• Structural Engineering and Mechanics
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• v.25 no.5
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• pp.519-540
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• 2007

In this paper, the energy-based plastic-damage model previously proposed by the authors [International Journal of Solids and Structures, 43(3-4): 583-612] is first simplified with an empirically defined evolution law for the irreversible strains, and then it is extended to its rate-dependent version to account for the strain rate effect. Regarding the energy dissipation by the motion of the structure under dynamic loadings, within the framework of continuum damage mechanics a new damping model is proposed and incorporated into the developed rate-dependent plastic-damage mode, leading to a unified constitutive model which is capable of directly considering the damping on the material scale. Pertinent computational aspects concerning the numerical implementation and the algorithmic consistent modulus for the unified model are also discussed in details, through which the dynamic nonlinear analysis of damping structures can be coped with by the same procedures as those without damping. The proposed unified plastic-damage model is verfied by the simulations of concrete specimens under different quasistatic and high rate straining loading conditions, and is then applied to the Koyna dam under earthquake motions. The numerical predictions agree fairly well with the results obtained from experimental tests and/or reported by other investigators, demonstrating its capability for reproducing most of the typical nonlinear performances of concrete under quasi-static and dynamic loading conditions.

• Journal of the Korean institute of surface engineering
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• v.41 no.6
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• pp.301-307
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• 2008

In this paper, nanocrystallization of CuNiTiZr bulk metallic glass (BMG) subjecting to a kinetic spraying, dependent on impact velocity, was investigated by numerical and experimental approaches. The crystallization fraction and nucleation activation energy of initial feedstock and as-deposited coating were estimated by DSC and Kissinger method, respectively. The results of numerical modeling and experiment showed that the crystalline fraction and nucleation activation energy in BMG coatings were depended on kinetic energy of incident particle. Upon impact, the conversion of particle kinetic energy leads to not only decreasing free energy barrier but also increasing the driving force for an amorphous to crystalline phase transformation. The nanocrystallization of BMGs is associated with the strain energy delivered by a plastic deformation with a high strain rate.

• Proceedings of the Korea Concrete Institute Conference
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• 1994.04a
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• pp.91-96
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• 1994

The growth and propagation of microcracks existed in concrete cause failure of concrete. This is called "damage". The concepts of two principles, equivalent strain principle and equivalent energy principle, are reviewed and compared in the case of uniaxial compressior to concrete. The damage evolution law and constitutive equation are derived by using the Helmholz free energy and the dissipation potential by means of the thermodynamic principles.rinciples.

• Korean Journal of Metals and Materials
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• v.50 no.12
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• pp.873-882
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• 2012

Dual Phase (DP) steel which has a soft ferrite phase and a hard martensite phase reveals both high strength and high ductility and has received increased attention for use in automotive applications. To conduct structural analysis to verify vehicle safety, highly credible experimental results are required. In this study, tensile tests were performed in a strain rate range from $10^{-4}/s$ to 300/s for Sink Roll-Less (SRL) hot-dip metal coated sheets. Collision properties were estimated through simulation by LS-DYNA using the stress-strain curve obtained from the tensile test. The simulation results were compared with the actual crash test results to confirm the credibility of the simulation. In addition, a tensile test and a crash test with 2% prestrain and a baking (PB) specimen were evaluated identically because automotive steel is used after forming and painting. The mechanical behaviors were improved with an increasing strain rate regardless of the PB treatment. Thus, plastic deformation with an appropriate strain rate is expected to result in better formability and crash characteristics than plastic deformation with a static strain rate. The ultimate tensile strength (UTS) and absorbed energy up to 10% strain were improved even though the total elongation decreased after PB treatment, The results of the experimental crash test and computer simulation were slightly different but generally, a similar propensity was seen.

• Transactions of Materials Processing
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• v.19 no.6
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• pp.329-336
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• 2010

Ring forging process is more appropriate for high-length and thin walled ring, because it utilizes the forging press and hence does not require heavy-duty ring rolling mill. Although ring forging process is very simple and economic for facilities, the process is not efficient because of multi-forging-step and low material utilization. An effective ring forging process is developed using a hollow ingot. When a hollow ingot is used with a workpiece, the ingot can be forged into a final ring without multi-stage pre-forging process, such as, cogging, upsetting, and piercing, etc.. Finally it has advantages of the material utilization and process improvement because a few reheating and forging process are not necessary to make workpiece for ring forging. The important design variables are the applied plastic deformation energy to eliminate cast structure and make uniform properties. In this study, the mechanical properties after forging of hollow cast ingot were investigated from the experiment using circumferential sectional model. Also, the effects of process variables were studied by FEM simulation on the basis of thermo-visco-plastic constitutive equation. Applied strain is different at each position in length direction because diameter of hollow ingot is different in length direction. The different strain distribution become into a narrow gap by additional plastic deformation during diameter extension process.

• Transactions of Materials Processing
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• v.23 no.3
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• pp.178-183
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• 2014

Continuous casting is a process where molten metal is solidified into a semi-finished billet on a large scale with either a rectangular or round cross section for subsequent processing. The use of continuous casting provides an opportunity for producing material on a practical industrial scale with lower cost than conventional casting. In the current study, the material was fabricated by continuous casting and subsequent extrusion. Tensile tests were conducted on continuous cast ZK60A after extrusion over a range of strain rates at temperatures from 473K to 623K. The alloy exhibits a quasi-superplastic behavior with a maximum recorded elongation of ~250% at 523K when tested with an initial strain rate of $10^{-5}/s$. The experiments give a strain rate sensitivity exponent of 0.3~0.4 and an activation energy of 108 kJ/mol. From the current investigation, it was found that the high-temperature plastic flow of the ZK60A is controlled by a dislocation viscous glide mechanism.

• Structural Engineering and Mechanics
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• v.58 no.5
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• pp.799-823
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• 2016

A finite element model for the non-linear dynamic analysis of a reinforced concrete (RC) containment shell of a nuclear power plant subjected to extreme loads such as impact and earthquake is presented in this work. The impact is modeled by using an uncoupled approach in which a load function is applied at the impact zone. The earthquake load is modeled by prescribing ground accelerations at the base of the structure. The nuclear containment is discretized spatially by using 20-node brick finite elements. The concrete in compression is modeled by using a modified $Dr{\ddot{u}}cker$-Prager elasto-plastic constitutive law where strain rate effects are considered. Cracking of concrete is modeled by using a smeared cracking approach where the tension-stiffening effect is included via a strain-softening rule. A model based on fracture mechanics, using the concept of constant fracture energy release, is used to relate the strain softening effect to the element size in order to guaranty mesh independency in the numerical prediction. The reinforcing bars are represented by incorporated membrane elements with a von Mises elasto-plastic law. Two benchmarks are used to verify the numerical implementation of the present model. Results are presented graphically in terms of displacement histories and cracking patterns. Finally, the influence of the shear transfer model used for cracked concrete as well as the effect due to a base slab incorporation in the numerical modeling are analyzed.

• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.953-960
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• 2012

This paper reports the irradiation effect on the deformation behavior and tensile fracture properties of A533B RPV steel. An inverse identification technique using iterative finite element (FE) simulation was used to determine those properties from tensile data for the A533B RPV steel irradiated at 65 to $100^{\circ}C$ and deformed at room temperature. FE simulation revealed that the plastic instability at yield followed by softening for higher doses was related to the occurrence of localized necking immediately after yielding. The strain-hardening rate in the equivalent true stress-true strain relationship was still positive during the necking deformation. The tensile fracture stress was less dependent on the irradiation dose, whereas the tensile fracture strain and fracture energy decreased with increasing dose level up to 0.1 dpa and then became saturated. However, the tensile fracture strain and fracture energy still remained high after high-dose irradiation, which is associated with a large amount of ductility during the necking deformation for irradiated A533B RPV steel.

• Nuclear Engineering and Technology
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• v.50 no.1
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• pp.190-202
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• 2018

Maintaining the integrity of the major equipment in nuclear power plants is critical to the safety of the structures. In particular, the soundness of the piping is a critical matter that is directly linked to the safety of nuclear power plants. Currently, the limit state of the piping design standard is plastic collapse, and the actual pipe failure is leakage due to a penetration crack. Actual pipe failure, however, cannot be applied to the analysis of seismic fragility because it is difficult to quantify. This paper proposes methods of measuring the failure strain and deformation angle, which are necessary for evaluating the quantitative failure criteria of the steel pipe elbow using an image measurement system. Furthermore, the failure strain and deformation angle, which cannot be measured using the conventional sensors, were efficiently measured using the proposed methods.

• Steel and Composite Structures
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• v.28 no.6
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• pp.767-778
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• 2018

The purpose of this study is to investigate how a fully restrained bolted beam splice affects the connection behavior as a column-tree connection in steel special moment frames under cyclic loading when located within the plastic hinge zone. The impacts of this attachment in protected zone are observed by using nonlinear finite element analyses. This type of splice connection is designed as slip-critical connection and thereby, the possible effects of slippage of the bolts due to a possible loss of pretension in the bolts are also investigated. The 3D models with solid elements that have been developed includes three types of connections which are the connection having fully restrained beam splice located in the plastic hinge location, the connection having fully restrained beam splice located out of the plastic hinge and the connection without beam splice. All connection models satisfied the requirement for the special moment frame connections providing sufficient flexural resistance, determined at column face stated in AISC 341-16. In the connection model having fully restrained beam splice located in the plastic hinge, due to the pretension loss in the bolts, the friction force on the contact surfaces is exceeded, resulting in a relative slip. The reduction in the energy dissipation capacity of the connection is observed to be insignificant. The possibility of the crack occurrence around the bolt holes closest to the column face is found to be higher for the splice connection within the protected zone.