• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.1481-1486
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• 2007

Frictional laws are criticized with emphasis on their application to bulk metal forming simulation in this paper. Coulomb frictional law and constant shear frictional law are investigated in detail in terms of their effect on metal forming process. A friction sensitive bulk metal forming process, a long-pipe simultaneously shrinking and expanding process, is introduced and the problems of the constant shear frictional law are revealed comparing the predictions obtained by the Coulomb frictional law and the constant shear frictional law with the experiments. It is shown that the constant shear frictional law is improper in the case that the normal stress varies very much from position to position and that the normal stress is low compared with flow stress of the adjacent material. It is also shown that the Coulomb frictional constant is more or less affected by the normal stress.

• Geomechanics and Engineering
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• 제7권3호
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• pp.297-316
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• 2014

This paper presents a detailed study focused on investigating the effects of silt content on the static and dynamic properties of sand-silt mixtures. Specimens with a low-plastic silt content of 0, 15, 30 and 50% by weight were tested in static triaxial, cyclic triaxial, and resonant columns in addition to consolidation tests to determine such parameters as compression index, internal friction angle, cohesion, cyclic stress ratio, maximum shear modulus, normalized shear modulus and damping ratio. The test procedures were performed on specimens of three cases: constant void ratio index, e = 0.582; same peak deviator stress of 290 kPa; and constant relative density, $D_r$ = 30%. The test results obtained for both the constant-void-ratio-index and constant-relative-density specimens showed that as silt content increased, the internal friction angle, cyclic stress ratio and maximum shear modulus decreased, but cohesion increased. In testing of the same deviator stress specimens, both cohesion and internal friction angle were insignificantly altered with the increase in silt content. In addition, as silt content increased, the maximum shear modulus increased. The cyclic stress ratio first decreased as silt content increased to reach the threshold silt content and increased thereafter with further increases in silt content. Furthermore, the damping ratio was investigated based on different silt contents in three types of specimens.

• 한국정밀공학회지
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• 제13권2호
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• pp.102-109
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• 1996

Effects of frictional laws on finite element solutions in metal forming were investigated in this paper. A rigid-viscoplastic finite element formulation was given with emphasis on the frictional laws. The Coulomb friction and the constant shear friction laws were compared through finite element analyses of compression of rings and cylinders with different aspect ratios, ring-gear forging, multi-stage cold extrusion and hot strip rolling under the isothermal condition. It has been shown that two laws may yield quite different results when the aspect ratio of a process and the fractional contact region are large.

• Smart Structures and Systems
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• 제24권3호
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• pp.345-360
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• 2019

The shear keys are set in a seismic isolation system to resist the long-term service loadings, and are cut off to isolate the earthquakes. This paper investigated the influence of shear keys on the seismic performance of a vertical spring-viscous damper-concave Coulomb friction isolation system by an incremental dynamic analysis (IDA) and a performance-based assessment. Results show that the cutting off process of shear keys should be simulated in a numerical analysis to accurately predict the seismic responses of isolation system. Ignoring the cutting off process of shear keys usually leads to untrue seismic responses in a numerical analysis, and many of them are unsafe for the design of isolated structure. And those errors will be increased by increasing the cutting off force of shear keys and decreasing the spring constant of shear keys, especially under a feeble earthquake. The viscous damping action postpones the cutting off time of shear keys during earthquakes, and reduces the seismic isolation efficiency. However, this point can be improved by increasing the spring constant of shear keys.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.445-449
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• 2009

With the recent increase in the demand for the net-shape forming, numerical simulations are being commonly adopted to increase the efficiency and effectiveness of design of bulk metal forming processes. Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in such simulations. In other words, lubrication and friction play important roles in metal forming by influencing the metal flow, forming load and die wear. In order to quantitatively estimate such friction condition or lubricant characteristic, the constant shear friction model is widely used for bulk deformation analyses. For this, new friction testing method based on the forward or backward extrusion process is proposed to predict the shear friction factor in this work. In this method, the tube-shaped punch pressurizes the workpiece so that the heights at the center and outer of punch (or mandrel) become different according to the friction condition. That is, the height at the center of punch is higher than that at the outer of the punch when the friction condition at the contact interface is severe. From this founding, the proposed friction testing method can be applied to effectively evaluate the friction condition in bulk metal forming processes.

• Geomechanics and Engineering
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• 제34권4호
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• pp.461-472
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• 2023

A box scale affects the shear behavior of soils in the direct shear test. The purpose of this study is to investigate the scale effect on the shear behavior of dilative granular materials by testing specimens of different heights placed in a type C shear box. Experimental tests were performed on specimens composed of glass beads with different heights and equal initial void ratios. Results showed that the peak friction and dilation angles linearly increased with the specimen height; however, the residual friction angle remained relatively constant. Similarly, the shear stiffness increased with the specimen height, rapidly reaching its peak state. Height does not have a significant effect on the total volume changes; nevertheless, a high aspect ratio can be assumed to result in global and homogeneous failure. The results and interpretations may be used as reference for recommending shear box scale in direct shear tests.

• 터널과지하공간
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• 제17권3호
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• pp.203-215
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• 2007

암반의 역학적 변형거동과 안정성은 불연속면의 역학적 특성에 크게 좌우되기 때문에 터널이나 암반구조물의 안정성 해석 및 설계를 위해서는 반드시 암반 불연속면의 역학적 성질을 규명할 필요가 있다. 지하암반 절리면의 실제 거동을 실내에서 정확히 모사하기 위해 본 연구에서는 일련의 일정 수직강성 조건하 직접 전단시험을 수행하여 초기 수직응력, 전단속도 그리고 절리면의 거칠기가 거친 화강암 절리면의 전단거동특성에 미치는 영향을 살펴보았다. 일정 수직강성 조건에서 거친 암석절리에 대한 시험 결과 전단거동은 일반적으로 1차 정점 전단응력에서의 전단응력 감소 정도에 따라 크게 두 가지 형태의 전단거동을 보이는 것으로 구분되었다. 초기 수직응력이 증가함에 따라 정점 전단변위와 1차 정점 전단응력은 증가하지만 마찰각과 정점 마찰계수의 경우 감소하는 것으로 나타났으며, 전단강성과 평균마찰계수의 경우는 초기 수직응력에 영향을 받지 않는 것으로 나타났다. 거친 절리에 대한 전단속도의 영향은 초기 수직응력이 낮은 경우 일부 전단변수들에서 약간 관찰되었으나 수직응력이 증가함에 따라 대부분의 전단시험 결과변수들에서 전단속도의 영향은 미미하였다. 거칠기에 따른 전단거동의 변화를 분석하였으나 명확한 관련성이 나타나는 경우보다 시료간의 편차가 심한 경우가 많았다.

• Journal of Advanced Marine Engineering and Technology
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• 제20권5호
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• pp.58-67
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• 1996

In the present study, the pressure distribution, wall shear stress distribution and friction factor of developing turbulent pulsating flows are investigated theoretically and experimentally in the entrance region of a square duct. The pressure distribution for turbulent pulsating flows are in good agreement with the theoretical values. The time-averaged pressure gradients of the turbulent pulsating flows show the same tendency as those of turbulent steady flows as the time-averged Reynolds number $(Re_{ta})$ increase. Mean shear stresses in the turbulent pulsating flow increase more in the inlet flow region than in the fully developed flow region and approach to almost constant value in the fully developed flow region. In the turbulent pulsating flow, the friction factor of the quasi-steady state flow $({\lambda}_{q, tu})$ follow friction factor's law in turbulent steady flow. The entrance length of the turbulent pulsating flow is not influenced by the time-averaged Reynolds number $(Re_{ta})$ and it is about 40 times as large as the hydraulic diameter.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2003년도 사면안정학술발표회
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• pp.145-154
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• 2003

When a nail pulled out in dense, granular soil, the soil in the vicinity of the nail tends to dilate, but its dilatancy results in a normal stress concentration at the soil/nail interface, thereby increasing the pull-out resistance of the inclusion. It is thought to be occurring within the resistance zone where the soil mass is at stationary state and the reinforcement are held in position by the soil, due to the friction or bond. In this paper, A series of direct shear and interface tests were conducted by using so called‘Constant Normal Stiffness Test Apparatus’which was modified and improved from the conventional direct shear box test rig. Unlikely the normal shear box test, this enables to simulate the different constraint effects of surrounding soil during shear under the conditions of constant stress and volume, constant normal stiffness. The aim of the research programme is to get better understanding of pull-out bond mechanism, thus to explore the possibility of evaluating the pull-out bond capacity of soil/reinforcement at the preliminary design stage from the laboratory test.

• Geomechanics and Engineering
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• 제17권3호
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• pp.253-259
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• 2019

Concrete-to-concrete bedding planes (CCBP) are observed from time to time due to the multistep hardening process of the concrete materials. In this paper, a series of direct/cyclic shear tests are performed on CCBP under static and dynamic normal load conditions to study the frictional behavior effect by the shear velocities, normal impact frequencies, horizontal shear frequencies, normal impact force amplitudes, horizontal shear displacement amplitudes and normal load levels. According to the experimental results, apparent friction coefficient k ($k=F_{Shear}/F_{Normal}$) shows different patterns under static and dynamic load conditions at the stable shear stage. k is nearly constant in direct shear tests under constant normal load conditions (DCNL), while it is cyclically changing with nearly constant peak value and valley value for the direct shear tests under dynamic normal load conditions (DDNL), where k increases with decreasing normal force and decreases with increasing normal force. Shear velocity has little influence on peak values of k for the DCNL tests, but increasing shear velocity leads to increasing valley values of k for DDNL tests. It is also found that, the valley values of k ascend with decreasing impact normal force amplitude in DDNL tests. The changing pattern of k for the cyclic shear tests under constant and dynamic normal load conditions (CCNL and CDNL tests) are similar, but the peak value of k is smaller in CDNL tests than that in CCNL tests. Normal load levels, shear displacement amplitudes, vertical impact frequencies, horizontal shear frequencies and normal impact force amplitudes have little influence on the changing pattern of k for the cyclic shear tests. The tests of this study provide useful data in understanding the frictional behavior of the CCBP under distinct loadings, and these findings are very important for analyzing the stability of the jointed geotechnical structures under complicated in situ stress conditions.