晶粒尺寸对高纯铝板材层裂特性的影响

采用不同热处理工艺制备了3种晶粒尺寸（60、100、500 μm）的高纯铝板材,利用平板撞击实验研究了其层裂行为。通过改变飞片击靶速度,在靶板中实现初始层裂状态和完全层裂状态。基于自由面速度时程曲线和微损伤演化及断口显微形貌分析,讨论了晶粒尺寸对高纯铝板材层裂特性的影响规律。实验结果显示:（1）晶粒尺寸对高纯铝板材层裂特性的影响强烈依赖于冲击加载应力幅值,在低应力条件下,层裂强度与晶粒尺寸之间表现出反Hall-Petch关系,而在高应力条件下,晶粒尺寸对层裂强度几乎没有影响;（2）随着晶粒尺寸的增大,靶板损伤区微孔洞的尺寸和分布范围均增大,但数量显著减少,在微孔洞周围还发现比较严重的晶粒细化现象;（3）随着晶粒尺寸的增大,层裂微观机制从韧性沿晶断裂向准脆性沿晶断裂转变,且在断口上观察到少量随机分布的小圆球,归因于微孔洞长大和聚集过程中严重塑性变形引起的热效应。     

纯铝在单轴应力循环作用下棘轮行为的试验研究

对纯铝进行了单轴应变控制和应力控制下的系统循环试验。对纯铝应变循环下的循环应变幅值、应变幅值历史、平均应变对循环特性的影响进行了揭示，对纯铝在非对称应力循环下的应力幅值、平均应力及其历史对循环蠕变〈即棘轮〉的影响进行了分析，得到了纯铝单轴循环行为的一些有意义的结果。     

激光选区熔化增材制造GP1不锈钢动态拉伸力学响应与层裂破坏

采用选择性激光熔化增材制造技术,制备了GP1不锈钢单轴拉伸板条试样和层裂圆片试样,并对材料微观结构进行了表征。借助Zwick-HTM5020高速拉伸试验机,并结合数字图像相关性全场应变测量技术,开展了增材制造GP1不锈钢材料的轴向拉伸力学性能实验研究,得到了不同应变率下材料的拉伸应力-应变曲线,结果显示:(1)GP1不锈钢流动应力具有比较显著的应变强化效应;(2)通过回收试样的电子背散射衍射表征,发现GP1不锈钢在拉伸变形过程中会发生奥氏体与马氏体之间的相变;(3)GP1不锈钢的屈服应力随着应变率呈幂指数增大,断裂应变在中低应变率下保持不变,但在高应变率下则显著减小。采用一级轻气炮实验装置和激光干涉粒子速度测量技术,开展了增材制造GP1不锈钢的层裂实验,发现GP1不锈钢的层裂强度随着飞片撞击速度增大而减小。单轴拉伸试样断口和层裂试样断口的显微分析结果表明:随着应变率增大,单轴拉伸断裂模式和断裂机理都发生了转变;层裂损伤易成核于激光熔池边界线的交汇处,断口韧窝形貌明显区别于单向拉伸断口。     

利用锰铜压力计确定材料的层裂厚度和累积损伤判据

我们知道,材料产生层裂不仅和拉伸应力有关,而且也和时间有关。最早Rinehart等提出动态断裂强度的判据,Skidmore又引出了应变率影响或应力梯度,Tuler提出累积损伤判据。我们利用锰铜压力计研究了铝(LY12)的层裂,确定了它的断裂判据,其实验装置见图1。     

填充硅橡胶的泡沫铝复合材料的力学性能

用渗流法向开孔泡沫铝-硅合金和泡沫纯铝中充填硅橡胶获得含硅橡胶的泡沫材料, 在材料试验机和SHPB上对含硅橡胶的复合材料进行动态与准静态压缩实验。实验结果表明:含硅橡胶的泡沫复合材料只有弹性段和塑性段两个阶段,具有更高的应变率敏感性,其应力-应变曲线抖动幅度比较大。     

脆性材料中应力波衰减规律与层裂实验设计的数值模拟

对混凝土、岩石类脆性材料的层裂实验进行了有限元模拟,研究了应力波在此类材料中传播的衰减规律,包括两类机制:弹性波因大尺寸试样的几何弥散产生的小幅度线性衰减、与应变率相关的黏塑性波因本构关系导致的指数衰减。在此基础上,提出了包含常数项的指数型应力波峰值拟合公式。建议采用可以忽略应力波衰减影响的细长形试样进行层裂实验。混凝土类脆性材料层裂破坏模拟结果显示,有限元模拟得到的层裂片厚度与一维应力波理论得到的结果非常吻合,验证了按一维应力波理论确定层裂强度的实验方法的有效性。通过对比3种不同入射波形下层裂片的形状和净拉应力波形,发现不对称的入射波形状更有利于实验获得平直的层裂断面和较准确的层裂强度。     

复杂应力状态下材料层裂特性探讨

在综述一维平面应变下层裂研究特点和研究方法的基础上,采用实验转换的方法,近似模拟球面层裂的受力特征,获得了不同预应变下LY12铝合金层裂信号,表明材料的层裂强度与所处应力状态密切相关。结合这些研究成果,探讨了复杂应力条件下层裂的新特征,讨论了可能用于层裂研究的方法及其改进措施。     

初始应力状态对材料层裂破坏特性影响研究

通过对球面飞片加载条件下的应力/应变状态分析表明采用施加径向应变方法可以近似模拟球面加载的受力过程. 采用过盈配合的热装配方法对平面样品施加了径向预应变,一维平面应变气炮实验结果显示初始预应力(变)明显降低了LY12铝层裂强度. 从空洞长大基本原理出发分析了各向异性受力条件下空洞长大的路径和所消耗能量不同于各向同性应力加载.通过数值模拟对含损伤的材料本构模型进行分析,得到了材料层裂强度与其外部宏观应力场密切相关,也间接的与构型相关的结论.      

双金属层状板中软界面层对裂纹扩展的影响

采用带软界面层的双金属层状板，分析了裂纹垂直于界面扩展过程中的力学行为。带单边裂纹的钢／铝层板的弯曲疲劳实验表明，裂尖接近软铝界面层时裂纹扩展速率da／dN变化很大；裂纹扩展到达界面层时发生偏折。利用有限元软件分析了裂纹尖端前沿应力和应变的变化规律，计算结果表明：软铝界面层出现较大塑性应变，增加了裂纹扩展阻力，并使裂纹扩展力△K减小，导致了da／dN降低．而软界面层中的T应力增高，造成裂纹偏折现象。     

预应力对LY12铝层裂强度的影响

通过过盈配合的热装配方法和机械夹紧方法对LY12铝合金平面样品施加了径向预应变,采用平面飞片撞击方法获得了预应力对层裂特性的影响。结果表明:当飞片以500 m/s的速度撞击样品,预应变为0（无预应变）、25110-6、1 10810-6、1 88610-6时,层裂强度分别为1.130、0.935、0.755、0.643 GPa,初始预应力（变）明显降低了LY12铝层裂强度,证实了材料的层裂强度与初始应力状态密切相关,可为以后的非一维平面应变下的层裂研究提供参考。     

High Strain Rate Torsion Properties of Ultrafine-Grained Aluminum

Mechanical properties of most metallic materials can be improved by reducing their grain size. One of the methods used to reduce the grain size even to the nanometer level is the severe plastic deformation processing. Equal Channel Angular Pressing (ECAP) is one of the most promising severe plastic deformation processes for the nanocrystallization of ductile metals. Nanocrystalline and ultrafine grained metals usually have significantly higher strength properties but lower tensile ductility compared to the coarse grained metals. In this work, the torsion properties of ECAP processed ultrafine grained pure 1070 aluminum were studied in a wide range of strain rates using both servohydraulic materials testing machines and Hopkinson Split Bar techniques. The material exhibits extremely high ductility in torsion and the specimens did not fail even after 300% of strain. Pronounced yield point behavior was observed at strain rates 500 s⁻¹ and higher, whereas at lower strain rates the yielding was continuous. The material showed slight strain softening at the strain rate of 10⁻⁴ s⁻¹, almost ideally plastic behavior at strain rates between 10⁻³ s⁻¹ and 500 s⁻¹, and slight but increasing strain hardening at strain rates higher than that. The tests were monitored using digital cameras, and the strain distributions on the surface of the specimens were calculated using digital image correlation. The strain in the specimen localized very rapidly after yielding at all strain rates, and the localization lead to the development of a shear band. At high strain rates the shear band developed faster than at low strain rates.     

A Micromechanics Study of Lamellar TiAl

Microdeformation patterns of lamellar TiAl specimens with various grain sizes under uniaxial tension are mapped using the micro/nano experimental mechanics technique called SIEM (Speckle Interferometry w ith Electron Microscopy). The stress–strain relationships were obtained from deformations within decreasing areas ranging from mm² to μm². We found that the stress–strain relationship of the material depends on the size of strain measuring area in relation to the grain size. The stiffness at a grain boundary can be as large as 7–10 times more than that of the grain itself. From the data obtained so far, it seems that the traditional way of using PST (polysynthetically twinned) single crystal to predict polycrystalline behavior may not be appropriate.     

The Effect of Grain Size on Deformation and Failure of Ti<Subscript>2</Subscript>AlC MAX Phase under Thermo-Mechanical Loading

An experimental investigation was performed to analyze the effects of grain size on the quasi-static and dynamic behavior of Ti₂AlC. High-density Ti₂AlC samples of three different grain sizes were densified using Spark Plasma Sintering and Pressureless sintering. A servo-hydraulic testing machine equipped with a vertical split furnace, and SiC pushrods, was used for the quasi-static experiments. Also, a Split Hopkinson Pressure Bar (SHPB) apparatus and an induction coil heating system were used for the dynamic experiments. A series of experiments were conducted at temperatures ranging from 25 °C to 1100 °C for strain rates of 10^?4 s^?1 and 400 s^?1. The results show that under quasi-static loading the specimens experience a brittle failure for temperatures below Brittle to Plastic Transition Temperature (BPTT) of 900–1000 °C and large deformation at temperatures above the BPTT. During dynamic experiments, the specimens exhibited brittle failure, with the failure transitioning from catastrophic failure at lower temperatures to graceful failure (softening while bearing load) at higher temperatures, and with the propensity for graceful failure increasing with increasing grain size. The compressive strengths of different grain sizes at a given temperature can be related to the grain length by a Hall-Petch type relation.     

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。     

A simple model for dislocation behavior,strain and strain rate hardening evolution in deforming aluminum alloys

In this work, modeling of the stress–strain behavior is carried out using a simple dislocation model. This model uses three variables to characterize the dislocation population: The average forest and mobile dislocation densities, ρ_f and ρ_m, and the average dislocation mean free path L. However, it is shown that within reasonable assumptions, only two of these variables are independent. The mathematical form derived from this dislocation-based model was applied to experimental stress–strain data determined at room temperature for pure aluminum, 3003-O, 2008-T4, 6022-T4, 5182-O and 5032-T4 aluminum alloy sheets. The evolution of the state variables was calculated for these materials from a single stress–strain curve. The average dislocation mean free paths at a strain of 0.5 were compared with TEM observations of dislocation cell sizes or inter-dislocation spacing for specimens deformed equal biaxially with the hydraulic bulge test. A very good agreement was obtained between predictions and experiments.     

Torsional split Hopkinson bar tests at strain rates above 104s−1

The torsional split Hopkinson bar is used for testing materials at strain rates above 10⁴s⁻¹. This strain rate, which is an order of magnitude higher than is typical with this technique, is obtained by using very short specimens. Strain rates of 6.4×10⁴s⁻¹ have been achieved with specimens having a gage length of 0.1524 mm. Results from tests on 1100 aluminum show an increase in rate sensitivity as the strain rate increases.     

An experimental investigation of a superplastic constitutive equation in Al–Mg–Si alloy composites reinforced with Si3N4 whiskers

Superplastic properties at 818 K were investigated by tensile tests for Al–Mg–Si alloy composites reinforced with Si₃N₄ whiskers whose volume fractions were 0–30%. The 20 and 30 vol.% composites exhibited large elongation of 615 and 285% at a high strain rate of 2×10⁻¹ s⁻¹, respectively. High strain rate superplasticity of the composites is attributed to the very small grain size of less than 3 μm. The stress–strain rate relation for the composites was almost the same as that of the alloy, taking into consideration the influences of threshold stress and grain size, and the relation was independent of the volume fraction of whisker. This is probably because grain boundary sliding was not hampered by the whiskers due to the presence of liquid phase for the composites.     

Quasi-Static and Dynamic Large Strain Shear-Tension Testing

A new shear specimen is designed, evaluated and tested quasi-statically and dynamically. The specimen consists of a long cylinder having a horizontal gauge section created by two diametrically opposed semi-circular slots machined parallel to the longitudinal axis. This geometry imposes a rather uniform stress state, close to pure shear in the gauge section. Quasi-static and dynamic tension-shear tests up to a strain rate of 10⁴ 1/s were carried out on 1020 cold-drawn steel specimens. The obtained stress–strain curves and ductility were validated numerically. The new specimen can be used to study the shear mechanical characteristics of a material using tensile testing.     

Effect of grain size on slow fatigue crack propagation and plastic deformation near crack tip

Fatigue crack growth and its threshold are investigated at a stress ratio of 0.5 for the three-point bend specimen made of Austenitic stainless steel. The effect of grain size on the crack tip plastic deformation is investigated. The results show that the threshold value Δk_th increases linearly with the square root of grain size d and the growth rate is slower for materials with larger grain size. The plastic zone size and ratio for different grain sizes are different at the threshold. The maximum stress intensity factor is k_max and σ_ys is the yield strength. At the same time, the characteristics of the plastic deformation development is discontinuous and anti-symmetric as the growth rate is increased from 2·10^—8 to 10⁻⁷ mm/cycle.A dimensionless relation of the form for collating fatigue crack starting growth data is proposed in which Δk_th represents the stress intensity factor range at the threshold. Based on experimental results, this relation attains the value of 0.6 for a fatigue crack to start growth in the Austenitic stainless steel investigated in this work. Metallurgical examinations were also carried out to show a transgranular shear mode of cyclic cleavage and plastic shear.     

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.