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．     

多台阶深筒形件成形工艺设计及仿真

根据塑性成形理论分析、计算确定某电器产品的阶梯圆筒件成形工艺,并建立有限元模型,实现多道次拉深过程的仿真.最后对仿真结果进行分析和试验验证.     

正交异性板料拉深件快速展开与成形模拟

针对板件初始毛坯形状计算以及正交异性板的快速有限元分析问题,依据拉深件的UG模型,导出了零件的三角网格数据,并采用几何映射方法得到了用于快速有限元分析的拉深件初始展开毛坯.在此基础上,基于理想变形假设以及Hill’48正交异性屈服准则,给出了用于拉深件成形过程快速分析的一步法数学公式和有限元表达,并在Unigraphics系统中进行了有限元分析的后置处理.此外,对TC1钛板筒型拉深件毛坯初始形状进行了优化,并对成形中拉深件厚向应变分布进行了分析,得到了满意结果.     

金属韧性断裂准则的数值模拟和试验研究

对不同外形的45钢试件进行了拉伸、压缩和扭转等材料试验,对工程中常用的5个韧性断裂准则的适用范围进行了对比研究,并采用Gurson-Tvergaard(GT)多孔材料本构模型对试验过程进行了数值模拟.指出目前使用的断裂准则都不可能对材料在多种变形条件下给出一个固定临界值.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.试验和数值计算结果证明了该准则的有效性和普适性,进而利用单向拉伸和扭转试验确定的材料常数合理地预测压缩过程中的韧性断裂现象.     

板料成形中韧性断裂准则应用研究进展

对板料成形中的成形极限应力图、最大变薄率、成形极限图以及韧性断裂准则等预测成形极限的方法,进行了综述和分析。指出利用韧性断裂准则,不但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。利用有限元方法模拟时,韧性断裂准则既可以应用到完全耦合的弹塑性损伤模型的增量方法中,也可以应用到一步有限元逆算法中。为了准确地预测成形极限,除了提高有限元模拟精度外,应找到一种本质地反映材料性能的韧性断裂准则。     

一种韧性断裂损伤力学建模方法及其应用

基于连续损伤力学理论,综合考虑变形历史（温度、应变速率）对损伤的累积演化和临界损伤值的影响,提出了一种新的韧性断裂预测方法。建立了相应的韧性断裂准则,准确地预测了钛合金的热锻成形、镁合金的温热冲压成形和不锈钢板的液压成形过程中的破裂行为,表明该方法可以很好地用于存在变形历史的体积成形和板成形的成形极限预测。     

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.     

GH4169集合器超塑成形有限元分析及预成形研究

为了成功超塑成形GH4169高温合金集合器并降低研制成本,采用有限元分析对GH4169合金集合器超塑成形过程进行了数值模拟,并采用拉深成形工艺对集合器超塑成形所需的预成形进行了研究.结果表明:超塑成形需筒形预成形件,且聚料工步是超塑成形出满足设计要求的零件所必需的一道工序;成功地拉深出集合器超塑成形预成形件;合金经预拉深成形后,在超塑成形温度(950℃)下保温30 min后仍是ASTM 10级以上的细晶组织,能够满足超塑成形所需的细晶组织的要求.     

板料成形拉延筋技术研究现状

拉延筋在汽车覆盖件拉深成形中起着十分重要的作用.利用拉延筋可以调节和控制板料的变形程度和变形分布,抑制破裂、起皱、面畸变等多种冲压质量问题的产生.在很多情况下,拉延筋的设置合理与否将决定覆盖件拉深的成败.较系统地总结了板料成形拉延筋技术以及拉延筋作用原理、阻力模型、影响因素等的研究现状.     

预胀对筒形件充液拉深变形和硬化的影响

为定量分析预胀对塑性成形的影响,以变形量差异较大的平底筒形件为研究对象,采用实验和数值模拟方法研究了预胀高度对双相钢DP340/590板材充液拉深的影响,分析了预胀对试件等效应变、壁厚减薄率和硬度分布的影响.研究表明,预胀显著增加平底筒形件底部的变形,即试件底部壁厚减薄率增加、硬度升高、等效应变增大.与普通充液拉深相比,相对预胀高度为20%时,试件等效应变差减小16.7%,底部维氏硬度值提高23.8%.     

基于韧性准则的金属板料冲压成形断裂模拟

该文基于Rice-Tracey韧性失效准则,探索金属板料冲压成形的断裂数值模拟方法。首先设计两种试验,并结合有限元分析结果确定双相钢板料的材料失效参数;然后,基于Abaqus软件的显式模块Explicit,编写采用Rice-Tracey韧性失效准则的用户自定义材料子程序VUMAT;最后对双相钢薄板深拉成形过程中的断裂行为进行数值模拟,并进行了试验验证。结果表明,数值模拟结果与试验结果吻合较好。可见,Rice-Tracey韧性准则可用于金属板料冲压成形的断裂预测。     

Mixed Mode Brittle and Ductile Fracture of a High Strength Rotor Steel at Room Temperature

The mixed mode fracture of a high strength rotor steel has been investigated at room temperature using single edge notched specimens. In mode I, and for limited amounts of shear loading, the steel exhibited cleavage fracture. For conditions near mode~II ductile fracture occurred. A transition from brittle to ductile fracture occurred for mixed mode loading. Finite element analysis provided estimates of the extent of near crack tip yielding and elastic-plastic stress intensity factors. Test results agreed with the maximum tensile stress (MTS) criterion for small scale yielding for limited amounts of shear loading. The load for mode II fracture was lower than predicted from the MTS criterion, but higher than predicted from plastic collapse predictions. Observed fracture angles where in broad agreement with the predicted fracture mechanisms. The load for the transition from brittle to ductile fracture was found to agree approximately with the predicted load. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of Cracks Generated in Columbium Alloy (C-103) Sheets During Deep Drawing Operation

Columbium (Niobium) alloy C-103 sheets were formed through deep drawing operations. Two sheets out of a lot of six failed (cracked) during deep drawing. A detailed technical review and microstructural analysis of the failed component was carried out to find out the reasons for cracking. Visual observation of the failed component revealed localized thinning (cup-cone type) at the crack locations. Fractographic observations revealed a predominantly ductile mode of failure. The detailed analysis carried out indicated the failure (crack) is due to nonuniform deformation during first stage deep drawing. This paper provides detail of the failure analysis methodology and present reasons for the failure.     

A critical shear displacement criterion for ductile–brittle transition under mixed mode I and II loading

Abstract

Micromechanisms producing ductile and brittle damage operate in parallel at a crack tip. The dominant mode of failure depends upon which of the two (ductile or brittle) damage parameters first reaches its critical value. This has been shown by a study of ductile–brittle transition behaviour in HY100 steel under mixed mode I and II loading. The transition from ductile to brittle behaviour in HY100 steel was found to be affected by mixed mode I and II ratio (ratio of imposed tensile and shear loading) in a manner such that with increasing shear the transition temperature decreased. In the present paper, a criterion is proposed based on the shear strain ahead of a notch tip, to predict the fracture behaviour at any given temperature and mixed mode ratio.     

COMBINED MODE FRACTURE OF A DUCTILE MATERIAL UNDER PLANE STRESS CONDITIONS

Abstract— On the basis of our experimental results on Ly12-CZ (similar to 2024-T3) aluminium alloy tensile sheet specimens and Tresca's theory, a fracture criterion for a combined mode crack in ductile materials under plane stress conditions is proposed.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

A study of fracture criteria for ductile materials

In order to establish a ductile fracture criterion, several potential fracture parameters were investigated by comparing numerical simulations of crack extension with available experimental data. Based on the comparison, a fracture criterion, the Global-Local Fracture Criterion (GLFC), was proposed. The J-integral is employed as a global parameter to characterize the initial stage of crack extension. In the subsequent steady state crack growth, the fracture criterion is switched to a local parameter characterizing the crack tip stress or strain. The accuracy of the proposed fracture criterion in predicting ductile fracture behavior was verified.