• Computational Structural Engineering
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• v.8 no.3
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• pp.59-66
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• 1995

The dynamic responses of structure under impulsive loading have been investigated according to its duration, based on the theory of viscoplasticity which can appropriately represent the effects of plasticity and rheology simultaneously. The viscoplastic model has been implemented into the two-dimensional finite element system to solve plane stress, plane strain or axi-symmetric problems, and the implicit integration scheme, of which solutions are unconditionally stable for relatively large time step length, has been developed to simulate visoplastic straining with deriving the explicit relationship between stress and strain at a material point level. After simulation, one carefully concludes that the duration as well as magnitude of impulsive loading plays an important role in design of structures.

• Advanced Composite Materials
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• v.18 no.3
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• pp.265-285
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• 2009

Off-axis compressive deformation behavior of a unidirectional CFRP laminate at high temperature and its strain-rate dependence in a quasi-static range are examined for various fiber orientations. By comparing the off-axis compressive and tensile behaviors at an equal strain rate, the effect of different loading modes on the flow stress level, rate-dependence and nonlinearity of the off-axis inelastic deformation is elucidated. The experimental results indicate that the compressive flow stress levels for relatively larger off-axis angles of $30^{\circ}$, $45^{\circ}$ and $90^{\circ}$ are about 50 percent larger than in tension for the same fiber orientations, respectively. The nonlinear deformations under off-axis tensile and compressive loading conditions exhibit significant strain-rate dependence. Similar features are observed in the fiber-orientation dependence of the off-axis flow stress levels under tension and compression and in the off-axis flow stress differential in tension and compression, regardless of the strain rate. A phenomenological theory of viscoplasticity is then developed which can describe the tension-compression asymmetry as well as the rate dependence, nonlinearity and fiber orientation dependence of the off-axis tensile and compressive behaviors of unidirectional composites in a unified manner. It is demonstrated by comparing with experimental results that the proposed viscoplastic constitutive model can be applied with reasonable accuracy to predict the different, nonlinear and rate-dependent behaviors of the unidirectional composite under off-axis tensile and compressive loading conditions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.6
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• pp.670-676
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• 2004

Most metallic materials and alloys show rate independence or negative rate sensitivity in some temperature region when dynamic strain aging occurs. It is generally recognized that negative rate sensitivity is an essential feature of dynamic strain aging that can depend on strain and/or strain rate. The unified viscoplasticity theory based on overstress is applied to reproduce a change of rate sensitivity type that depends on strain or strain rate. This is accomplished through the introduction of a single new term in the growth law of the equilibrium stress, which is a tensor valued state variable of the model. It is also shown that the new term can be used to reproduce a dramatic increase of rate sensitivity in dynamic plasticity.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.9 no.2
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• pp.31-42
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• 1989

An experimental and theoretical investigation was conducted in the present study to explore the mechanical behavior of concrete under dynamic loads. The stress-strain behavior and mechanical characteristics of concrete under various strain-rates were studied from several series of experiments. A new constitutive theory was derived on the basis of viscoplasticity to model the dynamic behavior of concrete. The theory is verified by the present test results as well as other previous test data. It is found from the present study that the strength of concrete is greatly increased with the increase of strain rate and that the rate of strength increase is greater in tension than in compression. The present theory as well as present test results will provide very useful basis to explore the dynamic behavior of concrete structures.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.190-200
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• 1991

Certain structural components are exposed to high temperatures. At high temperature, under thermal and mechanical loading, metal components exhibit both creep and plastic behavior. The unified constitutive theory is to model both the time-dependent behavior(creep) and the time-independent behavior(plasticity) in one set of equations. Microscopically both creep and plasticity are controlled by the motion of dislocations. A finite element method is presented encorporating a unified constitutive model for the transient analysis of viscoplastic behavior of structures exposed to high temperature.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.195-200
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• 2006

Laboratory creep experiments show that compaction of unconsolidated shale is an irrecoverable process caused by viscous time-dependent deformation. Using Perzyna's viscoplasticity framework combined with the modified Cam-clay theory, we found the constitutive equation expressed in the form of strain rate as a power law function of the ratio between the sizes of dynamic and static yield surfaces. We derived the volumetric creep strain at a constant hydrostatic pressure level as a logarithmic function of time, which is in good agreement with experimental results. The determined material constants indicate that the yield stress of the shale increases by 6% as strain rate rises by an order of magnitude. This demonstrates that the laboratory-based prediction of yield stress (and porosity) may result in a significant error in estimating the properties in situ.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.4 no.3
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• pp.43-52
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• 1984

This paper is concerned with the strain characteristics of the time effect on the remoulded saturated day sampled from the downstream of the Yeongsan river, and the constitutive equation that can generally explain time-dependent behaviors of norma1ly consolidated clay. This paper examines whether or not the afore-said constitutive equation can be applied to the remoulded Mooan-clay. Throughout this study, the conclusions obtained are as follows. 1. Throughout the isotropic consolidation test for 7 days and the isotropic relaxation test, the existence of the static and dynamic yielding surfaces is confirmed respectively. 2. The characteristics of time effect of the deformation, namely, the existence of a unique stress-strain-time relation, is conformed from the experimental result on the Mooan-clay. 3. The prodictions of the stress path and the strain on the Cam-clay theory is not consistent with those observed during the experiments. 4. Constitutive equation(2-3-12) obtained by applying Cam-clay theory to Perzyna's elastic-viscoplasticity theory can explain the behavior of pore water pressure during isotropic stress relaxation, concerned with time dependency under undrained condition. The equation can also explain the results of the undrained triaxial compression test for the clay with different strain rate under the same or different consolidation history. 5. This constitutive equation has eight material parameters which can be determined from triaxial compression tests.