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Micro/Nanotribology and Micro/Nanomechanics of Magnetic Storage Devices
Published in Bharat Bhushan, Handbook of Micro/Nano Tribology, 2020
If asperities in a sample surface have a preferential orientation, this directionality effect will be manifested in macroscopic friction data; that is, the coefficient of friction may be different in one sliding direction from that in the other direction. Such a phenomenon has been observed in rubbing wool fiber against horn. It was found that the coefficient of friction is greatest when the wool fiber is rubbed toward its tip (Mercer, 1945; Lipson and Mercer, 1946; Thomson and Speakman, 1946). Makinson (1948) explained the directionality in the friction by the “ratchet” effect. Here, the ratchet effect is the result of large angle θ, where instead of true sliding, rupture or deformation of the fine scales of wool fibers occurs in one sliding direction. We note that the frictional directionality can also exist in materials with particles having a preferred orientation.
Study of the deformation property of rock under cyclic loading based on the sub-loading surface theory
Published in Charlie C. Li, Xing Li, Zong-Xian Zhang, Rock Dynamics – Experiments, Theories and Applications, 2018
Y.Q. Zhou, Q. Sheng, N.N. Li, Y.B. Zhou
The deformation characteristics of rock materials under cyclic loading demonstrate that the unloading curve and loading curve does not coincide to form a closed plastic hysteresis loop, which is the Massing effect. Meanwhile, the ratchet effect is produced. As the number of cycles increases, the hysteresis loops move in the direction of increasing strain, and the irreversible plastic deformation of rocks increases with the increase of dynamic strain. At present, the researches on the mechanical properties of rock materials under cyclic loading are mostly based on experimental methods (Fuenkajorn & Phueakphum 2010, Ma et al. 2013, Bagde & Petros 2009) and thus seldom reflected by developing constitutive models and numerical models.
Digital public space for the evolved mind
Published in Naomi Jacobs, Rachel Cooper, Living in Digital Worlds, 2018
We do not simply copy others, but add our own ideas. Over time technology may be improved upon by individuals, leading to a ‘ratchet’ effect of cumulative culture (Tomasello, 1999). We also are very much influenced by what others around us do, and feel a strong urge to conform to group expectations and values. Sociality is a key part of our human behaviour, and is bound into such evolved traits as our long childhoods (and our helplessness as babies). Behaviours that may not have an obvious immediate advantage but are socially acceptable within the group may be copied and maintained within our social communities, leading to culture.
Optimisation of wheel profile of variable gauge high-speed trains
Published in Vehicle System Dynamics, 2023
Yayun Qi, Huanyun Dai, Feng Gan, Hao Gao
The surface fatigue index was proposed by Ekberg et al. [27] to evaluate the rolling contact fatigue (RCF) characteristics of the wheel and rail. Wheel-rail contact fatigue is also a common problem during the operation of high-speed trains, and the RCF index is used as one of the targets for optimising the wheel profile. The definition of the surface fatigue index is shown in the following equations. Where: a and b are the short and long semi-axes of the elliptical contact patch respectively; k is the pure shear yield strength of the material, taken as 303 MPa; is the wheel-rail normal force; is the traction coefficient, and is the wheel-rail longitudinal and lateral creep forces, respectively. When the wheel surface fatigue index is positive, the ratchet effect of the wheel occurs which leads to wheel surface cracking.
A multi-objective optimisation method of rail combination profile in high-speed turnout switch panel
Published in Vehicle System Dynamics, 2023
Jiasheng Fang, Rong Chen, Jiayin Chen, Jingmang Xu, Ping Wang
The wheel-rail RCF is closely related to the multiaxial stress field distribution of materials and the load conditions. It can usually be divided into three types [20], including surface-initiated fatigue, subsurface-initiated fatigue, and fatigue initiated at deep material defects. To improve the model calculation efficiency, only surface-initiated fatigue is considered. Surface-initiated fatigue is often caused by the ratchet effect of the surface material and low-cycle fatigue [21]. Formula 6 is used to determine whether surface-initiated RCF occurs. where μ is the traction coefficient, Fz is the normal contact force, a and b are the semiaxes of the elliptical contact patch, and k is the yield limit in shear for the work-hardened material. Formula 6 is based on the Hertzian contact theory. For non-Hertzian contact problems, the wheel-rail contact patch can be divided into n strips along the transverse direction of the track. Furthermore, the wheel-rail contact in the centre of each strip is considered to meet the Hertzian contact assumption so that the RCF discriminant formula for the non-Hertzian contact condition can be obtained using Formula 7 as: Where FIsurf(x,y) is the partial RCF index in each unit and Fz(x,y), Fx(x,y), and Fy(x,y) are the normal contact stress and tangential contact stresses in the longitudinal and lateral directions, respectively.
Effect of Elastic Contact Force on Tribological Characteristics of Current-carrying Roll Rings in Rotating Conductive Joints
Published in Tribology Transactions, 2023
Tianhua Chen, Chenfei Song, Zili Liu, Li Wang, Chao Sun, Xianjuan Pang, Yongzhen Zhang
The morphologies of the worn surfaces for all the tests were observed using a 3D surface profiler explorer. The surface profile becames coarser as the elastic force decreases, as shown in Fig. 6. At the lowest elastic force, the surface profile exhibited significant fluctuations, and the corresponding average surface roughness was 1.86 μm. When the elastic force increased to 13.6 N and 17.0 N, the surface profile tended to be smooth, and the corresponding surface roughness became slightly lower. The surface roughness, at the largest elastic force of 20.4 N, decreased considerably to 0.82 μm. It could be observed that a rougher surface can be obtained at the lower elastic force condition. For rough surfaces in contact with each other under current-carrying tribological conditions, a reduction in surface roughness could increase the real contact area and decrease the contact resistance. At the same time, owing to the ratchet effect of roughness asperities, a high roughness would lead to the high friction force. Surface roughness might be one of the reasons for high friction under low elastic force.