• Title/Summary/Keyword: negative friction

Search Result 135, Processing Time 0.032 seconds

Study on Shear Behavior Characteristics of Granular Material using DEM (DEM을 이용한 조립재료의 전단거동 특성에 관한 연구)

  • Jo, Seon-Ah;Jeong, Sun-Ah;Lee, Seok-Won;Cho, Gye-Chun;Chun, Youn-Chul
    • Proceedings of the Korean Geotechical Society Conference
    • /
    • 2009.03a
    • /
    • pp.136-145
    • /
    • 2009
  • Factors influencing shear behavior of granular material include particle size, shape, distribution, relative density, particle crushing, etc. In this study, these factors are characterized by viewpoint of shear behavior using numerical analysis based on DEM. Geometrical particle shape is represented by a combination of small circular particles and influence of particle shape on crushing is studied through relative comparisons between clump (uncrushable) and cluster (crushable) models which are modeled using DEM. Also, particle shape is quantified by the dimensionless parameters such as circularity and convexity. The results indicate that particle shape indexes have a negative association with internal friction angle. Also, internal friction angle becomes reduced and failure envelop curve becomes nonlinear due to the particle crushing. It is also found that numerical results are quite good agreement with the experimental test conducted in this study.

  • PDF

An Experimental Study on the Squeal Noise Generation due to Dynamic Instability of Brake Pad (브레이크 패드의 동적 불안정성에 따른 스퀼 소음 발생 원인의 실험적 연구)

  • Cho, Sangwoon;Lim, Byoungduk
    • Transactions of the Korean Society of Automotive Engineers
    • /
    • v.24 no.5
    • /
    • pp.520-526
    • /
    • 2016
  • Squeal noise is a typical brake noise that is annoying to both passengers and pedestrians. Its frequency range is fairly wide from 1 kHz to 18 kHz, which can be distressful to people. The brake squeal noise occurs due to various mechanisms, such as the mode coupling of the brake system, self-excited vibration, unstable wear, and others. In this study, several parameters involved in the generation of a squeal noise are investigated experimentally by using a brake noise dynamometer. The speed, caliper pressure, torque, and friction coefficient are measured as functions of time on the dynamometer. The contact pressure and temperature distributions of the disc and the pad are also measured by using a thermal imaging camera and a pressure mapping system. As a result of the simultaneous measurement of the friction coefficient and squeal amplitude as functions of the velocity, it is found that the onset of the squeal may be predicted from the μv curve. It is also found that a non-uniform contact pressure causes instability and, in turn, a squeal. Based on the analysis results, design modifications of the pad are suggested for improved noise characteristics.

Study on the Undrained Shear Strength Characteristics (반월지역 해성점토의 비배수 전단강도 특성에 관한 연구)

  • 장병욱;박영곤
    • Magazine of the Korean Society of Agricultural Engineers
    • /
    • v.36 no.3
    • /
    • pp.90-99
    • /
    • 1994
  • To investigate the undrained shear strength characteristics of marine soils with high water content, high compressibility and weak bearing capacity, a series of undrained triaxial tests with pore pressure measurements on undisturbed and disturbed Banwol marine clay in normally consolidated and overconsolidated states is carried out. The results and main conclusions of this study are summarized as follows : 1 . When the consolidation pressure is increased, the maximum deviator stress of disturbed and undistubed clay in normally consolidated state is increased. Pore pressure parameters and internal friction angle of undisturbed clay are greater than those of disturbed clay. 2. The relationship between pore pressure and axial strain of undisturbed clay in normally consolidated state can be expressed as a hyperbolic function like stress-strain relation proposed by Kondner. 3. In the pore pressure-axial strain relation of disturbed clay in normally consolidated state, failure ratio R'f is greatly deviated in the range of 0.7~0.9 proposed by Christian and Desai. 4. For overconsolided clay, when overconsolidation ratio (OCR) is increased, normalized maximum deviator stress is increased and maximum pore pressure is decreased gradually. 5. Cohesion of overconsolidated clay is greater than that of nomally consolidated clay and internal friction angle slightly is decreased. 6. Pore pressure parameter at failure (Af) of overconsolidated clay is varied with OCR, Af becomes negative values with increment in OCR

  • PDF

In-situ Characterization of Electrochemical and Frictional Behaviors During Copper CMP

  • Eom, Dae-Hong;Kang, Young-Jae;Park, Jin-Goo
    • Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
    • /
    • 2004.07a
    • /
    • pp.227-230
    • /
    • 2004
  • As the organic acids were added in the slurry, zeta potential of alumina was changed to negative value and IEP value was shifted from alkaline to acidic pH. In citric acid based slurry, Cu surface continuously dissolved and etching depth linearly increased. On the contrary, passivation layer was grown on Cu surface in oxalic acid based slurry. As the platen rotation speed increased, Preston coefficient decreased in both slurries. With oxalic acid based slurry, at low velocity, removal rate is high value because of high friction force compared to citric acid based slurry. As platen velocity increased, removal of Cu in citric acid based slurry became higher value than oxalic acid based slurry. Typical lubrication behaviors were observed in both slurries. As Sommerfeld number increased, COF values gradually decreased and then re-increased. It indicated that lubrication was changed to direct contact or semi-direct contact mode to hydro-lubrication mode.

  • PDF

Large eddy simulation of flow around a stay cable with an artificial upper rivulet

  • Zhao, Yan;Du, Xiaoqing;Gu, Ming;Yang, Xiao;Li, Junjun;He, Ping
    • Wind and Structures
    • /
    • v.26 no.4
    • /
    • pp.215-229
    • /
    • 2018
  • The appearance of a rivulet at the upper surface of a stay cable is responsible for rain-wind-induced vibration (RWIV) of cables of cable-stayed bridges. However, the formation mechanism of the upper rivulet and its aerodynamic effects on the stay cable has not been fully understood. Large eddy simulation (LES) method is used to investigate flow around and aerodynamics of a circular cylinder with an upper rivulet at a Reynolds number of 140,000. Results show that the mean lift coefficients of the circular cylinder experience three distinct stages, zero-lift stage, positive-lift stage and negative-lift stage as the rivulet located at various positions. Both pressure-induced and friction-induced aerodynamic forces on the upper rivulet are helpful for its appearance on the upside of the stay cable. The friction-induced aerodynamic forces, which have not been considered in the previous theoretical models, may not be neglected in modeling the RWIV. In positive-lift stage, the shear layer separated from the upper rivulet can reattach on the surface of the cylinder and form separation bubbles, which result in a high non-zero mean lift of the cylinder and potentially induces the occurrence of RWIV. The separation bubbles are intrinsically unsteady flow phenomena. A serial of small eddies first appears in the laminar shear layer separated from the upper rivulet, which then coalesces and reattaches on the side surface of the cylinder and eventually sheds into the wake.

Molecular Dynamics Simulation Studies of Benzene, Toluene, and p-Xylene in NpT Ensemble: Thermodynamic, Structural, and Dynamic Properties

  • Kim, Ja-Hun;Lee, Song-Hi
    • Bulletin of the Korean Chemical Society
    • /
    • v.23 no.3
    • /
    • pp.447-453
    • /
    • 2002
  • In this paper we have presented the results of thermodynamic, structural, and dynamic properties of model systems for liquid benzene, toluene and p-xylene in an isobaric-isothermal (NpT) ensemble at 283.15, 303.15, 323.15, and 343.15 K using molecular dynamics (MD) simulation. This work is initiated to compensate for our previous canonical (NVT) ensemble MD simulations [Bull. Kor. Chem. Soc. 2001, 23, 441] for the same systems in which the calculated pressures were too low. The calculated pressures in the NpT ensemble MD simulations are close to 1 atm and the volume of each system increases with increasing temperature. The first and second peaks in the center of mass g(r) diminish gradually and the minima increase as usual for the three liquids as the temperature increases. The three peaks of the site-site gC-C(r) at 283.15 K support the perpendicular structure of nearest neighbors in liquid benzene. Two self-diffusion coefficients of liquid benzene via the Einstein equation and via the Green-Kubo relation are in excellent agreement with the experimental measures. The self-diffusion coefficients of liquid toluene and p-xylene are in accord with the trend that the self-diffusion coefficient decreases with increasing number of methyl group. The friction constants calculated from the force auto-correlation (FAC) function with the assumption that the fast random force correlation ends at time which the FAC has the first negative value give a correct qualitative trends: decrease with increase of temperature and increase with the number of methyl group. The friction constants calculated from the FAC's are always less than those obtained from the friction-diffusion relation which reflects that the random FAC decays slower than the total FAC as described by Kubo [Rep. Prog. Phys. 1966, 29, 255].

Investigation on glass transition temperature of low density polyethylene by the characteristics of temperature dependent linear expansion (선팽창 온도특성에 의한 저밀도 폴리에틸렌의 유리 천이온도에 대한 고찰)

  • 김봉흡;강도열;김재환
    • 전기의세계
    • /
    • v.30 no.7
    • /
    • pp.441-447
    • /
    • 1981
  • As a preceeding work for the study on dielectric characterstics of a kind of low density polyethylene introduced morphological change by mechanical method, glass transition temperature which is regarded as a macroscopic aspect for relaxation of molecular chain segments has been observed by means of temperature dependent dilatometric measurement. The origina specimen clearly shows two knees which correspond to two peaks (.gamma. and .betha. peak) in the intenal friction measurement, suggesting the existence of separated glass transition temperatures at 150.deg.k and 260.deg.k respectively. On the specimen irradiated to 100 Mrad both glass transition temperatures tend to shift towards high temperature sides because of crosslinking by irradiation. furthemore an evidence can be seen that radiation effect, even in amorphous phase, is also slelctive depending on slight morphological differences. The specimen extended to four times in length shows a peculiar nature such as negative linear thermal expansion coefficient increasing with temperature between 220.deg.k and ambient temperature and that this fact is interpreted by considering that c axis of the lattice aligns along the extended direction by drawing, further c axis inherently possesses the characteristics of negative linear thermal expansion coefficient. For the observations that the relatively small positive linear expansion on the specimen extended to ca. two times as well as the part below 220.deg.k of the specimen extended to four times, it is considered for the reason of the facts that the incompletely oriented region indicated as the middle part of Peterlin's model tends to restore partially to orginal arrangement-a kind of phase transition-as increasing with temperature.

  • PDF

Interfacial Crack Propagation Under Various Mode-Mixes

  • Park, Byung-Sun;Chai, Young-Suck
    • Journal of Mechanical Science and Technology
    • /
    • v.16 no.1
    • /
    • pp.39-45
    • /
    • 2002
  • Initiation and propagation of interfacial crack along bimaterial interface are considered in this study. A biaxial loading device for a single specimen is used for obtaining a wide range of mode-mixities. The specimen is an edge-cracked bimaterial strip of glass and epoxy; the biaxial loading device, being capable of controlling displacements in two perpendicular directions, is developed. A series of interfacial crack initiation and Propagation experiments are conducted using the biaxial loading device for various mixed modes. Normal crack opening displacement (NCOD) is measured near crack front by a crack opening interferometry and used for extracting fracture parameters. From mixed mode interfacial crack initiation experiments, large increase in toughness with shear components is observed. The behavior of interfacial crack propagation analyzed as a function of mode-mix shows that initial crack propagation is delayed with increase of mode-mixity, and its velocity is increased with positive mode-mixity but decreased with negative case. However, it is found that crack propagation is less accelerated with positive mode-mixity than the negative mode-mixity, which may be caused by contact and/or effects of friction between far field and near-tip Held along the interfacial crack.

An Experimental Study of Turbulent Uniform Shear Flow in a Nearly Two-Dimensional 90 Curved Duct (I) - Mean Flow Field- (2차원 90 곡관에서 균일전단류의 특성에 대한 실험적 연구 (1) -평균유동장-)

  • 임효재;성형진;정명균
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.19 no.3
    • /
    • pp.834-845
    • /
    • 1995
  • An experimental study is made in a nearly two-dimensional 90.deg. curved duct to investigate the effects of interaction between streamline curvature and mean strain on turbulence. The initial shear at the entrance to the curved duct is varied by an upstream shear generator to produce five different shear conditions ; a uniform flow (UF), a positive weak shear (PW), a positive strong shear(PS), a negative weak shear (NW) and a negative strong shear(NS). With the mean field data of the case UF, variations of the momentum thickness, the shape factor and the skin friction over the convex(inner) surface and the concave (outer) surface are scrutinized quantitatively in-depth. It is found that, while the pressure loss due to curvature is insensitive to the inlet shear rates, the distributions of wall static pressure along both convex and concave surfaces are much influenced by the inlet shear rates.

The Effect of Compressibility Terms on the Simulation of the Flowfield around a Cylinderical Afterbody (실린더 후부 유동장 모사를 위한 압축성 수정항의 영향)

  • 김성훈;정명균
    • Journal of the Korean Society of Propulsion Engineers
    • /
    • v.3 no.1
    • /
    • pp.15-23
    • /
    • 1999
  • K-ω model is used for simulation of flowfield around the cylinderical afterbody. In addition to two-equation turbulence model, modification terms for the compressibility effects are applied to the simulation. Although the estimations of the skin friction and the surface pressure distribution at hypersonic ramp flowfield were satisfactory, the result of the simulation with the modifications for this flowfield is worse than that of the original K-ω model. The compressiblility modification terms do negative effects on the estimation. The basic research on the turbulence model for the compressible flowfield has to be further conducted.

  • PDF