• Computational Structural Engineering : An International Journal
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• 제3권1호
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• pp.9-17
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• 2003

This paper aims at investigating the damage process of steel members leading to the failure under strong repeated loading, proposing the damage index using various factors related to the damage, and developing the analysis method for evaluating the damage state. Cantilever-type steel members were analyzed under uniaxial load and combined with a constant axial load, considering a horizontal displacement history. In analyzing the models, loading patterns and steel types (SS400, SM570, Posten80) were considered as main parameters. From the analysis results, the effects of parameter on the failures mode, the deformation capacity, the damage process are also discussed. Each failure process was compared as steel types. Consequently, the failure of steel members under strong repeated loading was determined by loading. Especially it was seen that the state of the failure is closely related to the local strain.

• 한국분말재료학회지
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• 제19권3호
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• pp.204-209
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• 2012

In this work, powder metallurgy and severe plastic deformation by high-pressure torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. Pure Cu powders were mixed with 5 and 10 vol% diamonds and consolidated into disc-shaped samples at room temperature by HPT at 1.25 GPa and 1 turn, resulting in ultrafine grained metallic matrices embedded with diamonds. Neither heating nor additional sintering was required with the HPT process so that in situ consolidation was successfully achieved at ambient temperature. Significantly refined grain structures of Cu metallic matrices with increasing diamond volume fractions were observed by electron backscatter diffraction (EBSD), which enhanced the microhardness of the Cu-diamond composites.

• 한국안전학회지
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• 제27권3호
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• pp.15-19
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• 2012

The fatigue behavior of AM60 magnesium alloy produced by equal channel angular pressing(ECAP) process was investigated through fatigue lifetime and fatigue crack propagation rate tests. The grain structure of the material was refined from 19.2 ${\mu}m$ to 2.3 ${\mu}m$ after 6 passes of ECAP at 493 K. The yield strength(YS) and ultimate tensile strength (UTS) increase after two passes but decrease with further pressing, although the grain size becomes finer with increasing pass number. The softening effect due to texture anisotropy overwhelmed the strengthening effect due to grain refinement after 2 passes. A large enhancement in fatigue strength was achieved after two ECAP passes. The current finding suggests that two passed material is better than the multi-passed material in view of the static strength and fatigue performance.

• Steel and Composite Structures
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• 제2권1호
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• pp.35-50
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• 2002

A series of tests on simple-welded plate specimens (SWPS) and T-stub tension specimens simulating some of the joint details in moment frame connections were conducted in this investigation. The effects of weld strength mismatch and weld metal toughness on structural behavior of these specimens were considered under both static and dynamic loading conditions. Finite element analyses were performed by taking into account typical weld residual stress distributions and weld metal strength mismatch conditions to facilitate the interpretation of the test results. The major findings are as follows: (a) Sufficient specimen size requirements are essential in simulating both load transfer and constraint conditions that are relevant to moment frame connections, (b) Weld residual stresses can significantly elevate stress triaxiality in addition to structural constraint effects, both of which can significantly reduce the plastic deformation capacity in moment frame connections, (c) Based on the test results, dynamic loading within a loading rate of 0.02 in/in/sec, as used in this study, premature brittle fractures were not seen, although a significant elevation of the yield strength can be clearly observed. However, brittle fracture features can be clearly identified in T-stub specimens in which severe constraint effects (stress triaxiality) are considered as the primary cause, (d) Based on both the test and FEA results, T-stub specimens provide a reasonable representation of the joint conditions in moment frame connections in simulating both complex load transfer mode and constraint conditions.

• 한국해양공학회지
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• 제19권5호
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• pp.65-70
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• 2005

Fracture behavior and pipe strength are very important to the integrity of energy plants, ocean structures, and so forth. The pipes of energy plants and ocean structures are subject to local wall thinning, resulting from severe erosion-corrosion damage. Recently, the effects of local wall thinning on fracture strength and fracture behavior of piping systems have been the focus of many studies. In this paper, the elasto-plastic analysis is performed by FE code ANSYS on straight pipes with wall thinning. We evaluated the failure mode, fracture strength and fracture behavior, using FE analysis. Also, the effect of the axial strain on deformations and failure modes was estimated by FE analysis.

• 한국분말재료학회지
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• 제17권1호
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• pp.52-58
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• 2010

Bulk nanostructured metallic materials are generally synthesized by bottom-up processing which starts from powders for assembling bulk materials. In this study, the bottom-up powder metallurgy and High Pressure Torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. After the HPT process at 473K, the disk samples reached a steady state condition when the microstructure and properties no longer evolve, and equilibrium boundaries with high angle grain boundaries (HAGBs) were dominant. The well dispersed alumina particles played important role of obstacles to dislocation glide and to grain growth, and thus, reduced the grain size at elevated temperature. The small grain size with HAGBs resulted in high strength and good ductility.

• 한국재난정보학회 논문집
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• 제8권2호
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• pp.138-147
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• 2012

본 논문에서는 해상풍력발전타워와 선박충돌에 대한 다양한 매개변수에 해석을 수행하여 선박충돌시 버켓파일로 지지된 기초부와 상부타워의 극한하중에 대한 거동을 분석하였다. 또한 충돌에너지의 변화에 따른 버켓기초의 안정성 여부 및 풍력타워의 에너지 소산능력에 대해 파악하였다. 해석결과 선박이 충돌에너지는 주로 타워의 소성변형에너지에 의해 소산 되었으며 이러한 극한상태의 하중에도 기초부는 충분한 지지력을 보이는 것으로 나타났다.

• 한국해양공학회지
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• 제22권2호
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• pp.99-105
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• 2008

Ultrasonic nanocrystal surface modification (UNSM) is a method to induce severe plastic deformation to a material surface, so that the structure of the material surface becomes a nanocrystal structure from the surface to a certain depth. It improves the mechanical properties, namely hardness, compressive residual stress, and fatigue characteristics. Specimens of SKD61 were tested to verify the effects of the variation of UNSM static load level on fatigue characteristics. The results were as follows: the grain size of SKD61 treated with UNSM became very fine from the material surface to a $100{\mu}m$ depth. The surface hardness of SKD61 was increased up to 37% after UNSM. And fatigue strength at $10^7$ cycles was increased by 8.3, 11.2, and 17.9% respectively, when the static load levels of UNSM were 4, 6, 8 kgf.

• 대한금속재료학회지
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• 제49권2호
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• pp.104-111
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• 2011

An ultrafine grained Mg-9Al-1Zn magnesium alloy with the mean grain size less than $1{\mu}m$ was produced by using high-ratio differential speed rolling. The processed alloy exhibited excellent superplasticity at relatively low temperatures. The micro-forming tests were carried out using a micro-forging apparatus with micro V-grooved shaped dies made of silicon and the micro-formability was evaluated by means of micro-formability index, $R_f$ ($=A_f/A_g$, $A_f$: formed and inflowed area into the V-groove, $A_g$: area of the V-groove). The $R_f$ value increased with temperature up to $280^{\circ}C$ and then decreased beyond $300^{\circ}C$. The decrease of the $R_f$ value at $300^{\circ}C$ was attributed to the accelerated grain coarsening. Increasing the micro-forging pressure increased the $R_f$ values. At a given die geometry, die filling ability decreased as the die position moved away from the die center to the end. FEM simulation predicted this behavior and a method of improving this problem was proposed.

• 대한금속재료학회지
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• 제49권2호
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• pp.128-136
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• 2011

Equal channel angular pressing (ECAP) with intermediate heat treatment was employed to optimize the strength of Cu-15 wt.%Ag. Changes in microstructure, electrical properties and mechanical properties were studied as a function of pressing methods and heat treatment. ECAPed Cu-15wt.%Ag exhibited ultrafine-grained microstructures with the shape and distribution of Ag-rich lamellae dependent on the processing routes. For route A in which the sample was pressed without rotation between each pass, the initial dendrites of Ag-rich phase were elongated along the shear direction and developed into elongated filaments. For route C in which the sample was rotated by 180 degree after each pass, the morphology of initial dendrites of Ag-rich phase was not much modified and the networked structure remained even after 8 passes of ECAP. For route Bc in which the sample was rotated by 90 degree after each pass, the initial dendrites became finer by fragmentation with no pronounced change of the shape and distribution of Ag-rich lamellae. The strength of Cu-15wt.%Ag ECAPed using route Bc was found to be greater than those ECAPed using route A, suggesting that the substructural strengthening is more effective in strengthening than the interface strengthening.