• 대한기계학회논문집A
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• 제33권10호
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• pp.1091-1098
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• 2009

In this paper, the procedure to estimate fatigue crack initiation life has been established by considering fretting wear and multiaxial stress states on the contact surface of press-fitted shafts. And a method to calculate the local friction coefficient during the running-in period of fretting wear process has been proposed. The predicted result of worn surface profile in the press-fitted shaft with non-linear local friction coefficient can avoid excessive wear depth estimation compared with that for the case of constant local friction coefficient. Furthermore, the predicted fatigue crack initiation lives based on Smith-Watson-Topper model considering the fretting wear are in good agreement with the experimental data. Consequently, the present method is valid not only for predicting worn surface profile, but also for assessing fatigue crack initiation lives considering the fretting wear during the running-in period in press fits.

• 한국자동차공학회논문집
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• 제22권3호
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• pp.130-135
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• 2014

In this paper, a new estimation method is proposed to calculate steering rack axial force using a 3 dimensional tire mesh model when a car is standing on the road. This model is established by considering changes of camber angle and contact patch between the tires and the ground according to steering angle. The steering rack bar axial force is estimated based on the static equilibrium equations of forces and moments. A tire friction force is supposed to act on the center point of the contact patch, and the proportional coefficient of friction depending on contact patch is suggested. Using the proposed estimation method, rack axial force sensitivity analysis is evaluated according to changes of suspension geometry. Then optimal motor power of Motor Driven Power Steering(MDPS) is evaluated using suggested rack forces.

• 소성∙가공
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• 제22권2호
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• pp.86-92
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• 2013

When stamping ultra-high-strength steel (UHSS), the phenomenon of galling, which corresponds to a transfer of material from the sheet to the tool surface, occurs because of the high contact pressure between tool and workpiece. Galling leads to increased friction, unstable interface conditions, scratches on the sheet and the tool surfaces and, eventually, premature tool surface failures. Therefore, a simple and accurate evaluation method for tool scratching is necessary for the selection of tool material and coating, as well as for a better optimization of process conditions such as blank holder force and die radius. In this study, the pin-on-disc (PODT) and pin-on-flat surface (POFST) tests are conducted to quantitatively evaluate scratch-related tool life for stamping of UHSS. The variation of the friction coefficient is used as an indicator of scratch resulted from galling. The U-channel ironing test (UCIT) is performed in order to validate the results of the friction tests. This study shows that the POFST test provides a good quantitative estimation of tool life based on the occurrence of scratch.

• 설비공학논문집
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• 제15권12호
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• pp.1007-1017
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• 2003

An experimental study on the air-side pressure drop and heat transfer coefficient of slit fin-tube heat exchanger has been carried out. The data reduction methodology for air-side heat transfer coefficients in the literature is not based on a consistent approach. This paper focuses on new method of data reduction to obtain the air-side performance of fin-tube heat exchanger using R22 and recommends standard procedures for dry and wet surface heat transfer estimation in fin-tube heat exchanger having refrigerant on the tube-side. Results are presented as plots of friction f-factor and Colburn j -factor against Reynolds number based on the fin collar outside diameter and compared with previous studies. The data covers a range of refrigerant mass fluxes of 150∼250 kg/$m^2$s with air flows at velocity ranges from 0.3 m/s to 0.8 m/s.

• 설비공학논문집
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• 제15권3호
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• pp.220-229
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• 2003

An experimental study on the air-side pressure drop and heat transfer coefficient of slit fin-tube heat exchanger has been carried out. The data reduction methodology for air-side heat transfer coefficients in the literature is not based on a consistent approach. This paper focuses on new method of data reduction to obtain the air-side performance of fin-tube heat exchanger using R22 and recommends standard procedures for dry surface heat transfer estimation in fin-tube heat exchanger having refrigerant on the tube-side. Results are presented as plots of friction f-factor and Colburn j -factor against Reynolds number based on the fin collar outside diameter and compared with previous studies. The data covers a range of refrigerant mass fluxes of 150~250 kg/$m^2$s with air flows at velocity ranges from 0.6 m/s to 1.6 m/s.

• 한국안전학회지
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• 제26권3호
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• pp.1-7
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• 2011

Fatigue damage on the train wheel surface was estimated by considering the effect of friction coefficient of rolling on the contact surface between the wheel and rail during operation. From FEM analys, the maximum Tresca stress was 550.7 MPa at a depth of 2.07 mm under the maximum contact pressure ($P_{max}$ = 894.3 MPa) between wheel and rail. The maximum stress continued to increase along with the increase in the frictional coefficient. The fatigue initiation lifetime of the wheel by the rolling contact was predicted using the Smith-Watson-Topper (SWT) equation and the maximum principal strain equation (${\varepsilon}$-N).

• 한국전산구조공학회논문집
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• 제27권6호
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• pp.581-588
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• 2014

최근에 널리 사용되고 있는 PSC 교량은 콘크리트의 처짐과 균열 등의 취약점을 긴장재와 강봉을 사용하여 보완하고 성능을 향상시킨 구조물이다. 따라서 PSC 교량에서 긴장재에 작용하는 하중을 적절하게 산정하는 것은 구조물의 안전하고 효율적인 유지, 보수를 위하여 중요하다. 이 논문은 텐던에 작용하는 하중과 앵커헤드 변형과의 관계를 확인하기 위하여 멀티 텐던 앵커헤드의 변형률에 대한 수치해석을 수행하고 분석한 것이다. 정확한 해석을 위하여 재료의 물성, 접촉 문제의 비선형성 등을 모두 고려하였으며 해석은 범용 유한요소 프로그램인 Abaqus를 사용하여 수행되었다. 수치해석 결과로부터 텐던에 작용하는 하중을 추정하는 데에는 hoop 방향 변형률이 가장 유용하며, 마찰 계수, 경계조건, 그리고 배치 등에 따라 영향을 받는 것을 확인하였다.

• International Journal of Concrete Structures and Materials
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• 제9권3호
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• pp.369-379
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• 2015

Friction in a post-tensioning system has a significant effect on the distribution of the prestressing force of tendons in prestressed concrete structures. However, attempts to derive friction coefficients using conventional electrical resistance strain gauges do not usually lead to reliable results, mainly due to the damage of sensors and lead wires during the insertion of strands into the sheath and during tensioning. In order to overcome these drawbacks of the existing measurement system, the Smart Strand was developed in this study to accurately measure the strain and prestressing force along the strand. In the Smart Strand, the core wire of a 7-wire strand is replaced with carbon fiber reinforced polymer in which the fiber Bragg grating sensors are embedded. As one of the applications of the Smart Strand, friction coefficients were evaluated using a full-scale test of a 20 m long beam. The test variables were the curvature, diameter, and filling ratio of the sheath. The analysis results showed the average wobble and curvature friction coefficients of 0.0038/m and 0.21/radian, respectively, which correspond to the middle of the range specified in ACI 318-08 in the U.S. and Structural Concrete Design Code in Korea. Also, the accuracy of the coefficients was improved by reducing the effective range specified in these codes by 27-34 %. This study shows the wide range of applicability of the developed Smart Strand system.

• 한국ITS학회 논문지
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• 제9권6호
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• pp.54-62
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• 2010

차 대 보행자 교통사고는 다른 유형의 사고에 비해 피해가 매우 크고 사고의 비중도 높으나 사고의 특성 상 과학적 접근이 어렵다. 기존 보행자 사고에 관한 연구의 대부분은 차 대 보행자의 충돌 실험에 대한 실험식 도출에 집중되어 있고, 실험식에서 중요한 변수로 작용되는 인체 활주 마찰 계수에 대한 연구가 부족한 실정이다. 본 연구에서는 인체와 노면간의 마찰계수에 대한 현장실험을 실시하여 이에 대한 실험값을 측정하여 제시하였다. 본 연구에서 조사된 인체노면 마찰계수 값은 건조한 아스팔트 도로에서 0.59~0.62, 건조한 콘크리트 도로는 0.59~0.61로 측정되었다. 또한, 젖은 아스팔트와 콘크리트 도로에서는 각각 0.56~0.59, 0.51~0.54로 나타나 건조한 노면 상태와 비교하여 5.0%와 8.3% 감소하는 것으로 파악되었다. 도출된 마찰계수를 시뮬레이션 프로그램을 이용하여 검증한 바, 실험 수치와 시뮬레이션 결과가 유사함을 확인하였다. 본 연구결과를 활용하여 국내 교통사고 조사 시 사용되는 인체 마찰계수의 적용 값의 범위를 축소할 수 있고, 이는 사고 조사의 정확성 향상에 큰 도움을 줄 것으로 기대한다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집B
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• pp.565-570
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• 2001

Vehicle dynamics control (VDC) system requires more information on driving conditions compared with ABS and/or TCS. In order to develop the VDC system, tire slip angles, vehicle side-slip angle, and vehicle lateral velocity as well as road friction coefficient are needed. Since there are not any cheap and reliable sensors, recent researches on parameter estimation have given rise to a number of parameter estimation techniques. This paper presents new vehicle model to estimate vehicle's yaw rate. This model is improved from the conventional 2 degrees of freedom vehicle model, so-called bicycle model, taking nonlinear effects into account. These nonlinear effects are: (i) tyre nonlinearity; (ii) lateral load transfer during cornering; (iii) variable gear ratio with respect to vehicle velocity. Estimation results are validated with the experimental results.