Friction stir welding FSW） has achieved remarkable success in the joining and processing of aluminium alloys and other softer structural alloys. Conventional FSW, however, has not been entirely ...successful in the joining, processing and manufacturing of different desired materials essential to meet the sophis- ticated green globe requirements. Through the efforts of improving the process and transferring the existing friction stir knowledge base to other advanced applications, several friction stir based daughter technologies have emerged over the timeline, A few among these technologies are well developed while others are under the process of emergence. Beginning with a broad classification of the scattered fric- tions stir based technologies into two categories, welding and processing, it appears now time to know, compile and review these to enable their rapid access for reference and academia. In this review article, the friction stir based technologies classified under the categol~J of welding are those applied for join- ing of materials while the remnant are labeled as friction stir processing （FSP） technologies. This review article presents an overview of four general aspects of both the developed and the developing friction stir based technologies, their associated process parameters, metallurgical features of their products and their feasibility and application to various materials. The lesser known and emerging technologies have been emphasized.
Robust superlubricity (RSL), defined by concurrent superlow friction and wear, holds great promise for reducing material and energy loss in vast industrial and technological operations. Despite ...recent advances, challenges remain in finding materials that exhibit RSL on macrolength and time scales and possess vigorous electrical conduction ability. Here, the discovery of RSL is reported on hydrated NbB
films that exhibit vanishingly small coefficient of friction (0.001-0.006) and superlow wear rate (≈10
m
N
m
) on large length scales reaching millimeter range and prolonged time scales lasting through extensive loading durations. Moreover, the measured low resistivity (≈10
Ω m) of the synthesized NbB
film indicates ample capability for electrical conduction, extending macroscale RSL to hitherto largely untapped metallic materials. Pertinent microscopic mechanisms are elucidated by deciphering the intricate load-driven chemical reactions that generate and sustain the observed superlubricating state and assessing the strong stress responses under diverse strains that produce the superior durability.
This essay focuses on the current state of cross-strait relations and whether the issue may evolve from friction to war. In 2022, after Russia declared war on Ukraine, the world is again in the ...shadow of war, and cross-strait relations have been a powder keg in the international community. Taiwan, as a country with its legislature, political party and complete state system, but not recognized by the international community. It has long been at odds with the PRC, which claimed Taiwan as part of its own. Since the KMT stepped down and the DPP took power in 2016, Tsai Ing-wen (Taiwan’s de facto leader) has denied the 1992 Consensus, and with Pelosi’s visit to Taiwan, increasing friction in cross-strait relations, and PLA air forces operating across the centre line of the strait, there is growing concern among the world’s population that cross-strait relations will break down and degenerate into war. The purpose of this paper is to discuss the possibility of cross-strait relations from mutual condemnation to actual conflict in terms of the history of cross-strait relations, the current development of Taiwan and China, and the attitudes of the aborigines on both sides of the Taiwan Strait. It also discusses the current situation of cross-strait relations, the future development of the two places, and the attitudes of several Taiwanese and mainlanders toward the cross-strait conflict.
In this review, we discuss the current knowledge on the tribology of human skin and present an analysis of the available experimental results for skin friction coefficients. Starting with an overview ...on the factors influencing the friction behaviour of skin, we discuss the up-to-date existing experimental data and compare the results for different anatomical skin areas and friction measurement techniques. For this purpose, we also estimated and analysed skin contact pressures applied during the various friction measurements. The detailed analyses show that substantial variations are a characteristic feature of friction coefficients measured for skin and that differences in skin hydration are the main cause thereof, followed by the influences of surface and material properties of the contacting materials. When the friction coefficients of skin are plotted as a function of the contact pressure, the majority of the literature data scatter over a wide range that can be explained by the adhesion friction model. The case of dry skin is reflected by relatively low and pressure-independent friction coefficients (greater than 0.2 and typically around 0.5), comparable to the dry friction of solids with rough surfaces. In contrast, the case of moist or wet skin is characterised by significantly higher (typically >1) friction coefficients that increase strongly with decreasing contact pressure and are essentially determined by the mechanical shear properties of wet skin. In several studies, effects of skin deformation mechanisms contributing to the total friction are evident from friction coefficients increasing with contact pressure. However, the corresponding friction coefficients still lie within the range delimited by the adhesion friction model. Further research effort towards the analysis of the microscopic contact area and mechanical properties of the upper skin layers is needed to improve our so far limited understanding of the complex tribological behaviour of human skin.
Methods and means for studying the factor of plastic contact friction in the processes of volumetric plastic deformation of solid and hollow precision workpieces were discussed in the article. ...Estimation of the forces values required for overcoming friction resistance is important for the correct calculation and choice of technological variants for deformation processes, for assessing the properties of technological lubricants, as well as for the correct setting of initial conditions when simulating forming processes using various methods, including using FEM. An analysis of methods and means for assessing the conditions of contact friction in die forming processes was carried out, and a conclusion about the advantage of methods for directly measuring of friction forces in plastic deformation processes compared to ones for indirectly determining friction constants using deformation, kinematic, and force parameters was made. Designs of devices for measuring friction forces directly during the process of plastic deformation have been proposed and tested. Radial extrusion schemes have the advantages of identifying surfaces of directional influence by friction forces and recording them. Based on the results of measurements of friction forces, the effectiveness of several technological lubricants for cold and semi-hot extrusion of steel and aluminum alloys was assessed. The values of friction coefficients according to Siebel’s law have been refined. A correlation between friction stress and normalized pressure of extrusion was proposed, in which the flow stress of the near-contact metal layer was expressed through hardness indicators, and correction coefficients were found from experimental data of measuring friction forces. The proposed devices made it possible to expand the understanding of the contact friction factor in the processes of working by pressure, which is important for practice and for plastic deformation processes simulation.
The temperature and load dependence of the sliding friction behaviour of a racing tire tread compound on coarse and fine rough granite substrates is analysed by experimental and theoretical ...techniques. Based on dry friction measurements at different temperatures, friction master curves are constructed by shifting the data horizontally on the velocity axis using the same shifting factors as found from viscoelastic master curves. The obtained isothermal friction curves increase rapidly with increasing sliding velocities and show a more or less pronounced plateau over a broad velocity range, which decreases with increasing load. For analysing this behaviour, the Klüppel and Heinrich theory of rubber friction and contact mechanics is applied, which considers the multi-scale contacts and excitations of the rubber sliding on rough surfaces in the frame of a linear viscoelastic approach. The extension of this theory to more realistic surfaces with two or more scaling ranges is described in some detail. It takes adhesion and hysteresis contributions into account referring to the viscoelastic response of the rubber on different frequency scales. The theory predicts that under isothermal conditions the coefficient of friction decreases with load, which is more pronounced for the adhesion than for the hysteresis contribution. This result is found to be in fair agreement with the measure friction curves confirming the contact mechanical approach of the theory.
•New procedure for constructing viscoelastic and friction master curves is presented.•Basic concepts of the friction theory are described and input parameters are discussed.•Load dependent friction master curves are measured and created according to WLF.•Theoretical approach of rubber friction shows fair agreement with experimental data.•Theoretical results confirm the measured load dependence of the friction coefficient.
This paper proposes a new method to identify the mechanical parameters of a servo drive system, such as the moment of inertia and friction torque coefficients. Identification of the moment of inertia ...is essential for the design of a high-performance speed and position controller. Furthermore, the mechanical friction torque coefficients, such as the viscous and Coulomb friction torque coefficients, can be used to reduce the speed and position error without resorting to the use of a high gain for the speed controller. To simultaneously identify the moment of inertia and the friction torque coefficients, the proposed method uses the fact that the sinusoidal speed is in phase with the friction torque and out of phase with the torque for the moment of inertia. The proposed method is based on the speed control loop and the moment of inertia, and friction torque coefficients can be exactly obtained from a half-period integration of the torque reference of the low frequency sinusoidal speed control. Using computer simulations and experiments employing 600-W servo drive systems, the feasibility of the proposed method was verified.
There is growing interest in reducing friction in lubricated machine components to thereby increase the energy efficiency of machines. One important way to minimise friction is to employ friction ...modifier additives to reduce friction in thin film boundary lubrication conditions. There are currently three main types of friction modifier additive, organic friction modifiers, oil soluble organomolybdenum friction modifiers and functionalised polymers. In common practice, a single such additive is generally employed in a formulated lubricant, but it is of interest to explore whether combinations of two friction modifier additives may prove beneficial. In this study, the performance of eight commercial friction modifier additives spanning all three main types was first measured in three quite different friction tests. The aim was to identify the contact conditions under which each additive was most effective. Additive solutions in both a base oil and a formulated engine oil were investigated. In general, functionalised polymers were most beneficial in sliding–rolling contacts, while oil soluble organomolybdenum friction modifiers worked best in severe, reciprocating sliding conditions. However, all friction modifier additive response was strongly affected by the other additives present in formulated engine oils. The friction performance of combinations of friction modifier additives was then explored. When two different friction modifiers additives were combined in solution, several possible outcomes were observed. The most common was for one of the additives to predominate, to give friction that was characteristic of that additive alone, while in some cases friction lay between the values produced by either additive on its own. In a few cases the additives behaved antagonistically so that the combination gave higher friction than either additive by itself. In a few cases true synergy was observed, where a combination of two additives produced lower friction in a given test that either individual component at the same overall concentration. Another, and possibly more important synergy could also occur, however, when a pair of FMs worked more effectively than either individual additive over the range of test conditions present in different friction tests. This study suggests that optimal combinations of FMs may provide a means of reducing boundary friction and thus increasing the efficiency of machines, especially if the latter contain a range of lubricated machine components that operate with different types of tribological contacts.
This paper presents an exploration of friction modeling encompassing theoretical and practical aspects, utilizing a planar or 2D contact system. Various white-box friction models, including static ...and dynamic variants, are introduced, highlighting the superior capability of dynamic models in comprehensively capturing friction effects, substantiated through numerical simulation. Practical aspects of friction measurement and data-driven friction modeling are elucidated. The discourse extends to the development of grey-box and black-box friction models. A significant contribution lies in the proposition of a physics-informed neural network-based friction modeling approach, presenting it as an advanced and preferable alternative for friction estimation. To exemplify its efficacy, a case study of a torsion-based frictional contact scenario, employing Physics-Informed Neural Network (PINN) and the Nelder–Mead (N–M) algorithm for concurrent dynamics and friction model identification, was examined. Experimental data from a double torsion pendulum system, characterized by discontinuous dynamics, is employed for training. Results demonstrate the PINN’s superiority, providing more accurate representation of stick–slip phases at the contact zone and exhibiting faster performance compared to the N–M algorithm. The paper concludes by deliberating on challenges, prospects, and future directions in friction modeling.
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