Partition of plastic work into heat and stored energy in metals

This study investigates heat generation in metals during plastic deformation. Experiments were designed to measure the partition of plastic work into heat and stored energy during dynamic deformations under adiabatic conditions. A servohydraulic load frame was used to measure mechanical properties at lower strain rates, 10^–3 s^–1 to 1 s^–1. A Kolsky pressure bar was used to determine mechanical properties at strain rates between 10³ s^–1 and 10⁴ s^–1. For dynamic loading, in situ temperature changes were measured using a high-speed HgCdTe photoconductive detector. An aluminum 2024-T3 alloy and -titanium were used to determine the dependence of the fraction of plastic work converted to heat on strain and strain rate. The flow stress and for 2024-T3 aluminum alloy were found to be a function of strain but not strain rate, whereas they were found to be strongly dependent on strain rate for -titanium.     

6061-T6铝合金动态拉伸本构关系及失效行为

采用HMH-206高速材料试验机开展了6061-T6铝合金在0.001～100 s-1应变率范围内的静、动态拉伸力学性能实验,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对6061-T6铝合金流动应力和应变率敏感性指数的影响,并基于实验结果对Johnson-Cook本构模型进行了修正。结合缺口试件的实验结果和模拟数据,得到了材料的Johnson-Cook失效模型参数,并对模型的准确性和适用性进行了验证。结果表明,在拉伸载荷作用下,6061-T6铝合金表现出明显的应变硬化特征和应变率敏感性,其流动应力随应变率的升高而提高,修正的Johnson-Cook本构模型可以描述材料的动态塑性流动行为,建立的Johnson-Cook失效模型能够表征材料的断裂失效行为。     

Crack-tip stresses as computed from strains determined by stereoimaging

Measurements of the three in-plane elements of the strain tensor near a crack tip have been combined with the material constitutive behavior to compute the three inplane elements of the stress tensor and the mean stress. The method is illustrated with fatigue-crack plasticity results for 7075-T651 aluminum alloy.     

Tensile testing of materials at high rates of strain

A tension version of the split Hopkinson bar or Kolsky apparatus is developed for conducting tests in tension at high rates of strain up to 10³ s^?1. A number of aluminum, titanium, and steel alloys tested in tension show increasing degrees of rate sensitivity above 10 to 10² s^?1. Tests on 6061-T651 and 7075-T6 aluminum show measurable strain-rate sensitivity in tension at the highest strain rates, although similar tests in compression in the literature show essentially no strain-rate sensitivity. Details of the apparatus and instrumentation and guidelines for its use are presented.     

宽应变率范围下2A16-T4铝合金动态力学性能

为了研究2A16-T4铝合金的动态力学性能,利用电子万能试验机、高速液压伺服试验机及霍普金森压杆(SHPB)装置进行常温下准静态、中应变率和高应变率的动态力学性能实验,得到不同应变率下的应力应变曲线,基于修正的Johnson-Cook本构模型对它进行拟合,并分析材料中应变率力学特性对模型应变率敏感参量的影响。结果表明:2A16-T4铝合金在应变率10-4~102 s-1范围内应变率敏感性较弱,而在102~103 s-1范围内应变率敏感性较强,且应变率强化效应随塑性应变的增大而减小;同时,在10-4~103 s-1范围内具有较强的应变硬化效应,且应变硬化效应随应变率的增大而减小;此外,修正Johnson-Cook本构模型的拟合结果与实验结果吻合很好,能够很好表征材料的动态力学行为,且考虑材料中应变率力学特性可提高本构模型参量的准确性。     

Determination of the plastic work converted to heat using radiometry

An improved calibration method for infrared radiometers is developed that has been shown to increase the accuracy of temperature measurement. To validate this new calibration technique, high strain rate compression tests are performed on the aluminum alloy BS 2011 and high strain rate torsion tests are performed on the titanium alloy 90% Ti-6%Al-4%V. The adiabatic temperature rise that occurs during these tests is measured using an infrared radiometer and validated, in the case of the compression tests, using fast response thermocouples. The proportion of plastic work converted to heat, , is found to increase with plastic strain for both materials, which is similar to previous research. These results challenge the classical assumption that has a constant value of approximately 0.95.     

纵向冲击压缩下LY12铝合金圆环的塑性失稳

通过对铝合金圆环的纵向冲击压缩研究发现,一定条件下在试件的宏观塑性硬化阶段会出现明显的应力降过程。为揭示此应力降的发生机制,对润滑、细磨、粗磨3种端面粗糙条件下,外径、内径和高度比值为6:3:2的LY12铝合金圆环进行系统的Hopkinson压杆纵向冲击实验。结果表明:应力降主要发生在较大的应变和较高的应变率条件。进一步对实验样品的金相观察发现:应力降产生的内在机制为绝热剪切带的形成和发展,此现象是一种动态塑性失稳的过程。以上结果为金属材料在冲击条件下绝热剪切带产生的研究提供了参考。     

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.

     

A Shear Compression Disk Specimen with Controlled Stress Triaxiality Under Dynamic Loading

This paper presents an experimental and numerical study of the potential of the Shear Compression Disk specimen (SCD) to characterize the plastic flow and fracture of metals under various levels of stress triaxiality at strain rates of up to 10⁴ 1/s. The main loading mode in that specimen is shear with triaxiality ranging from 0 to -0.8. The specimen is relatively small and fits into a standard split Hopkinson pressure bar system. Aluminum 7075-T651 alloy was chosen for a test case study. Experimental and numerical investigations reveal the adequacy of the SCD specimen for the study of mechanical properties of materials under high strain-rates and low, though wide, range of stress triaxialities.     

The dependence of acoustic emission on strain rate in 7075-T6 aluminum

The dependence of RMS acoustic-emission output on strain rate was determined during tensile tests on 7075-T6 aluminum over a range of strain rates from 0.015 min^?1 to 0.230 min^?1. A special normalization technique was developed to eliminate the inherent differences in the acoustic emission from supposedly identical specimens tested under identical conditions. It was found that the RMS acoustic-emission output increases approximately linearly with strain rate, and that the slope of the linear dependence decreased with increased plastic strain. The effect of pinned dislocation breakaway on the acoustic-emission output vs. plastic-strain relationship is the subject of a preliminary theory.     

高速列车用6008铝合金动态变形本构与损伤模型参数研究

高速列车在实际服役过程中会经受复杂的应力状态和环境条件,铝合金型材以其优良的力学和加工性能被广泛应用于新型高速列车的吸能结构,其防撞性能对高速列车的安全运行至关重要。本文针对一种新型轨道车辆用材料6008-T4铝合金型材进行了多种力学性能测试,包括动静态拉压实验、准静态高低温实验、不同应力路径的断裂实验等,提出了一种计算局部断裂应变的新方法,进而标定和获取了Johnson-Cook本构和损伤模型参数。最后利用平板侵彻实验来对所获取的参数进行检验,发现模拟和实验结果吻合良好,说明本文所获取的参数和参数标定方法都是有效的。     

Thermo-mechanical response of tension panels under intense rapid heating

The temperature dependence of the fracture strength of 7075-T6 aluminum alloy was determined under rapid heating (0.02–.50 sec) conditions by exposing thin-section specimens to intense surface irradiation while under constant tensile load. When combined with a numerical thermal analysis and an appropriate limit analysis, these data enabled accurate prediction of the heating time required to produce ductile fracture in spot irradiated tension panels. The transient heat transfer model employed in the computations incorporated material removal due to melting, temperature-dependent material properties, and convection/radiation losses.     

新型铝锡硅合金高温塑性变形流变应力的研究

采用高温等温压缩变形方法,在温度为373-673K范围和应变速率为0.001-1.0s^-1范围内,测定了新型Al-10Sn-4Si合金的流变应力曲线,结果表明,该合金为正应变速率敏感材料并且具有稳态流变特征;稳态流变应力随变形速率的增加而增大,随变形温度的升高而降低,通过回归分析,建立了该合金高温塑性变形时稳态流变应力的半经验方程,这种稳态流变特征与动态回复、动态再结晶及局部晶界粘滞性流动行为有关,受热激活过程控制。     

Residual-stress determination by single-axis holographic interferometry and hole drilling—Part II: Experiments

Experiments to assess the ability of the holographic/hole-drilling technique to accurately determine uniaxial stresses are described. The experimental data are in the form of optical interference fringe patterns. Different patterns obtained by varying the direction of laser light illuminating a test specimen with respect to the direction of stress are shown. Stresses estimated by the technique are compared with known values in specimens of aluminum alloy 7075-T651 and hardened Type 304L stainless steel. D. Williams was Senior Technical Aide, Sandia National Laboratories.     

A Shear Compression Disk Specimen with Controlled Stress Triaxiality under Quasi-Static Loading

This paper introduces a double shear axisymmetric specimen (Shear Compression Disk) and the methodology to extract flow and fracture properties of ductile materials, under various stress triaxiality levels. A thorough numerical investigation of the experimental set-up is performed, which reveals that the stresses are quite uniformly distributed in the gauge section during all the stages of the test. The attainable level of stress triaxiality (with pressures of up to 1.9 GPa) ranges from −0.1 to 1, which can be adjusted by a proper choice of geometrical parameters of the specimen. The methodology is implemented to quasi-static experiments on 4340 Steel and Aluminum 7075-T651 specimens. The flow properties are compared to those obtained by upsetting cylinders and show a very good agreement. For these materials it is observed that, contrary to the fracture strain, the flow properties are quite insensitive to the level of stress triaxiality. The fracture strain of the aluminum alloy increases with triaxiality and may be fitted with an exponential polynomial of the type suggested by [27]. These examples demonstrate the potential of the new specimen to obtain flow and fracture properties of ductile materials under controlled triaxiality.     

磁驱动准等熵压缩下LY12铝的强度测量

高压高应变率加载下材料的强度研究一直是冲击动力学的一个难题,目前动态载荷下材料的高压强度测量主要是基于平板撞击技术,冲击温升和应变率效应对材料强度的影响难以分离. 基于小型磁驱动加载装置CQ-4,开展了磁驱动准等熵压缩下LY12 铝的声速和强度测量的实验研究,讨论了考虑加载-卸载过程时磁驱动压缩实验的负载电极设计、实验样品设计、数据处理与分析等内容,并获得了12 GPa 压力范围沿加载-卸载路径的声速变化和峰值压力点的强度数据.      

Low-cycle fatigue of 7075-T651 aluminum alloy in biaxial bending

The uniaxial and biaxial low-cycle-fatigue life of 7075-T651 aluminum alloy in completely reversed bending is investigated. Uniaxial data is obtained from a cantilever specimen, and round and elliptical simply supported plate specimens are used for nominal strain biaxialities of 1∶1, 1∶075 and 1∶0.50 in the plane of the specimen. Experimental data is correlated on the basis of the total octahedral shearing strain range in which the dependent component strains are calculated using the “effective value” of Poisson's ratio for elastic-plastic loading. Good agreement is obtained oetween the uniaxial and biaxial data when the total octahedral shearing-strain range is plotted against the number of cycles to failure in logarithmic coordinates. Also, it is shown that, in anisotropic materials, the directional uniaxial and biaxial low-cycle properties can be predicted from fatigue properties in any one direction if the anisotropy of fracture ductility is known.     

基于应力三轴度和罗德参数的6061和7075铝合金材料断裂失效分析

分别对6061铝合金和7075铝合金材料的缺口圆棒试件和凹槽平板试件进行准静态拉伸试验,并采用ABAQUS软件对拉伸过程进行数值模拟。模拟结果与试验测试结果吻合很好,验证了有限元模型的合理性和可靠性。通过有限元模拟,分别给出了不同试件的应力三轴度和罗德参数随等效塑性应变的变化曲线以及两种材料的失效轨迹,并对它们进行了分析讨论。结果表明:形状相同、材料性质不同的试件,应力三轴度的演化规律不同;材料的失效应变受应力三轴度和罗德参数的影响,并且不同性质的材料对罗德参数的敏感性不同。     

Viscoplastic constitutive modeling with one scalar state variable

State variables have been used to represent the material resistance to plastic deformation in the recent development of the viscoplastic constitutive equations. In a previous paper, an experimental method was suggested to identify the relative roles to be played by the scalar state variable (drag stress) and the tensorial state variable (back stress) in the state-variable based constitutive equations. The results on 2618-T61 aluminum alloy tested at 200°C suggested that the scalar state variable alone should be enough to model the experimental results of 2618-T61 aluminum alloy.

In the current work, an early version of the viscoplastic constitutive equation proposed by Bodner and Partom, which was formulated with one scalar state variable, was adopted to model the experimental results of 2618-T61 aluminum. Experiments included creep tests under stepwise loadings, controlled-strain-rate tests, and creep tests under nonproportional loadings. A constitutive equation based on strain hardening approach, which was developed in an earlier work, was also studied.

In order to improve the results of the Bodner and Partom's model, a recovery term which was an explicit function of the time exposed to the test temperature was suggested for the scalar state variable. Aging was discussed as one possible softening mechanism for the current material.     

Influence of strain rate on mechanical properties of 6061-T6 aluminum under uniaxial and biaxial states of stress

A technique is presented for determining mechanical properties of materials under dynamic tensile loads. A Dynapak metalworking machine was modified into a test fixture capable of producing the required dynamic loads for uniaxial and certain biaxial tensile tests. Results from uniaxial dynamic tests on 6061-T6 aluminum alloy are presented and compared to “static” data obtained from a universal testing machine. The dependence of tensile strength on strain rate and the augmenting effect of temperature on this dependence can be seen. The results of biaxial tests are described in terms of a modified form of the distortion-energy failure theory.