滚压诱导梯度超细晶铜的润滑微动磨损特性研究

通过滚柱对纯铜表面进行大载荷的多次滚压,使其表层累积不同应变水平的塑性变形,对所形成的约150μm的塑性变形层沿深度方向的晶粒形态和分布进行分析,并对其显微硬度进行测试,表明该工艺在纯铜表层诱导形成了一种梯度超细晶结构;最表层平均晶粒尺寸约为662 nm,并随深度逐渐增至基体晶粒尺寸94μm;梯度超细晶铜(简称梯度铜)最表层硬度较基体硬度提高约45%,并随深度逐渐下降至基体硬度.在SRV IV试验机上对梯度铜和粗晶铜在润滑条件下的微动磨损特性进行对比研究.结果表明:梯度铜摩擦系数先减小后增大,并在大载荷下与粗晶铜趋于一致;梯度铜的主要磨损形式为磨粒磨损,而粗晶铜为磨粒磨损和疲劳剥落,部分区域出现氧化磨损;梯度铜抗微动磨损能力比粗晶铜提高10倍以上.     

H13钢的高温拉伸性能研究

利用高温传感器在 MTS880试验机上系统地研究了 H13钢由室温到700℃温度范围内的应力应变特性,并给出其应力应变关系随温度的变化规律,同时对拉伸试件断口进行了微观分析。研究结果表明：由室温到600℃的温度范围内,H13钢的屈服强度、抗拉强度、弹性模量随温度的增加而线性地缓慢降低;室温拉伸试验的断口表现为多点起裂的撕裂断口,表明 H13钢的强度较高,但韧性较低;400℃时断口为典型的韧性断口形貌,强韧性很好;550℃和600℃时断口为韧窝状断口,具有一定的强度和韧性;当温度高于600℃时 H13钢的强度和韧性快速下降,失去了承载能力。     

混凝土类材料动态拉伸强度的微观力学模型

基于水泥砂浆试样动态劈裂拉伸实验,研究了不同加载速率下水泥砂浆材料动态劈裂时的裂纹发 生和扩展规律,提出一个微观力学模型。结果表明,微裂纹惯性是混凝土类材料动态拉伸实验中测量到的动 态拉伸强度随应变率的增加而提高的一种微观机制。     

PMMA动态拉伸破坏实验研究

本文利用Hopkinson压杆系统对PMMA变截面柱锥试样进行冲击加载,研究材料动态拉伸断裂特性和破坏机理.由于PMMA的拉压强度差别不大,采用圆柱状试件难以产生反射拉伸破坏,因此,将试样设计为圆柱锥状,实验时,将试样的圆柱端粘贴于输入杆上.实验结果表明,在一定的撞击速度下,由于应力波的汇聚作用,在试样锥段产生了反射拉伸破坏;当撞击速度较高时,在产生反射拉伸破坏的同时,试件中还产生了银纹区,随着打击速度的提高,银纹区域增大.同时根据断口的SEM观察,研究了材料的破坏机制.     

DH36钢拉伸塑性流动特性及本构关系

对DH36钢在温度从293~800 K、应变率为0.001和0.1 s-1的拉伸塑性流动特性进行实验研究,通过端口形貌图对变形前后的试样进行了微观分析,结果表明：(1)在实验温度范围内,0.001和0.1 s-1的应变率下,第三型应变时效现象出现,随应变率的增加,时效发生的温度区域移向更高温度;(2)第三型应变时效的发生与合金原子在晶界和晶粒中大量的第二相析出强化有关联;(3)建立包含第三型应变时效现象的统一本构模型,通过比较该模型能够较好的预测DH36的塑性拉伸流动应力。     

平面应变下紧凑拉伸试样的动态断裂韧性的实验研究

材料的动态断裂韧性是衡量材料在动载荷作用下抵杭裂纹扩展能力的重要指标,以往的材料动态断裂韧性测试多采用三点弯曲试样,而针对紧凑拉伸试样的动态断裂韧性研究很少.本文将紧凑拉伸试样(即CT试样)简化成等效弹簧质量模型,得到了CT试样动态应力强度因子的近似表达式.对Hopkinson压杆装置进行了改进,利用改进后的实验装置进...     

大理岩动态劈裂拉伸的SHPB实验研究

利用直径100 mm的Hopkinson压杆和薄圆形铝片作为波形整形器,用不同弹速径向冲击平台巴西圆盘试样以研究大理岩的动态拉伸强度。分析了试样的应变率、破坏时间、破坏模式,以及破坏过程中的载荷-应变关系,得到了关于大理岩在高应变率下拉伸强度及弹性模量的一些结论。考虑了试样的尺寸大小及两个平台附近应力的时间不均匀性与空间不均匀性对实验结果的影响。同时,利用有限元法对平台巴西圆盘试样的动态应力分布进行了数值模拟,验证了动态劈裂拉伸实验的有效性。     

一种改进的金属材料的高温动态拉伸实验技术

在旋转盘冲击拉伸实验装置上,利用金属材料自身的导电特性,对试样施加电流.使其在电流作用下发热,实现自加热,形成了试件快速加热而波导杆温升很小的金属材料的动态高温高应变率拉伸实验技术.应用该实验技术获取了45 #钢从室温到1000℃温度范围和应变率650s-1时的材料动态拉伸应力—应变曲线.实验结果表明,45 #钢具有明显的热软化效应,其流动应力和屈服应力随温度的升高而降低.     

玻纤增强复合材料高应变率拉伸实验研究

利用爆炸膨胀环实验技术,对玻璃纤维增强复合材料(GFRC)进行了高应变率(104s-1)下力学性能研究。实验中使用铜丝汽化引爆装药的方法对驱动环进行能量加载,并采用激光速度干涉仪(VISAR)测量试样环自由膨胀的质点速度,实现了玻璃纤维增强复合材料在104应变率下的一维拉伸破坏,并获取了该材料的速度时间曲线。经过计算得到了材料的应力应变关系,最后与该材料在准静态下、中高应变率下实验得到的数据进行了比较。     

20#钢动态拉伸断裂行为及其临界损伤度研究

首先对一维应变平面冲击载荷下20#钢的动态拉伸断裂行为进行了实验研究,考察了加载应力和拉伸应变率影响;然后基于损伤度函数模型,对实验进行了数值模拟研究,结果表明:模型中定义的特征物理量-断裂临界损伤度与外载荷条件(加载应力和拉伸应变率)的无关性或不敏感性;最后.采用Ls-dyna动力有限元程序并嵌入损伤度函数模型,对滑移爆轰下20#钢柱壳内爆驱动层裂进行二维数值模拟研究,结果表明:一维应变平面层裂实验和模型计算确定的损伤模型参数,也适用于描述柱壳构形复杂应力环境下的动态拉伸断裂问题.从而初步证实了断裂临界损伤度可以看作是一个表征延性金属动态拉伸断裂的内禀物性参数.     

Constitutive modeling of strain rate effects in nanocrystalline and ultrafine grained polycrystals

We present a variational two-phase constitutive model capable of capturing the enhanced rate sensitivity in nanocrystalline (nc) and ultrafine-grained (ufg) fcc metals. The nc/ufg-material consists of a grain interior phase and a grain boundary affected zone (GBAZ). The behavior of the GBAZ is described by a rate-dependent isotropic porous plasticity model, whereas a rate-independent crystal-plasticity model which accounts for the transition from partial dislocation to full dislocation mediated plasticity is employed for the grain interior. The scale bridging from a single grain to a polycrystal is done by a Taylor-type homogenization. It is shown that the enhanced rate sensitivity caused by the grain size refinement is successfully captured by the proposed model.     

Cracking in D6ac steel

This paper examines the results of an extensive test program undertaken to study crack growth in D6ac steel and shows that in each case the increment in the crack length per cycle (da/dN) conforms to the Generalised Frost-Dugdale crack growth law. This is found to be true for both constant K_max, constant R ratio load increasing, and compression-compression pre-cracking tests in the L-T, T-L and the S-T directions.     

Studies of effect of dynamic preloads on mechanical properties of steel

In order to determine the effect of load history upon the yield characteristics of a low-carbon steel, specimens of low-carbon steel have been subjected to initial dynamic compressive or tensile loads. The load-time function was approximately that of a sine wave of half-period duration with a half-period time of about 1 msec. The intensity of the load was varied; scme specimens were subjected to elastic strains for loads in excess of the static yield stress but less than the dynamic yield stress, while in others gross yield was initiated. These specimens were then aged for a short time at temperatures ranging from 68 to 250°F and then retested dynamically or in static tension. Upper and lower yield stresses and the inelastic microstrain which preceded gross yield on the retest have been used to evaluate the extent of damage and recovery. The results of these studies tend to confirm a yield phenomenon predicted by dislocation theory.     

An experimental study of dynamic tensile properties of glass fiber at low-temperatures

Tensile impact experiments of EC8.0−24×7 glass fiber bundles at different low temperaturesT(14°C, −40°C and −10°C) and strain rates ɛ were carried out, and complete stress-strain curves were obtained. Within the range of the experiment temperatures and strain rates, it is found that the initial modulusE, the ultimate strength σ_max and the unstable strain ɛ _b of the glass fiber bundles all increase with ɛ at an identicalT. At an identical ɛ, with the decrease ofT, E and σ_max increase; but ɛ _b increases when 10°C>T>−40°C and decreases when −40°C>T>−100°C. The strain-rate- and temperature-dependent bimodal Weibull statistical constitutive theory was adopted for the statistical analysis of the experimental results, and the Weibull parameters of single fiber were obtained. The results show that the bimodal Weibull distribution function is suitable to represent the strength distribution of the glass fiber at low temperature and different strain rates. The differences in the mechanical properties between EC8.0−24×7 and EC5.5−12 ×14 glass fiber bundles were also discussed. Project supported by the National Natural Science Foundation of China (No. 19772058).     

Analysis of dynamic growth of a tensile crack in an elastic-plastic material

The influence of inertia on the stress and deformation fields near the tip of a crack growing in an elastic-plastic material is studied. The material is characterized by the von Mises yield criterion and J₂ flow theory of plasticity. The crack grows steadily under plane strain conditions in the tensile opening mode. Features of the stress and deformation state at points near the moving crack tip are described for elastic-perfectly plastic response and for several crack propagation speeds. It is found that inertia has a significant effect on the elastic-plastic response of material particles near the crack tip, and that elastic unloading may occur behind the crack tip for higher speeds. The relationship between the applied crack driving force, represented by a remote stress intensity factor, and the crack tip speed is examined on the basis of a critical crack tip opening angle growth criterion. The calculated result is compared with dynamic fracture toughness versus crack speed data for a 4340 steel.     

45号钢的动态力学性能研究

对45号钢在不同环境温度(25 300℃)和不同应变率(10-4～103s-1)的 关系进行了研究。修正了Johnson Cook模型中的应变率强化系数C,确定了45号钢的本构关系。金相观察表明,与应变率强化相比,较高的环境温度使硬化速率降低,且占主要因素,其结果使材料的强度降低。透射电镜分析表明,高应变率在使位错运动的能量提高的同时,增加了位错在晶界处的阻力,而较高的环境温度则为位错提供了较多的滑移面和滑移方向,其结果是使材料更容易压缩。     

A technique for rapid two-stage dynamic tensile loading of polymers

A method for rapid two-stage dynamic-dynamic tensile loading of polymers, based on a tensile Hopkinson bar apparatus, is established. In this technique, the initial incident wave and its reflection are used to load a specimen in quick succession. Consequently, the specimen is stressed, momentarily unloaded, then reloaded until fracture. By adopting appropriate assumptions, a procedure to obtain the associated stress-strain curves for such double-stage loading is formulated. These assumptions are examined experimentally and analytically to substantiate their validity. To verify the proposed approach, a relatively rate-insensitive material, LEXAN 141 polycarbonate, was subjected to two-stage dynamic tension. The stress-strain curves obtained via the procedure established were compared with results from static loading. Favorable correlation between the two indicates that the proposed technique can be applied to the study of load history effects on the dynamic behavior of polymeric materials.