Modified Equivalent Radius Approach in Evaluating Stress-Strain Relationship in Torsional Test

배윤신 한국방재학회 2008 한국방재학회논문집 Vol.8 No.2

Determination of stress-strain relationship in torsional tests is complicated due to nonuniform stress-strain variation occurring linearly with the radius in a soil specimen in torsion. The equivalent radius approach is adequate when calculating strain at low to intermediate strains, however, the approach is less accurate when performing the test at higher strain levels. The modified equivalent radius approach was developed to account for the problem more precisely. This approach was extended to generate the plots of equivalent radius ratio versus strain using modified hyperbolic and Ramberg-Osgood models. Results showed the effects of soil nonlinearity on the equivalent radius ratio curves were observed. Curve fitting was also performed to find the stress-strain relationship by fitting the theoretical torque-rotation relationship to measured torque-rotation relationship. Determination of stress-strain relationship in torsional tests is complicated due to nonuniform stress-strain variation occurring linearly with the radius in a soil specimen in torsion. The equivalent radius approach is adequate when calculating strain at low to intermediate strains, however, the approach is less accurate when performing the test at higher strain levels. The modified equivalent radius approach was developed to account for the problem more precisely. This approach was extended to generate the plots of equivalent radius ratio versus strain using modified hyperbolic and Ramberg-Osgood models. Results showed the effects of soil nonlinearity on the equivalent radius ratio curves were observed. Curve fitting was also performed to find the stress-strain relationship by fitting the theoretical torque-rotation relationship to measured torque-rotation relationship.

잔류응력의 영향을 고려한 용접부 내진 평가

김종성(Jong-Sung Kim),장현수(Hyun-Su Jang) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

BDBE 와 같이 심각한 지진 하중 하에서 원자력 발전소의 안전성을 검증하기 위해 APR-1400의 가압기와 고온관을 연결하는 밀림배관의 탄소성 유한요소 해석을 수행하였다 [1~2]. 이전 연구에서 지진에 중요한 위치인 가압기 노즐과 핫레그 노즐의 용접부 모사할 때 3case (용접부의 기하학적 형상을 고려하지 않은 경우, 기하학적 형상과 용접 물성을 고려한 경우, 잔류응력을 고려한 경우)에 대해서 지진해석을 수행하였다. 잔류응력을 고려하여 지진해석을 수행한 경우, 누적소성변형률과 등가소성변형률이 다른 경우에 비해 상당히 크게 발생하였다. 잔류응력 해석시 용접부가 융착되는 시간은 매우 짧다. 따라서 본 연구에서는 잔류응력을 고려한 지진해석의 누적소성변형률과 등가소성변형률을 좀 더 현실적으로 시뮬레이션 하기 위해서 어닐링 온도를 고려하여 잔류응력 해석을 수행하였다. 용접 재료인 ER308l의 물성은 주기적인 테스트 결과를 가지고 있지 않기 때문에, 간단한 인장 실험의 결과를 통해 샤보세 경화 모델을 정의하였다. 또한 분석 변수(예: 지진 변위 시간 이력)는 이전 연구에 사용하였던 것과 동일한 값을 사용하였다. 잔류응력을 고려한 지진 해석시 핫레그 노즐과 용접부 사이에서 취약부위가 발생하였다. 취약 부위에 대해 ASME code [3]의 변형률 기반 허용 기준 평가를 수행하였고, 무결성이 만족되었다. 어닐링 온도는 잔류응력 해석에 의해 발생한 누적소성변형률과 등가소성변형률에 상당한 영향을 미치고, 그에 따라 잔류응력을 고려한 용접부의 내진 평가 시 여유도에 영향을 미치는 것을 확인하였다. To verify the safety of nuclear power plants under severe seismic loads, such as BDBE, the finite element elastic-plastic analysis of the surge line system connecting the pressurizer and the hot-leg of APR-1400 was conducted. In the previous study, seismic analysis was conducted on 3 cases (not considering geometry shape of weld, considering geometry shape of welds and material properties of welds, considering welding residual stress) when simulating welds in pressurizer nozzles and hot-leg nozzles, which are critical location for earthquakes. When seismic analysis was conducted in consideration of welding residual stress, accumulated plastic strain and equivalent plastic strain were significantly larger than those of other cases. When conducting welding residual stress, the time for weld to fuse is very short. Therefore, in this study, welding residual stress analysis was conducted considering annealing temperature to more realistic simulation of accumulated plastic strain and equivalent plastic strain of seismic analysis considering welding residual stress. The welding material was used as ER308l instead of TP316 which was previously used material. Since the properties of ER308l do not have cyclic test results, define the Chaboche hardening model through the results of a simple tensile experiment. Furthermore, the analytical variables (such as seismic displacement time history) used the same values as those used in previous study. The seismic analysis was conducted considering the effects of welding residual stress, and vulnerable point occurred between hot-leg nozzles and welds. The integrity was satisfied by performing strain-based acceptance criteria evaluation of ASME code. The annealing temperature was confirmed to have a significant effect on the accumulated plastic strain and the equivalent plastic strain caused by welding residual stress analysis. Accordingly, it was confirmed that the seismic evaluation of welds considering welding residual stress affects the margin.

Modified Equivalent Radius Approach in Evaluating Stress-Strain Relationship in Torsional Test

Bae. Yoon-Shin 한국방재학회 2008 한국방재학회논문집 Vol.8 No.2

Determination of stress-strain relationship in torsional tests is complicated due to nonuniform stress-strain variation occurring linearly with the radius in a soil specimen in torsion. The equivalent radius approach is adequate when calculating strain at low to intermediate strains, however, the approach is less accurate when performing the test at higher strain levels. The modified equivalent radius approach was developed to account for the problem more precisely. This approach was extended to generate the plots of equivalent radius ratio versus strain using modified hyperbolic and Ramberg-Osgood models. Results showed the effects of soil nonlinearity on the equivalent radius ratio curves were observed. Curve fitting was also performed to find the stress-strain relationship by fitting the theoretical torque-rotation relationship to measured torque-rotation relationship.

잔류응력을 고려한 굽힘 성형 시편 Master S-N 선도 구축

황진하(Jin-Ha Hwang),이한상(Han-Sang Lee),정연일(Youn-Il Chung),김윤재(Yun-Jae Kim) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11

In previous study, master S-N curve considering influence of loading condition and pre-strain was determined about simple bending and tension fatigue tests, using equivalent structural stress method and normalized stress. Unlike simple bending and tension specimen, bending process specimen exists residual stress of thickness direction. Therefore, it is necessary to apply stress calculation method reflecting residual stress effect. In this study, master S-N curve of bending process specimen was determined using mean stress effect function which considered influence of residual stress.

Development of Equivalent Stress- and Strain- Dependent Model for Jointed Rock Mass and Its Application to Underground Structure

정성훈,송기일,조계춘 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.12

Current design methods for underground structures surrounded by jointed rock masses fail to account for the deformational characteristics of these surrounding rock masses, employing only input parameters that are defined prior to construction. Thus, there is the need to investigate the effects on underground structures that result from the deformational characteristics of jointed rocks that undergo stress relaxation during their excavation. Comprehensive research is conducted in this study. First, we propose an equivalent stress- and strain-dependent model incorporating equivalent stiffness at small strains into a hyperbolic model from intermediate to large strains. Then, experimental tests are conducted to characterize the deformational characteristics of jointed rock masses and extract requisite model parameters such as joint properties and nonlinearity. Finally, two numerical simulations are conducted using the proposed model to determine its field applicability to an unlined circular tunnel. The nonlinear simulation results are compared with the conventional analysis method that uses a constant stiffness value. Our results show that the proposed equivalent jointed rock mass model, which considers stress-and strain-dependency, can be utilized for deformational analysis of underground structures involving excavations in jointed rocks.

유아기 자녀를 둔 어머니의 정신화가 양육 스트레스에 미치는 영향

고태순 한국산학기술학회 2023 한국산학기술학회논문지 Vol.24 No.11

The purpose of this study was to investigate the relationship between mentalization and parenting stress of mothers with young children and to explore the influence of mentalization on parenting stress. The participants were 199 mothers with young children attending a kindergarten in Seoul and a daycare center in Gyeonggi-do. The collected data were analyzed using frequency analysis, reliability analysis, Pearsons's correlation analysis, and stepwise regression on the SPSS 25.0 program. The results showed that there was a significant correlation between mentalization and parenting stress of mothers with young children. All the sub-factors of mentalization, excluding emotional awareness, were found to be negatively correlated with the sub-factors of parenting stress. Among the sub-factors of mentalization, regulation of affect and non-psychic equivalence mode were the significant variables explaining the parenting stress of mothers with young children. The regulation of affect was the largest predictive variable explaining parenting stress. The results of this study are expected to be used as basic data for the development of a mentalization-based program to reduce the parenting stress of mothers with young children and to positively change the relationship between mothers and young children.

조선 해양 구조물용 강재 파단 변형률에 관한 실험적 연구

정준모(Joonmo Choung),심천식(Chun-Sik Shim),김경수(Kyung Su Kim) 대한조선학회 2011 대한조선학회 학술대회자료집 Vol.2011 No.6

This is the third of several companion papers dealing with derivation of material constants for ductile failure criterion under hydrostatic stress. It is observed that ultimate engineering stresses and elongations at fracture from tensile tests for round specimens with various notch radii tend to increase and decrease, respectively, due to stress triaxiality. The engineering stress curves from tests are compared with numerical simulation results, and it is proved that curves from two approaches are very closely coincide each other. Failure strains are obtained from equivalent plastic strain histories from numerical simulations at the time when the experimental engineering stress falls down suddenly. After introducing a new concept of average stress triaxiality and accumulated average strain energy, material constants of Johnson-Cook failure criterion for critical energies of 100%, 50%, and 15% are presented. Experimental results conducted for EH-36 steel are relatively good agreement with 100% critical energy whereas literature states that aluminum fits with 15% critical energy. Therefore it is expected that a unified failure criterion, available for most kinds of ductile materials, of critical energy according to the used materials can be provided.

Experimental study on the compressive stress dependency of full scale low hardness lead rubber bearing

이홍표,조명석,김선용,박진영,장광석 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.1

According to experimental studies made so far, design formula of shear characteristics suggested by ISO 22762 and JEAG 4614, representative design code for Lead Rubber Bearing(LRB) shows dependence caused by changes in compressive stress. Especially, in the case of atypical special structure, such as a nuclear power structure, placement of seismic isolation bearing is more limited compared to that of existing structures and design compressive stress is various in sizes. As a result, there is a difference between design factor and real behavior with regards to shear characteristics of base isolation device, depending on compressive stress. In this study, a full-scale low hardness device of LRB, representative base isolation device was manufactured, analyzed, and then evaluated through an experiment on shear characteristics related to various compressive stresses. With design compressive stress of the full-scale LRB (13MPa) being a basis, changes in shear characteristics were analyzed for compressive stress of 5 MPa, 10 MPa, 13 MPa, 15 MPa, and 20 MPa based on characteristics test specified by ISO 22762:2010 and based on the test result, a regression analysis was made to offer an empirical formula. With application of proposed design formula which reflected the existing design formula and empirical formula, trend of horizontal characteristics was analyzed.

조선 해양 구조물용 강재의 파단 변형률

정준모(Joonmo Choung),심천식(Chun-Sik Shim) 대한조선학회 2011 대한조선학회 학술대회자료집 Vol.2011 No.11

This is the third of several companion papers dealing with derivation of material constants for ductile failure criterion under hydrostatic stress. It is observed that ultimate engineering stresses and elongations at fracture from tensile tests for round specimens with various notch radii tend to increase and decrease, respectively, due to stress triaxiality. The engineering stress curves from tests are compared with numerical simulation results, and it is proved that curves from two approaches are very closely coincide each other. Failure strains are obtained from equivalent plastic strain histories from numerical simulations at the time when the experimental engineering stress falls down suddenly. After introducing a new concept of average stress triaxiality and accumulated average strain energy, material constants of Johnson-Cook failure criterion for critical energies of 100%, 50%, and 15% are presented. Experimental results conducted for EH-36 steel are relatively good agreement with 100% critical energy whereas literature states that aluminum fits with 15% critical energy. Therefore it is expected that a unified failure criterion, available for most kinds of ductile materials, of critical energy according to the used materials can be provided.

Analysis on Slope Stability Considering Seepage Effect on Effective Stress

Yuanzhan Wang,Xufei Liu,Zhikai Zhang,Panbo Yang 대한토목학회 2016 KSCE JOURNAL OF CIVIL ENGINEERING Vol.20 No.6

Interaction between seepage field and soil stress field influences soil effective stress in general consideration. In addition, water erosion in soil skeleton can also affect soil effective stress. The current studies conducted triaxial compression experiments with seepage, Duncan-Chang model analysis, ABAQUS numerical simulation and followed by a case study analysis. Experiments show that the water erosion in soil skeleton reduces effective stress strength of soil, which is quantitatively described by the reduction of equivalent confining pressure. Through analyzing the characteristics of soil stress-strain curves, the equivalent confining pressure is applied to improve Duncan-Chang model. Using the modified Duncan-Chang model as well as considering the interaction between seepage field and soil stress field, ABAQUS software package is employed to simulate triaxial compression experiments with seepage. This method is validated by comparing simulation and experimental results. Furthermore, this method is applied in the case study of the overall stability of bank slope under seepage effect and finds that the modified model is more conservative on safety assessment of slope stability. The studies provided evidences to prove the importance of considering soil skeleton eroded by water flow in slope stability analysis quantitatively and proposed a modified Duncan-Chang model for engineering application with consideration of seepage effect.