Estimation of Gurson material parameters in bimetallic weldments for the nuclear reactor heat transport piping system
01 Dec 2008-Vol. 222, Iss: 12, pp 2331-2349
TL;DR: In this article, the estimation of Gurson material parameters of base metal and weld metal regions of a BMW was performed using the micromechanical modelling approach for addressing the structural integrity issues in BMWs.
Abstract: Bimetallic welds (BMWs) play a critical and indispensable role in the primary heat transport piping system of nuclear reactors. The primary heat transport system itself is the critical part of a nuclear reactor. Any failure of this system can lead to very grave consequences, not only speaking of huge monetary losses resulting from non-utilization of the reactor setup, but also immensely valuable and irreparable loss of human life. This paper describes the experimental efforts towards estimation of Gurson material parameters of base metal and weld metal regions of a BMW so as to use the micromechanical modelling approach for addressing the structural integrity issues in BMWs.
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TL;DR: In this article, many experimental tests and finite element model computation are performed to predict the damage evolution in notched tensile specimen of sheet metal using the Gurson-Tvergaard-Needleman (GTN) model.
Abstract: 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.
111 citations
Cites background from "Estimation of Gurson material param..."
...[31] demonstrated that for the SS304 the final volume fraction ff is less compared with the others steels, whereas the percentage change in void volume fraction values from initial void volume fraction to final void volume fraction it’ is height....
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TL;DR: In this paper, the authors used the Gurson-Tvergaard-Needleman (GTN) damage model to determine the forming limit curve of anisotropic sheet metals.
Abstract: In this paper, the Gurson–Tvergaard–Needleman (GTN) damage model is used to determine the forming limit curve of anisotropic sheet metals. The mechanical behavior of the matrix material is described using Hill’48 quadratic yield criterion and an isotropic hardening rule. For this purpose, a VUMAT subroutine has been developed and used inside the ABAQUS/Explicit finite element code. The implementation of the constitutive model in the finite element code is presented in detail. Finally, the forming limit curve of an AA6016-T4 sheet metal is constructed using the developed VUMAT subroutine and running numerical simulation of Nakjima tests. The quality of the numerical results is evaluated by comparison with an experimental forming limit curve. Furthermore, theoretical forming limit curves of the AA6016-T4 sheet are obtained using Marciniak–Kuczynski (M–K) and modified maximum force criterion (MMFC) models. The results show that the forming limit curve predicted by the anisotropic GTN model is in better agreement with the experimental results especially in the biaxial tension region. This fact indicates that the GTN model is a useful tool in analyzing the formability of anisotropic sheet metals.
100 citations
TL;DR: In this article, the void related parameters of GTN model for SAE 1010 plain carbon steel were identified by response surface method (RSM) through minimizing the difference between numerical and experimental results of tensile test on a standard specimen.
Abstract: The present work is devoted to experimental and numerical investigation of in situ tensile tests to recognize the mechanisms of ductile fracture under different stress states. The GTN model, which is a micromechanical based damage model, was used for numerical simulations. The void related parameters of GTN model for SAE 1010 plain carbon steel were identified by response surface method (RSM) through minimizing the difference between numerical and experimental results of tensile test on a standard specimen. The void related parameters of GTN model were determined 0.00107, 0.00716, 0.01 and 0.15 for f0, fN, fc and ff respectively. After calibrating the damage model for the studied material, the tensile tests were carried out on the in-situ specimens with different geometries. The fractographic analysis was performed to identify the ductile fracture under wide range of stress states and two failure mechanisms were observed. The calibrated damage model was applied to FE simulations of in-situ tensile tests for numerical study of the experimentally observed fracture phenomenon. The extracted numerical results showed a good agreement with experimental observations comparing load-displacement plots with a margin of error within 5%. A better ductile fracture predictions were captured in 90° specimens. The location of fracture initiation, crack growth orientation and the displacement at fracture zone in numerical studies also showed close correspondence with experiments.
40 citations
TL;DR: In this paper, a modified Gurson-Tvergaard-Needelman (GTN) model was used to numerically analyse S235JR steel elements under the action of complex stresses.
Abstract: The paper analyses the load-bearing capacity of S235JR steel elements subjected to complex stress states, taking into account the effect of microstructural damage. Assessing the material required conducting a microstructural analysis and standardized tensile strength tests. A modified Gurson–Tvergaard–Needelman (GTN) model was used to numerically analyse S235JR steel elements under the action of complex stresses. The results of the numerical analysis were reported to be consistent with those obtained during the strength tests. The method for the numerical calculations was described along with the admissible results and the criteria of failure for S235JR steel basing on the modified GTN model and the development of microdamage. As S235JR steel is a common structural material in Poland, the investigation results will be of use to a number of engineers and other specialists responsible for determining the load-carrying capacity and structural safety of elements or whole systems.
30 citations
TL;DR: In this paper, the role of dislocations during void nucleation in ductile deformation has been investigated and it is shown that dislocation boundaries facilitate the growth process of void growth.
Abstract: Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 nm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.
23 citations
References
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TL;DR: In this paper, the effect of microscopic voids on the failure mechanism of a ductile material is investigated by considering an elastic-plastic medium containing a boubly periodic array of circular cylindrical voids.
Abstract: The effect of microscopic voids on the failure mechanism of a ductile material is investigated by considering an elastic-plastic medium containing a boubly periodic array of circular cylindrical voids. For this voided material under uniaxial or biaxial plane strain tension the state of stresses and deformations is determined numerically. Bifurcation away from the fundamental state of deformation is analysed with special interest in a repetitive pattern that represents the state of deformation inside a shear band. Both in the fundamental state and in the bifurcation analysis the interaction between voids and the details of the stress distribution around voids are fully accounted for. Comparison is made with the shear band instabilities predicted by a continuum model of a ductile porous medium. Based on the numerical results an adjustment is suggested for the approximate yield condition in this model of dilatant, pressure sensitive plastic behaviour.
2,021 citations
TL;DR: In this article, an axisymmetric numerical model and a set of approximate constitutive equations for a voided material suggested by Gurson were used to analyse bifurcation into a localized mode.
Abstract: Macroscopic properties of a porous ductile medium are analysed on the basis of an axisymmetric numerical model and on the basis of a set of approximate constitutive equations for a voided material suggested by Gurson. Both models are used to analyse bifurcation into a localized mode. A number of predictions obtained by the two different approaches are in reasonable agreement; but it is found under several different loading conditions that the critical strain for localization is considerably overestimated by the approximate continuum model. A relatively simple modification of the constitutive equations for a voided medium results in considerably improved predictions.
1,259 citations
TL;DR: In this paper, a convected coordinate formulation of the field equations is used to describe the material failure by coalescence of microscopic voids, and a detailed micromechanical study of shear band bifurcation that accounts for the interaction between neighboring voids and the strongly nonhomogeneous stress distributions around each void has been carried out, and also elaborated in this chapter.
Abstract: Publisher Summary This chapter describes the material failure by coalescence of microscopic voids. The voids nucleate mainly at second phase particles, by decohesion of the particle-matrix interface or by particle fracture, and subsequently the voids grow because of plastic straining of the surrounding material. The growth of voids to coalescence by plastic yielding of the surrounding material involves so large geometry changes that finite strain formulations of the field equations are a necessary tool. A convected coordinate formulation of the governing equations is used. Convected coordinates are introduced, which serve as particle labels. The convected coordinate net can be visualized as being inscribed on the body in the reference state and deforming with the material. It is found that after nucleation, cavities elongate along the major tensile axis and that two neighboring cavities coalesce when their length has grown to the order of magnitude of their spacing. This local failure occurs by the development of slip planes between the cavities or simply necking of the ligament. A detailed micromechanical study of shear band bifurcation that accounts for the interaction between neighboring voids and the strongly nonhomogeneous stress distributions around each void has been carried out, and are also elaborated in this chapter.
938 citations
TL;DR: In this paper, a set of elastic-plastic constitutive relations that account for the nucleation and growth of microvoids is analyzed numerically, based on the set of constitutive relation for axisymmetric and plane strain notched tensile specimens.
Abstract: Ductile fracture in axisymmetric and plane strain notched tensile specimens is analyzed numerically, based on a set of elastic-plastic constitutive relations that account for the nucleation and growth of microvoids. Final material failure by void coalescence is incorporated into the constitutive model via the dependence of the yield function on the void volume fraction. In the analyses the material has no voids initially; but as the voids nucleate and grow, the resultant dilatancy and pressure sensitivity of the macroscopic plastic flow influence the solution significantly. Considering both a blunt notch geometry and a sharp notch geometry in the computations permits a study of the relative roles of high strain and high triaxiality on failure. Comparison is made with published experimental results for notched tensile specimens of high-strength steels. All axisymmetric specimens analyzed fail at the center of the notched section, whereas failure initiation at the surface is found in plane strain specimens with sharp notches, in agreement with the experiments. The results for different specimens are used to investigate the circumstances under which fracture initiation can be represented by a single failure locus in a plot of stress triaxiality vs effective plastic strain.
702 citations