杆杆型冲击拉伸试验装置的二维轴对称弹塑性有限元分析

对带有哑铃状圆柱形试件的杆杆型冲击拉伸试验装置,建立了含有多个物理和几何间断面的二维轴对称弹塑性有限元模型,采用动态增量非线性有限元程序ＡＤＩ－ＮＡ,进行了数值模拟模拟分析,揭示了应力波在试验系统中的传播规律,在弹性框架内初步论证了杆杆型冲击拉伸试验装置测试原理的有效性。     

Two-dimensional analysis of the split hopkinson pressure bar system

The split Hopkinson pressure bar is widely used to measure the dynamic properties of solid materials. This paper presents the results of the first comprehensive two-dimensional numerical analysis of the technique, and quantitatively describes the effects of realistic friction and of variations in both the specimen geometry and the imposed strain-rate on the validity of the assumptions used in analyzing experimental data. A two-dimensional axisymmetric numerical analysis is used to compute all components of the stress, strain and strain-rate tensors at each mesh point within the specimen and the elastic bars. The calculated response of the pressure bars is used to reconstruct the stress-strain behavior of the specimen and this is compared to both the input stress-strain curve and the actual calculated stress-strain states in the specimen. Thus, the validity of the assumptions and the corrections used in the analysis of the data is determined.Inertia and friction between the specimen and the elastic bars affect the response of the specimen differently for different length-to-diameter ratios. Inertia effects produce stress waves propagating radially and axially in the specimen and may result in an oscillating reconstructed stress-strain curve. If the ends of the specimen are well lubricated and care is taken to minimize the effects of inertia, the reconstructed stress-strain curve agrees with the input. However, serious stress and strain nonuniformity exists when the ends are not lubricated and this results in a reconstructed stress-strain curve where, for any given strain, the stress magnitude is larger than the correct value. A comparison of the calculations with experiment shows excellent agreement for various interface conditions. Finally, the inertia correction of E.D.H. Davies and S.C. Hunter (1963) is found to be reasonable.     

Stress analysis of an orthotropic material under diametral compression

The diametral compression test is commonly used to determine the tensile strength of brittle materials. For isotropic materials a simple relation based on specimen geometry and the applied load at failure is used to calculate the tensile strength. Previous to this work the effect of material orthotropy and material orientation on the specimen stress state had not been completely determined. In this study, both isotropic and orthotropic specimens were analyzed using a finite element analysis and experimentally verified by strain gage and photoelastic measurements. Further, this work investigated the effect of the applied load area on the specimen stress state. Results of this work show that there is a significant difference between the theoretical calculations based on the assumption of material isotropy when compared to an orthotropic material. This difference can be as much as 45 percent depending on the degree of orthotropy and the orientation. It was also determined that the applied load area and material orientation significantly influence the specimen stress state. An applied load area of 8 percent of the circumference was found to reduce the stresses in the applied load region.     

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…     

高导无氧铜杆高应变率拉伸下的颈缩与断裂

采用Hopkinson拉伸实验装置和一种高速拉伸断裂实验新装置,对高导无氧铜(OFHC)杆件进行了一系列高应变率拉伸断裂试验.实验结果表明,局部化的断裂应变随拉伸速度增大并不明显增大,其断裂位置有随机性.存在一种临界拉伸速度,当冲击拉伸速度大于此值时,断裂即发生在冲击拉伸端附近,杆的其它部分几乎无应变.采用典型的Johnson-Cook本构关系,使用LS-DYNA程序进行一系列数值模拟,提出颈缩处直径收缩率达极值的颈缩失效判据,由此计算所得的局部化颈缩应变及断裂位置与试验回收结果有一定差别.     

A finite thickness band method for ductile fracture analysis

We present a finite element method with a finite thickness embedded weak discontinuity to analyze ductile fracture problems. The formulation is restricted to small geometry changes. The material response is characterized by a constitutive relation for a progressively cavitating elastic–plastic solid. As voids nucleate, grow and coalesce, the stiffness of the material degrades. An embedded weak discontinuity is introduced when the condition for loss of ellipticity is met. The resulting localized deformation band is given a specified thickness which introduces a length scale thus providing a regularization of the post-localization response. Also since the constitutive relation for a progressively cavitation solid is used inside the band in the post-localization regime, the traction-opening relation across the band depends on the stress triaxiality. The methodology is illustrated through several example problems including mode I crack growth and localization and failure in notched bars. Various finite element meshes and values of the thickness of the localization band are used in the calculations to illustrate the convergence with mesh refinement and the dependence on the value chosen for the localization band thickness.     

Bilinear softening parameters and equivalent LEFM R-curve in quasibrittle failure

For composites and adhesive joints, the determination of the cohesive zone parameters from Double Cantilever Beam specimens loaded with pure moments is now well established and documented. However, for quasibrittle materials used in Civil Engineering such as concrete or wood, the difficulty to apply a pure bending moment lies inappropriated the method used for composites. Nevertheless, the one-to-one correspondence which exists between the R-curve and the softening curve is here revisited and adapted for any kind of specimen geometry and for the bilinear approximation of the softening function, well-known to successfully describe the failure of a wide group of quasibrittle materials. It is shown that even though the connections between the cohesive parameters and the ‘equivalent LEFM’ R-curve are geometry and material dependent, their trends are preserved whatever the specimen geometry and the material are. The outline of a general estimation procedure of the cohesive zone parameters funded on the equivalent LEFM R-curve is proposed.     

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.     

High-rate Characterization of 304L Stainless Steel at Elevated Temperatures for Recrystallization Investigation

An integrated experimental technique was developed for high-rate mechanical characterization of 304L stainless steel at elevated temperatures by using a modified split Hopkinson pressure bar (SHPB). A sandwich structure consisting of two platens and the specimen in between was heated before mechanical loading while the bars were maintained at room temperature to eliminate the temperature gradient effect on the wave propagation in the bars. Upon contacting the cold bars, temperature gradients form in the platens, leaving the temperature in specimen constant and uniform. Pulse shaping techniques were employed to maintain constant strain-rate deformation and dynamic stress equilibrium in the specimen. Dynamic compressive stress-strain curves at elevated temperatures for the 304L stainless steel were obtained. To relate recrystallization to impact loading, a momentum trapping system was employed to apply a single loading on the specimen during one dynamic experiment. We also controlled the quenching time to study its effect on recrystallization.     

软材料的SHPB实验设计

通过对SHPB实验中加载波波形进行控制设计 ,实现软材料试样在加载过程中的应力平衡和常应变率加载 ,从而保证SHPB实验的前提条件。采用这种方法研究了两种材料的高应变率本构 ,实验结果表明 :设计的方法是行之有效的。     

一种用于材料高应变率剪切性能测试的新型加载技术

高应变率下的冲击剪切实验技术是材料动态力学行为及其微观机理研究的重要基础.采用分离式霍普金森压杆(split Hopkinson pressure bar)装置一般可以获得材料在104s-1以内应变率的动态力学性能.在超过104s-1的应变率下对材料进行冲击剪切测试时,通常需要采用高速压剪飞片技术或由气炮发射子弹对试样进行直接加载.本文提出一种可用于传统霍普金森压杆技术的新型双剪切试样,可以在103~105s-1剪应变率范围实现对材料剪切性能的精确测量;同时,可以对材料的变形及失效过程进行直接观测.试样与压杆之间避免了复杂的界面或连接装置,通过转接头可以保证试样与压杆直接接触,提高测试精度,同时可以防止因试样的横向位移而导致的非均匀变形.获得了紫铜在1400~75000s-1应变率下的剪应力-剪应变曲线,并采用计算软件"ABAQUS/Explicit"对双剪切试样的动态加载过程进行了数值模拟和结果验证.分析表明,剪切区的主要区域内剪切成分占主导地位,其应力应变场沿厚度及宽度方向基本呈均匀分布.实验得到的剪应力-剪应变曲线与模拟结果吻合较好,说明所提出的基于分离式霍普金森压杆系统的双剪切试样可以为材料的高应变率力学性能测试提供一种方便有效的加载技术.      

铝合金 2A12韧性断裂机制的实验研究

用轴对称圆棒静拉伸实验研究了铝合金2A12韧性断裂的特点,并探讨了临界空穴扩张比参数-VGC在实验材料上的适用性。实验测试了实验材料的拉伸特性参数及VGC参数。实验证实实验材料在不同应力状态下会发生断裂形式的转变,光滑试样表现为剪切破坏,而缺口试样为拉伸破坏,VGC参数对材料的断裂形式较为敏感。用SEM观测了试样断口显示缺口试样为空穴聚合型断裂机制。VGC参数只适用于以拉伸型断裂为主的缺口试样,对以剪切断裂为主的光滑拉伸试样不适用。     

Optimum Specimen Geometry for Accurate Tensile Testing of Superplastic Metallic Materials

The high temperatures and large strain limits associated with superplastic materials amplify the possibility of the tensile test outcomes being sensitive to the shape and size of the specimen geometry. In spite of that, the disparities in the specimen geometries used throughout the numerous efforts on characterising this unique class of materials are rather astonishing. There is an urge to evaluate the dependency of a superplastic tensile test on specimen geometry, before a much-needed universally-adopted standard specimen can be designed; which is the main objective of this comprehensive experimental investigation. More than 20 geometries, covering multiple variations in gauge length, gauge width, grip length and grip width values, are tested at identical conditions, and the corresponding material behaviour is compared in terms of deformation uniformity, material flow and the extracted stress/strain curves. The results reveal the influences of each geometrical parameter, as well as their combined effects, and guide the selection of an optimum specimen geometry for accurate and unified tensile testing of superplastic metallic materials.     

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.     

A basic study of a strain gage made of nickel foil

Strain gages made of nickel foil are devised for measuring the elastic surface stress of a specimen subjected to repeated loads. Sticking nickel foil on the surface of a specimen, the elastic stress is measured by observing slip-bands in the foil resulting from repeated strains. Calibration studies with rotating-bending tests are performed on round steel bars with nickel foil to determine the applicable test temperatures for the gage, and to establish the relation between the first appearance of slip-bands and the magnitude of cyclic stress. The peak stresses in grooved shafts under bending are measured with the nickel foil gages. The accuracy of the results is examined.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Hopkinson拉杆试验的优化与高导无氧铜拉伸本构关系的确定

为有效测量试件中的应力、应变及应变率,Hopkinson拉伸试验(TSHB)必须作优化分析,所进行的数值模拟涉及试件与杆件等连接对于实验结果的影响.为减小上升前沿、惯性效应且使试件处于一维应力及均匀应力与应变状态,优化的试件具有一定的长度与形状要求.对于高导无氧铜,由准静态试验及不同应变率与温度的优化TSHB试验得到一...     

Disk-shaped compact tension test for asphalt concrete fracture

A disk-shaped compact tension (DC(T)) test has been developed as a practical method for obtaining the fracture energy of asphalt concrete. The main purpose of the development of this specimen geometry is the ability to test cylindrical cores obtained from in-place asphalt concrete pavements or gyratory-compacted specimens fabricated during the mixture design process. A suitable specimen geometry was developed using the ASTM E399 standard for compact tension testing of metals as a starting point. After finalizing the specimen geometry, a typical asphalt concrete surface mixture was tested at various temperatures and loading rates to evaluate the proposed DC(T) configuration. The variability of the fracture energy obtained from the DC(T) geometry was found to be comparable with the variability associated with other fracture tests for asphalt concrete. The ability of the test to detect changes in the fracture energy with the various testing conditions (temperature and loading rate) was the benchmark for determining the potential of using the DC(T) geometry. The test has the capability to capture the transition of asphalt concrete from a brittle material at low temperatures to a more ductile material at higher temperatures. Because testing was conducted on ungrooved specimens, special care was taken to quantify deviations of the crack path from the pure mode I crack path. An analysis of variance of test data revealed that the prototype DC(T) can detect statistical differences in fracture energy resulting for tests conducted across a useful range of test temperatures and loading rates. This specific analysis also indicated that fracture energy is not correlated to crack deviation angle. This paper also provides an overview of ongoing work integrating experimental results and observations with numerical analysis by means of a cohesive zone model tailored for asphalt concrete fracture behavior.     

金属材料三维断裂韧度厚度效应的有限元模拟

随着金属材料大壁厚结构件在工程中的广泛应用,对其断裂韧度的厚度效应研究具有重要的科学意义和工程价值。本研究基于有限元和实验相结合的方法,对金属材料断裂韧度的厚度效应进行预测。首先,通过一组薄壁厚金属材料标准三点弯曲试验得到试样失效时的临界载荷值,并利用内聚力模型与基于虚拟裂纹闭合技术的裂纹扩展模拟方法得到裂纹扩展时的单元临界能量释放率。随后,以此临界能量释放率作为裂纹扩展的启裂准则门槛值,通过有限元计算得到不同试样厚度下裂纹启裂时的裂尖断裂参数随着厚度的变化规律。最后,为了验证有限元模拟结果的准确性,本研究进行了另外两组不同厚度下三点弯曲试样的断裂韧度试验,并将试验结果与有限元结果进行了对比,验证了有限元所模拟的断裂韧度厚度效应的准确性。本研究旨在,通过薄壁厚三点弯曲试样的实验结果结合有限元模拟工作,即可实现金属材料断裂韧度的整个厚度效应曲线,为任意厚度下金属材料断裂韧度预测提供一种可靠的研究方法,有益于缩减试验成本,为大壁厚工程结构件的失效预测提供依据。     

钢筋混凝土结构改进型分离式数值模型

提出一种考虑粘结滑移效应的钢筋混凝土改进型分离式数值模型。在混合物理论基础上,该模型兼顾混凝土基体和钢筋的力学行为,且基于钢筋混凝土界面粘结滑移模型,获得了钢筋等效模型。改进型分离式数值模型由于对钢筋及其界面无显式离散要求,使得钢筋的运用完全独立于其几何形状,同时对混凝土网格没有约束,并且不增加计算成本,因此该模型可适用于钢筋混凝土宏观结构层面分析。通过钢筋混凝土构件-结构的爆炸实验,对改进型分离式数值模型进行层次化验证。对比结果表明,考虑粘结滑移效应的有限元模型能够更好地预测钢筋混凝土结构的宏观力学行为。     

Numerical investigation into the influence of the punch shape on the mechanical behavior of pharmaceutical powders during compaction

During the production of pharmaceutical tablets using powder compaction, certain common problems can occur, such as sticking, tearing, cutting, and lamination. In the past, the compressibility of the powder was calculated only along the axis of the device; consequently, critical areas of the material throughout the volume could not be identified. Therefore, finite element method （FEM） can be used to predict these defects in conjunction with the use of an appropriate constitutive model. This article summarizes the current research in the field of powder compaction, describes the Drucker-Prager Cap model calibration procedure and its implementation in FEM, and also examines the mechanical behavior of powder during compaction. In addition, the mechanical behavior of pharmaceutical powders in relation to changes in friction at the wall of the system is examined, and the dependence of lubrication effect on the geometry of the compaction space is also investigated. The influence of friction on the compaction process for the flat-face, fiat-face radius edge, and standard convex tablets is examined while highlighting how the effects of friction change depending on the shape of these tablets.