高压、高应变率与低压、高应变率实验的本构关联性

 指出Johnson-Cook（J-C）、Zerilli-Armstrong（Z-A）、Bodner-Parton（B-P）本构方程在一定条件下的适用性,表明对于低压、高应变率实验,单一曲线假定似乎可以采用。通过等效应力、等效应变,可以将不同应力状态下的流动应力函数采用统一的方程描述。然而,这些本构方程的确立,并不包括平面冲击波实验。对适合于平面冲击波实验的Steinberg-Cochran-Guinan（SCG）本构方程,讨论了其方程中所包含的高压与高应变率耦合效应。指出,以剪切模量度量的流动应力具有应变率相关性。基于温度效应的新发现以及直接测量平面冲击波流动应力的新进展,分别用J-C本构及SCG本构方程估算了钨材料在高压、高应变率加载下的流动应力。结果表明,采用J-C本构估算的流动应力仅在压力为10 GPa以下才能与实验数据相近,当压力高于10 GPa时,流动应力只能采用SCG本构估算。也指出了高压、高应变率本构方程与低压、高应变率本构方程所对应的不同物理背景。     

金刚石压砧内单轴应力场对物质状态方程测量的影响

金刚石压砧的几何结构使得在高压下封垫内的样品通常处于单轴应力场中：压砧轴向加载应力最大,径向应力最小.由于金刚石压砧内非静水压单轴应力场的影响,用传统的高压原位X射线衍射方法测得的物质压缩曲线一般位于理想静水压压缩曲线之上.利用金刚石压砧径向X射线衍射技术以及晶格应变理论,结合最近的钨、金刚石和硼六氧样品的高压原位同步辐射径向X射线衍射实验结果,从宏观差应力、样品强度、标压物质和待测物质强度的关系三个方面分析讨论了金刚石压砧内单轴应力场对物质状态方程测量的影响及解决方案. 关键词： 金刚石压砧 单轴应力场 高压原位X射线衍射 状态方程     

Effect of coherency strain on the deformation of In x Ga1− x As superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In _x Ga_{1? x} As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3?µm and 1.1?µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.     

某PBX炸药的动态力学性能研究

 PBX炸药作为现代武器的主装药,它的力学行为决定着武器的生存能力。为了研究PBX炸药的动态力学特性,采用分离式霍普金森压杆（Split Hopkinson Pressure Bar,SHPB）作为加载手段,结合半导体应变片测试技术和压电晶体监测技术,保证了实验数据的有效性。利用SHPB加载波整形技术,实现了材料两端应力平衡和常应变率加载,得到了不同应变率（90~410 s^-1）下材料的应力-应变曲线。根据材料的模量、破坏强度和破坏应变随应变率的变化规律,采用粘性修正的Sargin模型,得到了该PBX炸药在单轴压缩下的唯象本构模型,模拟结果与实验曲线符合较好。     

High‐resolution stress mapping of polycrystalline alumina compression using synchrotron X‐ray diffraction

The ability to achieve uniform stress in uniaxial compression tests of polycrystalline alumina is of significance for the calibration of piezospectroscopic coefficients as well as strength studies in ceramics. In this study high‐energy X‐rays were used to capture powder diffraction profiles over a half‐section of a polycrystalline alumina parallelepiped sample under an increasing uniaxial compressive load. The data were converted to strain and results were used for stress mapping of the sample. Stress maps from the study quantify the higher stresses at the sample–platen contact interface and reveal the evolution of the stress distribution in these specimens with load. For the geometry of the samples used, at the center section of the specimen the overall magnitudes of the compressive stresses were found to be 20% higher compared with the average expected theoretical stress based on the applied load and cross‐sectional area. The observed compressive stresses at the corners of the parallelepiped specimen were 62% higher and shear stresses were observed at the specimen interface to the load mechanism. The effects, seen at the interface, can lead to premature failure at these locations and can affect the accuracy of calibration of spectral peaks with stress as well as compression strength measurements. The results provide important information that can be used to establish guidelines on material and geometry considerations in developing compression tests on high‐strength ceramics.     

Strength, anisotropy, and preferred orientation of solid argon at high pressures

The elasticity and plasticity of materials at high pressure are of great importance for the fundamental insight they provide on bonding properties in dense matter and for applications ranging from geophysics to materials technology. We studied pressure-solidified argon with a boron-epoxy-beryllium composite gasket in a diamond anvil cell (DAC). Employing monochromatic synchrotron x-radiation and imaging plates in a radial diffraction geometry (Singh et al 1998 Phys.?Rev.?Lett.?80 2157; Mao et al 1998 Nature 396 741), we observed low strength in solid argon below 20?GPa, but the strength increases drastically with applied pressure, such that at 55?GPa, the shear strength exceeded 2.7?GPa. The elastic anisotropy at 55?GPa was four times higher than the extrapolated value from 30?GPa. Extensive (111) slip develops under uniaxial compression, as manifested by the preferred crystallographic orientation of (220) in the compression direction. These macroscopic properties reflect basic changes in van der Waals bondings under ultrahigh pressures.     

高导无氧铜的高压与高应变率本构模型研究

基于Y/G及G/B为常数的假设,构建了高导无氧铜的七种高压与高应变率本构模型.对于高导无氧铜进行了平面冲击波试验,采用纵向与横向锰铜应力计记录了试件中的纵向与横向应力,从而得到了屈服应力历史.用所构建的七种本构模型进行了数值模拟,并与高导无氧铜的平面冲击波试验结果进行比较.结果表明,平面冲击波载荷下高导无氧铜的屈服强度对于压力、密度、温度以及塑性应变的依赖性是本构描述的关键.而由Hopkinson试验取得的高导无氧铜高应变率本构模型,并不适合描述平面冲击波载荷下的本构特性. 关键词： 本构模型 高导无氧铜 平面冲击波试验 锰铜应力计     

Strength of magnesium oxide under high pressure: evidence for the grain-size dependence

X-ray diffraction patterns from magnesium oxide compressed in a diamond anvil cell up to 55 GPa have been recorded and the differential stress (a measure of compressive strength) and grain-size (crystallite size) determined as a function of pressure from the line-width analysis. The strength agrees well with the uniaxial stress component (another measure of compressive strength) derived earlier from the line-shift data. The strength increases while the crystallite size decreases steeply as the pressure is raised from ambient to ∼10 GPa. The increase in strength is much smaller at higher pressures. The strength-pressure data are explained by combining the grain-size dependence of strength and the shear-modulus scaling law. The dependence of strength on grain-size has not been considered in the past in the discussion of high-pressure strength data.     

Spall Strength of Resistance Spot Weld for QP Steel

The spall tests under the plane tensile pulses for resistance spot weld(RSW) of QP980 steel are performed by using a gun system.The velocity histories of free surfaces of the RSW are measured with the laser velocity interferometer system for any reflector.The recovered specimens are investigated with an Olympus GX71 metallographic microscope and a scanning electron microscope(SEM).The measured velocity histories are explained and used to evaluate the tension stresses in the RSW applying the characteristic theory and the assumption of Gathers.The spall strength(1977-2784 MPa) of the RSW for QP980 steel is determined based on the measured and simulated velocity histories.The spall mechanism of the RSW is brittle fracture in view of the SEM investigation of the recovered specimen.The micrographs of the as-received QP980 steel,the initial and recovered RSW of this steel for the spall test are compared to reveal the microstructure evolution during the welding and spall process.It is indicated that during the welding thermal cycle,the local martensitic phase transformation is dependent on the location within the fusion zone and the heat affected zone.It is presented that the transformation at high strain rate may be cancelled by other phenomenon while the evolution of weld defects is obvious during the spall process.It may be the stress triaxiality and strain rate effect of the RSW strength or the dynamic load-carrying capacity of the RSW structure that the spall strength of the RSW for QP980 steel is much higher than the uniaxial compression yield strength(1200 MPa) of the martensite phase in QP980 steel.Due to the weld defects in the center of the RSW,the spall strength of the RSW should be less than the conventional spall strength or the dynamic load-carrying capacity of condensed structure.     

Strength and equation of state of molybdenum triboride under 80 GPa pressure,using radial X-ray diffraction

The strength and equation of state of molybdenum triboride have been determined under nonhydrostatic compression up to 80?GPa, using an angle-dispersive radial X-ray diffraction technique in a diamond anvil cell (DAC). The RXD data yield a bulk modulus and its pressure derivative as K_0?=?342(6)?GPa with K₀′?=?2.11(17) at ψ?=?54.7°. Analysis of diffraction data using the strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.002 to 0.050 at pressures of 4–80?GPa. Together with theoretical results on the high pressure shear modulus, our results here show that molybdenum triboride sample under uniaxial compression can support a differential stress of ～10?GPa when it started to yield with plastic deformation at ～30?GPa. In addition, we draw a conclusion that MoB₃ is not a superhard material but a hard material.     

Al–Pd–Mn icosahedral quasicrystal: deformation mechanisms in the brittle domain

The extreme brittleness of Al–Pd–Mn quasi-crystalline alloys over a wide range of temperatures drastically restricts investigation of their plastic deformation mechanisms over a small high-temperature regime. Recently, plastic deformation of Al–Pd–Mn quasicrystal has been achieved in the brittle domain (20?≤?T?≤?690°C) using specific deformation devices, which combined a uniaxial compression deformation or a shear deformation with a hydrostatic pressure confinement (0.35–5?GPa). Results of these experimental techniques, which provide various deformation conditions giving rise to a range of Al–Pd–Mn plastic features in the brittle domain, are discussed. On this basis, we propose that low and intermediate temperature plastic properties of Al–Pd–Mn are controlled by non-planar dislocation core extensions specific to the non-periodic structure.     

Molecular dynamic simulations of elastomer structure and its influence on anisotropic stress under time-varying strain

We investigate the evolution of polymer structure and its influence on uniaxial anisotropic stress under time-varying uniaxial strain, and the role of external control variables such as temperature, strain rate, chain length, and density, using molecular dynamics simulation. At temperatures higher than glass transition, stress anisotropy in the system is reduced even though the bond stretch is greater at higher temperatures. There is a significant increase in the stress level with increasing density. At higher densities, the uncoiling of the chains is suppressed and the major contribution to the deformation is by internal deformation of the chains. At faster rates of loading stress anisotropy increases. The deformation mechanism is mostly due to bond stretch and bond bending rather than overall shape and size. Stress levels increase with longer chain length. There is a critical value of the functionality of the cross-linkers beyond which the uniaxial stress developed increases caused primarily by bond stretching due to increased constraint on the motion of the monomers. Stacking of the chains in the system also plays a dominant role in the behaviour in terms of excluded volume interactions. Low density, high temperature, low values of functionality of cross-linkers, and short chain length facilitate chain uncoiling and chain slipping in cross-linked polymers.     

不同应变率下两种岩石的压缩破碎特征试验研究

为了研究不同岩石在不同应变率下压缩时裂纹的产生规律及破坏模式,将石灰岩和红砂岩制成试件,研究其在不同应变率和受力模式下裂纹的形成模式。开展了两种岩石的准静态压缩和动态压缩试验,采用高速摄影机记录了裂纹的产生和破坏模式。对两种岩石试件的裂纹形态进行对比,基于岩石的物理性质、受力状态、能量演化分析,得到了在不同应变率下压缩时产生差异性的原因。结果表明:准静态压缩下岩石试件受压的破坏模式也会因应变率的不同而存在差异,并且破坏模式的差异对岩石试件的抗压强度将产生显著的影响;从能量演化的角度分析,入射能量的大小将会决定岩石试样动态抗压强度曲线是否出现起伏;动态压缩时,裂纹的周向扩展速度与岩石抗压强度呈正相关。     

Experimental analysis of the UHPFRCs behavior under tension at high stress rate

The results of experimental investigation on the tensile response of UHPFRCs under high stress rates are presented. The dynamic tests are carried out on notched cylindrical specimens using a Split Hopkinson Tensile Bar. The material behavior of UHPFRC and its matrix (UHPC) is investigated at four high stress rates (from 400 to 1000?GPa/s). The rate sensitivity of the UHPFRC and UHPC in tension is evaluated in terms of Dynamic Increase Factor (DIF) of their peak strength. Finally, two different coring orientations on UHPFRC having 3, 4 and 5% fiber content are considered.     

Atomistic simulations to characterize the influence of applied strain and PKA energy on radiation damage evolution in pure aluminum

Knowledge of defects generation, their mobility, growth rate, and spatial distribution is the cornerstone for understanding the surface and structural evolution of a material used under irradiation conditions. In this study, molecular dynamics simulations were used to investigate the coupled effect of primary knock-on atom (PKA) energy and applied strain (uniaxial and hydrostatic) fields on primary radiation damage evolution in pure aluminum. Cascade damage simulations were carried out for PKA energy ranging between 1 and 20 keV and for applied strain values ranging between ?2% and 2% at the fixed temperature of 300 K. Simulation results showed that as the atomic displacement cascade proceeds under uniaxial and hydrostatic strains, the peak and surviving number of Frenkel point defects increases with increasing tension; however, these increments were more prominent under larger volume changing deformations (hydrostatic strain). The percentage fraction of point defects that aggregate into clusters increases under tension conditions; compared to the reference conditions with no strain, these increases are around 13% and 7% for interstitials and vacancies, respectively (under 2% uniaxial strain), and 19% and 11% for interstitials and vacancies, respectively (under 2% hydrostatic strain). Clusters formed of vacancies and interstitials were both larger under tensile strain conditions, with increases in both the average and maximum cluster sizes. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their size distributions under expansion/compression strain conditions were higher for higher PKA energies. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Elastic and plastic deformation of NaCl and Ni3Al polycrystals during compression in a multi-anvil apparatus

Abstract

The compression behaviour in a multi-anvil apparatus of pure NaCl and of a foil of Ni₃Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction. At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. A saturation of microstrains observed in NaCl at 2 GPa is thus attributed to brittle fracture setting in at uniaxial stresses of around 0.3 GPa. Ni₃Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa. The compression behaviour of both Ni₃Al and NaCl is identical to that previously determined in a diamond anvil cell. While a multi-anvil device thus has the advantage, compared with a diamond anvil cell, of constant pressure and stress throughout the sample chamber, microstrains in poly-crystalline samples arise in both devices. Samples in a multi-anvil apparatus thus need to be mixed with a pressure medium and to consist of essentially single crystals just as in a diamond anvil cell. Annealing experiments at high pressures confirm that the release of the uniaxial stress component in the pressure medium does not cause a release of microstrains in the embedded sample if the latter has been plastically deformed. Annealing for the purpose of attaining hydrostatic conditions in compression studies thus has to be carried out with care.     

金属玻璃温度依赖的拉压屈服不对称研究

通过引入静水应力对自由体积演化的影响, 研究了金属玻璃在不同温度下的拉压屈服行为. 结果表明, 在拉伸和压缩载荷下, 屈服强度均满足(T/T_g)^1/2的温度依赖关系; 同时, 在不同温度下, 材料的压力敏感系数保持为常值0.1. 随着温度的升高, 压力对自由体积的影响逐渐降低, 从而导致材料的拉压屈服不对称性逐渐趋于不显著. 在高温下, 显著的结构弛豫减缓了自由体积增长速率从而抑制材料迅速屈服. 这些结果将有助于更深入的认识金属玻璃屈服及其拉压不对称性的内在机理.     

Effect of temperature on inhomogeneous elastic deformation and negative stiffness of NiAl and FeAl alloy nanofilms

The uniaxial tension of NiAl and FeAl intermetallic alloy nanofilms at different temperatures has been investigated by the molecular dynamics method. It was previously shown that nanofilms at 0 K are elastically deformed by almost 40% and that, under strain-controlled tension, there is a region in the stress—strain curves, where an increase in the strain is accompanied by a decrease in the tensile stress, i.e., the stiffness of nanofilms is negative. Deformation of the films in the thermal instability region is associated with the appearance of domains with different elastic strains. The influence of the temperature on these effects is investigated. Particularly, it is shown that as the temperature increases, both the elastic strain and the negative stiffness of nanofilms decrease. The inhomogeneous elastic strain and negative stiffness for FeAl films are observed in a broader temperature range (to 1000 K) than for NiAl films (to 300 K), which constitutes 0.16 and 0.65 of the melting point of these materials, respectively.     

Investigating mechanical properties of polymethylmethacrylate/silver nanoparticle composites by molecular dynamics simulation

The molecular dynamics simulation (MD) was carried out to investigate the mechanical properties of pristine polymethylmethacrylate (PMMA) and the composites of PMMA mixed with the silver nanoparticles (PMMA/AgNPs) at two AgNP weight fractions at 0.60 and 1.77 wt%. From the stress–strain profiles by the tensile process, it can be seen that the improvement on Young’s modulus is insignificant at these lower AgNP fractions. The tensile strength of pristine PMMA can be slightly improved by the embedded AgNPs at 1.77 wt%, because the local density and strength of PMMA in the vicinity of AgNP surface within about 8.2 Å are improved. For the temperature effect on the mechanical properties of pristine PMMA and PMMA/AgNP composite, the Young’s moduli and strength of pristine PMMA and PMMA/AgNP composite significantly decrease at temperatures of 450 and 550 K, which are close to the predicted melting temperature of pristine PMMA about 460 K. At these temperatures, the PMMA materials become more ductile and the AgNPs within the PMMA matrix display higher mobility than those at 300 K. When the tensile strain increases, the AgNPs tend to get closer and the fracture appears at the PMMA part, leading to the close values of Young’s modulus and ultimate strength for pristine PMMA and PMMA/AgNP composite at 450 and 550 K.

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Graphical abstract Stress–strain curves of pristine PMMA, polymethylmethacrylate (PMMA)/silver nanoparticles (AgNP) (0.60%), and PMMA/AgNP (1.77%). Inset images: local shear strain of pristine PMMA (red) and PMMA/AgNP (1.77%) (green).