Φ100mm SHPB应力均匀及恒应变率加载试验技术研究

大直径分离式霍普金森压杆(SHPB)试验装置被越来越多地应用于混凝土等脆性材料的高应变率力学性能研究,试件在破坏前,满足应力均匀分布要求,且保持恒应变率加载,是保证SHPB试验有效性及试验结果可靠性的关键.以提高试验结果准确性为研究目标,以C50素混凝土的SHPB试验为例,采用Φ100mm SHPB试验装置,对试验中的应力均匀及恒应变率加载技术进行研究,包括波形整形器的设计、近似恒应变率的估算、最佳近似恒应变率的确定以及应力均匀性检验.结果表明:厚度为1mm,直径分别为20mm、25mm、30mm、35mm的H62黄铜波形整形器对入射波形有明显的改善效果,可将入射脉冲上升沿的升时延长1～2倍,且波形呈半正弦状;应变率对于波形整形器的直径很敏感,随着整形器直径的增大,最佳近似恒应变率也在增加,一个几何尺寸的波形整形器只能对应于一个最佳近似恒应变率,且与整形器的直径之间满足线性函数关系.     

Ф100mmSHPB应力均匀及恒应变率加载试验技术研究

大直径分离式霍普金森压杆（SHPB）试验装置被越来越多地应用于混凝土等脆性材料的高应变率力学性能研究,试件在破坏前,满足应力均匀分布要求,且保持恒应变率加载,是保证SHPB试验有效性及试验结果可靠性的关键。以提高试验结果准确性为研究目标,以C50素混凝土的SHPB试验为例,采用Ф100mmSHPB试验装置,对试验中的应力均匀及恒应变率加载技术进行研究,包括波形整形器的设计、近似恒应变率的估算、最佳近似恒应变率的确定以及应力均匀性检验。结果表明：厚度为1mm,直径分别为20mm、25mm、30mm、35mm的H62黄铜波形整形器对入射波形有明显的改善效果,可将入射脉冲上升沿的升时延长1～2倍,且波形呈半正弦状;应变率对于波形整形器的直径很敏感,随着整形器直径的增大,最佳近似恒应变率也在增加,一个几何尺寸的波形整形器只能对应于一个最佳近似恒应变率,且与整形器的直径之间满足线性函数关系。     

SHPB测试中岩石试件应力均匀性分析研究

对花岗岩、千枚岩、磁铁石英岩三种岩石进行SHPB应力均匀性试验测试,研究表明:试件内达到应力均匀时间介于弹塑性状态假设条件下试件内达到应力均匀时间之间,且试件内达到应力均匀时间与二者的平均值具有一定的相关性。试验中子弹的冲击速度在一定范围内,能够使三种岩石满足应力均匀性的要求,且在断裂应变达到1%之前均有较长的恒应变率时间段。     

SHPB测试中岩石试件应力均匀性分析研究

对花岗岩、千枚岩、磁铁石英岩三种岩石进行SHPB应力均匀性试验测试,研究表明:试件内达到应力均匀时间介于弹塑性状态假设条件下试件内达到应力均匀时间之间,且试件内达到应力均匀时间与二者的平均值具有一定的相关性。试验中子弹的冲击速度在一定范围内,能够使三种岩石满足应力均匀性的要求,且在断裂应变达到1%之前均有较长的恒应变率时间段。     

基于脆性材料的SHPB实验研究与展望

本文介绍了岩石、混凝土、陶瓷等脆性材料的分离式霍普金森压杆(SHPB)实验研究进展。总结发现,脆性材料进行SHPB实验时主要存在应力波几何弥散、试件应力不均匀及非恒应变率加载、高强度脆性材料的二维效应和不完全平面接触等问题。结合SHPB实验原理和脆性材料自身特性,着重讨论了问题产生的原因及相应的改进技术,并就这一方面更深入的研究工作进行了展望。     

单脉冲加载技术及其在钛合金拉伸行为研究中的应用

为精确评价钛合金热率相关的力学行为,利用冲击拉伸试验系统和基于单应力脉冲加载的冲击拉伸复元试验技术分别获得TC11钛合金在高应变率(102~103s-1)范围内的绝热应力-应变曲线和等温应力-应变曲线,实现拉伸响应的热力解耦;利用冲击拉伸加卸载试验技术实施变温度和变应变率测试,研究历史效应对于本构行为的影响。结果表明:TC11的初始屈服行为呈现温度软化和应变率强化特性,而等温塑性应变硬化行为表现出温度和应变率不敏感特征,瞬态绝热温升是导致材料动态应变硬化率降低的主要原因;高应变率加载时材料内的热功转换系数约为0.9,且其拉伸力学行为无明显的温度和应变率历史效应。实验结果为建立钛合金的本构模型奠定试验基础。     

SHPB试验岩石试件应力平衡时间预估分析

运用一维应力波理论,对分离式Hopkinson压杆(SHPB)试验中弹性应力波的传播过程进行了分析,得到了试件应力分布相关计算公式,讨论了试件应力平衡时间的影响因素和变化规律。以变截面杆SHPB试验装置对煤矿岩石试件加载为例,计算分析了3种岩石试件在光滑的试验入射波和与其升时相同的理论梯形入射波加载情况下试件应力均匀性和应力平衡时间。发现采用变截面入射杆进行加载,能够实现岩石试件在应力峰值之前达到应力平衡,满足应力均匀性假定要求的有效条件。结果表明,采用理论梯形入射波可以近似代替与其升时相同的试验入射波,预估岩石试件应力均匀性和应力平衡时间,对类似脆性材料的SHPB试验设计具有一定的参考价值。     

端面不平行对岩石SHPB测试结果的影响分析

为探讨岩石试件加工端面不平行对岩石材料动力学特性测试结果的影响,采用有限元分析软件LS-DYNA对13种端面不平行度的岩石试件开展SHPB数值模拟,并分析了端面不平行对加载过程应力均匀性的影响。结果表明:随端面不平行度γ的增加,岩石试件动态应力-应变曲线逐渐向右和向下移动,且峰前加载段逐渐延长。当γ1.60%时,动态应力-应变曲线在加载初期出现一个微小的加载-卸载过程,且在升时为120μs的半正弦入射波加载阶段岩石试件达不到应力均匀状态。基于数值模拟结果,建立了端面不平行度与量纲一化的动态单轴抗压强度、平均应变率和峰值应变的定量表达式。     

对粘弹性材料力学性质的再认识

以[1]的线性理论为基础,分析粘弹性材料动态压缩试验的压力,变形及加卸卸载速率,据此绘制材料的应力应变曲线,证明粘弹性材料动态压缩试验的加载与卸载曲线不重合,指出卸载阶段约束时试件有不能立即恢复的粘弹性变形,并导出了与粘弹性回复曲线对应的指数衰减函数。     

冲击压缩载荷作用下杨木的力学性能研究

通过SHPB动态压缩实验,对杨木在高应变率条件下的力学特性进行了研究。基于一维应力波理论的分析表明,尽管杨木相比于金属压杆具有较低的波阻抗,其SHPB动态压缩实验中试样两端的应力均匀性条件仍可以得到满足,而不需要进行入射波整形。通过在透射杆上使用半导体应变片,可以获得具有较高信噪比的透射波信号。结合杨木的准静态实验结果发现,其应力-应变曲线具有多孔材料的典型特征。轴向试样具有Ⅱ型吸能结构特征,失效应力和平台应力都具有应变率敏感性,其平台应力的敏感性稍弱;弦向试样具有Ⅰ型吸能结构特征,没有明显的失效强度,其平台应力具有应变率敏感性,与通常的Ⅰ型吸能结构相比存在一定差异。通过对实验数据的拟合,发现应变率对杨木失效强度的影响符合对数率。     

An analysis of stress uniformity for concrete-like specimens during SHPB tests

As far as concrete-like brittle materials are concerned, whether stresses have been uniformly distributed along the specimen thickness before fracture occurs is a key problem with regard to the validity of SHPB data. By using characteristics method of wave propagation, the SHPB tests for cement mortar which is respectively regarded as the so-called viscoelastic material and the elastic material are numerically studied in the present paper, in order to see how the stress distribution is during both loading and unloading processes. It is found that different rise-times τ_s of incident waves evidently influence the stress uniformity, and the optimum rise-time looks like τ_s/t_L = 2, either shorter or longer rise-time will result in worse situation in stress uniformity; the stress amplitude has no influence on the stress uniformity, but markedly influences the value of strain and average strain rate in specimens. The error found in the determination of dynamic stress–strain curves of rate-dependent brittle materials by the traditional SHPB technique, either by three-wave method or two-wave method, is actually unacceptable, although the dynamic fracture stress can be correctly determined if the requirement of stress uniformity is satisfied before the specimen fails.     

Correlation between the accuracy of a SHPB test and the stress uniformity based on numerical experiments

Split Hopkinson pressure bar (SHPB) has become a frequently used technique to measure the uniaxial compressive stress–strain relation of various engineering materials at high strain rate. Using the strain records on incident and transmitter bars, the average stress, strain and strain rate histories within the specimen can be calculated by SHPB formulae based on one-dimensional wave propagation theory. The accuracy of a SHPB test is based on the assumption of stress and strain uniformity within the specimen, which, however, is not always satisfied in an actual SHPB test due to the existence of some unavoidable negative factors, e.g., friction and specimen size effects. Two coefficients are introduced in the present paper to measure the stress uniformity in axial and radial directions of the specimen in a numerical SHPB test. It is shown that the accuracy of a SHPB test can be correlated to these two stress uniformity coefficients. An assessment and correction procedure for SHPB test results is illustrated through a numerical example.     

Experimental study on dynamic damage evolution of concrete under multi-axial stresses

The effect of stress state on the dynamic compressive strength and the dynamic damage evolution process of concretes are investigated by use of a Spilt Hopkinson Pressure Bar (SHPB) and the ultrasonic technique. The columned concrete specimen is encircled by a steel sleeve. The multi-axial loading includes the axial and the radial loadings. The axial loading is supplied by the incidence bar, and the radial ones are produced by the steel sleeve. Analysis of the dynamic damage evolution of the samples is based on the measurement of the changes of ultrasonic wave velocities before and after the impact tests. The waveforms in the test bars, the stress strain curves, the confining pressure of the specimen, the dynamic compressive strength and other information about the samples are obtained during the SHPB experiments. The results of the tests show that the loading rate and stress states of the specimen apparently influence the damage evolution process in concretes. The dynamic damage evolutions are accelerated with the increase of the strain rate and are delayed significantly under the confined pressure.     

泡沫硅橡胶动态力学性能的实验研究

泡沫硅橡胶在缓冲减振方面有独特的优良性能,但由于它是一种柔性材料,波速和波阻抗均极低,因此实验研究其动态力学性能具有很大困难,本文首次实现了利用霍普金森压杆技术对泡沫硅橡胶作冲击压缩实验,得到了高应变率下的应力￣应变曲线,结果表明,该材料具有强烈的应变率敏感性。     

Effects of radial inertia and end friction in specimen geometry in split Hopkinson pressure bar tests: A computational study

The split Hopkinson pressure bar (SHPB) technique has been used widely for the impact testing of materials in the strain-rate range from 10² to 10⁴ s⁻¹. However, some specific problems still remain mainly concerning the effects of radial inertia and end friction in a cylindrical specimen on the accurate determination of dynamic stress–strain curves of materials. In this study, the basic principle of the SHPB technique is revisited based on energy conservation and some modifications are made considering radial momentum conservation. It is pointed out that the radial inertia and end friction effects are coupled to each other in the SHPB specimen. Computational simulations using the commercial finite element (FE) code ABAQUS/Explicit ver. 6.8 are conducted to check the validity of the modifications for ductile pure aluminum specimens. Both rate-independent and rate-dependent models are adopted for the test material. Simulations are performed by varying two different control parameters: a friction coefficient between the specimen and the pressure bars and a slenderness ratio of the specimen (or thickness to diameter ratio).     

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material under thermo-mechanical loading

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material (FGM) is investigated using a three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus in conjunction with induction coil heating system is used during elevated temperature dynamic loading experiments. A simple and accurate technique has been developed to identify the time corresponding to the load at which the fracture initiates. A series of experiments are conducted at different temperatures ranging from room temperature to 800 °C, and the effect of temperature and loading rate on the fracture initiation toughness is investigated. The material fracture toughness is found to be sensitive to temperature and the fracture initiation toughness increases as the temperature increases. Furthermore, the fracture initiation toughness is strain rate sensitive and is higher for dynamic loading as compared to quasi-static loading.     

Evaluation of stress–strain curve estimates in dynamic experiments

Accurate measurements of the forces and velocities at the boundaries of a dynamically loaded specimen may be obtained using split Hopkinson pressure bars (SHPB) or other experimental devices. However, the determination of a representative stress–strain curve based on these measurements can be challenging. Due to transient effects, the stress and strain fields are not uniform within the specimen. Several formulas have been proposed in the past to estimate the stress–strain curve from dynamic experiments. Here, we make use of the theoretical solution for the waves in an elastic specimen to evaluate the accuracy of these estimates. It is found that it is important to avoid an artificial time shift in the processing of the experimental data. Moreover, it is concluded that the combination of the output force based stress estimate and the average strain provides the best of the commonly used stress–strain curve estimates in standard SHPB experiments.