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.     

基于J-C模型的镁合金MB2动静态拉伸破坏行为

为了研究不同应力状态和应变率条件下镁合金MB2的拉伸破坏行为,利用材料试验机和分离式Hopkinson拉杆(SHTB),对镁合金MB2的光滑及缺口圆柱试件进行了动静态拉伸加载;拟合得到了镁合金MB2的动静态拉伸本构关系,建立了其修正的Johnson-Cook失效破坏准则,并对不同试件的拉伸破坏行为进行了数值模拟;利用SEM对宏观破坏模式对应的微观损伤机理进行了分析。结果表明,随着应力三轴度的增加,镁合金MB2的等效破坏应变先增大后减小,宏观破坏模式由剪切转为正拉断,微观损伤机制由混合断裂转变为韧窝断裂;而随着应变率的增加,等效破坏应变不断减小,破坏模式不发生改变。Johnson-Cook本构关系和修正后的Johnson-Cook失效破坏准则能较好地拟合动态静态拉伸实验结果并预测不同试件的杯锥形破坏特征。     

基于试验与有限元耦合技术的延性材料全程单轴本构关系获取方法

摘 要: 材料拉伸直至断裂的全程单轴本构关系对材料大变形和断裂机理研究具有重要意义。传统拉伸试验获取的材料真应力-真应变曲线在试样颈缩后不可测。借助可以精确测量三维变形的DIC(Digital image correlate) 技术和有限元分析技术(Finite element analysis),本文提出了基于漏斗试样拉伸试验获取材料全程单轴本构关系的新方法,即TF(Test and FEA)方法。该方法将TF方法获取的材料全程单轴应力应变关系曲线作为有限元软件中的材料本构关系对漏斗试样拉伸变形过程进行模拟,其模拟载荷-位移曲线、漏斗根部直径-位移曲线和漏斗变形轮廓线等均与试验结果吻合良好,试样表面模拟应变也与DIC测试结果吻合, 根据不同半径漏斗试样模拟获得的全程真应力-真应变曲线保持良好一致性。最后,还对试样颈缩断面的力学行为进行了讨论,并给出了304不锈钢、汽轮机叶片材料2Cr12Ni4Mo3VNBN和 1Gr12Ni3Mo2VN、汽轮机转子材料30Cr2Ni4MoV的全程单轴本构关系模型参数、破断应力和破断应变。     

轴向拉伸载荷作用下缺口圆棒的应力三轴度

应力三轴度是表达应力状态的参量,可作为变量表征材料的塑性和断裂损伤模型,在结构强度和失效分析中发挥重要作用.具有缺口的圆棒拉伸试验可用于标定塑性和损伤模型中的参数.然而,文献中却存在两个不同的公式计算拉伸载荷作用时缺口圆棒最小截面轴心处的三轴度,二者分别由国际著名学者Bridgman和Wierzbicki先后提出,其不同性往往造成应用时的困惑.本文通过精细化的有限元计算分析,意在澄清两个公式的有效性和适用性.结果表明,Bridgman公式仅弹性阶段和特定a/R范围内较为准确,Bao-Wierzbicki公式与数值模拟结果及实验数据吻合较好,可用于计算整个拉伸过程中三轴度的算术平均值.基于进一步的分析,本文提出了理想弹塑性条件下塑性阶段的应力三轴度新的修正公式,还讨论了缺口几何和应变强化效应：指出不同的缺口比例会影响到颈部应力场,缺口比例越小,弹性阶段的应力三轴度越接近1/3;而应变强化则会导致拉伸过程中应力三轴度的降低.     

考虑应力三轴度影响的30CrMnSiNi2A钢韧性断裂研究

30CrMnSiNi2A钢是一种在军工领域应用广泛的低合金高强度钢。针对结构完整性的评估问题,采用试验和数值计算结合的方法研究了30CrMnSiNi2A钢的韧性断裂特性。对光滑圆棒试件在不同温度下进行准静态和动态拉伸试验,并通过有限元迭代方法标定了材料的Johnson-Cook动态本构模型参数,分析了温度和应变率对30CrMnSiNi2A钢断裂行为的影响。开展了缺口圆棒拉伸、缺口平板剪切和圆柱压缩试验,计算了各试件对应的平均应力三轴度和断裂应变,给出了应力三轴度在?1/3～1.5区间内的断裂应变变化曲线,分别确定了Johnson-Cook和Bao-Wierzbicki失效模型参数。研究表明,30CrMnSiNi2A钢的断裂应变与应力状态密切相关,且在不同的应力三轴度区间内曲线单调性差异较大,Bao-Wierzbicki失效模型较好地描述了这种钢在不同应力状态下的断裂特性。     

应力三轴度的有限元计算修正

对某高强度钢制成的光滑圆棒和缺口圆棒进行了系列准静态拉伸实验,采用ABAQUS对每个试 件进行了数值模拟,得到了该材料的真实应力应变曲线,拟合出了J-C本构模型和失效模型的部分材料常数。 最后,对该高强度钢制成的平板进行了撞击实验,并用得到的J-C模型对平板撞击实验进行了数值模拟,计算 结果与实验结果吻合很好,证明利用数值模拟并修正应力三轴度的方法是可行的。     

镁合金MB2在高应力三轴度下的拉伸破坏行为研究

通过光滑试件及不同曲率半径缺口圆柱试件的拉伸试验,实现对镁合金MB2的单向及多向应力状态加载。结合数值模拟分析,研究了不同试件在拉伸加载过程中应力状态的变化。以应力三轴度为参数,给出了镁合金MB2等效破坏应变的变化规律,在应力三轴度-等效破坏应变空间建立了镁合金MB2的失效破坏准则。利用扫描电镜对试件断口形貌进行观察,分析了导致材料宏观延性变化的微观损伤机理,对不同应力状态下镁合金MB2的失效破坏行为做出了合理解释。     

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。     

混凝土单轴受拉的非局部本构模型

混凝土受拉本构行为存在很强的局部软化现象,使得单轴受拉试验无法给出应力-应变关系,而只能给出应力-位移关系。本文根据内变量理论和等效应变假设建立了基于真实应变的混凝土单轴受力本构方程,并根据Weibull分布可以描述混凝土等脆性材料断裂过程的试验现象,建立了关于弹性应变的损伤演化规律。然后,通过假设平均应变与真实弹性应变的函数关系,在应力-平均应变的本构关系中采用平均弹性应变以描述其非局部行为,而在材料的损伤演化规律中采用真实弹性应变以描述其局部行为,由此建立了单轴受拉荷载条件下的非局部本构模型。最后,对一个单调受拉试验和一个反复受拉试验的仿真结果表明所提出的非局部本构模型可以准确地模拟试验结果。     

基于BP神经网络与小冲杆试验确定在役管道钢弹塑性性能方法研究

为了能够在不停输油气工况下获得在役管道材料的弹塑性力学性能, 提出了一种人工智能BP (back-propagation)神经网络、小冲杆试验与有限元模拟相结合,通过确定材料真应力-应变曲线从而获得材料弹塑性力学性能的方法. 首先,通过系统改变Hollomon公式中的参数$K$, $n$值,获得457组具有不同弹塑性力学性能的假想材料本构关系, 其次,将得到的本构关系代入经试验验证的含有Gurson-Tvergaard-Needleman(GTN)损伤参数的小冲杆试验二维轴对称有限元模型,通过有限元计算得到了与真应力-应变曲线一一对应的457条不同假想材料的载荷-位移曲线,最终将两组数据作为数据库输入BP神经网络进行训练,建立了同种材料小冲杆试验载荷-位移曲线与真应力-应变曲线之间的关联关系.通过此关联关系,可利用试验得到的小冲杆载荷-位移曲线获取在役管道钢的真应力-应变曲线,从而确定其弹塑性力学性能.通过对比BP神经网络得到的X80管道钢真应力-应变曲线与单轴拉伸试验的结果以及引用现有文献中不同材料的试验数据对此关系进行验证,证明了该方法的准确性与广泛适用性.      

Evaluation of Ductile Damage Parameters under High Stress Triaxiality

The GTN model proposed by Gurson, Tvergaard and Needleman has been widely applied to predict ductile fracture. However, the evaluation of the GTN model under high stress triaxiality has only been reported in a few studies. In this paper, a series of tensile tests on round notched specimens were performed to evaluate the applicability of the GTN model parameters under high stress triaxiality. The evaluation was carried out by comparing the predicted load-displacement curves with experimental results. It was observed the GTN model parameters only depend on the material except the critical void volume fraction. The influence of stress triaxiality on the critical void volume fraction was discussed. A further discussion about the construction of a new void coalescence criterion for the GTN model was also presented in this paper.     

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.     

Simulation of damage evolution in ductile metals undergoing dynamic loading conditions

A set of constitutive equations for large rate-dependent elastic-plastic-damage materials at elevated temperatures is presented to be able to analyze adiabatic high strain rate deformation processes for a wide range of stress triaxialities. The model is based on the concepts of continuum damage mechanics. Since the material macroscopic thermo-mechanical response under large strain and high strain rate deformation loading is governed by different physical mechanisms, a multi-dissipative approach is proposed. It incorporates thermo-mechanical coupling effects as well as internal dissipative mechanisms through rate-dependent constitutive relations with a set of internal variables. In addition, the effect of stress triaxiality on the onset and evolution of plastic flow, damage and failure is discussed.Furthermore, the algorithm for numerical integration of the coupled constitutive rate equations is presented. It relies on operator split methodology resulting in an inelastic predictor-elastic corrector technique. The explicit finite element program LS-DYNA augmented by an user-defined material subroutine is used to approximate boundary-value problems under dynamic loading conditions. Numerical simulations of dynamic experiments with different specimens are performed and good correlation of numerical results and published experimental data is achieved. Based on numerical studies modified specimens geometries are proposed to be able to detect complex damage and failure mechanisms in Hopkinson-Bar experiments.     

Experimental Research into Fracture of EN-AW 2024 and EW-AW 2007 Aluminum Alloy Specimens with Notches Subjected to Tension

The paper presents the results of experimental tests of elasto-plastic fracture of axially symmetric specimens with circumferential notches with various round notch radii at the notch’s root. The tests were conducted for two materials: EN-AW 2024 and EN-AW 2007 aluminum alloys. The specimens were subjected to monotonic uniaxial tensile test. Special attention was paid to the shape of the fracture surface, the change of critical force value and the maximum displacement, depending on the shape of the notch. Additionally, hardening curves are presented for the adopted materials and the results of Finite Element calculations of stress and strain fields in the specimens with different notch radii.     

压电切口张开角和深度对其尖端力电损伤场的影响

基于三维各向异性压电损伤本构理论,导出了广义平面应力问题的损伤本构方程,并据此分析了压电薄板板边V形切口尖端附近的力电损伤,研究了切口张开角和深度对切口尖端力电损伤的影响规律．结果发现：和张开角对切口尖端损伤的影响相比,深度的影响更为明显;在张开角对切口尖端力损伤的影响规律方面,压电材料与一般弹塑性材料存在明显差异,原因在于压电切口尖端力电载荷比会随着深度的改变发生很大变化;不同深度下张开角与切口尖端力、电损伤关系曲线随着张开角的增大由发散逐渐会聚,不同张开角下深度与切口尖端力、电损伤关系曲线随着切口加深由会聚逐渐发散,并且电损伤曲线表现得更为明显．     

Damage in edge cracking of rolled metal slabs

Materials get damaged under shear deformations. Edge cracking is one of the most serious damage to the metal rolling industry, which is caused by the shear damage process and the evolution of anisotropy. To investigate the physics of the edge cracking process, simulations of a shear deformation for an orthotropic plastic material are performed. To perform the simulation, this paper proposes an elasto-aniso-plastic constitutive model that takes into account the evolution of the orthotropic axes by using a bases rotation formula, which is based upon the slip process in the plastic deformation. It is found through the shear simulation that the void can grow in shear deformations due to the evolution of anisotropy and that stress triaxiality in shear deformations of (induced) anisotropic metals can develop as high as in the uniaxial tension deformation of isotropic materials, which increases void volume. This echoes the same physics found through a crystal plasticity based damage model that porosity evolves due to the grain-to-grain interaction. The evolution of stress components, stress triaxiality and the direction of the orthotropic axes in shear deformations are discussed.