在氢环境中钢的应变速率效应

从静态、准静态直到冲击的应变速率范围内(.ε=10-7~103sec-1),研究了在氢环境中应变速率对10钢力学性能的影响。结果表明:对于10钢,应变速率在3.33×10-3~3.33×10-6sec-1范围,氢的影响最明显,随着应变速率的提高或降低,氢的影响逐渐减弱;对于渗氢和未渗氢试样,应变速率在10-4~103sec-1范围内,屈服应力都随着应变速率的增加而增加,在.ε<6.0×102sec-1区域Ⅰ内流动应力与应变速率呈线性关系,在.ε>6.0×102sec-1区域Ⅱ内流动应力对应变速率十分敏感。与准静态时相比较,高应变速率下未渗氢试样的流动应力增加了1.1倍,而渗氢试样则增加了1.5倍。文中给出了可以精确描述10钢在高应变速率下渗氢与未渗氢试样的应力应变关系表达式。     

阴极充氢对结构钢裂尖形变行为的影响

采用微机控制激光散斑干涉技术，原位研究了结构钢单边缺口预裂纹试样在３．５％Ｎａｄ水溶液中，－１４００ｍＶ（ＳＣＥ）电位下阴极完氢６小时前后，裂尖材料形变行为的变化。同时，还对充氢与未充氢条件下光滑圆柱试样的应力－应变行为进行了对比研究，用扫描电镜观察了断口形貌。结果表明：由于氢的进入，使材料的加工硬化程度加剧，从而导致了变形困难。     

位错载氢运动对材料氢脆行为的影响

位错载氢运动会导致金属结构材料中氢原子的再分布和氢损伤的加剧,但目前仍缺乏相关实验数据支撑。本文首先对比了高锰钢预充氢后在四种应变速率（1×10^–3 s^–1,1×10^–4 s^–1,1×10^–5 s^–1和1×10^–6 s^–1）下的断口特征,随后结合理论计算探究了不同应变速率下位错载氢对金属材料氢脆行为的影响。结果表明较慢应变速率（1×10^–5 s^–1 and 1×10^–6 s^–1）下拉伸的试样具有比较快应变速率（1×10^–3 s^–1 和 1×10^–4 s^–1）拉伸时更高的氢脆敏感性和更大的断口脆性区面积。这是因为较慢应变速率下拉伸时,氢可以随位错运动,导致在塑性变形过程中形成更大的氢原子扩散距离和脆性区深度。此外,在更慢的应变速率（1×10^–6 s^–1）下,晶界处可以富集更多的氢,导致断口具有最大的脆性区面积。但是由于塑性变形过程中形成大量对氢原子具有强束缚作用的缺陷,导致实际位错载氢运动距离远低于理论位错载氢运动距离。

     

氢对体心立方金属裂纹尖端变形过程的影响

利用扫描电镜(SEM)原位拉伸,观察了 Fe-3％Si 单晶体裂尖的变形和断裂过程。用网格法加计算机处理系统测出了裂纹尖端的应变场及氢对应变分布的影响。发现在Ⅰ型载荷下,主裂纹尖端应变呈不对称分布,形变带内应变梯度很大。形变带内应变分布为: ε_(yy)=f(x)exp(-Ar).氢不改变裂纹尖端塑性变形特征,但能明显促进裂纹尖端局部塑性变形的非均匀增加。     

基于ASME应变限制准则的小冲杆试验等效断裂应变

小冲杆试验(Small Punch Test,SPT)技术是一种新型微试样试验技术,通过极少量材料就能得到材料的许多力学性能。本文以压力容器常用钢材Q345R为研究对象,通过ABAQUS有限元软件模拟小冲杆试验过程,得到了小冲杆试样最先失效单元的应力三轴度;然后根据ASME防止局部失效的应变限制准则中求解多轴应变极限的方法,对小冲杆试验的等效断裂应变进行了研究。研究结果表明:基于ASME应变限制准则的等效断裂应变具有较大的保守性,作者所提出的修正应变限制准则参数的方法是可行的。此外,小冲杆试样的尺寸效应对等效断裂应变也有重要影响,使得计算的等效断裂应变比有限元模拟的应变值小。     

超高强度钢单轴拉伸下形变与断裂过程的研究

1.引言单轴拉伸试验是材料基础力学试验之一,研究它的形变与断裂过程,历来受到人们的重视。关于超高强度钢单轴拉伸的形变规律,一般来说,当外加负荷较小时,材料处在弹性阶段,这时试样形状为圆柱体,应力与应变服从虎克定律。随外加负荷增加,材料进入塑性状态。从屈服至最大负荷这一段,试样仍保持为圆柱体,应力与应变关系服从二段幂乘硬化律。从最大负荷到断裂这一段,试样发生了局部变形,即所谓颈缩现象,     

准静态条件下金属材料的临界断裂准则研究

本文针对9种金属材料完成了具有不同约束程度的10类试样的延性断裂试验, 获得了发生拉、压、扭和裂尖断裂等破坏形式构型试样的载荷-位移试验关系; 基于圆棒漏斗试样拉伸试验所得直至破坏的载荷-位移曲线, 采用有限元辅助试验(finite-element-analysis aided testing, FAT)方法得到了9种材料直至破坏的全程等效应力-应变曲线, 以此作为材料本构关系通过有限元分析获得了各类试样直至临界破坏的载荷-位移关系模拟. 载荷-位移关系模拟结果与试验结果有较好的一致性, 表明用于解决试样颈缩问题的FAT方法所获得的全程材料本构关系针对各向同性材料具有真实性和普适性. 对应9种材料、10类试样的36 个载荷-位移临界断裂点, 通过有限元分析获得了对应的材料临界断裂应力、应变与临界应力三轴度, 研究表明, 第一主应力在延性变形过程中为主控断裂的主导参量; 通过研究光滑、缺口、裂纹等构型试样的断裂状态, 提出了$-1$至3范围的应力三轴度下由第一主应力主控的统一塑性临界断裂准则.      

准静态条件下金属材料的临界断裂准则研究

本文针对9种金属材料完成了具有不同约束程度的10类试样的延性断裂试验,获得了发生拉、压、扭和裂尖断裂等破坏形式构型试样的载荷-位移试验关系;基于圆棒漏斗试样拉伸试验所得直至破坏的载荷-位移曲线,采用有限元辅助试验(finite-element-analysis aided testing, FAT)方法得到了9种材料直至破坏的全程等效应力-应变曲线,以此作为材料本构关系通过有限元分析获得了各类试样直至临界破坏的载荷-位移关系模拟.载荷-位移关系模拟结果与试验结果有较好的一致性,表明用于解决试样颈缩问题的FAT方法所获得的全程材料本构关系针对各向同性材料具有真实性和普适性.对应9种材料、10类试样的36个载荷-位移临界断裂点,通过有限元分析获得了对应的材料临界断裂应力、应变与临界应力三轴度,研究表明,第一主应力在延性变形过程中为主控断裂的主导参量;通过研究光滑、缺口、裂纹等构型试样的断裂状态,提出了-1至3范围的应力三轴度下由第一主应力主控的统一塑性临界断裂准则.     

TC25钛合金低周疲劳特性与断口分析

对TC25钛合金进行不同应力幅值下的低周疲劳试验,测试钛合金的低周疲劳寿命.在分析钛合金的低周疲劳应力应变迟滞回线的基础上,利用试验特性和应力应变本构关系,推导简化的Manson-Coffin公式,获取钛合金的应力-寿命曲线.最后,应用SEM对钛合金试样断口进行形貌观测,分析低周疲劳的断裂机理.结果表明,TC25钛合金的低周疲劳塑性变形,主要产生在加速断裂阶段.钛合金试样断面存在多处剪切唇,瞬时断裂由剪应力主导.钛合金的低周疲劳断裂以解理形核形成疲劳源,裂纹扩展存在多种模式,瞬断为准脆性断裂.     

皮下组织对羟基磷灰石力学性质的影响

将羟基磷灰石试样植入大白鼠的皮下组织20\,d后取出,然后采用数字图像相关方法研究纤维组织对人造羟基磷灰石的力学性质的影响. 测量了两种试样：植入大白鼠皮下组织的羟基磷灰石试样和未经植入的羟基磷灰石试样. 得到了羟基磷灰石完整的应力-应变曲线;还测量了压缩极限应力. 实验结果表明羟基磷灰石的应力-应变曲线在断裂前呈良好的线性关系;皮下纤维组织能使羟基磷灰石材料的弹性模量和强度增加, 20\,d后使羟基磷灰石的弹性模量增加了29.06{\%}, 压缩极限应力增加了28.71%.     

Effects of superimposed hydrostatic pressure on fracture in round bars under tension

The effect of superimposed hydrostatic pressure on fracture in round bars under tension is studied numerically using the finite element method based on the Gurson damage model. It is demonstrated that while the superimposed hydrostatic pressure has no noticeable effect on necking, it increases the fracture strain due to the fact that a superimposed pressure delays or completely eliminates the nucleation, growth and coalescence of microvoids or microcracks. The experimentally observed transition of the fracture surface, from the cup-cone mode under atmospheric pressure to a slant structure under high pressure, is numerically reproduced. It is numerically proved that the superimposed hydrostatic pressure has no effect on necking for a damage-free round bar under tension.     

An analysis of ductile rupture in notched bars

Ductile fracture in axisymmetric and plane strain notched tensile specimens is analyzed numerically, based on a set of elastic-plastic constitutive relations that account for the nucleation and growth of microvoids. Final material failure by void coalescence is incorporated into the constitutive model via the dependence of the yield function on the void volume fraction. In the analyses the material has no voids initially; but as the voids nucleate and grow, the resultant dilatancy and pressure sensitivity of the macroscopic plastic flow influence the solution significantly. Considering both a blunt notch geometry and a sharp notch geometry in the computations permits a study of the relative roles of high strain and high triaxiality on failure. Comparison is made with published experimental results for notched tensile specimens of high-strength steels. All axisymmetric specimens analyzed fail at the center of the notched section, whereas failure initiation at the surface is found in plane strain specimens with sharp notches, in agreement with the experiments. The results for different specimens are used to investigate the circumstances under which fracture initiation can be represented by a single failure locus in a plot of stress triaxiality vs effective plastic strain.     

Al2O3陶瓷动静态压缩下碎片形貌与破坏机理分析

为探究Al₂O₃陶瓷的宏观力学响应与破坏机理,分别利用材料试验机和分离式霍普金森压杆对其进行准静态和动态压缩实验,同时通过原位光学成像观测试样的破坏过程,并利用同步辐射CT和扫描电镜（SEM）对回收碎片的尺寸和形状以及微观破坏模式进行表征分析。宏观强度数据表明,Al₂O₃陶瓷的抗压强度符合Weibull分布,且与加载应变率呈现指数增长关系。原位光学成像和SEM回收分析共同揭示了动静态加载下裂纹成核与扩展模式存在明显差异。准静态加载时材料微观上更易发生沿晶断裂,宏观表现为劈裂裂纹较少,且倾向于沿加载方向传播并贯穿整个试样;而动态加载时穿晶断裂占主导地位,劈裂裂纹明显增加并发生相互作用,因此在传播过程中容易分叉而形成大量次生裂纹,提高了试样内裂纹密度。这与碎片的CT表征结果一致,即碎片平均球形度和伸长、扁平指数等均随应变率对数线性增加。破坏模式的改变最终导致高应变率下陶瓷材料应变率敏感性显著增强。

     

Modelling of dynamic ductile fracture and application to the simulation of plate impact tests on tantalum

A model of dynamic damage by void nucleation and growth is proposed for elastic-viscoplastic materials sustaining intense loading. The model is dedicated to ductile materials for which fracture is caused by microvoiding. The material contains potential nucleation sites where microvoids are generated when the local pressure overcomes the nucleation pressure. A probability density function is adopted to describe the fluctuation of the nucleation pressure within the material. The void growth is described by using a hollow sphere model where micro-inertia effects are accounted for. The matrix weakening due to void growth is also included.The model has been first tested under uniaxial deformation. When the strain rate is assumed constant, the pressure inside the material has nearly a linear response up to a maximum. An analytical expression for the maximum pressure is proposed.Finite element simulations of plate impact tests have been carried out and compared to experiments on tantalum. From simulations based on the proposed model, an increase of the spall strength is observed with higher shock intensities. Therefore, the relationship between the velocity pullback and spall strength usually assumed in the literature (based on the acoustic approach) seems to be inadequate. Velocity profiles are simulated for different flyer thicknesses and different impact velocities with close agreement with experiments.     

超细晶D6A钢动态拉伸力学特性实验研究

为了推进超细晶D6A钢在半穿甲战斗部壳体上的应用,研究了动态加载下其宏观力学行为和细观变形机理。运用旋转盘式Hopkinson拉杆技术,开展了超细晶D6A低合金钢（平均晶粒尺寸为510 nm）的动态拉伸实验,获得了不同应变率（500～1000 s?1）下超细晶钢的应力-应变曲线。运用TEM观测微观形貌,从细观层次研究了高应变率拉伸作用下超细晶钢的动态力学特性。结果表明,超细晶D6A钢具有较高的动态拉伸强度和良好的延展性。并且,晶粒细化和纳米析出相（渗碳体）是超细晶钢同时拥有高强度和较好韧性的重要因素;在动态拉伸过程中析出的大量纳米级渗碳体,与高密度晶界共同作用限制了位错运动,从而产生额外的塑性变形抗力,有效提升了超细晶钢的强度;在塑性变形阶段超细晶钢出现的明显应力下降现象,是可动位错密度增高的结果。     

Prediction of ductile fracture in tension by bifurcation,localization, and imperfection analyses

In this investigation, it is shown that the onset of ductile fracture in tension can be interpreted as the result of a supercritical bifurcation of homogeneous deformation and that this fact can be applied to predict ductile fracture initiation of materials with general imperfections or flaws. We focus on one dimensional quasi-static simple tension for rate-independent isotropic plastic materials. For deformation beyond the bifurcation point, multiple equilibrium paths appear. The homogeneous deformation, as one of the equilibrium paths, loses stability while the inhomogeneous paths are stable, thus indicating the occurrence of strain localization. This investigation also provides a physical example for the application of the Lambert W function in material localization analyses. Material instability is treated as the instability of a static system with dynamic perturbation. We also address the presence of microvoids in a power law plastic material as an unfolding of the supercritical pitchfork bifurcation. The imperfect system, idealized as spherical voids within the plastic matrix, is analyzed using the familiar Gurson model which is based on the presumption of a randomly voided material and characterized by the volume fraction of voids. If, in addition, the sizes of the microvoids are known, this then provides a length scale for the imperfection zone. In this manner, relevance to the sample size effects of strain-to-failure for ductile fracture initiation is addressed by considering separate zones with variations in void volume fractions. Fracture initiation predictions are presented and compare very well to existing experimental results.     

加载速率和几何尺寸对国产A508-III钢断裂行为影响的实验研究

由加载速率和几何约束改变而引起的压力容器钢韧脆转变问题是核能安全领域亟待解决的关键问题. 为了准确分析国产A508-III钢的动态断裂行为, 借助INSTRON VHS高速材料试验机, 开展了不同加载速率和几何尺寸条件下的国产A508-III钢的断裂韧性试验, 研究了加载速率和几何尺寸等因素对国产A508-III钢动态断裂韧性的影响. 研究表明, A508-III钢具有良好的抗冲击韧性, 随着加载速率的提高, 试样的总冲击吸收能基本保持恒定, 裂纹萌生吸收能量不断上升, 而裂纹扩展吸收能量呈下降趋势. J-Δa阻力曲线和条件起裂韧性J_Q随着几何约束的增加而降低, 随加载速率的增加而升高. 当达到某一临界速率时, 条件起裂韧性J_Q基本恒定, 试样断裂方式也由韧性断裂转变为韧?脆?韧混合断裂. 由于出现混合断裂模式, 发生脆性断裂时的最大J积分值J_max更适于描述国产A508-III钢的断裂韧性演化规律. 随着试样面外几何约束的降低, J_max随Δa_m的增加而线性增大. 试样面内几何约束越高, J_max与Δa_m之间的线性关系斜率越大. 随着试样几何约束的增加, 材料的韧脆转变速率增加, J_max值下降. 改变几何约束只能在有限的加载速率范围内改变材料的断裂方式, 当加载速率超过某个临界值时, 加载速率成为影响材料断裂方式的主要因素.      

Experimental investigation into plastic damage of microvoids with interaction

A mechanism of plastic flow localization in ductile matter near microvoids is studied. The voids with the size-scale of micromillimeter exist in sheet specimens under tensile loading, and the plastic strain field around voids is obtained by digital image processing of deformed grids. The size growth of the microvoids, the spacing change of the neighboring voids, and the development of shear bands in the ligament between the voids, are presented by experimental results accompanied with the plastic strain distribution, that gives good interpretation to the process of void growth and coalescence with the flow localization in the ligaments. The project supported by the National Natural Science Foundation of China     

氢在钢管壁内的扩散及氢腐蚀实验研究

运输含氢气氛的钢管会发生氢腐蚀而性能受损。本研究通过分析原子氢渗透扩散机理,选取常用的三种钢管（20#,16Mn,X52）进行Devanathane–Stachurski渗氢实验。采用扫描电子显微镜分析渗氢后钢材形貌,并以动电位极化曲线比较渗氢前后自腐蚀电位和电流的变化。结果表明：20#钢的氢扩散系数最大,X52最小。随着充氢电流密度增大与充氢时间增长,钢材腐蚀程度也加剧。动电位极化曲线测试表明,16Mn自腐蚀电流上升两个数量级,性能下降明显;X52钢的抗腐蚀性较好,性能下降最少。