6061-T6铝合金断裂应变与应力三轴度关系研究

研究材料断裂应变与应力三轴度的关系,从而建立其断裂失效模型,对分析6061-T6铝合金断裂失效过程具有重要理论意义和应用价值。分别借助数字图像相关(DIC)测量技术和Abaqus软件对6061-T6铝合金光滑圆棒试件与缺口圆棒试件准静态拉伸过程进行试验和数值模拟,基于所得结果对后者应力三轴度进行分析和修正,从而得到Johnson-Cook(J-C)失效模型的部分材料参数,并建立了6061-T6铝合金断裂应变与应力三轴度的关系模型。将建立的失效模型输入到Abaqus中进行断裂数值模拟,模拟结果与试验结果基本一致,验证了失效模型的正确性。研究表明:6061-T6铝合金的断裂应变随应力三轴度的增加而减小;采用J-C失效模型可以较好的描述6061-T6铝合金在不同应力三轴度下的断裂失效行为。     

韧性断裂阀值在冲压工艺分析中的应用研究

精冲工艺定型前,需对工艺可行性进行分析,确保冲压件没有缺陷。韧性断裂准则可用于钢材冲压成形过程中韧性断裂的判据,但预测的精确性依赖于断裂阀值的有效标定。对QSTE500TM钢料片试样进行拉伸试验,得到材料应力应变曲线;然后,基于有限元模型建立断裂起始点应力三轴度和真实应变演化曲线,并利用断裂阀值积分法求解常用七种韧性断裂准则的断裂阀值;最后,将断裂阀值应用到齿板件的精冲成形工艺分析,为精冲工艺预演及改进提供理论支撑。     

薄膜微电极在热键合过程中的断裂行为研究

为了实现真正意义上的芯片实验室,金属微电极正在成为微流控芯片的重要组成部分。针对铜薄膜微电极在热键合过程中的断裂问题,利用断口分析技术研究了其断裂行为。分析了多种制备断口的方法并利用精密的自动砂轮划片机制备出了理想的断口试样,对断口试样进行宏观分析,判定该断裂是由于剪切引起的韧性断裂,利用扫描电子显微镜进行微观分析,推断出电极在热键合过程中既承受剪切力又承受拉应力,判定铜薄膜微电极在PMMA芯片热键合过程中的断裂机理是一种典型的韧窝式韧性断裂,铜薄膜微电极的断裂是由较高的热键合温度引起的。     

金属剪切旋压成形时的韧性断裂准则

破裂是剪切旋压时最主要的失效形式。为实现对剪切旋压时破裂的精确预测,根据剪切旋压成形时变形区材料的受力特点,提出对基于材料压缩变形塑性本构模型的Oyane准则进行研究。以DP600高强钢为研究对象,通过ABAQUS软件的VUSDFLD子程序二次开发,将Oyane准则耦合到剪切旋压有限元模型进行断裂损伤的计算及断裂阈值的判断。通过椭球形芯模旋压试验对断裂预测结果进行验证。结果表明,基于Oyane准则模拟得到的断裂危险位置位于已变形区靠近变形区处并沿着切向均匀分布,与试验结果的分布规律一致;但基于模拟所得到的极限减薄率与试验结果之间的相对误差达到33.8%。在Oyane准则的基础上,通过考虑平均应力和最大剪切应力对韧性损伤过程的影响,构建出适用于剪切旋压成形的修正韧性断裂准则。结果表明,采用修正后的韧性断裂准则,模拟得到的极限减薄率与试验结果的相对误差仅为15.9%;与Oyane韧性断裂准则相比,相对误差减小了17.9%。获得适用于剪切旋压时的韧性断裂准则,为金属剪切旋压成形破裂的精确预测奠定理论基础。     

基于韧性断裂的汽车用铝合金板滚压包边成形开裂预测

通过对比铝合金平面直线翻边试验及基于集中性失稳模型得到的极限应变和开裂断口,研究了汽车用铝合金滚压包边的失效机理;基于韧性断裂、塑性增量法则和混合强化准则,理论推导得到了弯曲成形极限图,并通过试验对成形极限应力图进行了验证;最后,通过数值解析的方法,研究了韧性断裂准则在滚边成形中的适用范围。结果表明:基于韧性断裂准则的成形极限图,可以用来预测铝合金滚压包边过程中产生的开裂;包边变形过程中弯曲强化效应无法忽略,适用于拉弯成形极限预测的集中性失稳理论将无法应用于滚压包边成形。     

PCD车刀智能选择研究

以Al7075-T6为加工对象,以切削力、表面粗糙度和加工效率为优选目标,对PCD车刀切削过程中刀具智能选择进行研究。基于MySQL 8.0建立刀具智能选择平台数据库结构,确定PCD刀具优选目标,建立刀具优选模型和约束条件处理,完成PCD车刀车削Al7075-T6智能选刀平台的界面开发,并介绍了智能选刀过程与结果的应用实例。     

6000系铝合金薄壁结构压缩断裂行为的有限元模拟

针对汽车用6061和6063铝合金薄壁结构,先进行了准静态拉伸试验和180°弯曲试验,得到了铝合金的真应力-真应变曲线;然后采用线性回归拟合的方法建立了铝合金韧性断裂的本构模型,在此基础上,建立了铝合金薄壁结构压缩失效的有限元模型,并通过轴向压缩试验验证了有限元模型的准确性。结果表明:拉伸后,6061铝合金的表面呈橘皮形貌,6063铝合金的表面比较光滑;两种铝合金的断裂均为韧性断裂,6063铝合金具有更好的韧性;6061和6063铝合金的韧性断裂准则参数分别为104.81 MPa和179.91 MPa;有限元预测得到的铝合金薄壁结构的失效行为与试验结果比较吻合,证明了铝合金韧性断裂本构模型的正确性。     

基于韧性断裂准则用有限元方法预测镍涂层薄钢板的成形极限

首先通过拉伸试验,得到了镍涂层薄钢板和低碳钢薄板的材料参数,然后利用有限元软件Abaqus/Explicit对镍涂层薄钢板的冲压成形过程进行了有限元模拟,得到了涂层和基体在冲压变形过程中的应力、应变以及断裂的初始位置及厚度分布,并通过Oyane韧性断裂准则预测了镍涂层薄钢板的成形极限;并将韧性断裂准则的理论计算、试验结果与有限元计算结果进行对比。结果表明:Oyane韧性断裂准则能够较好地预测镍涂层薄板的成形极限和失效的初始位置,预测值与试验值相差较小。     

基于Lou-2013韧性断裂准则5182铝板成形极限研究

以Lou-2013韧性断裂准则为理论基础研究5182铝合金板材的韧性断裂力学性能,设计圆孔试件、平面应变试件和平面剪切试件的拉伸试验来获取准则中的材料参数。通过该准则确定"断裂应变?应力三轴度"曲线,为ABAQUS软件中的韧性损伤材料模型提供损伤判据,构建5182铝合金板材的韧性损伤仿真模型。利用此模型对上述三个试件的拉伸过程,以及半球形凸模胀形试验过程进行有限元仿真,并通过数据分析绘制基础拉伸试验与半球形凸模胀形仿真相结合的5182铝合金板材成形极限图。试验与仿真表明,所建立的有限元材料模型能够准确地再现三种试件的力程曲线和断裂特征;通过基础拉伸试验与半球形凸模胀形仿真相结合求解出的成形极限图包含两条曲线:损伤成形极限曲线(Damageforminglimitcurve,Damage FLC)和断裂成形极限曲线(Fracture FLC)。与成形极限试验数据对比表明,采用Damage FLC判定板材破裂失稳偏于安全,为实测成形极限数值的下限,而采用Fracture FLC判定则偏于危险。     

GCr15轴承钢韧性断裂阈值研究

在金属韧性断裂的数值模拟中,韧性断裂准则阈值的选取对模拟结果的精度有非常大的影响.通过对GCr15棒材试样进行单向拉伸、压缩试验,得到材料应力应变曲线及最大断面收缩率.通过有限元计算获得了三种常用韧性断裂准则的材料断裂阈值,研究结果可供进行GCr15轴承钢断裂模拟研究时应用.     

Comparison of Modified Mohr-Coulomb Model and Bai-Wierzbicki Model for Constructing 3D Ductile Fracture Envelope of AA6063

Ductile fracture of metal often occurs in the plastic forming process of parts.The establishment of ductile fracture cri-terion can effectively guide the selection of process parameters and avoid ductile fracture of parts during machining.The 3D ductile fracture envelope of AA6063-T6 was developed to predict and prevent its fracture.Smooth round bar tension tests were performed to characterize the flow stress,and a series of experiments were conducted to charac-terize the ductile fracture firstly,such as notched round bar tension tests,compression tests and torsion tests.These tests cover a wide range of stress triaxiality(ST)and Lode parameter(LP)to calibrate the ductile fracture criterion.Plas-ticity modeling was performed,and the predicted results were compared with corresponding experimental data to verify the plasticity model after these experiments.Then the relationship between ductile fracture strain and ST with LP was constructed using the modified Mohr-Coulomb(MMC)model and Bai-Wierzbicki(BW)model to develop the 3D ductile fracture envelope.Finally,two ductile damage models were proposed based on the 3D fracture envelope of AA6063.Through the comparison of the two models,it was found that BW model had better fitting effect,and the sum of squares of residual error of BW model was 0.9901.The two models had relatively large errors in predicting the fracture strain of SRB tensile test and torsion test,but both of the predicting error of both two models were within the acceptable range of 15％.In the process of finite element simulation,the evolution process of ductile fracture can be well simulated by the two models.However,BW model can predict the location of fracture more accurately than MMC model.     

Investigation of the viscous and thermal effects on ductile fracture in sheet metal blanking process

In this paper, a methodology is proposed to predict the ductile damage in the sheet metal blanking process using a coupled thermomechanical finite-element method. A constitutive material model combined with the ductile fracture criteria was used. The effect of material softening due to the heat generated during plastic work in a specimen was considered in blanking simulations. To verify the validity of the proposed model, several blanking simulations are performed and the results compared with those obtained from an experimental study. The interaction of fracture initiation and temperature distribution in the sheet metal during the process was studied. The effect of velocity and the clearance on the product shape were examined. It was seen that at high punch speeds the viscous and thermal effects have significant effects on product quality.     

An approximate upper bound approach for the single-grit rotating scratch with a conical tool on pure metal

An upper bound model has been proposed for the single-grit rotating scratch with a conical tool on a rigid perfectly-plastic material. The model was validated experimentally with a newly-developed technique to make scratches on annealed pure titanium. It has been realized that the built-up edge (BUE) governs the material removal process of the rotating scratch. The current model makes use of the modification of BUE to the tool configuration so that the discretization of plastic region in the vicinity of the tool becomes feasible. The features of side-ridge and front-ridge, BUE and sub-layer plastic zone have been found to be evolving in different ways during the rotating scratch. Three material removal mechanisms were uncovered to be involved with the rotating scratch process, namely, plastic (shear) deformation, contact friction and ductile fracture. In the first-half of the scratch, the first two mechanisms dominate the process, while in the second-half, the ductile fracture plays an active role as evidenced by the extensive tearing along the both banks of scratch.     

Experimental research on ductile fracture criterion in metal forming

Ductile fracture criterion is key limitation parameter in material forming. Accuracy predicting surface and internal failure in plastic deformation process affects on the technology design of workpiece and die greatly. Tension, compression, torsion and shearing test on 45# steel are utilized for providing the experimental values of the critical values at fracture, and 11 widely used ductile fracture criterion are selected to simulate the physical experiments and their relative accuracy for predicting and quantifying fracture initiation sites are investigated. The comparing results show that metal forming process under high triaxiality can be estimated successively using both Normalized Cockcroft-latham and the Brozzo ductile fracture criteria, but the Ayada and general Rice-Tracey model work very well for the low triaxiality cases.     

Ductile fracture and lifetime prediction of structural materials and components under high-temperature creep conditions

The problem of long-term strength prediction of structural materials under uniaxial and biaxial creep conditions, which give rise to ductile fracture, is considered. A new approach to long-term fracture modelling based on constitutive equations of the “isochronous creep theory” and criteria of ductile fracture is suggested. The change in the momentary tangential modulus specifies the materials rheological behaviour and a value tending to zero is assumed as the ductile fracture criterion. Using the model the times to fracture of rods, beams and thin-walled tubes made of high-temperature and heat-resistant materials are estimated and they show good agreement with experimental data.     

焊接接头脆性断裂评定的局部法研究进展

介绍了几种焊接结构断裂行为预测和评定方法的局限性。从理论基础、微观理论基础、评定方法等方面详细论述了焊接接头脆性断裂评定的局部法，并介绍了国内外研究状况及最新进展。指出研究各种塑性损伤模型在焊接接头中的可行性，探讨符合焊接接头特点的塑性损伤模型及控制参量的必要性。分析塑性损伤对Beremin解理断裂模型局部断裂参量的影响，进而利用局部法对焊接接头的脆性断裂行为进行科学预测。     

切口试样破坏过程试验与细观数值分析

对切口圆棒试样与切口板试样进行单向拉伸试验和有限元数值分析,以试样最小横截面上各点变化的应力应变场为体胞演化的载荷控制条件,采用三维含球形微孔洞体胞模型分别对各点微孔洞长大规律进行数值模拟。结果表明,在试样得到的断口韧性区域内,微孔洞的生长速度较快;试样失稳前微孔洞相对体积百分数最大的区域与试验时试样的起裂位置一致,因而证实微孔洞的演化是导致试样材料在拉伸条件下破坏的主要原因,同时也证实体胞模型用于局部破坏分析的有效性。     

铝合金管材无芯模冲切过程的有限元模拟

对铝合金管的冲切过程进行了有限元模拟,刀具采用刚体模型,工件采用弹塑性模型,材料断裂破坏采用Normalized Cockcroft&Latham破坏准则,分析了冲切过程中冲切力、等效应力的变化,得出了弹性变形、塑性变形、塑性变形至裂纹扩展、断裂始终贯穿于整个冲切过程之中。通过模拟切口与实际切口的对比以及模拟切屑形状与实际切屑形状的对比,说明了模拟过程的有效性。     

动态条件下Fe-24Mn-0.5C孪晶诱发塑性钢的断裂机制

对Fe-24Mn-0.5C孪晶诱发塑性钢进行了动态冲击拉伸试验,并采用扫描电镜、透射电镜等研究了该钢的断裂机制。结果表明:试验钢的抗拉强度为1 100 MPa,断后伸长率为50%;试验钢表现为典型的韧性断裂特征,断口主要由呈阶梯状的韧窝组成;其断裂机制为变形过程中塞积在晶界和孪晶界处的大量位错促使孔洞及裂纹的产生,继续变形则发生断裂。