冲击载荷下圆环纵向压缩力学行为研究

选用外径、内径和高度三者比值为6∶3∶2的铝合金圆环试件作为研究对象,在粗磨、细磨和润滑三种端面粗糙条件下,利用SHPB实验方法研究了其纵向动态压缩行为。实验结果表明:随着端面摩擦系数的增大,对应的屈服强度和硬化强度增大明显。同时,圆环纵向压缩应力应变曲线有一个明显的应力下降段。其原因是圆环在塑性压缩过程从内而外发生了动态塑性失稳。由于端面应力不均匀和压缩过程端面摩擦系数不均匀变化,试件前后端面的压缩变形明显不一致。数值模拟分析了不同表面粗糙情况下的圆环纵向压缩过程,得到了和实验相类似的现象。     

Ta2钛合金绝热剪切失稳起始温度研究

利用分离式霍普金森压杆加载TA2钛合金扁平帽型试样,结合高速红外测温与金相观察,分析动态加载下帽型试样受迫剪切力学响应以及绝热剪切带温度演化,确定绝热剪切带起始温度,并讨论绝热剪切失稳起始条件。结果显示,绝热剪切带起始温度约为470K,明显低于再结晶温度,以温度470K作为起始条件,对应剪切应力时程曲线起始时刻,并非剪切应力最大值,而是应力软化至504MPa,即发生应力“塌陷”处,应力下降8.86%,塑性应变继续发展     

复杂应力状态下GCr15轴承钢的动态剪切及熔融特性研究

本文探究了GCr15轴承钢压剪试样在冲击载荷下的绝热剪切和熔融破坏特性．首先,实验研究发现GCr15轴承钢的力学性能具有拉-压不对称性、应力状态及应变率敏感性．因此,通过引入应力三轴度、洛德角、应变率和绝热剪切温升机制扩展了经典J-C本构模型,并对GCr15轴承钢的动态单轴压缩和压剪试验结果进行了数值实现．研究结果表明,应力三轴度和洛德角是影响动态冲击下压剪试样的绝热剪切产生和熔融特性的重要因素．     

不同加载状态下TA2钛合金绝热剪切破坏响应特性

一般认为绝热剪切现象在宏观上表现为材料动态本构失稳,即热软化大于应变硬化.本文采用帽型受迫剪切试样研究TA2钛合金的动态力学特性和本构失稳过程.首先对剪切区加载应力状态进行理论和数值分析,通过合理设计帽型试样,剪切区变形可近似按剪切状态处理;结合二维数字图像相关法(two-dimensional digital image correlation,DIC-2D)直接测试试样剪切区应变演化,给出帽型受迫剪切实验的等效应力-应变响应曲线.进一步,利用Hopkinson压杆对TA2钛合金开展动态压缩及帽型剪切对比试验研究,比较压缩、剪切试验得到的等效应力-应变曲线,采用"冻结"试样方法分析试样中绝热剪切局域化演化过程,探讨不同加载状态下TA2钛合金的绝热剪切破坏现象及其动态力学响应特性.实验结果表明,在塑性变形初始阶段,动态压缩及剪切加载下的等效应力-应变曲线符合较好,但随塑性损伤发展及绝热剪切带形成,两者出现分离,表明损伤及绝热剪切演化过程与应力状态相关.剪切试样实验得到的本构"软化"特性能够反映绝热剪切带起始、破坏演化过程的力学响应特性,而在动态压缩实验中,即使试样中已出现双锥形的绝热剪切带及局部裂纹分布,其表观等效应力-应变曲线并不出现软化特征,动态压缩实验无法得到关于绝热剪切起始、发展以及破坏的本构软化响应特性.     

冲击载荷下钨合金圆台试件绝热剪切变形局部化的数值模拟

采用有限元计算编码ABAQUS模拟了钨合金圆台试件在冲击载荷下的变形和剪切局部化行为.计算采用二维轴对称应变条件下的绝热模型.钨合金的本构方程采用热粘塑性形式的Johnson-Cook模型.为了得到不同尺度的变形信息,计算中用了两种网格;先用粗糙网格分析试件变形局部化的概貌;接着,用细密网格(在变形局部化区域,网格尺寸达到10μm)分析绝热剪切带的形成和发展.有限元模拟得到的绝热剪切带位置和方向与实验一致.计算结果表明,绝热剪切带的形成和发展与试件的应力状态密切相关.     

冲击载荷下钨合金圆台试件绝热剪切变形局部化的数值模

采用有限元计算编码ABAQUS模拟了钨合金圆台试件在冲击载荷下的变形和剪切局部化行为。计算采用二维轴对称应变条件下的绝热模型。钨合金的本构方程采用热粘塑性形式的Johnson Cook模型。为了得到不同尺度的变形信息 ,计算中用了两种网格 ;先用粗糙网格分析试件变形局部化的概貌 ;接着 ,用细密网格 (在变形局部化区域 ,网格尺寸达到 10 m)分析绝热剪切带的形成和发展。有限元模拟得到的绝热剪切带位置和方向与实验一致。计算结果表明 ,绝热剪切带的形成和发展与试件的应力状态密切相关。     

高锰钢帽型样品在高速冲击下的剪切行为

绝热剪切带是材料在高速变形时一种典型的破坏形式,为了更好地理解高速冲击过程中绝热剪切带的形成和扩展,基于Johnson-Cook本构模型,利用ANSYS/LS-DYNA软件对高锰钢帽型样品高速冲击过程的剪切行为进行了二维数值模拟。结果表明:横穿剪切带方向,应力应变分布都是剪切带中心最高,然后向两边逐渐降低,类似于高斯分布; 平行于剪切带方向,应力应变分布则是呈两端高中间低的特点。然后利用模拟的应力应变场分布确定了剪切带和裂纹形成及扩展方向,即从剪切区两端形成并向中间扩展;最后通过编辑软件的k文件直接得到了剪切带内部及周围形变影响区和基体的温度分布,其和应力应变场分布规律一致,结果与实验结果基本吻合。     

TC4在动态载荷下的剪切行为研究

使用分离式霍布金森压杆(SHPB)对2种TC4(Ti-6Al-4V)试样(单边剪切试样与双边剪切试样)在应变率104 s-1下进行动态剪切加载，利用SIM D8高速照相系统捕捉了绝热剪切带扩展的整个历程，得到了TC4在拍照时刻的应力应变曲线；使用金相显微镜和SEM扫描电镜对TC4绝热剪切带的微观形貌进行观察，发现绝热剪切带宽度为5~12 μm，断口从韧窝断裂演变为解理断裂，可观测到韧窝状与河流花样断口形貌，但是并未看到相变的发生；对2种试样就产生绝热剪切带的形式与敏感性进行了分析，实验表明双边试样更易产生绝热剪切带；通过高速照相系统的标定换算，得到TC4绝热剪切带产生的临界剪切应变在78%~88%之间。在SHPB动态加载条件下，TC4绝热剪切带的扩展速度在460~1 250 m/s之间，且应变率越高，剪切带扩展越快，扩展平均速度与名义应变率近似呈线性关系；另外，在同一加载速率下，剪切带并不是匀速扩展，其扩展速度随载荷的增加而不断增加。     

双相高强钢FeNiAlC的动态剪切行为及微结构机理

绝热剪切带是金属材料在高应变率载荷下常见的一种失效模式。利用霍普金森压杆装置,对双相钢Fe-24.86Ni-5.8Al-0.38C不同微结构的帽形样品施加冲击载荷,研究它的动态剪切变形行为及微结构机理。先通过对固熔处理得到的粗晶态样品进行大应变冷轧获得冷轧态样品,再使用透射电子显微镜和扫描电子显微镜表征两种样品冲击前后微结构的变化差异。结果表明,双相钢FeNiAlC拥有较优异的动态剪切性能,剪切强度达1.3 GPa,均匀剪切应变达1.5。变形前,材料由奥氏体相和马氏体相构成,马氏体体积分数约为20%。变形过程由位错滑移和孪生变形主导,但因应变速率较高致使马氏体相变被抑制。不同微结构样品内均形成绝热剪切带,带内发生动态再结晶,形成超细晶粒,平均晶粒尺寸约300 nm,且剪切带内不发生相变;冷轧态剪切带宽度的实验值（14.6 μm）与理论计算值(12.3 μm)较好吻合,而粗晶态剪切带宽度的实验值（14.6 μm）与理论计算值（30 μm）相差甚远,初步分析可能是因为粗晶态样品应变较大基本不满足完全绝热的理论条件。在变形过程中,粗晶态因塑性变形做功产生的绝热温升高达720 K,而冷轧态的只有190 K。通过实验结果与热塑模型分析,得出绝热温升不是形成绝热剪切带的唯一因素,而应考虑材料的微观结构和局部化变形等的共同影响。     

快速加载下TNT材料中绝热剪切带的细观研究

本文对快加载后的国产ＴＮＴ材料，采用光学显微镜和扫描电镜观察，研究了其显微组织，结果表明：在快速加载条件下材料中有绝热剪切带生成。剪切带在细观上的表现形式为：（１）以应变高度集中，晶粒剧烈扭曲为特征的细观组织形貌；（２）在局部区域，晶粒被急剧拉长，形成粗亮而边界清晰的剪切带。在快速加载条件下ＴＮＴ材料中绝热剪切带的产生同时取决于应变和应变切带。在快速的加载条件ＴＮＴ材料中绝热剪切带的产生同时决于应     

脆塑性岩石破坏后区应力跌落效应数值模拟

采用三轴试验和数值模拟研究了岩石类脆塑性材料的应力跌落效应,并用塑性流动因子λ来描述应力跌落效应。为简化计算,给出了一种应力陡降过程中伴随的非零应变增量的工程近似处理方法,基于此针对性地编制了处理脆塑性材料应力跌落的有限元分析程序代码。数值模拟结果验证了塑性流动因子λ以及该近似处理方法的有效性。     

Effects of texture on shear band formation in plane strain tension/compression and bending

In this study, effects of typical texture components observed in rolled aluminum alloy sheets on shear band formation in plane strain tension/compression and bending are systematically studied. The material response is described by a generalized Taylor-type polycrystal model, in which each grain is characterized in terms of an elastic–viscoplastic continuum slip constitutive relation. First, a simple model analysis in which the shear band is assumed to occur in a weaker thin slice of material is performed. From this simple model analysis, two important quantities regarding shear band formation are obtained: i.e. the critical strain at the onset of shear banding and the corresponding orientation of shear band. Second, the shear band development in plane strain tension/compression is analyzed by the finite element method. Predictability of the finite element analysis is compared to that of the simple model analysis. Third, shear band developments in plane strain pure bending of a sheet specimen with the typical textures are studied. Regions near the surfaces in a bent sheet specimen are approximately subjected to plane strain tension or compression. From this viewpoint, the bendability of a sheet specimen may be evaluated, using the knowledge regarding shear band formation in plane strain tension/compression. To confirm this and to encompass overall deformation of a bent sheet specimen, including shear bands, finite element analyses of plane strain pure bending are carried out, and the predicted shear band formation in bent specimens is compared to that in the tension/compression problem. Finally, the present results are compared to previous related studies, and the efficiency of the present method for materials design in future is discussed.     

Effects of texture evolutions and point defects on ultrasonic waves under simple shear and pure shear

In this paper, longitudinal and transverse wave velocities propagating into aluminum alloy A6061 under simple shear and pure shear were studied experimentally. Compared with the same velocity change tendencies of transverse wave under simple shear and pure shear, longitudinal wave velocity shows different change tendencies under both shear states, regardless of the same shear strain states in two cases. Finite element analysis was performed and the analyzed results indicate that the transverse wave velocity depends on texture evolutions mainly, whereas the longitudinal wave velocity is sensitively influenced by point defects induced by cross-slips. Consequently, the longitudinal wave velocity showing a sensitive response to the point defects was examined by measuring longitudinal wave velocity changes propagating into Al single crystal subjected to the combination loads of equi-biaxial tension and compression.     

Investigation on the plastic work-heat conversion coefficient of 7075-T651 aluminum alloy during an impact process based on infrared temperature measurement technology

The plastic work-heat conversion coefficient is one key parameter for studying the work-heat conversion under dynamic deformation of materials. To explore this coefficient of 7075-T651 aluminum alloy under dynamic compression, dynamic compression experiments using the Hopkinson bar under four groups of strain rates were conducted, and the temperature signals were measured by constructing a transient infrared temperature measurement system. According to stress versus strain data as well as the corresponding temperature data obtained through the experiments, the influences of the strain and the strain rate on the coefficient of plastic work converted to heat were analyzed.The experimental results show that the coefficient of plastic work converted to heat of 7075-T651 aluminum alloy is not a constant at the range of 0.85–1 and is closely related to the strain and the strain rate. The change of internal structure of material under high strain rate reduces its energy storage capacity, and makes almost all plastic work convert into heat.     

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。     

Dynamic crush behaviors of aluminum honeycomb specimens under compression dominant inclined loads

The dynamic crush behaviors of aluminum 5052-H38 honeycomb specimens under compression dominant inclined loads are investigated by experiments. Different types of honeycomb specimens were designed for crush tests under pure compressive and compression dominant inclined loads with respect to the out-of-plane direction. A test fixture was designed for dynamic crush tests under inclined loads. The results of the dynamic crush tests indicate that as the impact velocity increases, the normal crush strength increases and the shear strength remains nearly the same. The trends of the normalized normal crush strengths under inclined loads for specimens with different in-plane orientation angles as functions of the impact velocity are very similar to each other. Based on the experimental results, a macroscopic yield criterion as a function of the impact velocity is proposed. The experimental results suggest that, as the impact velocity increases, the shape of the macroscopic yield surface changes, or more specifically, the curvature of the yield surface increases near the pure compression state. The experimental results also show similar microscopic progressive folding mechanisms in honeycomb specimens under pure compressive and inclined loads. However, honeycomb specimens under inclined loads show inclined stacking patterns of folds due to the asymmetric location of horizontal plastic hinge lines.     

高密度合金W-Ni-Mn的绝热剪切破坏研究

应用分离式霍普金森压杆对90W-Ni-Mn合金的圆柱体试样进行了动态压缩实验,并对试样的剖面和断面进行了扫描电镜观察,结合试样的真实应力-应变曲线,发现合金在真实应变为约45％时出现明显的绝热剪切现象,随着冲击力的进一步增加,试样发生剪切断裂。可见90W-Ni-Mn合金较传统钨合金更易出现剪切带,具有更高的绝热剪切敏感性。     

Instability and failure of internally pressurized ductile metal cylinders

Forlong, ductile, thick-walled tubes under internal pressure instabilities and final failure modes are studied experimentally and theoretically. The test specimens are closed-end cylinders made of an aluminum alloy and of pure copper and the experiments have been carried out for a number of different initial external radius to internal radius ratios. The experiments show necking on one side of the tubes at a stage somewhat beyond the maximum internal pressure. All tubes, except for one aluminum alloy tube, failed by shear fracture under decreasing pressure. The aluminum alloy tubes exhibited localized shear deformations in the neck region prior to fracture and also occasionally surface wave instabilities. The numerical investigation is based on an elastic-plastic material model for a solid that develops a vertex on the yield surface, using representations of the uniaxial stress-strain curves found experimentally. In contrast to the simplest flow theory of plasticity this material model predicts shear band instabilities at a realistic level of strain. A rather sharp vertex is used in the material model for the aluminum alloy, while a more blunt vertex is used to characterize copper. The theoretically predicted bifurcation into a necking mode, the cross-sectional shape of the neck, and finally the initiation and growth of shear bands from the highly strained internal surface in the neck region are in good agreement with the experimental observations.     

A perturbation analysis of the unstable plastic flow pattern evolution in an aluminum alloy

In the tensile loading of sheet metals made from some polycrystalline aluminum alloys, a single deformation band appears inclined to the elongation axis in the early stage of deformation, and symmetric double bands are observed in the later stage. This evolution of spatial characteristics of such an unstable plastic flow pattern in a polycrystalline aluminum alloy has been analyzed by a perturbation method. A small number of slip modes are taken to describe the tensile strain. A rate-dependent constitutive equation is used for each slip mode to account for the interaction between dislocations and solute atoms in dynamic strain aging. Unconstrained and constrained models are used to impose appropriate loading conditions at the early and later deformation stages, respectively. Both plane-strain and plane-stress cases are considered. It is found out that the change of boundary conditions and material inhomogeneity during the course of plastic deformation are closely related to the evolution of spatial characteristics of shear band (the Portevin–Le Chatelier band) patterns observed in experiments.