• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.350-352
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• 2009

The effect of porosity on the high-cycle fatigue properties of Al-Si-Mg casting aluminum alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both Al-Si-Mg casted (F) and heat-treated (T6) conditions. Porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The microstructure observation results showed that eutectic Si particles were homogeneously dispersed in the matrix of the Al-Si-Mg casting alloys, but there were porosities formed as cast defects. The high-cycle fatigue results indicated that the fatigue strength of the 356-T6 alloy was higher than that of the 356-F alloys because of the significant reduction in volume fraction of pores by heat treatment. The SEM fractography results showed that porosity affected detrimental effect on the fatigue life: 80% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. It was found that fatigue life decreased as the size of the surface pore increased. A comparison was made between surface pore and inner pore fur its effect on the fatigue behavior. The results showed that the fatigue strength with the inner pores was higher than that of the surface pore.

• 한국표면공학회지
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• 제48권6호
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• pp.260-268
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• 2015

The thermal barrier coating (TBC) for inner wall of liquid-fuel rocket combustor consists of NiCrAlY as bonding layer and $ZrO_2$ as a top layer. In most case, the plasma spray coating is used for TBC process and this process has inherent possibility of cracking due to large difference in thermal expansion coefficients among bonding layer, top layer and metal substrate. In this paper, we suggest crack-free TBC process by using a precise electrodeposition technique. Electrodeposited Ni-P/Cr double layer has similar thermal expansion coefficient to the Cu alloy substrate resulting in superior thermal barrier performance and high temperature oxidation resistance. We studied the effects of phosphorous concentrations (2.12 wt%, 6.97 wt%, and 10.53 wt%) on the annealing behavior ($750^{\circ}C$) of Ni-P samples and Cr double layered electrodeposits. Annealing temperature was simulated by combustion test condition. Also, we conducted SEM/EDS and XRD analysis for Ni-P/Cr samples. The results showed that the band layers between Ni-P and Cr are Ni and Cr, and has no formed with heat treatment. These band layers were solid solution of Cr and Ni which is formed by interdiffusion of both alloy elements. In addition, the P was not found in it. The thickness of band layer was increased with increasing annealing time. We expected that the band layer can improve the adhesion between Cr and Ni-P.

• 마이크로전자및패키징학회지
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• 제16권2호
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• pp.11-19
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• 2009

모아레 간섭계를 이용하여 와이어 본딩 플라스틱 볼 그리드 (WB-PBGA) 패키지의 열-기계적인 거동 특성을 연구하였다. 실시간 모아레 간섭계를 이용하여 각 온도단계 에서 변위분포를 나타내는 간섭무늬를 각각 얻고, 그로부터 굽힘변형 거동 및 솔더볼의 변형률에 대한 해석을 비교하여 수행하였다. 본 실험에서는 full grid와 perimeter with central connections 및 perimeter의 배열 형태를 갖는 세 가지 패키지를 사용하였으며, 이 배열 형태를 비교하여 굽힘변형 및 솔더볼의 평균변형률을 자세하게 해석하였다. 솔더볼의 유효변형률은 WB-PBGA-FG의 경우 칩 가장자리 바로 바깥쪽 솔더볼에서, WB-PBGA-P/C의 경우 가운데 연결 솔더볼의 가장 바깥 솔더볼에서, WB-PBGA-P의 경우는 칩과 가장 기까운 안쪽 솔더볼에서 최대값을 가지는 것으로 나타났다.

• Steel and Composite Structures
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• 제49권4호
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• pp.441-456
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• 2023

Aiming at the development trend of light weight and high strength of engineering structures, this paper experimentally investigated the seismic performance of circular-in-square high-strength concrete-filled double skin steel tubular (HCFDST) stub columns with out-of-code width-to-thickness (B/t) ratios. Typical failure mode of HCFDST stub columns appeared with the infill material crushing, steel fracture and local buckling of outer tubes as well as the inner buckling of inner tubes. Subsequently, the detailed analysis on hysteretic curves, skeleton curves and ductility, energy dissipation, stiffness degradation and lateral force reduction was conducted to reflect the influences of hollow ratios, axial compression ratios and infill types, e.g., increasing hollow ratio from 0.54 to 0.68 and 0.82 made a slight effect on bearing capacity compared to the ductility coefficients; the higher axial compression ratio (e.g., 0.3 versus 0.1) significantly reduced the average bearing capacity and ductility; the HCFDST column SCFST-6 filled with concrete obviously displayed the larger initial secant stiffness with a percentage 34.20% than the column SCFST-2 using engineered cementitious composite (ECC); increasing hollow ratios, axial compression ratios could accelerate the drop speed of stiffness degradation. The out-of-code HCFDST stub columns with reasonable design could behave favorable hysteretic performance. A theoretical model considering the tensile strength effect of ECC was thereafter established and verified to predict the moment-resisting capacity of HCFDST columns using ECC. The reported research on circular-in-square HCFDST stub columns can provide significant references to the structural application and design.

• Ocean Systems Engineering
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• 제4권4호
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• pp.309-325
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• 2014

Geotextile tubes are basically a huge sack filled with sand or dredged soil. Geotextile tubes are made of permeable woven or non-woven synthetic fibers (i.e., polyester or PET and polypropylene or PP). The geotextile tubes' performances in strength, dewatering, retaining solid particles and stacked stability have been studied extensively in the past. However, only little research has been done in the observation of the deformation behavior of geotextile tubes. In this paper, a large-scale apparatus for geotextile tube experiment is introduced. The apparatus is equipped with a slurry mixing station, pumping and delivery station, an observation station and a data station. For this study the large-scale apparatus was utilized in the studies regarding the stresses on the geotextile and the deformation behavior of the geotextile tube. Model tests were conducted using a custom-made woven geotextile tubes. Load cells placed at the inner belly of the geotextile tube to monitor the total soil pressure. Strain gauges were also placed on the outer skin of the tube to measure the geotextile strain. The pressure and strain sensors are attached to a data logger that sends the collected data to a desktop computer. The experiment results showed that the maximum geotextile strain occurs at the sides of the tube and the soil pressure distribution varies at each geotextile tube section.

• Journal of Advanced Marine Engineering and Technology
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• 제35권7호
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• pp.899-906
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• 2011

Generally, load conditions of machine or structure in fatigue destruction is occurred not under single load conditions but under mixed load conditions. However, the experiment under mixing mode is insufficient because of no having test standard to the behavior of crack under mixing mode and variety of test methods, and many tests are required. In this paper measured crack direction path by created figure capture system when a experiment. Also, we studied by comparison the behavior of crack giving the change of stress ratio and inserting beach mark. Through the test under mixing mode, advancing path of crack is indicated that advancing inclined angle ${\Theta}$ (direction of specimen length) has increased depending on the increase of mixed mode impaction. It is indicated that according to the increase of mixed mode loading condition impaction under mixing mode, advancing speed of crack gets slow. Also, we found that inner crack(cross section of specimen) is progressed more rapidly than outer crack based on data through beach mark.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 춘계 학술발표회
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• pp.1132-1139
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• 2010

The steel pipe of steel-concrete composite piles increases the pile strength and induces the ductile failure by constraining the deformation of the inner concrete. In this research, the load-movement relations and the reinforcement effect by the outer steel pipe in the steel-concrete composite pile were analyzed by performing three-dimensional numerical analyses, which can simulate the yielding behavior of pile material and the elasto-plastic behavior of soils. The parameters analyzed in the study include three pile materials of steel, concrete and composite, pile diameter, pile distance and loading direction. As the results, the axial capacity of the composite pile was about 73% larger than that of the steel pipe pile and about 14% larger than that of the concrete pile. In addition, the horizontal movement at the pile head of the composite pile was about 51% of that of the steel pile and about 19% of that of the concrete pile.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.908-913
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• 2000

In the process of integrity evaluation for nuclear power plant components, a series of fracture mechanics evaluation on surface cracks in reactor pressure vessel(RPV) must be conducted. These fracture mechanics evaluations are based on stress intensity factor, K. However, under pressurized thermal shock(PTS) conditions, the combination of thermal and mechanical stress by steep temperature gradient and internal pressure causes considerably high tensile stress at the inside of RPV wall. Besides, the internal pressure during the normal operation produces high tensile stress at the RPV wall. As a result cracks on inner surface of RPVs may experience elastic-plastic behavior which can be explained with J-integral. In such a case, however, J-integral may possibly lose its validity due to constraint effect. In this paper, in order to verify the suitability of J-integral, two dimensional finite element analyses were applied for various surface crack. Total of 18 crack geometries were analyzed, and Q stresses were obtained by comparing resulting HRR stress distribution with corresponding actual stress distributions. In conclusion, HRR stress fields were found to overestimate the actual crack-tin stress field due to constraint effect.

• 소성∙가공
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• 제17권1호
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• pp.46-51
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• 2008

Polycrystalline materials such as steels(BCC) and aluminum alloys(FCC) show the strain hardening and the strain rate hardening during the plastic deformation. The strain hardening is induced by deformation resistance of dislocation glide on some crystallographic systems and increase of the dislocation density on grain boundaries or inner grain. However, the phenomenon of the strain rate hardening is not demonstrated distinctly in the rage of $10^{-2}$ to $10^2/sec$ strain rate. In this paper, tensile tests for various strain rates are performed in the rage of $10^{-2}$ to $10^2/sec$ then, specimens are extracted on the same strain position to investigate the microscopic behavior of deformed materials. The extracted specimens are investigated by using the electron backscattered diffraction(EBSD) and transmission electron microscopy(TEM) results which show the effect of texture orientation, grain size and dislocation behavior on the strain rate hardening.

• 한국자동차공학회논문집
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• 제13권5호
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• pp.50-56
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• 2005

Aluminum member absorbs energy by stable plastic deformation under axial loading. While CFRP(Carbon Fiber Reinforced Plastics) member absorbs energy by unstable brittle failure but its specific strength and stiffness is higher than those of aluminum member. In this study, for complement of detects and synergy effect by combination with the advantages of each member, the axial collapse tests were performed for aluminum CFRP members which are composed of aluminum members wrapped with CFRP outside aluminum circular members. Based on the respective collapse characteristics of aluminum and CFRP members, crushing behavior and energy absorption characteristics were analyzed for aluminum CRRP members which have different CFRP fiber orientation angle and thickness Test results showed that aluminum CFRP members supplemented the unstable brittle failure of CFRP members due to ductile nature of inner aluminum members. It turned out that the CFRP fiber orientation angle and thickness influence energy absorption capability together with the collapse mode of the members.