A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

一种用于动态拉伸试验装置的新型试件装卡方式

针对杆杆型动态拉伸试验系统设计了一种新型的楔形卡口试件装卡方式.利用ANSYS/LS-DYNA软件,建立采用该新型试件装卡方式的直接式杆杆动态拉伸系统的三维有限元模型,并进行数值仿真试验.得到的波形与SHB(Split Hopkinson Bar)试验典型波形相符合,得到的动态应力应变曲线与输入材料模型曲线趋势是一致的.利用这种装卡方式对一种2024铝合金进行动态拉伸试验,得到的动态拉伸应力应变曲线与利用SHPB试验得到的动态压缩曲线基本一致,证明这种新型试件装卡方式是有效的.     

A Long Split Hopkinson Pressure Bar (LSHPB) for Intermediate-rate Characterization of Soft Materials

In this study, we developed a long split Hopkinson pressure bar (LSHPB) for mechanically characterizing soft materials at intermediate strain rates. Using a proper pulse shaper, a loading pulse over 3 ms was produced for compression experiments on a PMDI foam material at the strain rates in the order of 10/s. The pulse shaping technique minimized the dispersion effects of stress wave when propagating through such a long bar system. Consistency of stress–strain curves obtained from the LSHPB and an MTS in the same strain rate range shows that a gap currently existing in intermediate strain-rate range is closed by the introduction of the LSHPB.     

Dynamic and Quasi-Static Propagation of Compaction Waves in a Low-Density Epoxy Foam

We conducted dynamic and quasi-static compression experiments with low-density (ρ = 120 kg/m³) epoxy foam specimens. The specimens had a 10.0-mm-square cross-section and a length of 19.3 mm. Dynamic experiments were conducted with a modified split Hopkinson pressure bar (SHPB), and the quasi-static experiments were conducted with a hydraulic load frame device (MTS-810). In both cases, the specimens were loaded from one end at a constant velocity. Equally spaced grid lines were marked on the specimens to monitor the deformation history. Digital images taken at equally spaced time intervals gave the positions of each grid line. These images showed that a constant end-face velocity V produced a compaction wave front that traveled at a constant velocity C in both dynamic and quasi-static experiments. We described these results with a shockwave analysis that used a locking solid material model.     

Influence of Specimen Geometry on Split Hopkinson Tensile Bar Tests on Sheet Materials

In recent years numerous studies on the high strain rate behaviour of sheet materials using split Hopkinson tensile bar set-ups have been reported in literature. For these experiments mostly dogbone-shaped specimens are used. However, widely divergent specimen dimensions can be found. In the present study the influence of this specimen geometry on the test results is investigated experimentally. An extensive series of Hopkinson tests on a steel sheet material using different specimen geometries is performed. An advanced optical technique is used to obtain the true distribution of the deformation along the length of the specimen. Important issues such as the contribution of the deformation of the transition zones to the total deformation and the (non-)homogeneity of the strain in the specimen are thus determined. From the experiments it is clear that the influence of the specimen geometry on the observed behaviour cannot be neglected. It is shown that inconsistencies between the assumed and real specimen behaviour account for these differences. For the TRIP steel considered in the study, accurate deformation values are only guaranteed if the length to width ratio of the central zone is larger than 1.25 and if the radius of the transition zone is sufficiently small.     

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.     

Non-parametric identification of the non-homogeneous stress in high strain-rate uni-axial experiments

Stress and strain homogeneity, in tested samples, is a crucial assumption during a dynamic test. Whenever this assumption is true, the conventional recovery of the mean strain and stress is valid. However, when the stress and strain fields in the sample are not homogeneous, more sophisticated treatment must be considered. Inverse problem techniques are then proposed. Nevertheless, they may yield a non-physical result. In this paper, a non-parametric solution to the problem of non-homogeneity in dynamic tests is presented. The stress field is deduced from the displacement field measured via a Digital Speckle Photography (DSP) technique and a force boundary condition.     

Compression response of a thermoplastic elastomer gel tissue surrogate over a range of strain-rates

The mechanical response of a thermoplastic elastomer synthetic ballistic gel is studied over a range of strain-rates. Experiments were conducted at room temperature under uniaxial stress compression at rates ranging from 0.001 to 500/s. Low-rate experiments (<1/s) were conducted with a servo-hydraulic load frame. High-rate experiments (>100/s) were conducted with a polymeric Kolsky bar, along with several modifications to improve data quality. These modifications include the use of a commercial force transducer, a normal displacement interferometer, and a line laser extensometer. Because of the low shear strength and comparatively high compressibility of these materials, inertial effects are very pronounced. Specimen size is varied in an effort to study inertial effects at various loading rates. High speed photography is also used to demonstrate the presence of non-uniform deformation, due to both inertia and friction between the specimen and the loading surfaces. Finally, numerical simulation is used to verify trends observed in the experiments and further validate the data. It is concluded that this material is rate sensitive, with an almost three-fold increase in stiffness over the range of strain-rates studied.     

Torsional split Hopkinson bar tests at strain rates above 104s−1

The torsional split Hopkinson bar is used for testing materials at strain rates above 10⁴s⁻¹. This strain rate, which is an order of magnitude higher than is typical with this technique, is obtained by using very short specimens. Strain rates of 6.4×10⁴s⁻¹ have been achieved with specimens having a gage length of 0.1524 mm. Results from tests on 1100 aluminum show an increase in rate sensitivity as the strain rate increases.     

Full Field Strain Measurement in Compression and Tensile Split Hopkinson Bar Experiments

The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide a synchronized stereo view of the specimen. Using an array of 128 × 80 (compression tests) and 258 × 48 (tensile tests) pixels, the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout the surface of the specimen. The strains determined with the image correlation method are compared with those determined from analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the waves are not valid, but the image correlation method provides the accurate full field strain history.     

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。     

金属拉伸试验方法的关键及其对策

本文首先指出金属拉伸试验方法的关键问题是试验时的应力速率及应变速率控制,首次提出应用传感器及计算机技术,以一只比例节流阀为控制器构成旁路节流式微机控制电液系统,具有压力和速度双重控制功能,在液压式万能材料试验机上获得良好的应力速率及应变速率控制的试验结果,为我国液压材料试验机功能扩展和开发新一代机型奠定基础,以适应２１世纪静强度检测的需要。     

Radial Inertia Effects in Kolsky Bar Testing of Extra-soft Specimens

Impact responses of extra-soft materials, such as ballistic gelatins and biological tissues, are increasingly in demand. The Kolsky bar is a widely used device to characterize high-rate behavior of materials. When a Kolsky bar is used to determine the dynamic compressive response of an extra-soft specimen, a spike-like feature often appears in the initial portion of the measured stress history. It is important to distinguish whether this spike is an experimental artifact or an intrinsic material response. In this research, we examined this phenomenon using experimental, numerical and analytical methods. The results indicate that the spike is the extra stress from specimen radial inertia during the acceleration stage of the axial deformation. Based on this understanding, remedies in both specimen geometry and loading pulse to minimize the artifact are proposed and verified, and thus capture the intrinsic dynamic behavior of the specimen material.     

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dynamic fracture toughness(DFT) of high strength steel,using the split Hopkinson tension bar(SHTB) apparatus,combined with a hybrid experimental-numerical method.The center-cracked tension specimen is connected between the bars with a specially designed fixture device.The fracture initiation time is measured by the strain gage method,and dynamic stress intensity factors(DSIF) are obtained with the aid of 3D finite element analysis(FEA).In this approach,the dimensions of the specimen are not restricted by the connection strength or the stress-state equilibrium conditions,and hence plane strain state can be attained conveniently at the crack tip.Through comparison between the obtained results and those in open publication,it is concluded that the experimental data are valid,and the method proposed here is reliable.The validity of the obtained DFT is checked with the ASTM criteria,and fracture surfaces are examined at the end of paper.     

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.     

冲击拉伸实验装置的研制及其试验研究

材料动力学试验技术远比准静态力学中的复杂,为了模拟各种速率的冲击加载过程,试验装置设计就成为关键问题之一.特别是针对材料动态拉伸性能的测试,目前的冲击拉伸装置还没有统一标准,因此本文基于一维弹性应力波原理设计了一套双气室间接杆-杆型冲击拉伸试验装置.该装置采用了双气室对称布置的方式,通过气体转换器实现气路的转换,克服了现有气动式冲击拉伸设备结构复杂、密封要求严格的缺点.本文利用该装置对2A12T4铝合金试件的冲击拉伸性能进行了测试,并数值分析了应力波在杆系和试件中的传播效应.通过试验测试和数值分析论证了该冲击拉伸装置实验的可靠性和设计的合理性.     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.     

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。     

混凝土动态直接拉伸实验技术研究

霍普金森压杆(Split Hopkinson Pressure Bar,简称为SHPB)实验装置越来越多地被应用于混凝土等脆性材料动态压缩力学性能的研究之中。由于种种原因,混凝土动态拉伸的实验研究一般还是通过SHPB实验装置间接进行(例如层裂实验和巴西圆盘实验等)。本文以C30混凝土材料为例,根据霍普金森实验技术的两个基本假定(一维假定和均匀假定),探讨了利用大直径(Φ75mm)霍普金森拉杆(Split Hopkinson Tension Bar,简称为SHTB)实验装置对混凝土类试件进行动态直接拉伸的实验技术问题,提出了一种可行的对混凝土类材料进行直接动态拉伸实验的方法。