Quasi-Static and High Strain Rate Simple Shear Characterization of Soft Polymers

Experimental Mechanics - The simple shear response of soft polymers under large deformation (>50%) and strain rates spanning 10?3 – 103 s?1 is characterized by...     

Characterization of the High Temperature Strain Partitioning in Duplex Steels

A microgrid technique has been developed for the analysis of the high-temperature micro-scale strain distribution between ferrite and austenite into duplex stainless steels. The local strain is measured by micro-extensometry using square microgrids engraved on flat specimens by electro-lithography. The sample with microgrids on the surface and preliminary imaged with high definition scanning electron microscope (SEM), is inserted in a plane strain compression specimen to be deformed under conditions representative of hot rolling. After deformation, the sample is extracted from the compressed block and the surface is again analyzed by SEM and image processing to determine the strain field. The strain is heterogeneously distributed with a strong localization of the deformation, in the form of shear bands located within the ferrite and at the vicinity of the austenite/ferrite interphase boundaries. These strain maps provide useful informations about the rheology of the phases as well as about the local conditions at the origin of the damage process.     

Specimen and Grain Size Effects of Al1100 on Strain and Strain Rate Hardening at Various Strain Rates for Al1100

This paper is concerned with comparison of the tensile properties of Al1100 thin film in a micro-scale to that of Al1100 sheet in a macro-scale. The material properties of Al1100 film and sheet with a thickness of 96 μm and 1 mm respectively have been investigated at strain rates ranging from 0.001 to 100 s^?1. The experiments were conducted with Static Micro-Material Testing Machine (SMMTM) and High Speed Micro-Material Testing Machine (HSMMTM) for micro-specimens and with Instron 5583 and high speed material testing machine (HSMTM) for macro-specimens. A reliable jig system for SMMTM and HSMMTM has been newly developed for easy installation of a specimen and accurate alignment between a specimen and the jig system to enhance the reproducibility of tests. The digital image correlation (DIC) method is employed to measure the axial strain of the specimens. In order to obtain a fine speckle pattern for the DIC method, a novel technique is employed to print the speckle pattern with fine particles by blowing sprayed particles before printing. The grain sizes of two Al1100 specimens have been compared and the number of grains in the gauge cross-section has been calculated to obtain the grain number which is related to the specimen size effect. Electron Back Scattered Diffraction (EBSD) images were obtained for both micro-specimens and macro-specimens and analyzed to measure the grain size. The Al1100 film with a smaller average grain size shows larger strain hardening than the Al1100 sheet with a larger average grain size.     

Specimen Size and Strain Rate Effects on the Compressive Behavior of Concrete

Three high-performance concrete (HPC) materials with different specimen geometries were characterized using Kolsky compression bar techniques to study the strain rate and specimen size effects on their uniaxial compressive strength. A large-diameter Kolsky bar and recently established annular pulse shaping technique were used to achieve dynamic stress equilibrium and constant strain-rate deformation in the experiments. A complimentary effort was conducted using a 19-mm-diameter Kolsky compression bar to understand the strain rate and specimen size effects on failure strength and dynamic increase factor (DIF) for concrete. It was found that, for all three concrete materials investigated, the failure strength is highly dependent on the specimen geometry, however such a relationship is not apparent for the DIF. The DIF observed in this study shows significantly lower values compared to historical data, which may indicate the importance of well-controlled dynamic testing conditions on the accuracy and validity of experimental results for concrete materials.     

An Apparatus for Tensile Characterization of Materials within the Upper Intermediate Strain Rate Regime

Experimental Mechanics - A new “Drop-Hopkinson bar” apparatus has been successfully developed for material tensile property characterization at intermediate strain rates. This new...     

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。     

极端环境下连续碳纤维增韧的陶瓷基复合材料的力学行为

连续纤维增韧的碳化硅复合材料(以下简称C/SiC),作为超高速飞行器热结构使用时,有可能在高温环境下受到高速撞击的作用,因此,掌握其在极端环境(高温、高应变率)下的力学性能是进行结构安全设计的基础。本文采用具有高温实验能力的分离式Hopkinson杆,在293~1273K温度范围内进行了动态压缩力学性能测试,研究了环境温度和加载速率对材料力学性能的影响。结果表明:C/SiC复合材料的高温压缩力学性能主要受应力氧化损伤和残余应力的共同影响。实验温度低于873K时,应力氧化损伤的影响很小,而由于增强纤维和基体界面残余应力的释放使界面结合强度增大,复合材料的压缩强度随温度的升高而增大;当实验温度高于873K时,应力氧化损伤加剧,其对压缩强度的削弱超过残余应力释放对强度的贡献,材料的压缩强度随温度的升高逐渐降低。由于应力氧化损伤受应变率的影响很大,当温度由873K升高至1273K时,高应变率下压缩强度降低的程度要比应变率为0.0001/s时低得多。     

应变率及温度对FGH95粉末高温合金塑性变形性能影响的实验研究

在420℃～650℃的温度范围内,实验研究了FGH95粉末高温合金在应变率0．0001s^-1～0．01S^-1范围内的拉伸一断裂性能,分析了温度和应变率对该合金流动应力的影响,结果表明,应变率对杨氏模量、拉伸屈服强度和塑性模量的影响不是很大,随着应变速率的增大和温度的升高,合金的塑性流动应力有所提高,断裂强度和断裂韧性增强。并通过流动应力与应变、应变率和温度之间的函数关系,分别讨论了硬化指数咒、应变速率敏感系数m及应力相关系数K与温度ε和应变率;的函数关系。SEM断口分析表明FGH95合金是微缺陷敏感材料,在高温（420℃-650℃）应变率范围为10^-4s^-1～10^-1s^-1时的拉伸断裂都是韧性断裂。     

温度对高应变率下三元乙丙橡胶力学性能影响的实验研究

利用加装了自行研制的半导体高低温调控装置的特别加长的分离式Hopkinson杆,对三元乙丙橡胶在变温度(233K～323K)、变应变率(1.8×103/s～3×103/s)条件下进行力学性能实验.为贴近军事装备相关的自然环境要求,分别测定和拟合了三元乙丙橡胶在各个相同温度条件下不同应变率以及各个相同应变率条件下不同温度的应力-应变曲线.结果表明:三元乙丙橡胶的力学性能受温度和应变率影响明显,其刚度随着应变率的增加而增加,其柔性随着温度的增加而增强.温度和应变率对其力学性能的影响有一定的等效性.文中给出了三元乙丙橡胶材料高应变率条件下由橡胶态向玻璃态转变的温度值.     

Synchronous Full-Field Strain and Temperature Measurement in Tensile Tests at Low,Intermediate and High Strain Rates

Tensile tests with simultaneous full-field strain and temperature measurements at the nominal strain rates of 0.01, 0.1, 1, 200 and 3000 s^?1 are presented. Three different testing methods with specimens of the same thin and flat gage-section geometry are utilized. The full-field deformation is measured on one side of the specimen, using the DIC technique with low and high speed visible cameras, and the full-field temperature is measured on the opposite side using an IR camera. Austenitic stainless steel is used as the test material. The results show that a similar deformation pattern evolves at all strain rates with an initial uniform deformation up to the strain of 0.25–0.35, followed by necking with localized deformation with a maximum strain of 0.7–0.95. The strain rate in the necking regions can exceed three times the nominal strain rate. The duration of the tests vary from 57 s at the lowest strain rate to 197 μs at the highest strain rate. The results show temperature rise at all strain rates. The temperature rise increases with strain rate as the test duration shortens and there is less time for the heat to dissipate. At a strain rate of 0.01 s^?1 the temperature rise is small (up to 48 °C) but noticeable. At a strain rate of 0.1 the temperature rises up to 140 °C and at a strain rate of 1 s^?1 up to 260 °C. The temperature increase in the tests at strain rates of 200 s^?1 and 3000 s^?1 is nearly the same with the maximum temperature reaching 375 °C.     

Use of Spectral Conditions to Separate Strain and Temperature Effects in Fibre Bragg Grating Sensors Embedded in Load-Carrying Anisotropic Laminates

This work presents a further development of the methods of simultaneous determination of temperature and axial strain using a single fibre Bragg grating (FBG). The reflected spectrum of composite-embedded FBG sensors has been analyzed in order to separate temperature and load effects. We found out that during the curing process of the laminates, a superstructure has been introduced on the FBG. Analyzing this effect, a temperature and strain separation was implemented by simply calculating and comparing the integrated intensity of the spectral response signal. A mathematical four-parametric model has been developed to calculate the reflected spectrum of a superstructured fibre Bragg grating. The mathematically achieved data has been evaluated with experimentally determined data from fibre Bragg gratings embedded in glass-fibre reinforced composite materials.     

A Study of Strain Rate Effects for Turbulent Premixed Flames with Application to LES of a Gas Turbine Combustor Model

Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low-NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the linear-eddy model with a one-dimensional counterflow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable, the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results obtained demonstrate that the effects of the resolved strain rate are not dominant for the considered premixed flame configuration and the unstrained turbulent flame speed model is found to perform as well as the one that allows for the strain rate effects.     

Effect of Particle Morphology,Compaction, and Confinement on the High Strain Rate Behavior of Sand

The effect of grain shape, size distribution, intergranular friction, confinement, and initial compaction state on the high strain rate compressive mechanical response of sand is quantified using Long Split Hopkinson Pressure Bar (LSHPB) experiments, generating up to 1.1 ms long load pulses. This allowed the dynamic characterisation of different types of sand until full compaction (lowest initial void ratio) at different strain rates. The effect of the grain morphology and size on the dynamic compressive mechanical response of sand is assessed by conducting experiments on three types of sand: Ottawa Sand with quasi-spherical grains, Euroquartz Siligran with subangular grains and Q-Rok with polyhedral grain shape are considered in this study. The adoption of rigid (Ti64) and deformable (Latex) sand containers allowed for quasi-uniaxial strain and quasi-uniaxial stress conditions to be achieved respectively. Additionally, the effect of intergranular friction was studied, for the first time in literature, by employing polymer coated Euroquartz sand. Appropriate procedures for the preparation of samples at different representative initial consolidation states are utilized to achieve realistic range of naturally occurring formations of granular assembly from loose to dense state. The results identify material and confining sample state parameters which have significant effect on the mechanical response of sand at high strain rates and their interdependency for future integration into rate dependent constitutive models.     

Application of a Special Yield Stress Strain Rate Law to Structural Impulse Problems

ABSTRACT

This paper presents approximate solutions to the dynamic response of three impulsively loaded structures: a wire with an impulsively loaded end mass, an impulsively loaded circular ring, and a cantilever beam with a tip mass subjected to an impulsive load at its tip. The material is assumed to be rigid, perfectly plastic with strain rate sensitivity. A proposed power law form of yield stress strain rate relationship is used to simplify the theoretical development. Numerical solutions are presented for mild steel and are compared with previously published results. Elastic effects and wave propogations are ignored.     

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.     

高温流体与变物性固体耦合系统温度场及热变形的数值计算

在腔体内部有高温流体,其外壳等固体材料的物性温度而为化的瞬态非线性问题中,其边界条件的确定十分困难。本文采用流一固“混合流”模式进行温度场的计算,由于方程计及了瞬态、复杂形状、浮动外边界条件等因系,采用变步长的控制容积法及时域有限元进行计算。对某个高温燃气配气阀的温度场及热变形进行了数值计算,得到与实测相符的结果。     

Some Misunderstandings on Rotation of Crystals and Reasonable Plastic Strain Rate

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Full-Field Strain Measurement and Identification of Composites Moduli at High Strain Rate with the Virtual Fields Method

The present paper deals with full-field strain measurement on glass/epoxy composite tensile specimens submitted to high strain rate loading through a split Hopkinson pressure bar (SHPB) device and with the identification of their mechanical properties. First, the adopted methodology is presented: the device, including an Ultra-High Speed camera, and the experimental procedure to obtain relevant displacement maps are described. The different full-field results including displacement, strain and acceleration maps for two mechanical tests are then addressed. The last part of the paper deals with an original procedure to identify stiffnesses on this dynamic case only using the actual strain and acceleration maps (without the applied force) by using the Virtual Fields Method. The results provide very promising values of Young’s modulus and Poisson’s ratio on a quasi-isotropic glass-epoxy laminate. The load reconstructed from the moduli and strains compares favourably with that from the readings.     

Single and Dual-Domain Models to Evaluate the Effects of Preferential Flow Paths in Alluvial Sediments

Typical features of preferential flow paths were evidenced by numerical tests of convective transport of conservative solutes performed in three blocks of alluvial sediments at the scale of depositional elements. The numerical experiments are analysed with standard single-domain models (SDMs) and with dual-domain models (DDMs): the model parameters are identified by minimisation of the misfit between the ??experimental?? and the modelled cumulative breakthrough curves (BTCs) and between the ??experimental?? and the modelled temporal moments of the BTCs. The results for the SDMs show different behaviours for the three model blocks and for the different flow directions, in good agreement with their hydrostratigraphic characteristics. The results for the DDMs sometimes correspond to cases for which one of the two domains is dominant and its values of diffusivity and average velocity are close to those obtained for the SDM; in some cases the DDM performs much better than the SDM and correctly represents the effects of preferential flow paths. Finally the relevance of the DDM is analysed in the framework of multi-objective optimisation: a proper choice of the objective-functions yields Pareto sets whose geometries are different for single- and dual-domain media.