• Title, Summary, Keyword: viscoplasticity

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A Study on Strain Rate Sensitivity by Unified Viscoplasticity (점소성 이론에 의한 변형률 속도 민감도에 대한 연구)

  • 호괄수
    • Transactions of Materials Processing
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    • v.13 no.7
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    • pp.600-607
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    • 2004
  • This paper addresses a viscoplastic constitutive model that allows a consistent way of modeling positive and negative rate sensitivities of flow stress concerned with dynamic strain aging. Based on the concept of continuum mechanics, a phenomenological constitutive model includes the use of a yield surface within the framework of unified viscoplasticity theory. To model negative rate sensitivity, rate-dependent back stress is introduced and flow stress in fully developed inelastic deformation regime is thus decomposed into the plastic contribution of rate independency and the viscous one of rate dependency.

A Generalized Viscoplasticity Theory Based on Overstress (과응력에 기초하여 일반화된 점소성 이론)

  • Ho, Kwang-Soo
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.26 no.10
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    • pp.1953-1960
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    • 2002
  • The viscoplasticity theory based on overstress, one of the unified state variable theories, is generalized to model zero (no influence of loading rate) and negative (flow stress decreases with loading rate) as well as positive (flow stress increases with loading rate) rate sensitivity in a consistent way. On the basis of the long-time asymptotic solution the different types of rate sensitivity are classified with respect to an augmentation function that is introduced in the evolution law fur a state variable equilibrium stress. The theory predicts normal relaxation and creep behaviors even if unusual rate sensitivity is modeled. The constitutive model fir the behavior of a modified 9Cr-1 Mo steel at various temperatures is then compared with experimental data found in the literature.

A coupled damage-viscoplasticity model for the analysis of localisation and size effects

  • Georgin, J.F.;Sluys, L.J.;Reynouard, J.M.
    • Computers and Concrete
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    • v.1 no.2
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    • pp.169-188
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    • 2004
  • A coupled damage-viscoplasticity model is presented for the analysis of localisation and size effects. On one hand, viscosity helps to avoid mesh sensitivity because of the introduction of a length scale in the model and, on the other hand, enables to represent size effects. Size effects were analysed by means of three-point bending tests. Correlation between the fracture energy parameter measured experimentally and the density fracture energy modelling parameter is discussed. It has been shown that the dependence of nominal strength and fracture energy on size is determined by the ligament length in comparison with the width of the fracture process zone.

An Overstress Model for Non-proportional Loading of Nylon 66 (Nylon 66의 무비례 하중에 대한 과응력 모델)

  • Ho, Gwang-Su
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.25 no.12
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    • pp.2056-2061
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    • 2001
  • Non-proportional loading tests of Nylon 66 at room temperature exhibit path dependent behavior and plasticity-relaxation interactions. The uniaxial formulation of the viscoplasticity theory based on overstress (VBO), which has been used to reproduce the nonlinear strain rate sensitivity, relaxation, significant recovery and cyclic softening behaviors of Nylon 66, is extended to three-dimensions to predict the response in strain-controlled, comer-path tests. VBO consists of a flow law that is easily written for either the stress or the strain as the independent variable. The flow law depends on the overstress, the difference between the stress and the equilibrium stress that is a state variable in VBO. The evolution law of the equilibrium stress in turn contains two additional state variables, the kinematic stress and the isotropic stress. The simulations show that the constitutive model is competent at modeling the deformation behavior of Nylon 66 and other solid polymers.

Analysis of AA6061 Wheel Forging Processes by the Thermo-Viscoplastic Finite Element Method (AA6061 휠 성형공정의 열-점소성 유한요소해석)

  • 김영훈;황병복
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • pp.11-16
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    • 1997
  • In this study, the finite element analysis of AA6061 wheel forging processes over hot working range is performed and a thermo-viscoplasticity theory applicable to hot forging is applied for simulation. Aluminum alloy has frequently been utilized to manufacture automobile and aircraft parts due to its various advantages such as lightness, good forgeability, and wear resistance. Several forging conditions are applied to the simulation, such as die speeds, rib thicknesses, and depth of die cavity. The effectiveness of the simulation results is summarized in terms of metal flow, strain distributions, temperature distributions, forging load, which are essential to over all process design.

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A Phenomenological Constitutive Model for Pseudoelastic Shape Memory Alloy (의탄성 형상기억합금에 대한 현상학적 구성모델)

  • Ho, Kwang-Soo
    • Transactions of Materials Processing
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    • v.19 no.8
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    • pp.468-473
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    • 2010
  • Shape memory alloys (SMAs) have the ability to recover their original shape upon thermo-mechanical loading even after large inelastic deformation. The unique feature is known as pseudoelasticity and shape memory effect caused by the crystalline structural transformation between two solid-state phases called austenite and martensite. To support the engineering application, a number of constitutive models, which can be formally classified into either micromechanics-based or phenomenological model, have been developed. Most of the constitutive models include a kinetic law governing the crystallographic transformation. The present work presents a one-dimensional, phenomenological constitutive model for SMAs in the context of the unified viscoplasticity theory. The proposed model does not incorporate the complex mechanisms of phase transformation. Instead, the effects induced by the transformation are depicted through the growth law for the back stress that is an internal state variable of the model.

The Rate Dependent Deformation Behavior of AISI Type 304 Stainless Steel at Room Temperature (304 스테인리스강의 점소성 특성에 관한 연구)

  • Ho, Kwang-Soo
    • Transactions of Materials Processing
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    • v.16 no.2
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    • pp.101-106
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    • 2007
  • Uniaxial displacement controlled tests were performed on annealed Type 304 stainless steel at room temperature. A servo-controlled testing machine and strain measurement on the gage length were employed to measure the response to a given input. The test results exhibit that the flow stress increases nonlinearly with the strain rate and the relaxed stress at the end of the relaxation periods depends strongly on the strain rate preceding the relaxation test. The rate-dependent inelastic deformation behavior is simulated using a new unified viscoplasticity model that has the rate-dependent format of nonlinear kinematic hardening rule, which plays a key role in modeling the rate dependence of relaxation behavior. The model does not employ yield or loading/unloading criteria and consists of a flow law and the evolution laws of two tensor and one scalar-valued state variables.

Viscoplastic Constitutive Equations for Ratchetting Behavior (라체팅 거동에 대한 점소성 구성방정식)

  • Ho, Kwang-Soo
    • Transactions of Materials Processing
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    • v.14 no.5
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    • pp.466-472
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    • 2005
  • Inelastic deformation behavior of metals and alloys is considered rate dependent. Uniaxial ratcheting experiments performed by Ruggles and Krempl, and Hassan and Kyriakides exhibited that higher mean stress for a fixed stress amplitude resulted in higher ratchet strain within a rate independent framework and higher stress rate resulted in lower ratchet strain, respectively. These phenomena are qualitatively investigated by numerical experiments through unified viscoplasticity theory. The theory does not separate rate-independent plasticity and rate-dependent creep, and thus uses only one inelastic strain to describe inelastic deformation processes with the concept of the yield surface. The growth law for the kinematic stress, which is a tensor valued state variable of the constitutive equations, is modified to predict the linear evolution of long-term ratchet strain.

A Temporal Finite Element Method for Elasto-Viscoplasticity through the Extended Framework of Hamilton's Principle (확장 해밀턴 이론에 근거한 탄점소성 시스템의 시간유한요소해석법)

  • Kim, Jin-Kyu
    • Journal of the Korean Association for Spatial Structures
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    • v.14 no.1
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    • pp.101-108
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    • 2014
  • In order to overcome the key shortcoming of Hamilton's principle, recently, the extended framework of Hamilton's principle was developed. To investigate its potential in further applications especially for material non-linearity problems, the focus is initially on a classical single-degree-of-freedom elasto-viscoplastic model. More specifically, the extended framework is applied to the single-degree-of-freedom elasto-viscoplastic model, and a corresponding weak form is numerically implemented through a temporal finite element approach. The method provides a non-iterative algorithm along with unconditional stability with respect to the time step, while yielding whole information to investigate the further dynamics of the considered system.

A Constitutive Model for the Rate-dependent Deformation Behavior of a Solid Polymer (속도 의존적인 폴리머 거동에 대한 구성적 모델)

  • Ho, K.
    • Transactions of Materials Processing
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    • v.22 no.4
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    • pp.216-222
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    • 2013
  • Solid polymers exhibit rate-dependent deformation behavior such as nonlinear strain rate sensitivity and stress relaxation like metallic materials. Despite the different microstructures of polymeric and metallic materials, they have common properties with respect to inelastic deformation. Unlike most metallic materials, solid polymers and shape memory alloys (SMAs) exhibit highly nonlinear stress-strain behavior upon unloading. The present work employs the viscoplasticity theory [K. Ho, 2011, Trans. Mater. Process. 20, 350-356] developed for the pseudoelastic behavior of SMAs, which is based on unified state variable theory for the rate-dependent inelastic deformation behavior of typical metallic materials, to depict the curved unloading behavior of polyphenylene oxide (PPO). The constitutive equations are characterized by the evolution laws of two state variables that are related to the elastic modulus and the back stress. The simulation results are compared with the experimental data obtained by Krempl and Khan [2003, Int. J. Plasticity 19, 1069-1095].