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．     

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。     

Evaluation of ductile fracture in sheet metal forming using the ellipsoidal void model

The ductile fracture in the simulation of sheet-metal-forming processes is evaluated by the ellipsoidal void model previously proposed by the author. In the present study, the simulation and experiment of the hole expansion test are performed using six types of metals. For an alloy, the relationship between prestrain and hole expansion ratio calculated using the ellipsoidal void configuration and ellipsoidal void shape and that calculated using the ellipsoidal void configuration and circular void shape agree with the relationship obtained experimentally. For a pure metal, the relationship between prestrain and hole expansion ratio calculated using the average void configuration and average void shape agrees with that obtained experimentally. Furthermore, the method of introducing prestrain to an as-rolled sheet is proposed, and the prestrain in this sheet is estimated.     

Springback investigation of anisotropic aluminum alloy sheet with a mixed hardening rule and Barlat yield criteria in sheet metal forming

A mixed hardening model has been implemented based on Lemaitre and Chaboche non-linear kinematic hardening theory to consider cyclic behavior and the Bauschinger effect. The Chaboche isotropic hardening theory is incorporated into the non-linear kinematic hardening model to introduce a surface of nonhardening in the plastic strain space. The bending and reverse bending case study has verified the effectiveness of the mixed hardening model by comparison with the proposed experiment results. Barlat’89 yielding criterion is adopted for it does not has any limitation while Hill’s non-quadratic yield criterion is for the case that the principal axes of anisotropy coincides with principal stress direction. The Backward–Euler return mapping algorithm was applied to calculate the stress and strain increment. The mixed hardening model is implemented using ABAQUS user subroutine (UMAT). The comparisons with linear kinematic hardening model and isotropic hardening model in NUMISHEET’93 benchmark show that the mixed hardening model coupled with Barlat’89 yield criteria can well reflect stress and strain distributions and give a more favorable springback angle prediction.     

On the numerical prediction of the ductile fracture in metal forming

In this paper, fully coupled constitutive equations accounting for combined isotropic and kinematic hardening as well as the isotropic ductile damage are implemented into the general purpose finite element code for metal forming simulation. The associated numerical aspects concerning both the local integration of the coupled constitutive equations as well as the global (equilibrium) integration schemes are presented. Various 2D and 3D examples are given in order to show the capability of the proposed numerical methodology to predict the ductile fracture initiation and growth during metal forming processes.     

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.     

Local criteria for ductile fracture

Strain distributions in specimens suitable for studying the initiation of fracture are reviewed, and distributions are developed for the steady-state propagation of cracks in plane strain lension of fully plastic materials. The functional forms of local fracture criteria are discussed for different metallurgical mechanisms. It is concluded that:

1. pure Mode I (normal) fracture is unlikely to exist except in cleavage.
2. there is both theoretical and experimental evidence for the development of both: sharp and flat-bottomed cracks.
3. simultaneous diffuse and concentrated (Dugdale-Muskhelishvili) flow fields can occur in torsion of longitudinally grooved bars if the stress-strain curve has a maximum which causes band formation, so that a displacement criterion becomes appropriate for final fracture.

     

Parameter identification of a mechanical ductile damage using Artificial Neural Networks in sheet metal forming

In this paper, we report on the developed and used of finite element methods, have been developed and used for sheet forming simulations since the 1970s, and have immensely contributed to ensure the success of concurrent design in the manufacturing process of sheets metal. During the forming operation, the Gurson–Tvergaard–Needleman (GTN) model was often employed to evaluate the ductile damage and fracture phenomena. GTN represents one of the most widely used ductile damage model. In this investigation, many experimental tests and finite element model computation are performed to predict the damage evolution in notched tensile specimen of sheet metal using the GTN model. The parameters in the GTN model are calibrated using an Artificial Neural Networks system and the results of the tensile test. In the experimental part, we used an optical measurement instruments in two phases: firstly during the tensile test, a digital image correlation method is applied to determinate the full-field displacements in the specimen surface. Secondly a profile projector is employed to evaluate the localization of deformation (formation of shear band) just before the specimen’s fracture. In the validation parts of this investigation, the experimental results of hydroforming part and Erichsen test are compared with their numerical finite element model taking into account the GTN model. A good correlation was observed between the two approaches.     

Modelling direction-dependent hardening in magnesium sheet forming simulations

In this work, a model capturing anisotropic hardening during plastic deformation under monotonic loading is proposed. For this purpose, the anisotropic plastic potential coefficients are assumed to be functions of a measure of the accumulated plastic strain. This model is applied to describe the plastic behavior of a magnesium alloy (ZM21) sheet at room temperature. The selected plastic potential accounts for the main features of Mg alloy plasticity, i.e., anisotropy and strength-differential (SD) effects. All the accumulated plastic strain dependent coefficients of the phenomenological model are determined from input data generated with a crystal plasticity approach. They are optimized to best capture the accumulated strain dependent potentials computed with crystal plasticity. The R-value (Lankford coefficient) anisotropy is used as an independent measure for the assessment of the approximation quality. This model is implemented into a finite element (FE) code and successfully validated through the numerical simulations of the cup drawing test. The calculated earing profile obtained with the proposed hardening model is compared to results assuming isotropic hardening for various plausible shapes of the plastic potential. Although the ear and valley numbers and positions are similar in all cases, the height differences between peaks and valleys are strongly dependent on the type of constitutive approach used in the simulation.     

Modelling of ductile failure in aluminium sheet forming simulation

In the present article novel approaches to account for localized necking, post-necking and ductile fracture in the numerical simulation of aluminium sheet forming are presented. The modelling concept was implemented in a commercial finite element software and is applied to an aluminium alloy sheet subjected to different forming operations. A validation of the simulation results by means of experiments is employed. It is shown that the proposed modelling framework is capable of accurately predicting the experimental results.     

On energy balance criteria in ductile fracture

This paper is concerned with a formulation and statement of energy balance criteria during slow crack growth and at inception of catastrophic rupture in relatively high strength ductile metals. The phenomena of necking in a tensile test and slow crack growth in a uniaxially loaded, transversely cracked sheet are taken as models for a general, conceptual analysis. The various mechanistic criteria of crack instability that have been suggested are critically evaluated, in light of recent experimental data, through simple mathematical-physical examples. A more general energy balance criterion for ductile fracture is proposed which is consistant with experimental observations and which reduces to the classical Griffith theory (both conceptually and mathematically) in the case of purely elastic, brittle fracture.

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Zusammenfassung Diese Abhandlung bezicht sich auf die Formulierung and Erklaerung von Energie-Gleichgewichts — Kriteria waehrend des langsamen Wachsens von Rissen und beim ploetzlichen Reissen relativ hoch belastbarer Metalle grosser Ducktilitaet. Die Erscheinungen, dass sich eine Einschnuerung waehrend einer Zerreissproble bildet und dass eine Bruchstelle langsam in einem einachsig beanspruchten Blech mit querlaufendem Riss waechst, werden als Ausgangspunkte fuer eine grundsaetzlich allgemeine Analyse verwendet. Die verschiedenen mechanistischen, bereits vorgeschlagenen Kriteria der Riss-Instabilitaet werden kritisch im Lichte experimentell gewonnener Daten an Hand einfacher mathematisch-physikalischer Beispiele gewertet. Ein allgemeineres Energie-Gleichgewichts-Kriterium fuer Ducktilitaetsrisse wind vorgeschlagen, das mit experimentellen Beobachtungen uebereinstimmt und das sich (sowohl begrifflich wie auch mathematisch) in den Faellen rein elastisch, sproeder Rissibildung auf die klassische Griffith Theorie reduzieren laesst.

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Résumé Cette étude traite d' une formulation et constatation du critére d' équilibre d' énergie pendant la croissance lente d'une fissure et le commencement d' une rupture catastrophique dans les métaux ductiles de grande resistance. Les phénomènes d'étroitement dans un essai de traction et croissance lente d'une fissure dans une feuille soumise á une tension uniaxiale et fissurée transversalement, sont utilisés comme modéles pour une analyse conceptuelle et générals. Les différents critères mécaniques de l'ínstabilités d'une fissure qui ont été jusque ici recommendés, sont critiquement evalués dans la lumière de récentes données expérimentales par de simples exemples physico-mathématic. Un critère plus général pour l'équilibre d' énergie d'une fissure ductile est recommendé qui se conforme avec les observations expérimentales et qui se réduit á la théorie classique de Griffith (tant en conception que mathématiquement) dans le cas d'une rupture par fragilité, purement élastique.

     

Tolerance transfer in sheet metal forming

A literature review of sheet metal forming errors as well as geometrical dimensions and tolerances (GD&T) shows that the theoretical means for the allocation of process tolerances with respect to GD&T are insufficient. In order to judge the influence of geometrical process errors (e.g., angular errors of bends), two typical sheet metal designs with parallelism and a position tolerance are studied. These case studies comprise a detailed analysis of tolerance chains including angular errors of bends and their positions. The resulting errors are compared with those resulting from length dimensional process errors and conclusions are drawn.     

Approximate yield criteria for anisotropic porous ductile sheet metals

An approximate macroscopic yield criterion for anisotropic porous sheet metals is developed under plane stress conditions in this paper. The metal matrices are assumed to be rigid perfectly plastic and incompressible. The Hill quadratic and non-quadratic anisotropic yield criteria are used to describe the matrix normal anisotropy and planar isotropy. The voids in sheet metals are assumed in the form of through-thickness holes. Under axisymmetric loading, a closed-form upper bound macroscopic yield criterion is derived as a function of the anisotropy parameter R, defined as the ratio of the transverse plastic strain rate to the through-thickness plastic strain rate under in-plane uniaxial loading conditions. The plane stress upper bound solutions for different in-plane strain ratios can be fitted well by the closed-form macroscopic yield criterion.     

Out-of-plane stretching and tearing fracture in ductile sheet materials

Pre-cracked ductile steel sheets are fractured by combined in-plane streching and bending. The deformation mimics the mode of fracture when plating is dented and torn as in ship grounding. Fracture toughness is determined for this mode of tearing. Values are greater than those obtained with DENT testpieces on the same material because of the different mode of crack opening. This revised version was published online in August 2006 with corrections to the Cover Date.     

On continuum models of ductile fracture

Several fracture criteria are reviewed with respect to ductile fracture. It is suggested that both critical crack-tip displacement, 2V _c ^*, and critical fracture strain,∃ ^*, criteria may describe the fracture of a ductile second phase rod in a ductile matrix. As a first approximation, this is experimentally verified by observations of ductile stainless steel fibres fracturing in an age-hardened aluminium matrix. For 0.05, 0.10 and 0.20 volume fraction composites, the average fracture strains are calculated to be 1.15 as compared to a measured average of 0.93 while the average critical crack-tip displacement is calculated to be 0.50 mm as compared to an “observed” average of 0.40 mm. The statistical variation in the fracture strain was not sufficiently small to allow any choice between these proposed criteria. In fact, both the experimental and theoretical evidence point to the equivalency of these criteria as given by 2V _c ^*=π/^*∃^* where /^* is the microstructural unit in front of the crack over which the strain is greater than or equal to∃ ^*.