• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.766-773
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• 2002

The failure phenomenon of overlaid concrete structures, such as surface crack, and peel-off failure, shear bond failure in the end contact zone, was investigated due to humidity changes. To investigate this failure phenomenon, the surface tensile stress, and the shear stress, the vertical tensile stress in the contact zone were analysed using the non-linear stress-strain relationship of material such as strain-hardening- and strain-softening diagrams. Overlay thickness and overlay material were the main variables in the analyses. It is assumed that the initial surface humidity of overlaid concrete structures was 100％ r.H. With a atmospheric humidity of 55％ r.H. and two load cases for drying(LCI), curing and drying(LC2), the stress states of overlaid concrete structures were calculated. The result shows that only fictitious cracks occurred in the overlay surface of CM2O, ECM25, and no shear bond failure occurred in the contact zone without CM2O. The peel-off failure was proved to be the main cause of the damage in the overlaid concrete structures. Only for overlay thickness of 1cm occurred no peel-off failure in the case of drying after a long-term public use(LC1). In the case of curing and drying during overlay work(LC2) occurred the peel-off failure within 1.5days for all the overlaid concrete structures.

• Journal of the Korea Concrete Institute
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• v.13 no.5
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• pp.491-498
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• 2001

The failure phenomenon of overlaid concrete structures, such as surface crack and peel-off failure in the contact zone, was investigated due to temperature shock(rainfall). To investigate this failure phenomenon, the surface tensile stress, and the shear stress, the vertical tensile stress in the contact zone were analysed using the non-linear stress-strain relationship of material such as strain-hardening- and strain-softening diagrams. Rainfall intensity, overlay thickness and overlay material were the main variables in the analyses. It is assumed that the initial temperature of overlaid concrete structures was heated up to 55$\^{C}$ by the solar heat. With a rain temperature 10$\^{C}$ and the rainfall intensity of nR=1/a, tR=10min, 60min, the stress states of overlaid concrete structures were calculated. The result shows that only fictitious cracks occurred in the overlay surface and no shear bond failure occurred in the contact zone. The vortical tensile stress increasing with overlay thickness was proved to be the cause of peel-off failure in the contact zone. The formulae for relationship between the vertical tensile stress and overlay thickness, material properties were derived. Using this formulae, it is possible to select proper material and overlay thickness to prevent failure in the contact zone due to temperature shock caused by rainfall.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.421-424
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• 2008

Ultra high strength steel fiber-reinforced concrete is characterized with high tensile strength and ductility. This paper revealed the influence of fiber volume fraction on the tensile softening behaviour of ultra high strength steel fiber-reinforced concrete and developed tensile softening model to predict the deformation capacity by finite element method analysis with experimental results. The initial stiffness of ultra high strength steel fiber-reinforced concrete was constant irrespective of fiber volume fraction. The increase of fiber volume fraction improved the flexural tensile strength and caused more brittle softening behaviour. Finite element method analysis proposed by Uchida et al. was introduced to obtain the tensile softening curve from three point notched beam test results and we proposed the tensile softening model as a function of fiber volume fraction and critical crack width.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 1999.03a
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• pp.25-31
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• 1999

암석 내에 포함된 불연속면으로 인하여 암석은 압축하중 하에서 복잡한 거동을 한다. 이러한 복잡한 거동은 변형률강화(strain hardening), 변형률연화(strain softening), 부피팽창(dilatancy)등의 비선형 거동을 포함하는데, 이들은 균열의 성장, 상호작용(interaction), 연합(coalescence) 등을 통하여 발생한다(Brace et al., 1972; Kranz, 1983). 이와 같은 암석의 비선형적 거동을 설명하기 위하여 여러 형태의 균열모형이 개발된 바 있다(Costin, 1985; Nemat-Nasser ＆ Horii, 1982; Wang & Kemeny, 1993; Jeon, 1998). (중략)

• Journal of the Korea Concrete Institute
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• v.21 no.2
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• pp.121-127
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• 2009

The failure behavior of RC structure was exceedingly affected by the size and the local strain distribution of the failure zone due to the strain localization behavior on the tension softening materials. However, it is very difficult to quantify and assess the local strain occurring in the failure zone by the conventional test method. In this study, image processing technology, which is available to measure the strain up to the complete failure of RC structures, was used to estimate the local strain distribution and the size of failure zone. In order to verify the reliability and validity for the image processing technology, the strain transition acquired by the image processing technology was compared with strain values measured by the concrete gauge on the uniaxial compressive specimens. Based on the verification of image processing technology for the uniaxial compressive specimens, the size and the local strain distribution of the failure zone of deep beam was measured using the image processing technology. With the results of test, the principal tensile/compressive strain contours were drawn. Using the strain contours, the size of the failure zone and the local strain distribution on the failure of the deep beam was evaluated. The results of strain contour showed that image processing technology is available to assess the failure behavior of deep beam and obtain the local strain values on the domain of the post-peak failure comparatively.

• Analytical Science and Technology
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• v.9 no.2
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• pp.179-191
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• 1996

In order to understand the high temperature deformation behavior of superalloy 718, a rotating grade 718 alloy has been compression tested to about 0.7 upset ratio at $927{\sim}1066^{\circ}C$ temperature range and $5{\times}10^{-4}{\sim}5{\times}10^0sec^{-1}$ strain rate. The maximum flow stress was increased with increasing strain rate, and similar behavior was observed with decreasing temperature. At low temperature and high strain rates other than $5{\times}10^{-1}sec^{-1}$, strain softening was occurred mainly by dynamic recovery and deformation twinning processes, while at high temperature and low strain rates strain softening was offseted by dynamic recrystallization. At $5{\times}10^{-1}sec^{-1}$, strain hardening was occurred due to work hardening of the dynamic recrystallized grains. Strain rate sensitivity, m, was varied with strain rates. In the case of lower strain rate tests, m was measured as 0.3 and it was observed that the deformation was mainly controlled by dynamic recrystallization. At higher strain rate, m was lowered to 0.1 and the deformation was controlled by the dynamic recovery and the deformation twinning processes.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.307-315
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• 1989

In recent years, the fatigue design method by analysis for the mechanical components and the welded structures has much increased, instead of the fatigue design method by rule that has been widely used from the past days. When a fatigue design is conducted by that method, the basic informations, fatigue life curves are mainly obtained from the results of the strain controlled low cycle fatigue test. From these point of views, the low cycle fatigue test is coming to be given a much importance lately. In this paper, the strain controlled low cycle fatigue properties at room temperature in air environment were investigated for the low carbon forged steel, SF45A, and the rolled steel for the welded structure, SM 41B. Throughout the test, strain ratio, R, was maintained constant with the fully reversed condition, -1. As the experimental results, the cyclic stress-strain behaviours of the test materials were different each other, but the low cycle fatigue life-time of them appeared to show little difference in the region of this test conditions.

• Korean Journal of Materials Research
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• v.7 no.11
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• pp.1007-1011
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• 1997

영활화혁발전소 보일러헤다 재질인 1Cr0.5Mo강의 파형에 따른 저주기 피로특성을 규명하고자 상온(298K) 및 고온(177K)의 삼각파와 사인파형 저주기 피로시험을 수행하였고 소성에너지법을 이용하여 파형에 따른 소성변형에너지와 피로수명과의 +관계를 분석하였다. 저주기 피로시 재료내부의 소성변형에너지를 히스테리시스루프의 면적으로 계산하여 구하였으며 이를통해 저주기 피로수명을 예측하였고 Coffin-Manson법 및 변형률분할법을 이용한 저주기 피로수명 결과와 서로 잘 일치하였다. 또한 상온 및 고온에서 피로반복수의 증가와 함께 재료가 반봅연화됨을 알 수 있었다.

• Magazine of the Korea Concrete Institute
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• v.1 no.2
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• pp.121-132
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• 1989

유한요소법을 바탕으로 한 프리스트레스트 콘크리트 평면 보-기둥 구조의 후좌굴 거동에 대한 수직해석법을 제시하였다. 콘크리트의 균열, 변형연화 및 PS강재의 항복과 같은 재료 비선형성을 고려하였다. 좌굴 거동 연구에 필수적 요소인 기하학적 비선형성을 Updated Lagraugian Formulation에 의하여고려하였다. 현재의 재료성질 및 변형상태에 부합하는 단분형 평형방정식을 수립하고 이것을 불평형 가중보정에 의한 Newton-Raphson 반복법으로 푼다. 좌굴후 발생하는 하중변형 곡선의 하련부는 비선형 평형 방정식의 해법중 일반적으로 많이 사용되는 가중 단분법이 아니라 변위단분법을 사용함으로써 올바르게 추적한다. 요소내의 재료성질변화는 층적분법에 의하여 고려한다. 본 논문에서는 콘크리트 균열에 의한 중립축이동의 영향을 정확히 고려하기 위하여 추가적으로 축방향변위에 대한 내부자유도를 설정하였다. 본 논문에서 제안하는 방법의 정당성과 응용성을 나타내 보일 수 있는 수직해석 예제를 제시하였다.

• Journal of Welding and Joining
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• v.8 no.3
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• pp.79-88
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• 1990

Welded joint often contains soft or softened regions such as the HAZ of TMCP steel welded with high heat input. In this study, the equivalent yield strength of plate structure containing softened region was predicted by FEM analysis, and its incremental behavior was explained with the results of the analysis. The calculated results of yield strength indicated the following for the plate structures. 1) As the softened region starts to yield, shear stress begins to build up along the boundary between base metal and softened region. This results in multi-axial stress condition which gives restraint on the softened region. 2) Restraint effect has a significant influence on the distribution of the shear stress, the nominal stress, and the strain. 3) The yielding behavior of softened region becomes the same as that of base metal when both ratios of length to width and thickness to width of softened region are larger than 30 and 13 respectively.