• Proceedings of the KSR Conference
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• 2007.05a
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• pp.340-350
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• 2007

The major objective of this study is to investigate the fatigue effects of rail pad on High Speed Railway with concrete slab track system. It analyzed the mechanical behaviors of HSR concrete slab track with applying rail pad stiffness based on fatigue effect(hardening and increasing stiffness) on the 3-dimensional FE analysis and laboratory test for static & dynamic characteristics. As a result, the hardening of rail pad due to fatigue loading condition are negative effect for the static & dynamic response of concrete slab track which is before act on fatigue effect. The analytical and experimental study are carried out to investigate rail pad on fatigue effected increase vertical acceleration and stress and decrease suitable deflection on slab track. And rail pad based on fatigue effect induced dynamic maximum stresses, the increase of damage of slab track is predicted by adopting fatigue effected rail pad. after due consideration The servicing HSR concrete slab track with resilient track system has need of the reasonable determination after due consideration fatigue effect of rail pad stiffness which could be reducing the effect of static and dynamic behavior that degradation phenomenon of structure by an unusual response characteristic and a drop durability.

• Transactions of Materials Processing
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• v.17 no.8
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• pp.579-585
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• 2008

Numerous experimental investigations on metallic materials and solid polymers have shown that relaxation behavior is nonlinearly dependent on prior strain rate. The stress drops in a constant time interval nonlinearly increase with an increase of prior strain rate. And the relaxed stress associated with the fastest prior strain rate has the smallest stress magnitude at the end of relaxation periods. This paper deals with the performance of three classes of unified constitutive models in predicting the characteristic behaviors of relaxation. The three classes of models are categorized by a rate sensitivity of kinematic hardening rule. The first class uses rate-independent kinematic hardening rule that includes the competing effect of strain hardening and dynamic recovery. In the second class, a stress rate term is incorporated into the rate-independent kinematic hardening rule. The final one uses a rate-dependent format of kinematic hardening rule.

• Earthquakes and Structures
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• v.25 no.3
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• pp.161-175
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• 2023

High-damping rubber bearings (HDRB) are commonly used as seismic isolation devices to protect civil engineering structures from earthquakes. However, the nonlinear hysteresis characteristics of the HDRB, such as their dependence on material properties and hardening phenomena, make predicting their behavior during earthquakes difficult. This study proposes a hysteretic model that can accurately predicts the behavior of shear deformation considering the nonlinearity when designing the seismic isolation structures using HDR bearings. To model the hysteretic characteristics of the HDR, dynamic loading tests were performed by applying sinusoidal and random waves on scaled-down specimens. The test results show that the nonlinear characteristics of the HDR strongly correlate with the shear strain experienced in the past. Furthermore, when shear deformation occurred above a certain level, the hardening phenomenon, wherein the stiffness increased rapidly, was confirmed. Based on the experimental results, the dynamic characteristics of the HDR, equivalent stiffness, equivalent damping ratio, and strain energy were quantitatively evaluated and analyzed. In this study, an improved bilinear HDR model that can reproduce the dependence on shear deformation and hardening phenomena was developed. Additionally, by proposing an objective parameter-setting procedure based on the experimental results, the model was devised such that similar parameters could be set by anyone. Further, an actual dynamic analysis could be performed by modeling with minimal parameters. The proposed model corresponded with the experimental results and successfully reproduced the mechanical characteristics evaluated from experimental results within an error margin of 10%.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.4
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• pp.427-434
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• 2004

Ferritic stainless steel is recently used in high temperature structures because of its good properties of thermal fatigue resistance, corrosion resistance, and low price. Tensile and low-cycle fatigue (LCF) tests on 429EM stainless steel used in exhaust manifold were performed at several temperatures from room temperature to 80$0^{\circ}C$. Elastic Modulus, yield strength, and ultimate tensile strength monotonically decreased when temperature increased. Cyclic hardening occurred considerably during the most part of the fatigue life. Dynamic strain aging was observed in 200~50$0^{\circ}C$, which affects the cyclic hardening behavior. Among the fatigue parameters such as plastic strain amplitude, stress amplitude, and plastic strain energy density (PSED), PSED was a proper fatigue parameter since it maintained at a constant value during LCF deformation even though cyclic hardening occurs considerably. A phenomenological life prediction model using PSED was proposed considering the influence of temperature on fatigue life.

• Journal of the Korean Society for Heat Treatment
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• v.8 no.4
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• pp.266-278
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• 1995

The high strain-rate dynamic plastic behavior of Fe-X%Mn alloys was investigated. The strain rate did not have an effect when tested under quasi-static strain rates($2{\times}10^{-3}/sec$ and $2{\times}10^{-1}/sec$). However, the true stress increased at all strain levels when the strain rate increased to $6{\times}10^3/sec$. Based on the experimental results, an constitution equation to calculate the dynamic strength for strain rates over $10^4/sec$ was determined. The Fe-5%Mn alloy containing athermal ${\alpha}^{\prime}$ martensite initially did not show work hardening. The work hardening increased with Mn content showing a maximum at 20% Mn. The high work hardening of Fe-20%Mn and Fe-30%Mn alloys appears to be closely related not only to the initial amounts of ${\varepsilon}$ martensite but to the strain induced transformation (${\gamma}{\rightarrow}{\varepsilon}$ and ${\varepsilon}{\rightarrow}{\alpha}^{\prime}$) occurring during each stages of deformation.

• Transactions of Materials Processing
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• v.30 no.1
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• pp.16-21
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• 2021

A new concept of ammunition without the use of explosive gunpowder has been recently studied, which achieves performance equal to or higher than that of high explosives. Frangible Armor Piercing (FAP) is one of the concepts, which utilizes a tungsten alloy penetrator specialized for fragmentation. To investigate the fracture behavior of the tungsten alloy penetrator, Taylor impact tests were conducted at various impact velocities. Additionally, finite element analysis was performed to predict the fracture behavior of the tungsten alloy. Compression tests were also carried out at six strain rates for dynamic material properties and the dynamic hardening behavior was successfully predicted with the Lim-Huh model. Finally, the Mohr-Coulomb fracture model based on the mean stress was adopted to predict impact failure in Taylor impact simulation. The analysis predicts the deformation and fracture behaviors of the tungsten alloy successfully.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.928-929
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• 2006

Tensile stress-strain and dynamic acoustic resonance tests were performed on Fe-C-Ni-Cu-Mo high-strength steels, characterized by a heterogeneous matrix microstructure and the prevalence of open porosity. All materials display the first yielding phenomenon and, successively, a continuous yielding behavior. This flow behavior can be described by the Ludwigson equation and developes through three stages: the onset of localized plastic deformation at the pore edges; the evolution of plastic deformation at the pore necks (where the austenitic Ni-rich phase is predominant); the spreading of plastic deformation in the interior of the matrix. The analytical modeling of the strain hardening behavior made it possible to obtain the boundaries between the different deformation stages.

• Journal of the Korean Society for Railway
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• v.10 no.5
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• pp.499-505
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• 2007

The major effective of this study is to investigate the fatigue effects of rail pad on High Speed Railway with concrete slab track system. It analyzed the mechanical behaviors of HSR concrete slab track with applying rail pad stiffness based on fatigue effect (hardening and increasing stiffness) on the 3-dimensional FE analysis and laboratory test for static & dynamic characteristics. As a result, the hardening of rail pad due to fatigue loading condition are negative effect for the static & dynamic response of concrete stab track which is before act on fatigue effect. The analytical and experimental study are carried out to investigate rail pad on fatigue effected increase vertical acceleration and stress and decrease suitable deflection on slab track. And rail pad based on fatigue effect induced dynamic maximum stresses, the increase of damage of slab track is predicted by adopting fatigue effected rail pad. after due consideration. The servicing HSR concrete slab track with resilient track system has need of the reasonable determination after due consideration fatigue effect of rail pad stiffness which could be reducing the effect of static and dynamic behavior that degradation phenomenon of structure by an unusual response characteristic and a drop durability.

• Transactions of Materials Processing
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• v.22 no.8
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• pp.450-455
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• 2013

In the current study, a new dynamic recrystallization model for predicting high temperature flow stress is developed based on a physical model and the mean field theory. In the model, the grain aggregate is assumed as a representative volume element to describe dynamic recrystallization. The flow stress and microstructure during dynamic recrystallization were calculated using three sub-models for work hardening, for nucleation and for growth. In the case of work hardening, a single parameter dislocation density model was used to calculate change of dislocation density and stress in the grains. For modeling nucleation, the nucleation criterion developed was based on the grain boundary bulge mechanism and a constant nucleation rate was assumed. Conventional rate theory was used for describing growth. The flow stress behavior of pure copper was investigated using the model and compared with experimental findings. Simulated results by cellular automata were used for validating the model.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.377-382
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• 2004

The dynamic material characteristics on some mild steel sheets were observed. The dynamic tests were conducted on the ESH servo-hydraulic test machine. It was observed that the mechanical properties of mild steel are highly sensitive to the value of strain rate. The well known Cowper-Symonds constitutive equation was used to generalize the strain rate sensitivity effect. Modified constitutive equations were suggested to couple the strain hardening to the strain rate sensitivity. The dynamic stress-strain relationships for the mild steel sheets used in the present study were reasonably predicted using these modified constitutive equations.