• 소성∙가공
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• 제13권1호
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• pp.21-26
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• 2004

Instrumented indentation tests have been used for estimating material properties. In order to analyze deformation characteristics with various factors, the unloading stiffness should be properly determined from the elastic behavior. The unloading stiffness is generally obtained from the shifted power functions fitting with the experimental unloading data. However, the functions often give rise to a poor representation of actual data, and also the unloading stiffness is governed by unloading condition. In this study, both numerical and experimental conditions to obtain proper unloading stiffness were investigated. The result showed that the amount of unloading ratio and hold time played an important role in fitting the unloading curves. The current efforts can successfully provide the unloading stiffness for indentation material properties.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 추계학술대회논문집
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• pp.168-171
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• 2003

Instrumented indentation tests have been used for estimating material properties. In order to analyze deformation characteristics with various factors, the unloading stiffness should be properly determined from the elastic behaviour. In general, the unloading stiffness is obtained from shifted power functions fitting to indentation unloading curves. But, the functions give often a poor representation of actual data. In this study, control conditions for fitting unloading curves by shifted power functions were investigated. The current efforts may provide useful information about unloading process and valid unloading stiffness.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.378-383
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• 2002

In previous study, the decrease and recovery of total stiffness in welded structure was discussed on the basis of experimental examination through tensile loading and unloading test of welded specimen. The recovery of structure stiffness was caused by the release of welding residual stress through mechanical loading. In this study, analysis model that is indispensable for the effective application of MSR method was established on the basis of test and measurement result. Thermal elasto-plastic analysis for welding process was performed by non-coupled analysis. Analysis results of welding process were transfer to elasto-plastic model for tensile loading & unloading by restart technique. In elasto-plastic analysis model for mechanical loading & unloading, hardening appearance of weld metal was considered by rezoning technique and tying technique was used for JIG condition of test machine.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.29-32
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• 2002

In previous study, the decrease and recovery of total stiffness in welded structure was discussed on the basis of experimental examination through tensile loading and unloading test of welded specimen. The recovery of structure stiffness was caused by the release of welding residual stress through mechanical loading. In this study, analysis model that is indispensable for the effective application of MSR method was established on the basis of test and measurement result. Thermal elasto-plastic analysis for welding process was performed by non- coupled analysis. Analysis results of welding process were transfer to elasto-plastic model for tensile loading & unloading by restart technique. In elasto-plastic analysis model for mechanical loading & unloading, hardening appearance of weld metal was considered by rezoning technique and tying technique was used for JIG condition of test machine.

• 전산구조공학
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• 제8권4호
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• pp.159-171
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• 1995

한 cycle의 이력곡선 loop을 완전히 표현하기 위해서는 pinch force, drift offset, effective stiffness, unloading, reloading, tangential stiffness 등의 변수가 필요하게 된다. 각 이력 loop에 대해 이들 변수들은 에너지 소산정도에 따라 변위와 축력의 함수로 표현될 수 있다. 본 논문에서는 먼저 16개의 전단벽 실험에서 얻어진 이력곡선 데이타를 분석하여 앞에 기술된 모든 변수를 표준화된 변위(.DELTA/.DELTA.y)의 함수로 표현했으며 이를 바탕으로 이력곡선의 포락선으로 표현되는 힘-변위관계를 예측할 수 있는 6개의 step을 제시하였다. 제시된 기법으로 구해진 비탄성 힘-변위관계는 실험곡선과 비교되었으며 내진설계에 있어서 가장 중요한 요소중 하나인 구조물의 비탄성 힘-변위관계를 예측하는 편리한 기법으로 이용될 수 있음을 보였다.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.372-377
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• 2002

Residual stress by welding should be reduced because that decreases the reliability on strength of welded structure. The reason is that the total stiffness of structure decreases by non-linear behavior of weldment under external load. The release of residual stress by mechanical loading and unloading is often performed in the fabrication of box structure for steel bridge. The proper degree of loading and unloading is significant at release method of residual stress by mechanical loading because that degree is changed by material and geometric shape of welded structure. Therefore, the simulation model that could exactly analyze the release of residual stress by mechanical loading is to be necessary. This simulation model should be established on the based of variable and accurate measurement data. In this study, the non-linear behavior of weldments under external loading and unloading, such as the decrease and increase of structure stiffness, was investigated by monitoring of nominal stress and strain. Tensile loading and unloading test under variable load was performed and the proper degree of stress relaxation was measured by sectioning technique using strain gauge.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.40-44
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• 2002

Residual stress by welding should be reduced because that decreases the reliability on strength of welded structure. The reason is that the total stiffness of structure decreases by non-linear behavior of weldment under external load. The release of residual stress by mechanical loading and unloading is often performed in the fabrication of box structure for steel bridge. The proper degree of loading and unloading is significant at release method of residual stress by mechanical loading because that degree is changed by material and geometric shape of welded structure. Therefore, the simulation model that could exactly analyze the release of residual stress by mechanical loading is to be necessary. This simulation model should be established on the based of variable and accurate measurement data. In this study, the non-linear behavior of weldments under external loading and unloading, such as the decrease and increase of structure stiffness, was investigated by monitoring of nominal stress and strain. Tensile loading and unloading test under variable load was performed and the proper degree of stress relaxation was measured by sectioning technique using strain gauge.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2005년도 춘계학술대회 논문집
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• pp.299-307
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• 2005

The 180 km/h Korean Tilting Train(TTX) which is now developing as a part of the Korean National R&D project, was elaborately designed. As the tilting trains run curve track with the $30\%$ higher speed than normal trains, the higher centrifugal and dynamic force are expected. Furthermore the complex tilting system increase the probability of failure. Therefore it is very important for tilting train to ensure safety against derailment under the various kind of failed condition in the middle of running as well as normal operating condition. The TTX train have the relatively high roll stiffness to improve the lateral ride comfort and to limit the roll displacement on the curve. But the higher roll stiffness increase the risk of derailment on the twisted track. This paper describes the study to review the safety against derailment caused by the wheel unloading on the severely twisted track. The worst combination of maximum cant change with maximum twist defect was established by numerical simulation. And also it was assumed that the air bag deflated and still the train run its speed limit. Those kind of assumption might be the worst case from the view point of wheel unloading derailment on the twisted track. The dynamic simulation was done by means of VAMPIRE S/W and non-linear transient analysis. We found that derailment quotients Q/P was only slightly influenced by track twist but the wheel unloading was greatly influenced. And we ascertained that the higher roll stiffness the higher wheel unloading. In case of air bag deflated situation, the wheel unloading reached up to $100\%$ which means the wheel lift or jumped. Therefore it was concluded that the design need to be improved to ensure the safety against derailment on the maximum twisted track in case of air bag deflated and tilting train's speed limit.

• Structural Engineering and Mechanics
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• 제77권2호
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• pp.179-196
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• 2021

Experimental investigations on the seismic behaviors of the PVC-FRP Confined Reinforced Concrete (PFCRC) columns under low cyclic loading are carried out and two variable parameters including CFRP strips spacing and axial compression ratio are considered. The PFCRC column finally fails by bending and is characterized by the crushing of concrete and yielding of the longitudinal reinforcement, and the column with a high axial compression ratio is also accompanied by the cracking of the PVC tube and the fracture of CFRP strips. The hysteretic curves and skeleton curves of the columns are obtained from the experimental data. With the increase of axial compression ratio, the stiffness degradation rate accelerates and the ductility decreases. With the decrease of CFRP strips spacing, the unloading sections of the skeleton curves become steep and the ductility reduces significantly. On the basis of fiber model method, a numerical analysis approach for predicting the skeleton curves of the PFCRC columns is developed. Additionally, a simplified skeleton curve including the elastic stage, strengthening stage and unloading stage is suggested depending on the geometric drawing method. Moreover, the loading and unloading rules of the PFCRC columns are revealed by analyzing the features of the skeleton curves. The quantitative expressions that are used to predict the unloading stiffness of the specimens in each stage are proposed. Eventually, an analytical model for the PFCRC columns under low cyclic loading is established and it agrees well with test data.

• Journal of Welding and Joining
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• 제21권1호
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• pp.66-71
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• 2003

The release of residual stress by mechanical loading and unloading is often performed in the fabrication of box structure fur steel bridge. The proper degree of loading and unloading is significant at release method of residual stress by mechanical loading because that degree is changed by material and geometric shape of welded structure. Therefore, the simulation model that could exactly analyze the release of residual stress by mechanical loading is to be necessary. In this study, the non-linear behavior of weldments under external loading and unloading, such as the decrease and increase of structure stiffness, was investigated by monitoring of nominal stress and strain. Tensile loading and unloading test and the proper degree of stress relaxation was measured by sectioning technique using strain gauge. Analysis model that is indispensable for the effective application of MSR method was established on the basis of test and measurement result.