通孔泡沫铝的动态压缩行为

在SHPB装置上对渗流法制备的通孔泡沫铝进行了动态压缩实验,研究了相对密度为0.341~0.419的通孔泡沫铝在10-3~2000 s-1应变率范围内的压缩响应特征和应变率相关性,并用扫描电镜（scanning electron microscope,SEM)分析了泡沫铝的压缩变形特征。实验结果表明,通孔泡沫铝有明显应变率效应,随应变率上升,泡沫铝流动应力提高。SEM观察结果揭示,在动态压缩下,通孔泡沫铝宏观上均匀变形,微观变形机制以泡孔横向伸展坍塌为主。     

Uniaxial compression tests at various loading rates for reactive powder concrete

Concrete is a material that is sensitive to the rate of loading. Understanding the dynamic behavior of concrete under various circumstances is an issue of great significance for applications in civilian and military engineering. Hence, an experimental investigation on the dynamic mechanical properties of the reactive powder concrete (RPC) was conducted using the split-Hopkinson pressure bar (SHPB). The specimens were made with different steel fibre volume fractions and the strain rate ranged from 10¹ s⁻¹ to 10³ s⁻¹. The results show the obvious rate-dependent mechanical behavior exists for RPC. Moreover, the different of the characteristic of energy absorbed are compared.     

The compressive failure of aluminum nitride considered as a model advanced ceramic

Uniaxial quasi-static compression, uniaxial dynamic compression and confined dynamic compression experiments were performed to characterize the failure of Aluminum Nitride (AlN) using a servo hydraulic machine and a modified Kolsky bar set-up respectively. High-speed digital cameras are used to visualize the failure processes. A summary of the available experimental results, including that in the literature, shows that the compressive strength of the AlN is sensitive to strain rate in the range from 10⁻³ to 10³ s⁻¹, and that the deviatoric strength of AlN is linearly dependent on pressure at low pressures and nearly independent of pressure above a transitional pressure (about 2 GPa). TEM characterization of fragments obtained after dynamic loading is used to characterize the deformation mechanisms in the AlN for varying confinement. The transition in the pressure dependent behavior is shown to be the result of a change of deformation mechanism. Classical wing crack micromechanics is used to predict the transition in the deformation mechanism, and to explain the observed behavior at low pressure.     

Mechanics of the rate-dependent elastic–plastic deformation of glassy polymers from low to high strain rates

A combined experimental and analytical investigation has been performed to understand the mechanical behavior of two amorphous polymers—polycarbonate and poly(methyl methacrylate)—at strain rates ranging from 10⁻⁴ to 10⁴ s⁻¹. This range in strain rates was achieved in uniaxial tension and compression tests using a dynamic mechanical analyzer (DMA), a servo-hydraulic testing machine, and an aluminum split-Hopkinson pressure bar. DMA tension tests were used to characterize the viscoelastic behavior of these materials, with focus on the rate-dependent shift of material transition temperatures. Uniaxial compression tests on the servo-hydraulic machine (10⁻⁴ to 1 s⁻¹) and the split-Hopkinson pressure bar (10³ to 10⁴ s⁻¹) were used to characterize the rate-dependent yield and post-yield behavior. Both materials were observed to exhibit increased rate sensitivity of yield under the same strain rate/temperature conditions as the β-transition of the viscoelastic behavior. A physically based constitutive model for large strain deformation of thermoplastics was then extended to encompass high-rate conditions. The model accounts for the contributions of different molecular motions which become operational and important in different frequency regimes. The new features enable the model to not only capture the transition in the yield behavior, but also accurately predict the post-yield, large strain behavior over a wide range of temperatures and strain rates.     

An appraisal of rheological models as applied to polymer melt flow

Summary An attempt is made at giving an appraisal of some representative rheological models of both differential and integral type, using the standard rheological measurements of six polymer melts. Experimental data obtained were the steady shear viscosity and the first normal stress difference by means of aWeissenberg rheogoniometer over the range of shear rates: 10^–2 ~ 10 sec^–1, and by means of aHan slit/capillary rheometer over the range of shear rates: 10 ~ 10³ sec^–1. Also measured by means of theWeissenberg rheogoniometer were the dynamic viscosity and dynamic elastic modulus over the range of frequencies: 0.3 × 10^–2 ~ 3 × 10² sec^–1. Rheological models chosen for an appraisal are theSpriggs 4-constant model, theMeister model, and theBogue model.It is found that the capability of the three models considered is about the same in their prediction of the rheological behavior of polymer melts in simple shearing flow. It is pointed out however that, due to the ensuing mathematical complexities, the usefulness of these models is limited to the study of flow problems associated with simple flow situations. Therefore, in analysing the complex flow situations often encountered with various polymer processings, the authors suggest use of the empirical models of the power-law type for both the viscosity and normal stress functions.With 11 figures, 4 schemas and 1 table     

Effects of strain rate and moisture content on the behaviour of sand under one-dimensional compression

The influence of strain rate and moisture content on the behaviour of a quartz sand was assessed using high-pressure quasi-static (10^?3 s^?1) and high-strain rate (10³ s^?1) experiments under uniaxial strain. Quasi-static compression to axial stresses of 800 MPa was carried out alongside split Hopkinson pressure bar (SHPB) experiments to 400 MPa, where in each case lateral deformation of the specimen was prevented using a steel test box or ring, and lateral stresses were recorded. A significant increase in constrained modulus was observed between strain rates of 10^?3s^?1 and 10³s^?1, however a consistently lower Poisson’s ratio in the dynamic tests minimised changes in bulk modulus. The reduction in Poissons ratio suggests that the stiffening of the sand in the SHPB tests is due to additional inertial confinement rather than an inherent strain-rate dependence. In the quasi-static tests the specimens behaved less stiffly with increasing moisture content, while in the dynamic tests the addition of water had little effect on the overall stiffness, causing the quasi-static and dynamic series to diverge with increasing moisture content.     

Atomic physics of relativistic high contrast laser-produced plasmas in experiments on Leopard laser facility at UNR

The results of the recent experiments focused on study of x-ray radiation from multicharged plasmas irradiated by relativistic (I > 10¹⁹ W/cm²) sub-ps laser pulses on Leopard laser facility at NTF/UNR are presented. These shots were done under different experimental conditions related to laser pulse and contrast. In particular, the duration of the laser pulse was 350 fs or 0.8 ns and the contrast was varied from high (10^?7) to moderate (10^?5). The thin laser targets (from 4 to 750 μm) made of a broad range of materials (from Teflon to iron and molybden to tungsten and gold) were utilized. Using the x-ray diagnostics including the high-precision spectrometer with resolution R ～ 3000 and a survey spectrometer, we have observed unique spectral features that are illustrated in this paper. Specifically, the observed L-shell spectra for Fe targets subject to high intensity lasers (～10¹⁹ W/cm²) indicate electron beams, while at lower intensities (～10¹⁶ W/cm²) or for Cu targets there is much less evidence for an electron beam. In addition, K-shell Mg features with dielectronic satellites from high-Rydberg states, and the new K-shell F features with dielectronic satellites including exotic transitions from hollow ions are highlighted.     

Shear thickening and shear-induced agglomeration of chemical mechanical polishing slurries using electrolytes

Chemical mechanical polishing is a fundamental technology used in the semiconductor manufacturing industry to polish and planarize a wide range of materials for the fabrication of microelectronic devices. During the high-shear (～1,000,000 s^?1) polishing process, it is hypothesized that individual slurry particles are driven together to form large agglomerates (≥0.5 µm). These agglomerates are believed to trigger a shear-induced thickening effect and cause defects during polishing. We examined how the addition of various monovalent salts (CsCl, KCl, LiCl, and NaCl) and electrostatic stabilizing bases (KOH, NaOH, or CsOH) influenced the slurry’s thickening behavior. Overall, as the added salt concentration was increased from 0.02 to 0.15 M, the shear rate at which the slurry thickened (i.e., the critical shear rate) decreased. Slurries with added CsCl, NaCl, and LiCl thickened at comparable shear rates (～20,000–70,000 s^?1) and, in general, followed ion hydration theory (poorly hydrated ions caused the slurry to thicken at lower shear rates). However, slurries with added KCl portrayed thickening behavior at higher critical shear rates (～35,000–100,000 s^?1) than other chloride salts. Also, slurries stabilized with CsOH thickened at higher shear rates (～90,000–140,000 s^?1), regardless of the added salt cation or concentration, than the slurries with KOH or NaOH. The NaOH-stabilized slurries displayed thickening at the lowest shear rates (～20,000 s^?1). The thickening dependence on slurry base cation indicates the existence of additional close-range structure forces that are not predicted by the Derjaguin–Landau–Verwey–Overbeek colloidal stability theory.     

A piezoelectric stress gauge for measuring dynamic stress in soil and rock media

A piezoelectric stress gauge is described in this paper. Its major performance data are: measuring range 10⁶–10⁸ Pa, response time less than 7μs; non-linearity within ±1% and total stress measurement error within ± 10%. It can be used for measuring dynamic stress in soil, rock and concrete media as well as dynamic force and dynamic pressure in fluids.     

PP/PA共混高聚物在高应变率下的热粘弹性本构关系和时温等效性

采用Instron1342电液伺服试验机和改进的SHPB技术对以113材料为例的PP/PA共混高聚物进行了应变率在10-4~103 s-1宽广范围,温度为25、40、60、80 ℃下的一维应力力学性能试验。结果表明,这类共混高聚物的力学响应对温度和应变率都是敏感的。以朱-王-唐非线性粘弹性本构方程来描述这类PP/PA共混高聚物的力学响应,并拟合得到了其热粘弹性本构参数,理论预言与试验结果在应变小于7%时吻合良好。对113材料20~80 ℃温度范围内不同应变率下的试验结果进行分析,证明PP/PA共混高聚物存在率温等效关系,提高温度和增加作用时间（减小应变率）的效果相当,反之,降低温度与减少作用时间（提高应变率）的效果相当。通过引入量纲一参量Z,使应变率d/dt、温度T这2个参量归结为统一的Z参量,从而得到了体现时温等效性的统一曲线。     

Hopkinson拉杆的入射波波形整形实验研究

目前,分离式Hopkinson杆实验技术已经被广泛用于测试材料在10~2~10~4s~(-1)应变率范围内的动态力学特性。为了抑制入射波的高频振荡,实现恒定应变率加载,本文利用分离式Hopkinson拉杆(SHTB)实验装置,研究了加载金属短杆(2A12T4铝合金)及整形垫片(纸板、PVC软塑料及带磁性胶皮)对入射波波形的影响。实验结果表明,整形垫片降低了入射应力脉冲的高频振荡,获得了比较平滑的入射应力脉冲,延长了上升时间。同时,利用所得的波形整形结果,对2A12T4铝合金进行了拉伸应力波脉冲加载的拉伸和断裂实验测试。     

Analysis of a pressure averaging device for measuring aerodynamic forces on a circular cylinder

A pneumatic spatial pressure averaging technique for measuring pressure forces has been analysed and calibrated for measuring steady and fluctuating forces on a circular cylinder. The technique is based on using two manifolds, each with one pressure output and eight inputs that feed into a small averaging chamber. This method has several advantages over using an array of pressure transducers due to its relative simplicity and cheapness. The pressure averager's dynamic characteristics and its accuracy in averaging pressure are investigated. It is shown to give reliable results for a circular cylinder over a range of Reynolds numbers from 1×10⁴ to 1.5×10⁵.     

A Combined Theoretical/Experimental Approach for Reducing Ringing Artifacts in Low Dynamic Testing with Servo-hydraulic Load Frames

One of the most challenging tasks facing computer-aided engineering (CAE) analysis is the acquisition of accurate tensile test data that spans quasi-static to low dynamic (10^?5/s?≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s) strain rates ( $ \overset{.}{\varepsilon } $ ). Critical to the accuracy of data acquired over the low dynamic range is the reduction of ringing artifacts in flow data. Ringing artifacts, which are a consequence of the inertial response of the load frame, are spurious oscillations that can obscure the desired material response (i.e. load vs. time or load vs. displacement) from which flow data are derived. These oscillations tend to grow with increasing strain rate and peak at the high end of the low dynamic range on servo-hydraulic tensile test frames. Common practices for addressing ringing are data filtering, which is often problematic since filtering introduces distortion in smoothed material data, or trial-and-error design of test specimen geometries. This renders techniques for reducing ringing based upon the mechanics of the load frame and optimization of tensile specimen geometry quite attractive. In the present paper, relationships between load, stress wave propagation, and specimen geometries are addressed, to both quantify ringing and to develop specimen designs that will reduce ringing. A combined theoretical/experimental approach for tensile specimen design was developed for reducing ringing in flow data over the low dynamic range of strain rates (10^?5/s≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s). The single camera digital image correlation (DIC) method was used to measure the displacement fields and strain rates with specimens resulting from the combined theoretical/experimental approach. While the approach was developed on a specific commercial load frame with a TRIP steel subject to a two-step quenching and partitioning heat treatment (Q&P980), it is readily adaptable to other servo-hydraulic load frames and metallic alloys. The developed approach results in a 90 % reduction in ringing artifact (with no filtering) in a tensile flow curve for Q&P980 at $ \overset{.}{\varepsilon}\kern-4pt $ = 5?×?10²/s. Results from split Hopkinson bar tests of Q&P980 were performed at $ \overset{.}{\varepsilon } $ = 500/s and compare favorably with the test data generated by the developed testing approach. Since the Q&P980 steel represents a new generation of advanced high strength steels, we also evaluated its strain rate sensitivity over the low dynamic range.     

2D-C/SiC复合材料应变率相关的动态本构模型

分别在电子万能实验机和SHPB（split Hopkinson press bar）实验装置上对2D-C/SiC复合材料进行了静态、动态实验,探讨了该材料在10-4~2.8103 s-1的应变率范围内的层向压缩力学性能。实验结果表明,在动态加载条件下,2D-C/SiC复合材料的应力应变呈非线性关系。随着应变率的提高,破坏强度提高、失效应变减小,弹性模量增加。弹性模量与对数应变率基本呈线性关系。提出了一个含有与应变率相关的损伤变量的动态本构方程,该方程与实验结果吻合较好。     

Mechanically alloyed nanocrystalline iron and copper mixture: behavior and constitutive modeling over a wide range of strain rates

A comprehensive study on the response of a nanocrystalline iron and copper mixture (80% Fe and 20% Cu) to quasi-static and dynamic loading is performed. The constitutive model developed earlier by Khan, Huang & Liang (KHL) is extended to include the responses of nanocrystalline metallic materials. The strain rate and grain size dependent behaviors of porous nanocrystalline iron-copper mixture were determined experimentally for both static and dynamic loading. A viscoplastic model is formulated by associating the modified KHL model (representing the fully dense matrix behavior), and Gurson's plastic potential which provides the yield criteria for porous material. Simulations of uniaxial compressive deformations of iron-copper mixture with different initial porosity, grain size and at a wide range of strain rate (10⁻⁴ to 10³ s⁻¹) are made. The numerical results correlate well with the experimental observations.     

An experimental platform for creating white dwarf photospheres in the laboratory

We present an experimental platform for measuring hydrogen Balmer emission and absorption line profiles for plasmas with white dwarf (WD) photospheric conditions (T_e ～1 eV, n_e ～10¹⁷ cm^?3). These profiles will be used to benchmark WD atmosphere models, which, used with the spectroscopic method, are responsible for determining fundamental parameters (e.g., effective temperature, mass) for tens of thousands of WDs. Our experiment, performed at the Z Pulsed Power Facility at Sandia National Laboratories, uses the large amount of X-rays generated from a z-pinch dynamic hohlraum to drive plasma formation in a gas cell. The platform is unique compared to past hydrogen line profile experiments in that the plasma is radiation-driven. This decouples the heating source from the plasma to be studied in the sense that the radiation temperature causing the photoionization is independent of the initial conditions of the gas. For the first time we measure hydrogen Balmer lines in absorption at these conditions in the laboratory for the purpose of benchmarking Stark-broadened line shapes. The platform can be used to study other plasma species and to explore non-LTE, time-dependent collisional-radiative atomic kinetics.     

Dynamic measurements on polymer solutions and melts

Summary An apparatus for the measurement of the dynamic viscosity is described. In a coaxial cylinder geometry forced vibrations are executed in the frequency region from 2·10^–4 to 50 cycles/sec. Fluids with a wide range of viscosities (10^–1 to 10⁷ poise) can be measured. A number of experimental results obtained on dilute solutions and melts of polymers are given and compared with results of the molecular theories.

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Zusammenfassung Es wird ein Gerät für die Messung der dynamischen Viskosität beschrieben. Innerhalb eines koaxialen Zylindersystems werden im Frequenzgebiet von 2 · 10^–4 bis 50 Hz erzwungene Schwingungen ausgeführt. Messungen an Flüssigkeiten sind in einem weiten Viskositätsbereich (10^–1 bis 10⁷ Poise) möglich. Einige Messungen an verdünnten Lösungen oder Schmelzen von Hochpolymeren werden mitgeteilt und mit Voraussagen molekularer Theorien verglichen.

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Paper read at the Annual Meeting of the German Rheologits, Berlin-Dahlem, May 20–21, 1968).     

Study of the aerodynamic stability of a blunt cone with a side flap in a hypersonic flow

The stationary and time-dependent aerodynamic coefficients of a slender blunt cone with a flap located near the base section of the model are experimentally investigated. The freestream parameters (M_∞ = 6, Re _L = 0.88 × 10⁷, and γ = 1.4) ensured a turbulent regime of flow over the conical surface and the flap. At high angles of attack (α ～ 10°) laminar-turbulent transition is observable in the separation zone on the leeward side of the body. Emphasis is placed on the determination of the trimming angles of attack for different positions of the center of rotation and the static and dynamic stability coefficients (the model oscillation damping coefficient).     

Finite element analysis of vortex shedding using equal order interpolations

An operator splitting and element‐by‐element conjugated gradient solver, and equal order interpolations are applied for solving time dependent Navier–Stokes (NS) equations to simulate flow induced vortex shedding in the present study. In addition, the convection term is corrected by balanced tensor diffusivity, which can stabilize the numerical simulation and overcome the numerical oscillations. The evolution of the interested flowing properties with time is analyzed by using spectral analysis. The developed code has been validated by the application of two examples: a driven cavity flow and a flow induced vortex vibration. Results from the first example for Reynolds number Re=10³ and Re=10⁴ are compared with other numerical simulations. Results from the second example, uniform flow past a square rod over a wide range of high Reynolds numbers from Re=10³～10⁵, are compared with experimental data and other numerical studies. Copyright © 2002 John Wiley & Sons, Ltd.