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More than 20 years after the adoption of UN Security Council Resolution 1325 on Women, Peace and Security, the international community is concerned with taking stock of its implementation in countries undergoing transitions from war to peace. This article contributes to a better understanding of the dynamics involved in implementing the Women, Peace and Security agenda through a focus on the frictional interactions that take place between different actors promoting women's participation in the peace process in Mali. Based on extensive fieldwork in Bamako between 2017 and 2019, it analyses interactions between different international and local actors in the Malian peace process through a discussion of vertical (between international and local actors) and horizontal (between local actors) friction. It finds that the way different actors respond to friction shapes relationships and impacts norm trajectories by triggering feedback loops, which in turn trigger new responses and outcomes.
Researchers in the Physics Department of St. Olaf College are using a uniquely designed, integrated nanoindenter-quartz microbalance apparatus to bridge the gap between the fundamental science of friction and the engineering of practical micromechanical systems. This level of micro-research requires extreme stability for the microbalance instrumentation. Since 2001, the lab has used negative-stiffness vibration isolation to achieve a high level of isolation in multiple directions, custom tailoring resonant frequencies to 0.5 Hz vertically and horizontally.
Here, we add damping to the harmonic oscillator, and explore the role of the resulting new time scale in the solutions to the equations of motion. Specifically, the ratio of damping to oscillatory time scale can be used to identify very different regimes of motion: under-, critically-, and over-damped. Then driving forces are added, we consider the effect those have on the different flavors of forcing already in place. The main physical example (beyond springs attached to masses in dashpots) is electrical, sinusoidally driven RLC circuits provide a nice, experimentally accessible test case. On the mathematical side, the chapter serves as a thinly-veiled introduction to Fourier series and the Fourier transform.
We report on enhanced mechanical, tribological, and surface-wettability characteristics of polymeric films dispersed with inorganic fullerene (IF)-type tungsten disulfide (WS2) nanoparticles derived through a two-step hydrothermal route. Imaging through transmission electron microscopy suggests the occurrence of polyhedral cage-like structures with a visibly nonspherical hollow ranging 55–75 nm. The mechanical stability of IF-type WS2 dispersed in polyvinyl alcohol (PVA) gets improved with increasing nano-inclusions, and upto 6 wt% loading. As compared with nanosheets, the IF-WS2 in PVA at the critical loading offers nearly 28.6, 33.6, and 42% respective improvements as regards, breaking stress, elongation at break, and toughness. Moreover, Stribeck curves in the mixed lubricating regime have revealed a nearly ∼80% reduction of coefficient of friction (COF) due to inclusion of IF-type WS2 in PVA. In the hydrodynamic region, the COF is drastically lowered from a typical value of 0.55 to 0.15 at the maximal sliding velocity with nanoparticle loading and despite the fact that the tribo feature gives a rising trend for a particular curve. Furthermore, exhibiting a progressive increase in water contact angle, a clear transition from the hydrophilic (∼64°) to hydrophobic (∼107°) surface of the nanocomposite films has been witnessed after inclusion of nano IF-WS2. An increased hydrophobicity and lowered surface adhesion and COF values along with marginal drop in surface energy are ensured in the investigated specimens. Investigation of responsive tribological and wetting–dewetting transition would find scope not only in coating and textile industry but also in smart miniaturized components.
Two preferred textures were observed in the Alhama de Murcia Fault rocks: (a) foliated bands (>100 µm thick) rich in well-crystallized dioctahedral micas, quartz, hematite and dolomite; and (b) ultrafine-grained bands (<100 µm thick) made of patches composed of small mica crystals (<15 µm) and dispersed Fe-oxides. In both textures, kaolinite forms intergrowths or patches of randomly oriented crystals filling gaps or opening layers of presumably inherited detrital mica crystals, which is interpreted as an epitaxial growth from fluids. The Na/K ratio of mica crystals in the thin ultrafine-grained bands shows a wider range than the micas from the foliated bands including muscovitic, intermediate Na/K and paragonitic compositions. The absence of the 0.98 nm intermediate peak in the diffractograms indicates that the small micas are submicroscopically paragonite and phengite intergrowths. The d001 values of the K-dioctahedral micas in the <2 µm and whole fractions are clearly different from each other. The d001 values of micas of the <2 µm fraction are larger, indicating a higher K and lower Na content in the small micas. Their composition corresponds to lower temperatures, suggesting their growth during a genetic episode in the fault. The textural relationships indicate a late growth of kaolinite, probably due to the fluid–rock interaction along fault planes and fractures. The neoformed clay minerals might alter the stability of the fault plane. The absence of expandable clay minerals and the relatively high frictional strength of kaolinite under wet conditions might explain the observed velocity-neutral behaviour of this gouge and earthquake propagation towards the surface.
In this paper, an integrated approach to turbine overspeed analysis is presented, taking into account the secondary air system dynamics and mechanical friction in a turbine assembly following an unlocated high-pressure shaft failure. The axial load acting on the rotating turbine assembly is a governing parameter in terms of overspeed protection since it governs the level of mechanical friction which acts against the turbine acceleration due to gas torque. The axial load is dependent on both the force coming from secondary air system cavities surrounding the disc and the force on the rotor blades. It is highly affected by secondary air system dynamics because rotor movement modifies the geometry of seals and flow paths within the network. As a result, the primary parameters of interest in this study are the axial load on the turbine rotor, the friction torque between rotating and static structures and the axial position of the rotor.
Following an initial review of potential damage scenarios, several cases are run to establish the effect of each damage scenario and variable parameter within the model, with comparisons being made to a baseline case in which no interactions are modelled. This allows important aspects of the secondary air system to be identified in terms of overspeed prevention, as well as guidelines on design changes in current and future networks that will be beneficial for overspeed prevention.
The dynamics of moving solids with unilateral contacts are often modelled by assuming rigidity, point contacts, and Coulomb friction. The canonical example of a rigid rod with one endpoint slipping in two dimensions along a fixed surface (sometimes referred to as Painlevé rod) has been investigated thoroughly by many authors. The generic transitions of that system include three classical transitions (slip-stick, slip reversal, and liftoff) as well as a singularity called dynamic jamming, i.e., convergence to a codimension 2 manifold in state space, where rigid body theory breaks down. The goal of this paper is to identify similar singularities arising in systems with multiple point contacts, and in a broader setting to make initial steps towards a comprehensive list of generic transitions from slip motion to other types of dynamics. We show that – in addition to the classical transitions – dynamic jamming remains a generic phenomenon. We also find new forms of singularity and solution indeterminacy, as well as generic routes from sliding to self-excited microscopic or macroscopic oscillations.
In this paper, a new model for joint dynamic friction of industrial robot manipulators is presented. In particular, the effects of the temperature in the joints are considered. A polynomial-based model is proposed and the parameter estimation is performed without the need of a joint temperature sensor. The use of an observer is then proposed to compensate for the uncertainty in the initial estimation of the temperature value. A large experimental campaign show that the model, in spite of the simplifying assumptions made, is effective in estimating the joint temperature and therefore the friction torque during the robot operations, even for values of velocities that have not been previously employed.
This study investigates the effect of bracket bonding-debonding on the tribological properties of human tooth enamel, when two different adhesives are applied. The nano-scratch experiment was performed on the enamel at untreated and under the bracket regions to obtain wear resistance, scratch hardness and friction coefficient. The results indicated that the tribological properties of the enamel vary significantly after bracket bonding-debonding. However, no significant difference exists between the effects of bracket bonding by the nano-composite and the composite adhesives on the tribological properties. Therefore, considering the mechanical and physical properties of the nano-composite adhesive, its bond strength for orthodontic bracket bonding and the influences on the enamel after removing the bracket, the use of nano-composite adhesive is suitable for orthodontic treatments.
Metal working tools are generally exposed to hard conditions, and the control of their excessive wear is of a crucial importance for the metal working process. Indeed, tribo-layers as mechanically mixed layers and wear debris are completely involved in the wear behavior. This paper undertakes the study of the frictional behavior and wear of X40CrMoV5 (AISI H13) tool steel as a function of speed rotation at room temperature. The utmost objective of this research work is to assess some wear mechanisms of this tool steel used at room temperature. The tribological experiments were accomplished on high temperature pin-on-disc tribometer with an open sliding contact. The pin material was X40CrMoV5 steel and the disc material was Fe360B steel. The investigations were accomplished for different rotatory speeds of the disc ranging from 25 rpm to 100 rpm, and different nominal pressure. SEM and EDS explored the development surface damage and oxides tribo-layers. It was concluded that the increase of the rotation speed of the disc and the nominal pressure reduce the friction coefficient by the creation of a wear protective layer.
In a hot forming process, the study of the interface tool/product proves important. This study focuses on the influence of the third body in the case of pin-on-disc in an open contact. The objective of this work was to identify the third body-particle circulation mechanisms at high temperatures. The “wear and friction” tests were conducted with an open sliding contact on pairs of X40CrMoV5/Fe360B steels under a normal force of 70 N at 600 °C and with a speed of rotation of the disc of 50 rev/min. The pin material was X40CrMoV5 (AISI H13) steel and the disc material was Fe360B steel. Scanning electronic microscope, energy-dispersive spectroscopy (EDS), and X-ray diffractometer explored the development surface damage and oxides tribo-oxides. It was concluded that various types of the third body particles were present in the contact. The wear mechanism on the X40CrMoV5 pin in a high temperature contact is proposed.
In this study, surface texturing and hydrogenated amorphous carbon (a-C:H) diamond-like carbon (DLC) coating was combined to evaluate the coating performance at various temperatures in oil lubricated reciprocating sliding tests. Micro dimples were created by laser surface texturing on M2 steel using a Pico second laser. DLC coating was deposited by hybrid magnetron sputtering on textured substrates. Textured a-C:H showed stable coefficient of friction at 30, 80, and 125 °C compared to un-textured a-C:H. At 30 °C, graphitization was not observed for both textured and un-textured DLC coating. Graphitization was more pronounced in the case of un-textured a-C:H at 80 and 125 °C. Results show that, at all temperatures tested (30–125 °C), DLC textured samples showed higher wear resistance compared to un-textured DLC coating. The improvement in wear resistance can be explained by the lower graphitization of textured DLC coating. Lower graphitization in the case of textured DLC might be due to the wear particle capturing and lubricant retention ability of textures.
Multilayered thin films of Al/Cu/Fe have been prepared by magnetron sputtering and annealed into quasicrystalline and approximant phases on Al2O3 and Si substrates, respectively. The nanomechanical and nanotribological properties, such as hardness, elastic modulus, friction, and toughness, have been measured using a triboindenter and analytical methods. The approximant phase was proved to be slightly harder than the quasicrystalline phase with a hardness of about 15.6 GPa, and with a similar elastic modulus of about 258 GPa. These values however decreased rapidly with an increasing amount of Si in the approximant. The indentation toughness of the approximant, <0.1 MPa/m½, was however inferior to that of the quasicrystals with 1.5 MPa/m½. Friction coefficients were measured in a range of 0.10–0.14 for both the quasicrystalline and approximant thin films.
This paper proposes a new approach for slip prediction of walking biped robots. The slip prediction is a measurement-based and friction behavior-inspired approach. A measurement-based online algorithm is designed to estimate the Coulomb friction which is regarded as a slip threshold. To predict the slip, a safety margin is introduced in the negative vicinity of the estimated Coulomb friction. The estimation algorithm concludes that if the applied force is outside the safety margin, then the foot tends to slip. The proposed approach depends on the available type of measurements. Three options of measurements are discussed. Among them, the foot acceleration and ankle force measurements scenario is validated by experiments on the humanoid SURALP (Sabanci University Robotics Research Laboratory Platform). The results demonstrate the effectiveness of the proposed approach for slip prediction and detection.
In this study, a model for wheeled mobile robots that includes a static friction model in the force balance at the robot's center of mass is presented. Additionally, a least-squares method to linearly combine functions is proposed to estimate the friction coefficients. The experimental and simulation results are discussed to demonstrate the effectiveness of this approach in indoor environments for two floor types.
In this work, direct potential measurements during cold rolling of zinc and X20Cr13
stainless steel were carried out in the rolling slit to follow the tribologic and galvanic
mechanisms of hydrogen formation and absorption on the surface of the working rolls made
of DHQ1 grade steel. An Ag/AgCl in 3.5 M KCl reference microelectrode was used to record
the open circuit potential of the electrochemical system roller-product immersed into
commercially relevant electrolyte (rolling emulsion) with a pH value of 4.5 and an
electric conductivity 46 mS cm-1. The potential shift into either negative or positive
direction of the rolls-product system gives information on the processes taking place at
the surface in the course of the friction. A detailed discussion of the in-situ
potentiometry experiments reveals a stationary situation established between the
destruction and repassivation of the surface structures during continuous cold rolling
accompanied with intensive hydrogen evolution. Galvanic coupling of the working rolls with
the product significantly intensifies the hydrogen embrittlement related problems of the
rolls. Atomic hydrogen is adsorbed on the surface and exhibits a pressure supported
absorption into the rolls during their whole lifetime.
The design of control laws for flexible manipulators is known to be a challenging problem, when using a conventional actuator, i.e., a motor with gear. This is due to the friction of the nonlinear actuator, which causes torque dead zone and stick-slip behavior, thereby hampering the good performance of the control system. The torque needed to attenuate the vibrations, although calculated by the control law, is consumed by the friction inside the actuator, rendering it ineffective to the flexible structure control. Nonlinear friction varies with different operational conditions of the actuator and a friction compensation mechanism based on these models cannot always keep a good performance. This study proposes a new control strategy using wavelet network to friction compensation. Experimental results obtained with a flexible manipulator attest to the good performance of the proposed control law.
This article works through ideas of bad/mis-translation in order to explore some of the cultural politics that undergird the construction of the global stage with specific reference to the idea of contemporary choreography in Asian dance. Bound by contesting flows toward heterogeneity and homogeneity, the global stage begs questions about the categories that pose as neutral signifiers—categories that are deployed in organizing “global” programming, and that ultimately over-determine our reception of difference. The article works through examples of artists working with Asian dance, touring their work on the global circuit, and often engaging in some kinds of mis-alignments or bad translations, thus allowing space for complicated questions of difference to emerge.
Among the many challenges to deal with, when a robot is interacting with its environment, friction at the contact surface and/or at the joints is one of the most important to be considered. In this paper we propose a control algorithm for the tracking of position and force (unconstrained orientation case only) of a manipulator end-effector that does not require the robot model for implementation. This characteristic has the advantage of making it capable to compensate friction effects without any previous estimation. Furthermore, no velocity measurements are needed, and the unit quaternion is employed for orientation control. Experimental and simulation results are provided.
Thermoplastic based composites containing different weight fractions of molybdenum
disulfide (MoS2) solid lubricant were developed by injection molding.
Polymethyl methacrylate (PMMA) and polycarbonate (PC) were chosen for the thermoplastic
matrices. In order to characterize the interfacial adhesion between the matrices and the
filler, we observed the fracture morphologies of selected composites. Micrographs of the
fractured surfaces showed removal of MoS2 particles by microcraking as well as
the presence of voids in the case of PMMA/MoS2 composites. These observations
were confirmed by complementary images obtained using the X-ray tomography. The addition
of an appropriate coupling agent may improve the adhesion between the MoS2
particles and the polymer matrix. Tribological behavior of the composites was also
investigated using a ball-on-flat microtribometer with a high chromium steel ball
antagonist. It was found that the addition of MoS2 particles didn’t improve the
tribological performance of the composite in the case of PMMA matrix unlike the case of PC
matrix where the friction coefficient was considerably reduced.