考虑材料微观塑性损伤的焊接接头延性裂纹扩展行为分析

通过考虑材料微观塑性损伤，定量研究了焊接接头中强度匹配、试件几何形状对接头抗延性裂纹扩展行为的影响。对管线用钢焊接接头，测试了标准三点弯曲试样、同种接头及不同匹配状态的双边裂纹拉伸试样的阻力曲线。通过考虑材料塑性损伤的数值模拟，由标准三点弯曲试样的阻力曲线测试结果，得出反映材料微观损伤的特殊单元模型控制参量，再根据该参量对同种接头及不同接头强度匹配下双边缺口拉伸试件的阻力曲线进行了定量预测，其结果与试验结果相当吻合。结果表明，考虑塑性损伤的特殊单元模型能很好地描述接头的抗延性裂纹扩展阻力特性。     

韧性断裂准则及损伤模型在冲裁有限元模拟中的应用研究

在冲裁有限元模拟中,韧性断裂准则的选择会对冲裁件断面质量与尺寸精度产生很大影响。为了获得符合实际的模拟结果,进而优化冲裁工艺,重点研究了一些常用的韧性断裂判定依据,并从物理学角度阐述了韧性断裂机制。基于试验的韧性断裂准则,考虑了变形历史中的应力应变关系,使用由反求法确定的临界值来判定韧性断裂的发生与否。基于连续损伤力学建立的损伤模型,考虑了变形过程中损伤累积对材料本构关系的影响,能够更准确地描述断裂过程。此外,还分析了冲裁有限元模拟中的关键技术,如采用任意拉格朗日欧拉方法来解决网格畸变问题,使用单元分裂、单元分离与单元删除等技术来处理裂纹的萌发与扩展。探讨了目前韧性断裂模拟中存在问题以及未来发展方向。     

一种基于应力三轴度的断裂准则及其在剪切工艺中的应用

将拉伸均匀变形阶段应力-应变的外推曲线作为初始本构关系,通过反复迭代拟合载荷-位移曲线来确定真实本构关系,很大程度上提升了材料模型的可靠性。采用试验与有限元模拟相结合的方法来研究缺口试样的断裂情况,获得了不同应力状态下应力三轴度与断裂应变间的关系曲线,并以此作为材料发生韧性断裂的判据。将基于拉伸试验获得的材料模型应用于剪切有限元模拟中来预测断面质量,采用ALE方法来提高单元网格质量,利用单元删除法来模拟裂纹的萌生与扩展,分析了剪切机制与损伤分布。最后,对冲裁断面形貌进行试验验证,模拟结果与剪切试验结果吻合程度高。     

6016铝合金冲压损伤演化和韧性断裂的细观力学表征

为了预测铝合金板料在冲压过程中的损伤演化和韧性断裂,在Gurson-Tvergaard-Needleman (GTN)细观损伤模型中引入Hill'48各向异性屈服准则,借助有限元软件LS-DYNA编写了能考虑材料各向异性的用户自定义子程序VUMAT。采用沿不同轧制方向的准静态拉伸试验获取6016铝合金材料的力学参数,并对样件断口微观形貌进行分析。为了准确表征6016铝合金在冲压过程中的损伤演化和韧性断裂性能,基于传统有限元标定法,新增样件断裂方式及断后颈缩量作为评判标准,利用修正的有限元标定法确定了一组材料的GTN损伤参数。运用Hill'48-GTN模型对6016铝合金杯突冲压过程进行了数值仿真并分析了其损伤演化机理,通过与试验结果的对比,验证了Hill'48-GTN损伤模型及修正有限元标定法的可行性。     

韧-脆转变温度区间内焊接接头断裂韧度预测

首先对采用同种焊接材料施焊的X80及X65管线用钢焊接接头,在韧脆转变温度区间测试了其断裂韧度,试验研究了强度匹配、试样几何形式对接头断裂行为的影响。然后根据局部法由标准三点弯曲试样的断裂韧度值,得出反映材料的脆性断裂控制参量m、σu,再根据该参量对同种接头及不同强度匹配接头双边裂纹拉伸试样的断裂行为进行了定量预测,当考虑韧脆转变温度区间内的延性裂纹扩展后,预测结果与试验结果相当吻合。表明,局部法能很好地描述材料的断裂行为。     

基于应力的断裂准则对AA7075-T6韧性断裂行为的预测研究

通过试验研究了AA7075-T6在剪切、单轴拉伸和平面应变拉伸应力状态下的塑性变形和韧性断裂行为。然后采用Swift-Voce硬化定律和pDrucker屈服函数表征了板材在不同加载状态下的塑性变形特性。最后，采用基于应力的pDrucker韧性断裂准则表征了不同应力状态下塑性变形过程中的韧性断裂行为。为了提高数值模拟预测精度，塑性本构模型和pDrucker韧性断裂准则参数均采用试验-模拟相结合的逆向工程方法进行优化标定。将有限元预测的韧性断裂位移与不同试件载荷-行程曲线进行了比较。结果表明，通过逆向工程优化的pDrucker韧性断裂准则能够准确描述AA7075-T6不同应力状态下的韧性断裂行为。     

铝合金板材胀形极限的数值模拟研究

应用韧性断裂准则与有限元数值模拟相结合的方法预测了铝合金板料的胀形极限.将有限元模拟获得板材的应力、应变值代入考虑应力三轴度的Oyane韧性断裂准则进行断裂判断,预测出初始断裂点.准则中的材料常数通过单向拉伸和平面拉伸试验确定.计算了三种铝合金板的半球形凸模胀形极限,计算结果表明,应用Oyane韧性断裂准则能有效地预测铝合金板材的胀形极限.     

基于MMC韧性断裂准则的高强双相钢成形极限研究北大核心CSCD

以高强双相钢CR450/780DP板材为研究对象,设计4种拉伸试样进行单向拉伸试验,获得了载荷-位移曲线与表面全场应变结果;采用有限元仿真分析手段,对4种试样的试验结果进行了对标分析,得到了材料硬化本构模型及应力三轴度、Lode角参数、等效塑性应变等历程数据;采用曲面拟合优化方法标定MMC韧性断裂准则的断裂参数。基于Keeler公式及简化的MMC韧性断裂准则分别绘制了高强双相钢CR450/780DP板材的理论和预测成形极限图,并通过半球形刚模胀形试验对预测结果进行验证。由对比结果可知,基于MMC韧性断裂准则预测的成形极限曲线与试验数据的吻合程度较高,验证了韧性断裂准则对高强双相钢CR450/780DP板材损伤与断裂预测的准确性与适用性。     

负间隙精冲的大变形弹塑性有限元分析

负间隙精密冲裁是一个复杂的力学过程,涉及弹性变形、塑性变形、损伤软化和韧性断裂等现象.本文使用DEFORM-2D软件对AISI-1045钢进行了弹塑性大变形有限元数值模拟,将McClintock断裂准则应用于负间隙精冲韧性断裂的预测中,预测了材料变形过程中的静水应力、等效应力和等效应变的分布以及发展趋势.并依据仿真条件和模拟结果分析,通过实验的方法获得了不同负间隙下的无毛刺冲裁件.     

考虑材料韧性损伤的中厚板半冲孔成形过程分析

为研究半冲孔成形过程中韧性损伤的演化以及部分工艺参数对成形质量的影响规律,本文在ABAQUS有限元模拟软件中建立了半冲孔轴对称有限元模型,并通过VUMAT用户子程序引入GTN(Gurson-Tvergaard-Needle-man)损伤模型,结合同时考虑空穴形状与体积变化影响的韧性断裂准则,进行弹塑性大变形有限元分析.基于该有限元模犁,预测了半冲孔工艺中静水压力、等效应力、等效应变、应力三轴度以及损伤断裂的产生和发展趋势,分析了反顶力、压边力和冲裁间隙对零件的影响规律,并与实验结果进行对比分析,验证了数值模拟的准确性.     

NUMERICAL MODELING OF DUCTILE CRACK EXTENSION USING COMPUTATIONAL CELL ELEMENTS FOR WELDED JOINTS

1.IntroductionConstrainteffectsonfracturetoughnessofferritesteelsremainakeyissueforthesafetyaJssessmentofstructures.Strllcturalandpressurevesselsteelsgenerallyexhibitincreaseinfracturetoughnessoverthefirstfewdistance0fstablecrackgr0wth.Laborat0rytestingoffracturespecimenstomeasureresistancecurves(R-curves)c0nsistentlyrevealsamarkedeffect0fabsolutespecimensize,ge0metry,andloadingmode0nRcurvesI1-3].Forthesamematerial,deep-notchbendspecimensyieldl0wRcurvewhiletheshallow-notchbendspecimenortensi…     

PROBABILISTIC MODELING OF BRITTLE FRACTURE WITH PRIOR DUCTILE CRACK EXTENSION

1.AnreductionRecently,tWoproghssivemethodshavebeenproposedtoaddressthet~sferfmmthefracturemechanicstestresultstothefractllrebehaviorofrealisticstructllralcomponentsL'--4).OneistheJ--TandJ--Qth..ri.,[sj,WheretheT--stressandQ--packeterhavebeendevelopedtoqUantifythedifferencebetweenthefull--fieldsolutionfortheneaxcracktipstressdistributionandK--controlledortheHRRstressfields.Anotheroneisaprobabilisticmethod,localapproach"l"inWhichtheWiibLlllstress6Wisdefinedasthenewcrackdrivingforce,atbrit…     

A NOVEL PARAMETER FOR EVALUATING THE FATIGUE CRACK GROWTH RATE IN CARBON STEELS

1. Introduction Estimation of the fatigue life of a component under cyclic loading is of great interest in the modern engineering design against plastic deformation or conventional fracture. The fatigue process of materials may be approximately divided into two stages, i.e. the fatigue crack initiation and its propagation. The latter is more significant in service, because most of the fatigue life of many engineering components is spent on the crack growth, especially the small crack growth. …     

残余应力对管线钢韧性断裂的影响

以弹塑性断裂力学理论为基础,应用ABAQUS有限元软件,采用固有应变法引入残余应力,并基于细观塑性损伤模型研究了残余应力对管线钢韧性裂纹扩展阻力曲线的影响.模型中选择了具有不同深浅裂纹的单边缺口弯曲（SENB）试样和DNV推荐准则中用于管线钢断裂评估的单边缺口拉伸（SENT）试样,并对这两种断裂力学试样在大范围屈服条件下的裂纹扩展阻力曲线（CTOD-R曲线）行为进行了对比分析.结果表明,对于深裂纹试样,残余应力降低了韧性裂纹扩展阻力曲线,而浅裂纹试样受残余应力的影响可忽略不计.     

Ductile fracture model in the shearing process of zircaloy sheet for nuclear fuel spacer grids

Features of sheared edges are predicted based on material properties of Zircaloy obtained from the tensile test and ductile fracture model such as the Gurson-Tvergaard-Needleman (GTN) and Johnson-Cook models. The sheared edges formations are numerically analyzed in each ductile model. An appropriate ductile fracture model is selected to study the relative depth of sheared edges with respect to process parameters. The tendency of failure parameters that are affected by sheared edges and fracture duration is investigated. We applied changes on parameters of failure models to show that the punch force curve and the ratio of characteristic lengths could be coincided, which led us to conclude that the GTN and Johnson-Cook models are equivalent. In the Johnson-Cook model, however, the characteristic length of the sheared edges does not change as each failure parameter reaches a critical value. Hence, the FE prediction model for forming defects is developed using the GTN failure model. Finally, the characteristic length of sheared edges have been measured using the FE prediction model for shearing process parameters such as punch velocities, clearance, and tool wear. Our results showed that the punch-die clearance is the most significant factor that affects forming defects when compared to other factors.     

Experimental investigation of void coalescence in metallic sheets containing laser drilled holes

Although important, ductility remains difficult to predict and there is a tremendous need for more precise modelling. Progress in this field is hampered by a lack of quantitative experimental results to assess the validity of these models due to the stochastic nature of ductile fracture. In this paper, tensile tests have been carried out in a scanning electron microscope on model materials made of thin metallic sheets containing laser drilled holes. Depending on the material and hole configuration, different failure modes and strains are observed. The results show the importance of void spacing and orientation, constraining effects, materials yield stress and work hardening rate, and the competition between ductile fracture and shear localization. Finally, it is shown that the Thomason model for void coalescence is not appropriate for predicting fracture of the model material. However, the McClintock model for void growth, and the Brown and Embury and the McClintock models for void coalescence provide relatively good predictions.     

Study on ductile crack initiation limit from notch root of pipeline girth welded joints with strength mismatch

This study investigated ductile crack initiation limit of pipeline girth welded joints with different strength mismatches. The ductile crack initiation limit for the girth welded joints was evaluated by conducting three-point bending fracture toughness tests and wide plate tensile tests with a surface notch. In addition, effect of heat input on the ductile crack initiation limit of weld metal (WM) was evaluated on the assumption that a welding condition would be varied in the field in the actual pipeline construction. As the results, the equivalent plastic strain at the notch tip for the ductile crack initiation of the three-point bending tests was consistent with those of the wide plate tests, and the heat input hardly affected the ductile crack initiation limit within the range of this study. This meant that the ductile crack initiation limit of the pipeline girth welded joints with strength mis-match was able to be estimated using the equivalent plastic strain obtained from the three-point bending tests. Based on these results, we proposed a procedure to determine the rational fracture toughness requirements which took into account the difference in the plastic constraint between standard fracture toughness test and pipeline girth welded joints. This procedure was also possible to determine the required strength matching level for a strain-based design for girth welded joint containing surface notch in the centre of the WM in terms of preventing the ductile crack initiation.     

Material characterization for the prediction of ductile fracture occurrence: An inverse approach

The most advanced approach to the prediction of ductile fracture occurrence in metal forming processes is represented by a damage mechanics formulation. Such an approach requires a suitable evaluation of the parameters which appear in the yield condition for the damaging material and in the model which governs the evolution of microvoids (nucleation, growth and coalescence), which allow to calibrate the model with respect to the actual state and properties of the material.

In the paper this aim is pursued employing an inverse identification approach based on an optimization technique which permits to adjust the material variables so that the numerical response matches the experimental one. In particular the load vs. displacement curve during a tensile test has been used to optimise the comparison between the numerical and the experimental results and to develop a full damage characterisation of the material.     

汽车用先进高强钢韧性断裂模型的研究与应用进展

韧性断裂预测对汽车轻量化产品设计与成形工艺优化有着重要的意义.全面综述了强耦合型与弱(非)耦合型韧性断裂模型的发展与研究现状;重点围绕考虑加载路径与应力状态的断裂失效与成形极限曲线预测、各向异性耦合的失效模型拓展、应变速率与温度效应对材料断裂的影响等3个方面的研究现状及应用效果进行了分析介绍,其中,MMC模型、Lou-...