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.     

套管式Hopkinson冲击拉伸实验相关问题的探讨

基于套管式Hopkinson冲击拉伸实验装置,对板状、棒状材料的冲击拉伸试验进行了简要介绍,重点讨论和解决材料冲击拉伸试验过程中出现的一些问题。通过提出的一种辅助式螺母连接方法,改进了棒状材料试件连接装置,解决了棒状试件试验过程中因连接拧紧度对实验结果的影响;通过建立其有限元模型,运用ABAQUS软件数值模拟分析入射杆端部拉伸螺母厚度对入射波形的影响,给出合理选择拉伸螺母的依据。     

高导无氧铜杆的冲击拉伸断裂系列实验以及基于样本体积单元的分析

采用Hopkinson装置和一种基于一级气体炮的高速冲击拉伸断裂装置,研究了无刻槽高导无氧铜 (OFHC)杆在一系列冲击拉伸速度下的断裂。当冲击拉伸速度大于40m/s时,断裂位置总在冲击拉伸端附 近,此速度被确定为OFHC的实验临界冲击拉伸速度。一种受单轴冲击拉伸荷载的、中心含椭球空穴的样本 体积单元被用于数值模拟所含空穴的增长与失稳的过程。OFHC的J-C与Z-A 本构关系用于描述基体材料 的动态响应。讨论了空穴失稳条件并提出以空穴形状演化为判据,比较了空穴失稳时的样本体积单元平均径 向应变与无刻槽杆的冲击断裂应变。也用这种样本体积单元模型分析了OFHC的实验临界冲击拉伸速度。     

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。     

套管式冲击拉伸实验装置的研制

介绍了几种常见的冲击拉伸实验装置,并做了简要的评述.提出了一种改进后的套管式冲击拉伸实验装置,该装置直接产生拉伸脉冲,消除了反射式冲击拉伸实验装置中两种干扰信号的影响,得到了远比反射式冲击拉伸实验更为理想的透射波形.运用改进后的套管式冲击拉伸试验装置对几种板材材料进行了动态拉伸实验,得到了这几种板材在600/s,1200/s,1800/s应变率下的应力应变曲线,与静态应力应变曲线相比较,反映了这几种板材具有明显的应变率效应.根据实验结果中动态与静态应力应变曲线的一致性,分析了这种套管式冲击拉伸实验装置的优越性与有效性,它能更好地反映材料的动态力学性能.     

一种基于SHPB的冲击膨胀环实验技术

设计了一种基于分离式Hopkinson压杆(SHPB)的冲击膨胀环实验装置,实验装置包括一个液压腔,一侧为驱动活塞,另一侧为圆环试件封闭。对活塞施加轴向冲击,利用液体体积近似不可压缩的特性,通过液压腔截面积的大比例缩小,将较低速度的对活塞冲击转化为圆环试件沿径向的高速膨胀,驱动试件发生拉伸变形直至断(碎)裂。使用这种冲击膨胀装置,获得了LY12铝环在不同撞击速度下碎裂过程的初步结果。实验结果显示,随着撞击速度增大,圆环试件碎裂产生的碎片的尺度减小,试件的表观断裂应变增加。这为研究材料的动态拉伸碎裂问题提供了一种加载方式。     

弹塑性材料滤波作用的研究--间接杆杆型冲击拉伸试验装置产生加载脉冲的三维弹塑性动力学有限元分析

本文对旋转盘式间接杆杆型冲击拉伸试验装置建立了产生拉伸加载脉冲的三维弹塑性动力学模型 ,使用ANSYS/LSDYNA程序模拟了该装置产生加载脉冲的过程 ,分析了弹塑性材料的滤波作用。数值模拟得到的入射脉冲与实验结果吻合 ,表明该数值模型和方法是有效的。分析结果表明 ,相对于直接连接系统 ,间接连接系统中的前置金属短杆的弹塑性变形直至断裂使锤头与撞块只发生一次撞击 ,即滤去了多次撞击 ,同时滤去了金属短杆与撞块界面扰动力的高幅抖动 ,使扰动力变成幅值较低的平稳脉冲。总之 ,前置金属短杆的作用相当于一个低阈值的低通滤波器。本模型还可为合理确定试验方案以及为改进和设计新的冲击拉伸试验装置提供依据和方法     

一种绸布的冲击拉伸性能实验

采用改进的分离式Hopkinson拉杆测试技术对低强度材料进行动态拉伸。该装置采用铝杆作为透射杆,并且透射杆采用半导体应变片。针对一种芳纶绸布材料,进行了试样两端应力平衡、实验速度、半导体应变片与电阻应变片测量的验证,获得了不同速度下被测材料的本构曲线,并通过高速照相机清楚地观察了试样的整个破坏过程。实验结果表明,设计的实验方法行之有效。     

冲击载荷下瞬态温度的实时测量方法

将红外瞬态测温装置引入SHPB冲击实验,确定了不同材料试件的温度标定曲线,并实时测量了冲击下Al合金和伪弹性TiNi合金试样的表面温度。结果表明,2种试样温度变化都经历了加载过程的温度升高,主要不同在于卸载过程,Al合金卸载过程中温度保持最大加载温度不变,而TiNi合金试样卸载过程中温度降低,这反映了2种材料不同的物理变形过程和温度变化机制。直接红外测温的实验结果与根据能量守恒理论计算的温度较好吻合,说明采用的红外测温方法实时测量冲击瞬态温度是可行的。     

高导无氧铜的临界冲击拉伸速度

基于气炮系统,提出了一种新的临界冲击拉伸速度实验方法。实验装置主要由2部分组成:气体炮系统驱动弹丸和弹丸击靶牵引多根拉伸杆试件。采用该实验方法对高导无氧铜进行了临界冲击拉伸速度实验。对于一维应力的临界冲击拉伸速度实验,探讨了一种完全热耦合的在颈缩区域计及塑性约束系数以及空穴增长与聚集效应的数值模拟方法。高导无氧铜的临界冲击拉伸速度实验的数值模拟结果表明,采用Zerilli-Armstrong本构关系预估的实验临界冲击拉伸速度比采用Johnson-Cook本构关系预估的更符合实验。     

Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events

Monitoring of acoustic emission activity is employed to characterize the initiation and progression of local failure processes during nanoindentation-induced fracture. Specimens of various brittle materials are loaded with a cube-corner indenter and acoustic emission activity is monitored during the entire loading and unloading event using a transducer mounted inside the specimen holder. As observed from the nanoindentation and acoustic emission response, there are fundamental differences in the fracture behavior of the various materials. Post-failure observations are used to identify particular features in the acoustic emission signal that correspond to specific types of fracture events. Furthermore, analysis of the parametric and transient acoustic emission data is used to establish the crack-initiation threshold, crack-arrest threshold, and energy dissipation during failure. It is demonstrated that the monitoring of acoustic emission signals yields both qualitative and quantitative information regarding highly localized failure events in brittle materials.     

Crack spacing effect for a piezoelectric cylinder under electro-mechanical loading or transient heating

This paper investigates the fracture problem of a piezoelectric cylinder with a periodic array of embedded circular cracks. An electro-mechanical fracture mechanics model is established first. The model is further used to the thermal fracture analysis of a piezoelectric cylinder subjected to a sudden heating on its outer surface. The temperature field and the associated thermal stresses and electric displacements are obtained and are added to the crack surface to form a mixed-mode boundary value problem for the electro-mechanical coupling fracture. The stress and stress intensities are investigated for the effect of crack spacing. Strength evaluation of piezoelectric materials under the transient thermal environment is made and thermal shock resistance of the medium is given.     

Mechanical modeling and transient anti-plane fracture analysis for the graded inter-diffusion regions in a bonded structure

An inter-diffusion interface model (IDIM) is put forward for a bonded structure. Laplace and Fourier integral transforms are applied to reduce the transient anti-plane fracture problem of the structure as a Cauchy singular integral equation. Lobatto-Chebyshev collocation method and Laplace numerical inversion transform are employed to evaluate transient stress intensity factors (TSIFs). The effects of geometrical and physical parameters on TSIFs are studied. Specially discussed are the influences of the weak/micro-discontinuity of the interfaces. Comparison between IDIM and the graded interlayer model indicates that if the inter-diffusion between the two original materials is prominent, the former should be applied instead of the latter in fracture analyses of bonded structures.     

Q235钢细观损伤声发射源动态观测实验研究

金属材料细观损伤过程伴随有瞬态应力波的释放,利用声发射检测技术可采集到应力波的信息,但由于缺少直观可视的手段,利用声发射信息定量评价金属材料的细观损伤存在困难。本文将SEMtester1000原位拉伸试验机、MZ1000正置显微镜和PCI-2型声发射检测系统相结合,搭建了声发射源动态观测实验系统。以Q235钢可视化原位拉伸实验为例,获取了晶体的滑移和夹杂物的断裂两种典型声发射源图像及对应的声发射信号。通过对不同声发射源信号的分析可知,滑移是一个持续的过程,复杂的滑移活动会产生多类型的声发射信号。夹杂物断裂是瞬态过程,产生典型的突发型声发射信号,能量较滑移信号更高。依据声发射源动态观察结果及图像分析,建立了基于声发射机制的Q235钢细观损伤模型。     

Mode-3 spontaneous crack propagation in symmetric functionally graded materials

The effects of spatially varying the material properties on the mode-3 planar crack propagation characteristics are numerically investigated. The spectral scheme that is available for homogeneous materials is modified to account for the symmetrically varying material properties. Crack propagation in hardening, softening and unsymmetric type of functionally graded have been simulated. A parametric study was performed by systematically varying the material inhomogeneity length scale. Our study indicated that softening and unsymmetric graded materials reduce the resistance to fracture, while a hardening material offers higher fracture resistance with increase in inhomogeneity. Only the transient phase of crack propagation speed was affected by the material property variation, irrespective of whether the material was hardening, softening or an unsymmetric type. The crack always reached a quasi-steady-state velocity, which remained unaffected by the material property inhomogeneity.     

Mixed-mode fracture analysis of orthotropic functionally graded materials under mechanical and thermal loads

Mixed-mode fracture problems of orthotropic functionally graded materials (FGMs) are examined under mechanical and thermal loading conditions. In the case of mechanical loading, an embedded crack in an orthotropic FGM layer is considered. The crack is assumed to be loaded by arbitrary normal and shear tractions that are applied to its surfaces. An analytical solution based on the singular integral equations and a numerical approach based on the enriched finite elements are developed to evaluate the mixed-mode stress intensity factors and the energy release rate under the given mechanical loading conditions. The use of this dual approach methodology allowed the verifications of both methods leading to a highly accurate numerical predictive capability to assess the effects of material orthotropy and nonhomogeneity constants on the crack tip parameters. In the case of thermal loading, the response of periodic cracks in an orthotropic FGM layer subjected to transient thermal stresses is examined by means of the developed enriched finite element method. The results presented for the thermally loaded layer illustrate the influences of the material property gradation profiles and crack periodicity on the transient fracture mechanics parameters.     

爆炸加载下金属壳体膨胀断裂过程中的关键物理问题

爆炸加载下金属壳体膨胀断裂过程是武器研制领域关注的重要课题,该过程包含着丰富的力学与材料学基础科学问题,吸引着众多学者的长期关注。本文中通过分析爆炸加载下金属壳体膨胀断裂过程,明确了其中蕴含的3个关键物理问题:材料动态拉伸本构、壳体膨胀断裂机理和破片尺寸控制机理,综合分析了这3个关键物理问题的研究现状与趋势。     

Discrete fracture modeling of asphalt concrete

A heterogeneous fracture approach is presented for modeling asphalt concrete that is composed of solid inclusions and a viscous matrix, and is subjected to mode-I loading in the fracture test configuration. A heterogeneous fracture model, based on the discrete element method (DEM), is developed to investigate various fracture toughening mechanisms of asphalt materials using a high-resolution image processing technique. An energy-based bilinear cohesive zone model is used to model the crack initiation and propagation of materials, and is implemented as a user-defined model within the discrete element method. Experimental fracture tests are performed to investigate various fracture behavior of asphalt concrete and obtain material input parameters for numerical models. Also, bulk material properties are necessary for each material phase for heterogeneous numerical models; these properties are determined by uniaxial complex modulus tests and indirect tensile strength tests. The main objective of this study is to integrate the experimental tests and numerical models in order to better understand the fracture mechanisms of asphaltic heterogeneous materials. Experimental results and numerical simulations are compared at different test conditions with excellent agreement. The heterogeneous DEM fracture modeling approach has the potential capability to understand various crack mechanisms of quasi-brittle materials.     

金属材料脆性断裂机理的实验研究

材料的脆性断裂有许多准则和模型,但对脆断机理和变化规律缺乏合理的描述,给工程应用带来不便。本文对典型脆性材料球墨铸铁、灰铸铁分别进行了拉扭双轴断裂实验和常规拉伸、扭转破坏实验;对韧性金属材料合金钢进行了单轴拉伸颈缩破坏实验。通过上述实验分析了脆性材料和韧性材料发生脆性断裂的机理特征并选择应力三维度作为应力状态参数描述危险点的应力状态,同时考察了脆性材料和韧性材料发生脆性断裂的主导因素。结果表明:韧性材料45#钢和14CrNiMoV合金钢在颈缩断面心部应力三维度值较大时发生脆性拉断,而在颈缩断面边缘处应力三维度值较小时发生剪断;脆性材料球墨铸铁在应力三维度值为0.0～0.33之间变化时均发生脆性断裂;灰铸铁在应力三维度值大于0.0时发生脆性拉断,而在应力三维度值小于0.0时发生剪断。因此可以认为,材料的细观组织结构和危险点应力状态是影响断裂机理及变化规律的主要因素。对于同种材料,随着应力三维度代数值从小向大变化材料的断裂机制由塑性剪切断裂逐渐转变为脆性断裂。本文通过对几种材料的脆性断裂危险点和断裂方向的研究给出了脆断宏观破坏条件。     

Combined analysis of fracture under stresses acting along cracks

A joint approach to the study of two non-classical fracture mechanisms, namely fracture of cracked materials with initial (residual) stresses acting along the crack planes and fracture under compression along parallel cracks, is considered in the framework of three-dimensional linearized solid mechanics. Mathematical statements of problems for pre-stressed solids that contain interacting circular cracks are given. Problems for an infinite solid containing two parallel co-axial cracks and for a space with the periodical set of co-axial parallel cracks as well as for a half-space with near-the-surface crack are solved. Several patterns of loading on the crack faces (normal loading, radial shear and torsion) are considered. The effects of initial stresses on stress intensity factors are analyzed for highly elastic materials with some types of elastic potentials. Formulation of fracture criteria accounting effect of initial (residual) stresses is given. Critical parameters of fracture of solids containing interacting cracks under compression along the cracks are calculated. The influence of geometrical parameters of the problems as well as physical and mechanical properties of materials on these critical parameters is analyzed.