• Title/Summary/Keyword: plastic instability

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The effect of plastic anisotropy on wrinkling behavior of sheet metal (소성 이방성이 박판의 주름 발생에 미치는 영향)

  • 양동열
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 1999.03b
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    • pp.14-17
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    • 1999
  • The wrinkling behavior of a thin sheet with perfect geometry is a kind of compressive instability. The compressive instability is influenced by many factors such as stress state mechanical properties of the sheet material geometry of the body contact conditions and plastic anisotropy. The analysis of compressive instability in plastically deforming body is difficult considering all the factors because the effects of the factors are very complex and the instability behavior may show wide variation for small deviation of the factors. In this study the bifurcation theory is introduced for the finite element analysis of puckering initiation and growth of a thin sheet with perfect geometry. All the above mentioned analysis and the post-bifurcation behavior is analyzed by introducing the branching scheme proposed by Riks. The finite element formulation is based on the incremental deformation theory and elastic-plastic material modeling. in order to investigate the effect of plastic anisotropy on the compressive instability a square plate that is subjected to compression in one direction and tension in the other direction is analyzed by the above-mentionedfinite element analysis. The critical stress ratios above which the buckling does not take place are found for various plastic anisotropic modeling method and discussed. Finally the effect of plastic anisotropy on the puckering behavior in the spherical cup deep drawing process is investigated.

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A Prediction of Bursting Failure in Tube Hydroforming Process Based on Plastic Instability (소성불안정성에 의한 관재 하이드로포밍 공정에서의 터짐 불량 예측)

  • Kim S. W.;Kim J.;Park H. J.;Kang B. S.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 2004.05a
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    • pp.210-213
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    • 2004
  • Based on plastic instability, analytical prediction of bursting failure on tube hydroforming processes under combined internal pressure and independent axial feeding is carried out. Bursting is irrecoverable phenomenon due to local instability under excessive tensile stresses. In order to predict the bursting failure, three different classical necking criteria such as diffuse necking criterion for sheet and tube, local necking criterion for sheet are introduced. The incremental theory of plasticity fur anisotropic material is adopted and then the hydroforming limit and bursting failure diagram with respect to axial feeding and hydraulic pressure are presented. In addition, the influences of the material properties such as anisotropy parameter, strain hardening exponent on bursting pressure are investigated. As results of the above approach, the hydroforming limit in view of bursting failure is verified with experimental results.

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Closed-Form Plastic Collapse Loads of Pipe Bends Under Combined Pressure and In-Plane Bending (압력과 모멘트의 복합하중을 받는 곡관의 소성 붕괴하중 예측식 개발)

  • Oh Chang-Sik;Kim Yun-Jae
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.30 no.8 s.251
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    • pp.1008-1015
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    • 2006
  • Based on three-dimensional (3-D) FE limit analyses, this paper provides plastic limit, collapse and instability load solutions for pipe bends under combined pressure and in-plane bending. The plastic limit loads are determined from FE limit analyses based on elastic-perfectly plastic materials using the small geometry change option, and the FE limit analyses using the large geometry change option provide plastic collapse loads (using the twice-elastic-slope method) and instability loads. For the bending mode, both closing bending and opening bending are considered, and a wide range of parameters related to the bend geometry is considered. Based on the FE results, closed-form approximations of plastic limit and collapse load solutions for pipe bends under combined pressure and bending are proposed.

STRAIN LOCALIZATION IN IRRADIATED MATERIALS

  • Byun, Thaksang;Hashimoto, Naoyuki
    • Nuclear Engineering and Technology
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    • v.38 no.7
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    • pp.619-638
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    • 2006
  • Low temperature irradiation can significantly harden metallic materials and often lead to strain localization and ductility loss in deformation. This paper provides a review on the radiation effects on the deformation of metallic materials, focusing on microscopic and macroscopic strain localization phenomena. The types of microscopic strain localization often observed in irradiated materials are dislocation channeling and deformation twinning, in which dislocation glides are evenly distributed and well confined in the narrow bands, usually a fraction of a micron wide. Dislocation channeling is a common strain localization mechanism observed virtually in all irradiated metallic materials with ductility, while deformation twinning is an alternative localization mechanism occurring only in low stacking fault energy(SFE) materials. In some high stacking fault energy materials where cross slip is easy, curved and widening channels can be formed depending on dose and stress state. Irradiation also prompts macroscopic strain localization (or plastic instability). It is shown that the plastic instability stress and true fracture stress are nearly independent of irradiation dose if there is no radiation-induced phase change or embrittlement. A newly proposed plastic Instability criterion is that the metals after irradiation show necking at yield when the yield stress exceeds the dose-independent plastic instability stress. There is no evident relationship between the microscopic and macroscopic strain localizations; which is explained by the long-range back-stress hardening. It is proposed that the microscopic strain localization is a generalized phenomenon occurring at high stress.

Elasto-Plastic Dynamic Analysis of Solids by Using SPH without Tensile Instability (인장 불안정이 제거된 SPH을 이용한 고체의 동적 탄소성해석)

  • Lee, Kyoung Soo;Shin, Sang Shup;Park, Taehyo
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.31 no.2A
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    • pp.71-77
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    • 2011
  • In this paper elasto-plastic dynamic behavior of solid is analyzed by using smoothed particle hydrodynamics (SPH) without tensile instability which caused by a clustering of SPH particles. In solid body computations, the instability may corrupt physical behavior by numerical fragmentation which, in some cases of elastic or brittle solids, is so severe that the dynamics of the system is completely wrong. The instability removed by using an artificial stress which introduces negligible errors in long-wavelength modes. Applications to several test problems show that the artificial stress works effectively. These problems include the collision of rubber cylinders, fracture and crack of plate.

The Effects of Thickness on the Plastic Instability under Uniaxial Tension in Sheet Metal (판재의 일축인장 소성불안정에 미치는 두께의 영향)

  • Han, K. T.;Kang, D. M.;Koo, Y.;Baek, N. J.
    • Journal of the Korean Society for Precision Engineering
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    • v.6 no.2
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    • pp.58-64
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    • 1989
  • Plastic instability in uniaxial tension of commercial purity Al has been studied with the emphasis of effects of thickness in cold worked specimens and recrystallized specimens. The thickness change gave rise to change in stress state and the amount of strain localization in specimen after diffuse necking. Therefore the thickness of speci- men could control modes of plastic instability. Regardless of recrystallized or cold worked state, the necking mode changed from diffuse necking to local necking, at about 1.5 .approx. 2 mm in thickness.

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A Prediction of Bursting Failure in Tube Hydroforming Process Based on Necking Conditions (네킹발생조건에 의한 관재 액압성형 공정에서의 터짐 불량 예측)

  • 김상우;김정;박훈재;강범수
    • Transactions of Materials Processing
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    • v.13 no.7
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    • pp.629-634
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    • 2004
  • Based on plastic instability, analytical prediction of bursting failure on tube hydroforming processes under combined infernal pressure and independent axial feeding is carried out. Bursting is irrecoverable phenomenon due to local instability under excessive tensile stresses. In order to predict the bursting failure, three different classical necking criteria such as diffuse necking criterion for sheet and tube, local necking criterion for sheet are introduced. The incremental theory of plasticity for anisotropic material is adopted and then the hydroforming limit and bursting failure diagram with respect to axial feeding and hydraulic pressure are presented. In addition, the influences of the material properties such as anisotropy Parameter, strain hardening exponent and strength coefficient on bursting Pressure are investigated. As results of the above approach, the hydroforming limit in view of bursting failure is verified with experimental results.

Nonlinear instability problems including localized plastic failure and large deformations for extreme thermo-mechanical loads

  • Ngo, Van Minh;Ibrahimbegovic, Adnan;Hajdo, Emina
    • Coupled systems mechanics
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    • v.3 no.1
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    • pp.89-110
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    • 2014
  • In this work we provide the theoretical formulation, discrete approximation and solution algorithm for instability problems combing geometric instability at large displacements and material instability due to softening under combined thermo-mechanical extreme loads. While the proposed approach and its implementation are sufficiently general to apply to vast majority of structural mechanics models, more detailed developments are provided for truss-bar model. Several numerical simulations are presented in order to illustrate a very satisfying performance of the proposed methodology.

Inelastic general instability of ring-stiffened circular cylinders and cones under uniform external pressure

  • Ross, C.T.F.
    • Structural Engineering and Mechanics
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    • v.5 no.2
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    • pp.193-207
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    • 1997
  • Experimental tests are described on three ring stiffened machined circular cylinders and three ring stiffened machined circular cones, which were tested to destruction under uniform external pressure. All six vessels failed by inelastic general instability. The experiments showed that the vessels initially deformed plastically at mid-bay in the circumferential direction, and this caused the circumferential tangent modulus to become much less than the elastic Young's modulus, causing the vessels to fail through plastic general instability at pressures much less than that predicted by elastic theory. Based on a thinness ratio, two semi-empirical design charts are provided, which are intended to be used for design purposes in conjunction with the finite element method and a plastic reduction factor.

Injection Molded Microcellular Plastic Gear (I) - Process Design for the Microcellular Plastic Gear - (초미세발포 플라스틱 기어에 관한 연구 (I) - 초미세발포 플라스틱 기어의 공정설계 -)

  • Ha Young Wook;Chong Tae Hyong
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.29 no.5 s.236
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    • pp.647-654
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    • 2005
  • This research Proposes a Process design of injection molded microcellular plastic gears for enhancing the fatigue strength/durability and accuracy of the gears applying thermodynamic instability to microcellular foaming process. To develop the injection molded plastic gears by way of microceliular process, it is absolutely necessary the following two process design. The first is microcellular forming process for enhancing the strength/durability of plastic gears. To be microcellular process succeeded, based on the microcellular principle, mechanical apparatus is designed where nucleation and cell growth are to be generated renewably. The second is the counter pressure process which is mainly fur improving the tooth surface roughness and the accuracy of microcellular gears. For the former process, screw, nozzle and gas equipment are newly designed, and for the latter, counter pressure by nitrogen gas is intentionally brought about into mold cavity when injecting plastic gears. Based on the proposed process design, using gear mold, experiments of injection molding show that, in internal space of plastic gears, microcellular nuclear cells less than 5 lim in diameter have been generated homogeneously via electron microscope photos.