A DUCTILE FRACTURE CRITERION BASED ON THREE-DIMENSIONAL VOID MODEL

A fracture criterion derived from a microscopic point of view is proposed and has proved to be effective in the analysis of uniaxial tension. On the one hand, a method of predicting a ductile fracture is proposed using a three-dimensional void model and the assumption of velocity discontinuity. The relationship between the void volume fraction and the critical strain to fracture, calculated with the help of the new model, shows the same tendency as that obtained from the modified Thomason model. On the other hand, the mechanical and metallographic analyses of the uniaxial tension experiment are performed using four kinds of carbon steel. The relationship between the void volume fraction and the critical strain to fracture, calculated from the new model, agrees better with the result obtained from the experiment, rather than that calculated by the modified Thomason model, which confirms the validity of the ductile fracture criterion based on the three-dimensional void model.     

基于微观损伤机理的延性裂纹扩展行为分析

在考虑材料内部塑性损伤的基础上,通过有限元数值方法(特殊单元模型法),编制了求解韧性断裂特征参量的程序,模拟材料的韧性断裂过程.通过单轴拉伸试验的模拟结果,得出反映材料微观损伤的控制参量,再根据该参量对同种材料的标准三点弯曲试验进行定量预测,即实现从一种试验结果预测同种材料的其它试验结果.结果表明,在韧性裂纹扩展时,考虑塑性损伤的特殊单元模型能够很好地描述材料的韧性断裂过程,可以实现同种材料不同试验间相互预测的目的.     

非线性路径下板料拉深成形过程中破裂预测

在优选模型参数和简化孔洞形核规律的基础上,采用Gurson—Tvergaard（GT）多孔材料本构模型分析圆筒件拉深过程;根据金属成形工艺特点,综合考虑拉伸型和剪切型2种不同韧性断裂机制,提出一个统一的韧性断裂准则形式。对于未经过预变形和经过预变形的圆筒件拉深试验和数值模拟进行了比较,结果表明：相对于成形极限图,新的韧性断裂准则可以更加准确地预测非线性路径下圆筒件的拉深破裂。     

基于韧性断裂准则的铝合金板材成形极限预测

为了准确地预测铝合金板材成形极限，将韧性断裂准则引入到数值模拟中。在数值模拟获得的应力应变值基础上，采用简单拉伸试验和数值模拟相结合的方法确定了韧性断裂准则中的材料常数，并应用该韧性断裂准则预测了铝合金LYl2(M)的圆筒件拉深和半球形凸模胀形的成形极限。预测结果与实验值吻合较好，该韧性断裂准则能够预测铝合金板材成形极限。     

精密冲裁成形仿真技术研究进展

精密冲裁是在普通冲裁基础上发展起来的一种先进的金属精密塑性成形工艺,是一个局部剧烈塑性大变形的过程.材料在狭小的凸凹模间隙附近形成塑性剪切带,发生剧烈的剪切流动,使成形过程的数值模拟存在许多难点.本文从延性损伤、韧性断裂准则及裂纹萌生与扩展仿真3个方面回顾了相关技术在精密冲裁成形过程数值模拟中的研究进展.     

T2紫铜-45钢电子束焊双材料界面性能与裂纹偏转

焊接接头性能与缺陷一直是焊接材料相关课题的重要方面,大量的研究描述了焊接材料的接头性能与特征,但异种焊接材料的界面与开裂问题研究并不充分. 文中对T2紫铜-C45钢电子束焊接双材料界面强度和裂纹开裂的问题,首先采用标准拉伸和三点弯曲试验,得到该材料焊缝区抗拉强度、屈服强度、弹性模量参数,并依此计算判断出断裂韧性值;其次对拉伸和三点弯曲试验试样的宏观和微观断口分析,表征其断口形貌特征,得出断口断裂类型为准解理和沿晶脆性复合断裂模式;基于试验分析结果对裂纹开裂偏转路径进行讨论,得出裂纹沿焊缝区扩展并偏向T2紫铜端;最后以实测参数为基准,基于ABAQUS有限元分析得出三点弯曲试验下焊缝区的裂纹扩展和偏转方向结果,为该种异种金属焊接材料接头性能的提升提供了依据.     

A conical mandrel tube drawing test designed to assess failure criteria

Cold tube drawing is a metal forming process which enables to produce tubes with high dimensional precision. It consists in reducing tube dimensions by pulling it through a die. Tube outer diameter is calibrated by a die and the tube inner diameter and thickness are calibrated by a mandrel. One of the major concern of metal forming industry is the constant improvement of productivity and product quality. In the aim of pushing the process to the limit the question is how far the material can be processed without occurrence of failure. In the present study, a long conical mandrel with a small cone angle was designed in order to carry out drawing tests up to fracture with experimental conditions very close to the industrial process. The FEM of the process was built in order to access the local stress and strain data. A specific emphasis was put on the friction characterisation. For that purpose force measurement during the conical mandrel experiments enabled to characterise a pressure dependent friction coefficient constitutive law by means of an inverse analysis. Finally, eleven failure criteria were selected to study the drawability of cobalt–chromium alloy tubes. The assessment of failure criteria based on damage variables or damage accumulation variables involved their calibration on uniaxial tensile tests. The experimental studies were completed by SEM fractography which enabled to understand the fracture locus and the propagation direction of the fracture.     

Two-Point Incremental Forming with Partial Die: Theory and Experimentation

This paper proposes a new level of understanding of two-point incremental forming (TPIF) with partial die by means of a combined theoretical and experimental investigation. The theoretical developments include an innovative extension of the analytical model for rotational symmetric single point incremental forming (SPIF), originally developed by the authors, to address the influence of the major operating parameters of TPIF and to successfully explain the differences in formability between SPIF and TPIF. The experimental work comprised the mechanical characterization of the material and the determination of its formability limits at necking and fracture by means of circle grid analysis and benchmark incremental sheet forming tests. Results show the adequacy of the proposed analytical model to handle the deformation mechanics of SPIF and TPIF with partial die and demonstrate that neck formation is suppressed in TPIF, so that traditional forming limit curves are inapplicable to describe failure and must be replaced by fracture forming limits derived from ductile damage mechanics. The overall geometric accuracy of sheet metal parts produced by TPIF with partial die is found to be better than that of parts fabricated by SPIF due to smaller elastic recovery upon unloading.     

Revisiting the fundamentals of single point incremental forming by means of membrane analysis

Knowledge of the physics behind the fracture of material at the transition between the inclined wall and the corner radius of the sheet is of great importance for understanding the fundamentals of single point incremental forming (SPIF). How the material fractures, what is the state of strain and stress in the small localized deformation zone and how these two subjects are brought together in order to explain the overall formability of SPIF in terms of ductile damage are still not well understood. However, they are of great importance for improving the robustness and enhancing the predictability of currently existing numerical models and for extending the scope of industrial applications of the process. This paper attempts to provide answers to these questions by means of a new theoretical model for rotational symmetric SPIF that was developed under membrane analysis with bi-directional in-plane contact friction forces.     

On fracture toughness in tearing of sheet metal

An approximate analysis is presented for predicting the specific fracture toughness,R, in the tearing fractures of ductile sheet from the stress-strain relationships. The model assumes the fracture work to be the plastic work consumed by localized necking. Predictions are compared to experimental findings.     

铝合金圆管空拉拔过程中温度场仿真预测

阐述了塑性成形中热力耦合效应的数值分析方法,建立了铝合金空拉拔过程热耦合计算的有限元模型,设计了两组模拟方案,一组以拉拔速度为变量,速度变化范围为50～200mm/s,一组以摩擦系数为变量,变化范围为0.02～0.4。通过模拟仿真获得了拉拔过程中模具和管子温度场的分布,并依据拉拔速度方案和摩擦系数方案得到了拉拔结束后模具最高温度分布和拉拔速度及摩擦系数的关系。结果表明,模具最高温度随拉拔速度的增加和摩擦系数的增加都是近似线性的增加;对坯料及模具温度场的仿真预测为润滑剂的选取、拉拔工艺及模具设计提供了依据。     

金属材料冲裁过程的有限元模拟

介绍了利用弹性有限元对金属冲裁加工模拟的基本原理,包括弹塑性大变形木匠,裂破坏模拟以及网格重划分等技术,利用商业化有限元程度DEFORM－2D^TM对不锈钢（AISI304）板料的冲裁工艺进行了模拟,模具采用刚体模型,工件采用弹塑性模型,材料断裂破坏采用Cockroft-Latham破坏准则,分析了冲裁过程中裂纹生成、扩展以及材料断裂分离的过程,冲载力的变化等,模拟结果中的材料断裂和冲裁力的变化与已发表文献中的实验结果一致。     

延性金属圆柱棒的拉伸断裂数值模拟

216 9钢在工业产品中经常用到 ,是一种奥氏体不锈钢 ,属于FCC金属。 2 16 9钢为延性金属 ,在拉伸状态下表现出延性断裂 ,从试验分析看 ,断口主要为韧窝形式。本文应用Gurson细观损伤模型 ,采用显式动力法分析了 2 16 9钢圆柱棒在拉伸状态下的损伤、紧缩和断裂。分析 2 16 9钢的静态拉伸表明 ,采用Gurson模型可以较好地模拟这种材料的延性断裂形式。分析结果与试验现象相符。     

Study of workability limits of porous materials under different upsetting conditions by compressible rigid plastic finite element method

Workability limits must be considered when designing powder metallurgy (PM) forging processes. This research successfully applied the general upsetting experiment method to the deformation of porous materials. Based on the plastic theory of porous materials, the compressible rigid plastic finite element method is used to simulate the deformation processes of cold upsetting of disks and rings for porous metal materials with a full account of contact friction boundary conditions, the height-to-diameter ratio, the initial relative density, and the die and workpiece geometry. Furthermore, a successful analysis of the cold forging process results in the prediction of the stress, the strain, and the density field. By coupling with the ductile fracture criterion, which is a strain-based criterion obtained by Lee and Kuhn, possible defects leading to material failure have been checked. This research reveals that larger height to diameter and a lesser friction factor can delay the local strain locus to intersect with the Lee and Kuhn’s fracture line and restrain formation of the surface crack. Meanwhile, it reveals that the initial relative density has only a very small influence on the strain to fracture in compression, and it shows the forming behavior of the ring and disk with the curved die. According to Lee and Kuhn’s results, the calculated results agree well with the experimental results.     

Analytical model to determine the critical feed per edge for ductile-brittle transition in milling process of brittle materials

Brittle materials like glass are considered difficult-to-machine because of their high tendency towards brittle fracture during machining. The technological challenge in machining such brittle materials is to achieve material removal by plastic deformation rather than characteristic brittle fracture. In ductile mode machining, the material is removed predominantly by plastic deformation and any cracks produced due to possible fracture in the cutting zone are prevented from extending into the machined surface. This is achieved by selecting an appropriate cutting tool and suitable machining parameters. In ductile machining by milling process, fracture induced cracks are diverted away from final machined surface by selecting a suitable feed per edge less than a critical threshold value. Hence determination of critical feed per edge is of paramount importance to achieve ductile mode machining by milling process. This paper presents an analytical model based on fracture mechanics principles to predict the critical feed per edge in milling process of glass. The size and orientation of cracks originating from brittle fracture during machining have been quantified by using indentation test results and the critical value of feed per edge has been determined analytically as a function of intrinsic materials properties governing brittle fracture and plastic deformation. Furthermore, an equivalent tool included angle has been suggested for machining operation as against the indenter included angle to correlate the indentation and machining test results with improved degree of accuracy. Experimental results validated the proposed model fairly accurately. It has been established that if the longest cracks oriented in radial direction to the cutting edge trajectory are prevented from reaching the final machined surface by selecting a feed per edge less than or equal to a critical value, a crack-free machined surface can be achieved.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

高强钢纵梁落料冲孔方案的研究

探索采用700L高强钢作为卡车纵梁材料来实现制件减重。为解决纵梁落料冲孔模生产时常见的冲孔凸模断裂问题,以现有某量产车型的纵梁落料冲孔模来进行700L高强钢的生产实验,记录生产过程的机床工作台震动量、冲裁力等数据,为落料冲孔方案确定提供研究方向。通过优化制件结构以及对纵梁落料冲孔凸模进行阶梯型布置,减少冲裁力,校核模具冲裁中心与机床中心,减少设备震动对凸模寿命的影响。最终解决了冲孔凸模断裂频发的问题。     

WC-Co硬质合金精密级进模断裂失效研究

对精密级进模常用材料WC-Co硬质合金进行了研究,对高速冲裁过程中磨损断裂的硬质合金级进模样件进行了金相观测和能谱分析,对比了模具工作表面在断裂前后组织结构和Co元素含量的不同,分析了硬质合金级进模磨损断裂的微观过程,得出的结论是Co元素的丢失是造成高速冲裁中硬质合金级进模磨损断裂的重要微观原因.     

天线外壳连续拉深模设计

通过对天线外壳的工艺分析及计算,提出了落料、冲孔、拉深成形工艺。为了增加内外切开凸模的使用寿命,简化了这种结构形状及尺寸,并用拉料形式代替送料方式,可防止冲底孔凸模易于损坏。经多年来生产验证,该模具结构清晰、可靠,加工质量好,生产效率高。