system were 0.003 mm and 0.003, respectively. LGG was used to monitor the expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation revealed that the tensile strain at the center of the specimen is proportional to the average tensile strain and that the ratio is not sensitive to the material elastic parameters. The strain measured through LGG was compared with that measured by the DIC method using photos captured with a synchronized high-speed camera. The result of the LGG method was 20 % smaller than that of the DIC process. However, the latter was limited by the number of frames of the high-speed camera. The feasibility of this methodology was demonstrated using a polymer-bonded explosive (PBX).     

Specimen Size and Strain Rate Effects on the Compressive Behavior of Concrete

Three high-performance concrete (HPC) materials with different specimen geometries were characterized using Kolsky compression bar techniques to study the strain rate and specimen size effects on their uniaxial compressive strength. A large-diameter Kolsky bar and recently established annular pulse shaping technique were used to achieve dynamic stress equilibrium and constant strain-rate deformation in the experiments. A complimentary effort was conducted using a 19-mm-diameter Kolsky compression bar to understand the strain rate and specimen size effects on failure strength and dynamic increase factor (DIF) for concrete. It was found that, for all three concrete materials investigated, the failure strength is highly dependent on the specimen geometry, however such a relationship is not apparent for the DIF. The DIF observed in this study shows significantly lower values compared to historical data, which may indicate the importance of well-controlled dynamic testing conditions on the accuracy and validity of experimental results for concrete materials.     

EXPERIMENTAL AND NUMERICAL STUDIES ON DYNAMIC COMPRESSIVE BEHAVIOR OF REACTIVE POWDER CONCRETES

Split Hopkinson pressure bar （SHPB） technique is used to determine the dynamic strength of reactive powder concretes （RPCs） with different steel-fiber contents. Two types of pulse shapers with different thicknesses are considered to reduce the high-frequency-oscillation effect and achieve a nearly constant strain rate over a certain deformation range. It is known that the compressive strength of concrete-like materials is hydrostatic-stress-dependent, and the apparent dynamic strength enhancement comes from both the effects of the hydrostatic stress and strain rate. In order to differentiate them, numerical method is used to calculate the contribution of the hydrostatic stress, and then the genuine strain-rate effect on dynamic compressive strength of RPCs is determined. In addition, the effect of steel-fibers on dynamic strength and failure mode of RPCs is discussed.     

确定材料在高温高应变率下动态性能的 Hopkinson杆系统

描述了一种利用Hopkinson杆装置确定在高温（温度可高达1 173 K）、高应变率下材料动态性能的试验方法。在试样加温过程中,试样不与入射杆及透射杆接触。当试样加热到预定温度时,气压驱动同步组装系统,推动透射杆及试样,使得应力波到达入射杆与试样接触面时,入射杆、试样及透射杆紧密接触。利用以上系统,完成了连铸单晶铜及上引法连铸多晶铜从室温到1 085 K范围内的应力应变曲线。测试结果表明,不论是上引法连铸多晶铜还是连铸单晶铜,流动应力随温度的升高而下降,在温度低于585 K时,材料的应变硬化率明显大于在温度高于585 K时的应变硬化率。     

高应变率下多孔未极化PZT95/5铁电陶瓷的非线性力学行为

采用添加造孔剂的方法制备了4种不同孔隙率的未极化PZT95/5铁电陶瓷。采用基于超高速相机与数字图像相关性方法的试样全场应变测量技术以及分离式霍普金森压杆（SHPB）技术,对多孔未极化PZT95/5铁电陶瓷进行高应变率单轴压缩实验研究。全场应变测量结果显示:轴向应变仅在试样中部分布较均匀,将该区域的平均应变作为应力-应变关系中的试样应变测量值较为合理,而由SHPB原理计算的试样应变值明显偏大,需要摒弃或修正传统的SHPB数据处理方法。通过波形整形技术实现了恒应变率加载,弱化了径向惯性效应的影响,揭示出多孔未极化PZT95/5铁电陶瓷的压缩强度具有显著的应变率效应。通过分析试样轴向应变和径向应变随着加载应力的变化,阐明多孔未极化PZT95/5铁电陶瓷的非线性变形行为的物理机制是畴变和相变共同作用,并发现畴变临界应力和相变临界应力都随着应变率升高而增大。保持加载应变率不变,讨论了孔隙率对多孔未极化PZT95/5铁电陶瓷动态力学行为的影响,发现随着孔隙率的升高,动态压缩强度呈非线性衰减,而畴变临界应力和相变临界应力则基本呈线性衰减。     

Loading and unloading split hopkinson pressure bar pulse-shaping techniques for dynamic hysteretic loops

Pulse-shaping techniques are developed for both the loading and unloading paths of a split Hopkinson pressure bar (SHPB) experiment to obtain valid dynamic stress-strain loops for engineering materials. Front and rear pulse-shapers, in association with a momentum trap, are used to precisely control the profiles of the loading and unloading portions of the incident pulse. The modifications, ensure that the specimen deforms at the same constant strain rate under dynamic stress equilibrium during both loading and unloading stages of an experiment so that dynamic stress-strain loops can be accurately determined. Dynamic stress-strain loops with a constant strain rate for a nickel-titanium shape memory alloy and polymethyl methacrylate are determined using the modified SHPB. The modified momentum trap prevents repeated loading on a specimen without affecting the amplitude of the desired loading pulse and without damaging the bar at high stress levels.     

Comparison of Low Impedance Split-Hopkinson Pressure Bar Techniques in the Characterization of Polyurea

The split-Hopkinson pressure bar (SHPB) technique has been widely employed for over fifty years in characterizing the high strain-rate properties of many common engineering materials. Historically, however, this technique has had limited success in characterizing soft materials, since their low mechanical impedances can increase delays in attaining dynamic equilibrium and result in transmission pulses with extremely low signal-to-noise ratios. Due to interest in improving characterization of soft materials at high strain rates, numerous modifications to the traditional SHPB technique have been proposed. These include: using more sensitive piezoelectric gauges, employing hollow transmission bars, utilizing lower impedance polymeric pressure bars, and the use of pulse shaping techniques. To date, there has been no comparative studies or consensus within the SHPB community as to which approach is most advantageous. The goal of this investigation is to compare a number of these techniques, specifically the use of PMMA pressure bars and a hollow aluminum transmission bar (both with and without pulse shaping), alongside more traditional solid aluminum pressure bars in the characterization of polyurea, a common low impedance polymer. The advantages and disadvantages of each technique in generating high strain-rate stress-strain curves are discussed.     

波形整形器在酚醛树脂的霍普金森压杆实验中的应用

利用霍普金森压杆在室温下进行了应变为 10 2 ～ 10 3 s-1的冲击压缩实验 ,同时采用波形整形器使入射波的上升沿变宽 ,更好地满足试件中应力应变均匀分布的条件 ,使实验更接近常应变率加载的条件。结果表明 ,酚醛树脂是一种应变率敏感材料 ,在室温下和高应变率下表现出冲击脆化的特征。