韧性断裂准则在高强钢板料成形中的应用

 针对板料成形中的韧性断裂准则预测成形极限的方法,进行了综述和分析,提出了利用韧性断裂准则能够较好地预测塑性差的板料成形极限,而且还能考虑应变路径的变化．将Cockroft和Latham准则应用到高强度钢板DP590的成形预测中．对高强钢DP590进行了单向拉伸试验,获得了相应的物性参数．同时对该高强钢进行了方盒件成形试验,并进行了相应的有限元模拟．通过对高强钢的极限试验,利用有限元模拟获得了该材料的Cockroft和Latham准则常数．最后利用该常数对方盒件的拉深过程进行了缺陷的预测,模拟结果和试验结果完全吻合．表明韧性断裂准则是可以应用到高强度钢板的成形中的．     

锈损冷弯薄壁钢板的断裂机制与断裂模型

为研究锈蚀对冷弯薄壁钢板断裂机制的影响,本工作选取在工业环境中服役多年的C形钢檩条,从其平板部位和弯角部位截取标准试件进行单调拉伸试验,并通过有限元数值分析研究了锈坑深度、深径比和截面损失率对其应力三轴度和等效塑性应变的影响.拉伸试验结果表明:随锈蚀程度增大,试件的塑性硬化阶段和颈缩阶段逐渐缩短,屈服阶段逐渐消失;试件的屈服强度、极限强度和断裂应变逐渐减小.有限元分析结果表明:应力三轴度随锈坑深度和深径比增加逐渐增大,随截面损失率的增加变化不显著;等效塑性应变随锈坑深度和截面损失率增加明显增大,随锈坑深径比增加呈现先增大后减小的趋势.本工作还建立了应力三轴度和等效塑性应变相对值与锈坑深度、深径比和截面损失率的拟合公式,根据拟合结果,对空穴增长模型(VGM模型)和应力修正临界应变模型(SMCS模型)进行修正,从而推导出点蚀损伤和全面腐蚀损伤下冷弯薄壁钢板的断裂模型.     

应力状态在球形弹丸撞击6061-T6铝薄靶弹道行为数值预报的作用

针对部分金属材料延性断裂、应力三轴度及Lode参数同时相关问题,将两个含Lode参数影响的断裂准则写入有限元程序ABAQUS,用于1.6 mm厚6061-T6铝合金靶板在7.9 mm直径球形弹丸撞击下断裂行为及弹道极限数值预报。为揭示应力状态影响,用两种不考虑Lode参数影响的断裂准则进行有限元计算,并与试验对比。结果表明,引入Lode参数可提升6061-T6铝合金靶板断裂行为及抗侵彻性能的数值预报效果。     

基于Oyane韧性断裂准则的板料成形极限预测

本文基于Oyane韧性断裂准则,结合数值模拟方法,预测板料不同应变状态下的极限应变.准则中的材料参数通过单向拉伸和平面应变拉伸试验确定.在模拟胀形试验获得每一时间步应力、应变值的基础上,应用韧性断裂准则预测板料的成形极限.模拟结果表明用韧性断裂准则和数值模拟相结合的方法能成功获得板料的成形极限图.     

汽车用先进高强钢本构模型与韧性断裂模型研究进展

轻量化是当前汽车行业全产业链共同面对的课题,提高先进高强钢使用比例是实现汽车轻量化的有效手段。对先进高强钢本构模型与韧性断裂模型的充分研究有助于提高先进高强钢开裂分析和预测的准确性,从而推动先进高强钢工程的应用进程。目前,在先进高强钢的研究过程中,学者们通常通过多种应变强化模型的线性组合,或结合微观结构与宏观力学行为进行多尺度分析来建立本构模型;通过多种应力状态下的准静态拉伸实验以及使用仿真与实验混合的方法来标定韧性断裂模型的参数。以第三代先进高强钢中的淬火配分（QP）钢为重点讨论对象,介绍了制备工艺与材料特性及其相关研究进展,并介绍了QP钢本构模型的研究现状、新近发展的非耦合韧性断裂模型以及考虑了应力三轴度和罗德角参数影响的韧性断裂模型在先进高强钢上的应用现状,最后指出了先进高强钢本构模型和韧性断裂模型未来的研究方向。     

应力三轴度和应变率对6063铝合金力学性能的影响及材料表征

对6063铝合金试样在不同应力三轴度和不同应变率下进行拉伸试验,得到了该合金在这两种情况下的力学性能.研究结果表明:随着应力三轴度的减小,材料的等效弹性模量、等效屈服应力减小,但等效断裂应变增大;随着应变率逐渐增大,材料的屈服强度和断裂强度略有增大;断裂应变明显减小;抗拉强度基本不变.Johnson-cook本构模型及其断裂应变模型可以用来描述6063铝合金在不同三轴应力度和不同应变率下的本构及失效关系.通过材料表征,得出了Johnson-cook本构模型及其断裂应变模型中各个参数,为有限元(ABAQUS)模拟提供帮助.     

基于弯曲实验的DP980力学性能研究

目的 针对仅通过单向拉伸实验无法准确表征金属板材在弯曲成形过程中的力学性能变化的问题,研究通过弯曲实验获取材料力学性能参数.方法 对高强钢DP980展开力学性能测试研究,主要通过弯曲实验对材料弯曲变形过程中形成的弯矩曲率进行测试,将得到的弯矩曲率转化为应力-应变.分别将弯曲和拉伸得到的应力-应变数据导入到三点弯和辊弯成形有限元仿真中,预测板材的成形角度.结果 DP980弯曲变形时的屈服强度要大于轴向拉伸时的屈服强度;分别利用弯曲和拉伸实验测得的应力-应变数据进行仿真,与三点弯实验结果对比发现,采用弯曲实验得到的应力-应变数据对回弹量的预测偏大,而利用拉伸实验得到的应力-应变数据进行仿真,仿真得到的回弹量则偏小,弯曲实验下变形过程中的应变变化数据更加符合真实过程,与辊弯实验对比发现,利用弯曲实验数据进行仿真可以更准确地预测V形板的最终成形角度.结论 相较于单向拉伸实验,通过弯曲实验获取的材料力学性能参数可以更准确地描述材料在三点弯、辊弯成形过程中的力学性能变化.     

5182-O铝合金塑性成形性能表征

目的 结合复杂加载状态试验、塑性和损伤断裂本构模型及有限元应用,实现AA5182-O铝合金在复杂加载状态下塑性变形和损伤断裂行为的精确表征。方法 通过拉伸、剪切等试验,研究5182-O在剪切、单向拉伸、平面应变拉伸等复杂应力状态下的力学性能,应用pDrucker方程来表征其复杂加载状态下的塑性变形和损伤断裂特性。采用逆向工程方法实现pDrucker屈服方程和pDrucker断裂准则的精确标定。将标定后的塑性本构模型和损伤断裂准则应用到ABAQUS/Explicit中,预测不同试件的塑性变形和损伤断裂情况。结果 通过有限元模拟与试验结果的对比,发现有限元仿真准确预测了5182-O在复杂加载状态下的力-位移曲线和损伤断裂情况。结论 有限元模拟与试验结果的对比表明,pDrucker方程可以实现5182-O铝合金在复杂加载状态下塑性成形性能的精确表征。标定的pDrucker方程可应用于5182-O冲压成形过程的有限元分析、模具设计和工艺优化中。     

7075-T651铝合金靶板剪切冲塞的试验和数值模拟研究

为揭示在断裂准则中引入Lode角的必要性,在一级轻气炮上开展了刚性平头弹侵彻7075-T651铝合金靶板试验,获得了靶板的断裂行为和弹道极限;通过圆棒试样拉伸试验和平板试样剪切试验,标定了Johnson-Cook(JC)本构模型和断裂准则参数和一种Lode角相关的断裂准则;运用ABAQUS软件,分别采用Lode角相关和无关断裂准则对试验进行了三维数值模拟,并对Lode角的影响进行了分析。结果表明:在刚性平头弹撞击下,7075-T651铝合金靶板发生剪切冲塞破坏;7075-T651铝合金的断裂与Lode参数相关,剪切带中初始失效材料的应力三轴度低于0且Lode角接近0,用圆棒拉伸试验标定的JC断裂准则高估了断裂应变;采用JC断裂准则预报的断裂行为与试验有较大差别,得到的弹道极限比试验值高大约42%;采用Lode角相关断裂准则得到的弹道极限与试验结果吻合较好。     

基于SMCS模型的高强螺栓及节点火灾全过程断裂性能模拟

高强螺栓广泛应用于钢结构节点连接,火灾高温会影响其基本材性和断裂行为,从而影响螺栓节点抗火性能甚至整体结构抗倒塌性能。基于10.9级高强螺栓火灾全过程(升温段、降温段、火灾后)单轴拉伸试验结果,结合有限元模拟,对不同温度历程和应力三轴度对应的螺栓SMCS断裂模型进行校准,并与螺栓材性试验和T-stub节点试验结果对比验证;对T-stub节点火灾全过程断裂行为进行参数分析,研究损伤准则和温度历程对节点失效模式和变形特征的影响。结果表明：校准的SMCS模型能够有效、准确地预测螺栓和节点在火灾全过程和高应力三轴度(0.3～1.2)下的受拉断裂行为,适用预测误差在12%以内;拉伸温度和峰值温度是影响高强螺栓抗断能力的主要因素,螺栓抗断能力随温度升高而提高;不同温度历程下T-stub节点可能发生翼缘板屈服断裂、翼缘板和螺栓同时屈服断裂、螺栓屈服断裂三种失效模式,且节点的变形能力(延性系数)与失效模式有关,确定钢板母材和螺栓的断裂模型是准确预测节点失效模式的关键。     

Application of extended Mohr–Coulomb criterion to ductile fracture

The Mohr–Coulomb (M–C) fracture criterion is revisited with an objective of describing ductile fracture of isotropic crack-free solids. This criterion has been extensively used in rock and soil mechanics as it correctly accounts for the effects of hydrostatic pressure as well as the Lode angle parameter. It turns out that these two parameters, which are critical for characterizing fracture of geo-materials, also control fracture of ductile metals (Bai and Wierzbicki 2008; Xue 2007; Barsoum 2006; Wilkins et al. 1980). The local form of the M–C criterion is transformed/extended to the spherical coordinate system, where the axes are the equivalent strain to fracture [`(e)]<sub>f</sub>{\bar \varepsilon_f} , the stress triaxiality η, and the normalized Lode angle parameter [`(q)]{\bar \theta} . For a proportional loading, the fracture surface is shown to be an asymmetric function of [`(q)]{\bar \theta}. A detailed parametric study is performed to demonstrate the effect of model parameters on the fracture locus. It was found that the M–C fracture locus predicts almost exactly the exponential decay of the material ductility with stress triaxiality, which is in accord with theoretical analysis of Rice and Tracey (1969) and the empirical equation of Hancock and Mackenzie (1976), Johnson and Cook (1985). The M–C criterion also predicts a form of Lode angle dependence which is close to parabolic. Test results of two materials, 2024-T351 aluminum alloy and TRIP RA-K40/70 (TRIP690) high strength steel sheets, are used to calibrate and validate the proposed M–C fracture model. Another advantage of the M–C fracture model is that it predicts uniquely the orientation of the fracture surface. It is shown that the direction cosines of the unit normal vector to the fracture surface are functions of the “friction” coefficient in the M–C criterion. The phenomenological and physical sound M–C criterion has a great potential to be used as an engineering tool for predicting ductile fracture.     

Stress based fracture envelope for damage plastic solids

Recent development of damage plasticity theory shows the critical plastic strain at fracture for ductile solids depends on the pressure and the Lode angle on the octahedral plane along the loading path. The determination of the fracture strain envelope is usually a difficult and time consuming process. This is due to the experimental difficulties in maintaining a constant pressure and Lode angle at the fracture site, which is further complicated by the coupled nature of the parameters to be calibrated and the geometrical localization of the deformation. The fracture strain envelope is one of the key ingredients of the damage plasticity theory and relates to the accuracy of predicted results. In the present paper, the Lode angle dependence and the pressure sensitivity functions for the fracture strain envelope are derived from the hardening rule of the matrix using Tresca type fracture condition and Drucker-Prager formula, respectively. Quantitative analyses of Clausing’s and Bridgman’s test data are presented. Then a pressure modified maximum shear stress condition is adopted as fracture initiation condition to examine their joint effects on the fracture strain envelope. The relationship of the strain hardening, the pressure sensitivity and the Lode angle dependence are examined and verified by existing experimental results. We show that within the moderate range of stress triaxiality, the pressure modified maximum shear condition can be used as the fracture stress envelope for ductile metals within the framework of damage plasticity. The present method reduces significantly the amount of work to calibrate the material parameters for ductile fracture.     

Extension of the modified Bai‐Wierzbicki model for predicting ductile fracture under complex loading conditions

The ductile fracture behaviour of metallic materials is strongly dependent on the material's stress state and loading history. This paper presents a concept of damage initiation and failure indicators and corresponding evolution laws to enhance the modified Bai‐Wierzbicki model for predicting ductile damage under complex loading conditions. The proposed model considers the influence of stress triaxiality and the Lode angle parameter on both damage initiation and the subsequent damage propagation. The model parameters are calibrated for C45E + N steel using a series of mechanical tests and numerical simulations. The enhanced approach is applied to the modelling of various mechanical tests under proportional and non‐proportional loading conditions and successfully predicts the ductile damage behaviour in these tests.     

Continuum damage mechanics modelling incorporating stress triaxiality effect on ductile damage initiation

Micromechanical modelling of void nucleation in ductile metals indicates that strain required for damage initiation reduces exponentially with increasing stress triaxiality. This feature has been incorporated in a continuum damage mechanics (CDM) model, providing a phenomenological relationship for the damage threshold strain dependence on the stress triaxiality. The main consequences of this model modification are that the failure locus is predicted to change as function of stress triaxiality sensitivity of the material damage threshold strain and that high triaxial fracture strain is expected to be even lower than the threshold strain at which the damage processes initiate at triaxiality as low as 1/3. The proposed damage model formulation has been used to predict ductile fracture in unnotched and notched bars in tension for two commercially pure α‐iron grades (Swedish and ARMCO iron). Finally, the model has been validated, predicting spall fracture in a plate‐impact experiment and confirming the capability to capture the effect of the stress state on material fracture ductility at very high stress triaxiality.     

Optimized butterfly specimen for the fracture testing of sheet materials under combined normal and shear loading

The present paper is concerned with multi-axial ductile fracture experiments on sheet metals. Different stress-states are achieved within a flat specimen by applying different combinations of normal and transverse loads to the specimen boundaries. The specimen geometry is optimized such that fracture initiates remote from the free specimen boundaries. Fracture experiments are carried out on TRIP780 steel for four different loading conditions, varying from pure shear to transverse plane strain tension. Hybrid experimental–numerical analyses are performed to determine the stress and strain fields within the specimen gage section. The results show that strain localization cannot be avoided prior to the onset of fracture. Through-thickness necking prevails under tension-dominated loading while the deformation localizes along a band crossing the entire gage section under shear-dominated loading. Both experimental and simulation results demonstrate that the proposed fracture testing method is very sensitive to imperfections in the specimen machining. The loading paths to fracture are determined in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain. The experimental data indicates that the relationship between the stress triaxiality and the equivalent plastic strain at the onset of ductile fracture is not unique.     

Analysis of fracture limit curves and void coalescence in high strength interstitial free steel sheets formed under different stress conditions

Void formation, which is a statistical event, depends on inhomogeneities present in the microstructure. The analysis on void nucleation, their growth and coalescence during the fracture of high strength interstitial free steel sheets of different thicknesses is presented in this article. The analysis shows that the criterion of void coalescence depends on the d-factor, which is the ratio of relative spacing of the ligaments (δd) present between the two consecutive voids to the radius of the voids. The computation of hydrostatic stress (σ_m), the dominant factor in depicting the evolution of void nucleation, growth and coalescence and the dimensional analysis of three different types of voids namely oblate, prolate and spherical type, have been carried out. The ratio of the length to the width (L/W) of the oblate or prolate voids at fracture is correlated with the mechanical properties, microstructure, strains at fracture, Mohr’s circle shear strains and Triaxiality factors. The Lode angle (θ) is determined and correlated with the stress triaxiality factor (T), ratio of mean stress (σ_m) to effective stress (σ_e). In addition, the Void area fraction (V _a), which is the ratio of void area to the representative area, is determined and correlated with the strain triaxiality factor (T_o).     

Stress triaxiality effect on void nucleation in ductile metals

The stress triaxiality effect on the strain required for void nucleation by particle‐matrix debonding has been investigated by means of micromechanical modelling. A unit‐cell model considering an elastic spherical particle embedded in an elastic‐plastic matrix was developed to the purpose. Particle‐matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix‐particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime.     

Role of stress triaxiality in elastoplastic characterization and ductile failure prediction

The triaxiality of the stress state is known to greatly influence the amount of plastic strain which a material may undergo before ductile failure occurs.During tensile load histories, the necking induces significant stress triaxiality modifications which in turn affect the experimental stress-strain measurements needed for the characterization of ductile metals.In this paper, the recently proposed “MLR” model of necking effect is used to obtain the flow curves of various metals by correcting the experimental data of tensile tests. Finite elements simulations of the experimental tests are performed to calculate the stress triaxiality evolution on various notched and unnotched specimens. A ductile failure criterion, due to Bao and Wierzbicki, is then applied to evaluate the material damage and predict failure. This procedure is applied to a set of 20 specimens series made of six metals with 10 different notch shapes.The damage calculations also indicate the material points where failure initiates. These predictions are confirmed by micrographic observation of voids on polished fragments of the broken specimens.     

On the Effect of Stress Triaxiality on Void Coalescence

Failure of ductile materials is often related to coalescence of microscopic voids. The stress triaxiality is one of the primary factors that influence the coalescence. In the present work, a 3D unit cell model is employed to investigate this effect. The cell model contains two aligned voids. A coalescence criterion is proposed in which the critical void volume fraction is expressed in terms of stress triaxiality.