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.     

A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis

Tensile tests were conducted on dual-phase high-strength steel in a Split-Hopkinson Tension Bar at a strain-rate in the range of 150–600/s and in a servo-hydraulic testing machine at a strain-rate between 10^?3 and 10⁰/s. A novel specimen design was utilized for the Hopkinson bar tests of this sheet material. Digital image correlation was used together with high-speed photography to study strain localisation in the tensile specimens at high rates of strain. By using digital image correlation, it is possible to obtain in-plane displacement and strain fields during non-uniform deformation of the gauge section, and accordingly the strains associated with diffuse and localised necking may be determined. The full-field measurements in high strain-rate tests reveal that strain localisation started even before the maximum load was attained in the specimen. An elasto-viscoplastic constitutive model is used to predict the observed stress–strain behaviour and strain localisation for the dual-phase steel. Numerical simulations of dynamic tensile tests were performed using the non-linear explicit FE code LS-DYNA. Simulations were done with shell (plane stress) and brick elements. Good correlation between experiments and numerical predictions was achieved, in terms of engineering stress–strain behaviour, deformed geometry and strain fields. However, mesh density plays a role in the localisation of deformation in numerical simulations, particularly for the shell element analysis.     

Theory of necking localization in unconstrained electromagnetic expansion of thin sheets

Certain sheet metal alloys of industrial interest show a significant increase in ductility, over conventional forming methods, when high speed electromagnetic processes are used. The present work models the necking localization of a metal sheet during an electromagnetic process and examines the factors that influence this process. A Marciniak–Kuczynski “weak band” model is used to predict the onset of necking of a thin sheet under plane stress, an idealization of the local conditions in a thin sheet subjected to unconstrained electromagnetic loading. It is found that electromagnetic forming (EMF) increases ductility over quasistatic techniques due to the material’s strain-rate sensitivity, with ductility increasing monotonically with applied strain rates. The electric current also increases onset of necking strains, but the details depend on thermal sensitivity and temperature-dependence of the strain-rate sensitivity exponent. Given the insensitivity of the results to actual strain profiles, this local type analysis provides a useful tool that can be used for ductility predictions involving EMF processes.     

Development of testing systems for dynamic assessment of sheet metal specimens

The work presented herein regards the analysis of an experimental technique for the execution of dynamic tensile tests on structural material sheet specimens. Dynamic tensile testing of sheet is becoming more important due to the need for more optimized vehicle crashworthiness analysis in the automotive industry. Positive strain-rate sensitivity, i.e. the strength increases with strain-rate, offers a potential for improved energy absorption during a crash event. Tests have been carried out in the Reliability and Safety Laboratory of the 2^nd Engineering Faculty of the Politecnico di Torino. Different types of testing techniques have been used to generate data under dynamic conditions. However, no guidelines are available for the testing method, specimen dimensions, measurement devices, and other important issues which are critical for the quality of the results. Accurate signal processing and curve smoothing are often necessary to make the testing data usable.     

Dynamic tensile characterization of pig skin

The strain-rate dependent response of porcine skin oriented in the fiber direction is explored under tensile loading. Quasi-static response was obtained at strain rates in the range of 10-3s-1to 25 s-1. Characterization of the response at even greater strain rates is accomplished by measuring the spatio-temporal evolution of the particle velocity and strain in a thin strip subjected to high speed impact loading that generates uniaxial stress conditions. These experiments indicate the formation of shock waves; the shock Hugoniot that relates particle velocity to the shock velocity and the dynamic stress to dynamic strain is obtained directly through experimental measurements, without any assumptions regarding the constitutive properties of the material.     

A New Methodology for Uniaxial Tensile Testing of Free-Standing Thin Films at High Strain-Rates

The investigation of strain-rate effects on the mechanical response and characteristics of thin free-standing films is crucial for the design and fabrication of more reliable MEMS devices. It is also of high interest from the scientific and materials engineering views. In this paper we present a novel apparatus and a procedure for tensile testing of thin free-standing films under a wide range of strain rates from quasistatic to high, almost comparable with those obtained in Hopkinson bar tests. To provide this capability, a unique displacement measurement method is applied and a micro device which meets several strict requirements is implemented. We describe recent results of quasistatic experiments performed on pure aluminum free-standing thin films. A high rate experiment which demonstrates the setup capabilities is also presented. The micro-device measured properties are compared with finite element analysis results.     

Optimizing material strength constants numerically extracted from taylor impact data

Advanced design requirements have dictated a need for the mechanical properties of materials at high strain rates. Mechanical testing for these data poses a significant problem for experimentalists. High-speed testing machines have a limited capability at rates approaching 10²/s. The split Hopkinson pressure bar is the most reliable alternative for rates approaching 10⁴/s. Plate impact experiments are capable of generating strain rates of 10⁸/s and higher. The Taylor impact test occupies a place of particular importance by providing data at strain rates on the order of 10⁴/s–10⁵/s. The issue at present is extracting the data. This paper provides a method for obtaining dynamic strength model material constants from a single Taylor impact test. A polynomial response surface is used to describe the volume difference (error) between the deformed specimen from the Taylor test and the results of a computer simulation. The volume difference can be minimized using an optimizer, with the result being an optimum set of material constants. This method was applied to the modified Johnson-Cook model for OFHC copper. Starting from a nominal set of material constants, the iterative process improved the relative volume difference from 23.1 percent to 4.5 percent. Other starting points were used that yielded similar results. The material constants were validated by comparing numerical results with Taylor tests of cylinders having varying aspect ratios, calibers and impact velocities.     

实现材料高应变率拉伸加载的爆炸膨胀环技术

设计了新型的爆炸膨胀环实验加载装置,加载装置中采用爆炸丝线起爆方式,避免了传统装置中对碰爆轰波加载时的应力不均匀性。利用新型的爆炸膨胀环实验技术研究了无氧铜材料的动态性能,利用激光位移干涉仪测量了试样环的径向速度历史,处理数据获得了无氧铜材料的流动应力-塑性应变-应变率的关系,为进一步利用爆炸膨胀环实验技术研究材料在高应变率拉伸加载时的本构关系奠定了基础。     

Experimental Investigation of Strain Rate Dependence of Nanocrystalline Pt Films

A new microscale uniaxial tension experimental method was developed to investigate the strain rate dependent mechanical behavior of freestanding metallic thin films for MEMS. The method allows for highly repeatable mechanical testing of thin films for over eight orders of magnitude of strain rate. Its repeatability stems from the direct and full-field displacement measurements obtained from optical images with at least 25 nm displacement resolution. The method is demonstrated with micron-scale, 400-nm thick, freestanding nanocrystalline Pt specimens, with 25 nm grain size. The experiments were conducted in situ under an optical microscope, equipped with a digital high-speed camera, in the nominal strain rate range 10⁻⁶–10¹ s⁻¹. Full field displacements were computed by digital image correlation using a random speckle pattern generated onto the freestanding specimens. The elastic modulus of Pt, E = 182 ± 8 GPa, derived from uniaxial stress vs. strain curves, was independent of strain rate, while its Poisson’s ratio was v = 0.41 ± 0.01. Although the nanocrystalline Pt films had the elastic properties of bulk Pt, their inelastic property values were much higher than bulk and were rate-sensitive over the range of loading rates. For example, the elastic limit increased by more than 110% with increasing strain rate, and was 2–5 times higher than bulk Pt reaching 1.37 GPa at 10¹ s⁻¹.     

Experimental Investigation of Plastic Strain Recovery and Creep in Nanocrystalline Copper Thin Films

Nanocrystalline metals exhibit a phenomenon called plastic strain recovery whereby plastic strain introduced through a load cycle is gradually recovered under no external loading over a time period of hours and days. In this study, we experimentally explore the diffusive mechanisms and the strain rates for nanocrystalline thin films of copper with an average grain size of about 35 nm during plastic strain recovery and creep. The experiments are performed via the plane strain bulge test and the thin film samples are deposited using thermal evaporation and sputtering. The specimens recover their residual strain in a period of time with two characteristic strain rates, a transient strain recovery rate of the order of 10^?7/s and a steady-state strain recovery rate of the order of 10^?9/s and there is a characteristic time at which the transition occurs between the two rates. The results suggest that a diffusive mechanism in conjunction with voids within the nanocrystalline material can explain the two plastic strain recovery rates and the transition between the two.     

A Discontinuous Elastic Interface Transfer Model of Thin Film Nanoindentation

A new model of thin film indentation that accounted for an apparent discontinuity in elastic strain transfer at the film/substrate interface was developed. Finite element analysis suggested that numerical values of strain were not directly continuous across the interface; the values in the film were higher when a soft film was deposited on a hard substrate. The new model was constructed based on this discontinuity; whereby, separate weighting factors were applied to account for the influence of the substrate in strain developed in the film and vice-versa. By comparing the model to experimental data from thirteen different amorphous thin film materials on a silicon substrate, constants in each weighting factor were found to have physical significance in being numerically similar to the bulk scale Poisson’s ratios of the materials involved. When employing these material properties in the new model it was found to provide an improved match to the experimental data over the existing Doerner and Nix and Gao models. Finally, the model was found to be capable of assessing the Young’s modulus of thin films that do not exhibit a flat region as long as the bulk Poisson’s ratio is known.     

A shear-compression specimen for large strain testing

A new specimen geometry, the shear-compression specimen (SCS), has been developed for large strain testing of materials. The specimen consists of a cylinder in which two diametrically opposed slots are machined at 45° with respect to the longitudinal axis, thus forming the test gage section. The specimen was analyzed numerically for two representative material models, and various gage geometries. This study shows that the stress (strain) state in the gage, is three-dimensional rather than simple shear as would be commonly assumed. Yet, the dominant deformation mode in the gage section is shear, and the stresses and strains are rather uniform. Simple relations were developed and assessed to relate the equivalent true stress and equivalent true plastic strain to the applied loads and displacements. The specimen was further validated through experiments carried out on OFHC copper, by comparing results obtained with the SCS to those obtained with compression cylinders. The SCS allows to investigate a large range of strain rates, from the quasi-static regime, through intermediate strain rates (1–100 s⁻¹), up to very high strain rates (2×10⁴s⁻¹ in the present case).     

High-temperature pressure-shear plate impact experiments using pure tungsten carbide impactors

The pressure-shear plate impact experiment has been modified to test materials at high temperatures (up to 700°C). Together with the high strain rates characteristic of this experiment (10⁶ s^–1), the high-temperature capability allows the shearing resistance of materials to be measured under conditions unattainable with other testing equipment. The compressive and shear responses of pure tungsten carbide at different temperatures are presented, as well as the results of one test on OFHC copper at a temperature of 691°C and a shear strain rate of 1.4×10⁶ s^–1.     

Fiber Bragg Grating Based Stress Measurement for Films Deposited onto Cylindrical Surfaces

Shape of a substrate directly influences the residual stress in thin film coatings. In this study, a method involving Fiber Bragg Grating (FBG) was used to measure residual stress in a film deposited on a cylindrical surface. An FBG has a cylindrical surface and its Bragg wavelength shifts continuously when a film is being deposited on the sensor’s surface. Herein, we calculated the residual strain in the film from the wavelength shift of the Bragg grating by studying the transfer of the residual strain of the cylindrical film to the core of the optical fiber substrate during deposition. By employing the energy method, we derived expressions that related the strain in the core of fiber to the residual strain in single layer films, bilayer films, and multilayer cylindrical films. As an example, we demonstrated a detailed process for testing the stress and the strain distribution across a nickel (Ni) film electrodeposited on the surface of a nickel-phosphorus (Ni-P) alloy-coated optical fiber. The results indicated that the measured strain repeatability was less than 500 μ? and the strain sensitivity was more than ?2 × 10^?3 pm/μ?, when the thickness of the film was less than 5 μm. The negative sign on the strain sensitivity indicated that the tensile strain in the film produced compressive strain in the core of the optical fiber. The FBG sensor system has high test speed, and integrates measurement and signal transmission. This method provides an effective and convenient approach to measure stress in a film deposited on a cylindrical surface.     

Constitutive modeling of the dynamic fracture of smooth tensile bars

A recently developed viscoplastic-damage type of constitutive theory for high strain-rate flow processes and ductile fracture is used to model the deformation and fracture of dynamically loaded smooth cylindrical tensile bars. The analysis assumes polycrystalline materials which usually contain microvoids with an average density of the order of 10⁶ per cm³ that are dispersed homogeneously throughout. It is shown that for dynamically imposed loading that produce nominal strain rates ranging between 5 × 10² − 5 × 10³ sec ⁻¹, the inhomogeneous fields of stress and deformation caused by wave propagation and wave reflection induce necking at different locations along the gauge section, depending upon the strain-rate imposed. This occurs without imposition of any geometrical or material irregularity to preposition the location of the necking. The imposed rate of strain is also shown to affect the magnitude of the strain at which necking initiates, as well as the strain required for fracture.     

聚碳酸酯的高应变率拉伸实验

为了解应变率对聚碳酸酯拉伸力学行为的影响,在旋转盘式间接杆杆型冲击拉伸试验机和MTS809材料试验机上,对聚碳酸酯棒材进行了高应变率和准静态加载下的单向拉伸试验,应变率分别为380 s-1、800 s-1、1750 s-1和0.001 s-1、0.05 s-1,得到了聚碳酸酯的拉伸应力应变曲线.试验结果表明:聚碳酸酯的拉伸力学性能具有明显的应变率相关性,其屈服应力和失稳应变随应变率的增加而增大.依据试验结果,采用朱王唐粘弹性本构模型来描述聚碳酸酯的非线性粘弹性拉伸力学行为.模型结果显示,在本文实施的应变率范围内,朱王唐模型可以较好地表征聚碳酸酯的拉伸应力应变响应.     

内爆和柱壳条件下无氧铜的层裂特性

利用任意反射面位移干涉系统(DISAR)激光测速技术,成功地获得了滑移内爆加载和柱壳结构条件下无氧铜的内表面（自由面）速度剖面,并对其层裂特性进行了初步分析。结果表明:(1)在固定炸药和改变无氧铜圆管壁厚条件下,层裂片厚度随着圆管壁厚h的减小而增加;以圆管壁厚h为参照进行归一化,则相对层裂片厚度(/h)随相对装药厚度(he/h)的增大而增加,这种规律与以往对20钢的研究结果一致,但圆管发生层裂的临界条件,却显示出明显的材料相关性。(2)初步来看,无氧铜的层裂强度对结构的依赖性不明显,而与加载脉冲的幅值和宽度相关。(3)受无氧铜粘性和Taylor波衰减的影响,无氧铜的层裂强度随管壁厚度的增加而略有降低;同时,材料分散性也对此有一定影响。     

无氧铜平面冲击波实验的横向应力测试及屈服强度确定

为直接确定平面冲击波实验中无氧铜的流动应力,测量了试件中的纵向应力和横向应力,得到了试件中流动应力随时间的变化。采用构建的7种本构模型对无氧铜平面冲击波实验进行数值模拟,比较了实验结果与理论计算结果。研究指出,由SHPB等确定的中等应变率本构并不能拓宽应用于平面冲击波实验。     

Indentation of a hard film on a soft substrate: Strain gradient hardening effects

The strain gradient work hardening is important in micro-indentation of bulk metals and thin metallic films, though the indentation of thin films may display very different behavior from that of bulk metals. We use the conventional theory of mechanism-based strain gradient plasticity (CMSG) to study the indentation of a hard tungsten film on soft aluminum substrate, and find good agreement with experiments. The effect of friction stress (intrinsic lattice resistance), which is important in body-center-cubic tungsten, is accounted for. We also extend CMSG to a finite deformation theory since the indentation depth in experiments can be as large as the film thickness. Contrary to indentation of bulk metals or soft metallic films on hard substrate, the micro-indentation hardness of a hard tungsten film on soft aluminum substrate decreases monotonically with the increasing depth of indentation, and it never approaches a constant (macroscopic hardness). It is also shown that the strain gradient effect in the soft aluminum substrate is insignificant, but that in the hard tungsten thin film is important in shallow indentation. The strain gradient effect in tungsten, however, disappears rapidly as the indentation depth increases because the intrinsic material length in tungsten is rather small.