Coefficients of Friction: Static Versus Dynamic

This study was undertaken to 1. develop an apparatus to rapidly measure coefficient of friction (COF) on soft contact lenses; 2. determine if COFs measured on two daily-disposable lens models before and after wear are consistent with changes in lens surface morphology observed in parallel atomic force microscopy (AFM) images. Methods: A stress rheometer was adapted to measure COF on a soft contact lens by custom fabrication of a rapid-mount sample stage for increased throughput. Five subjects were randomly assigned to wear daily disposable nesofilcon A and delefilcon A contact lenses bilaterally for 4 hours, after which time lenses were removed. Static and kinetic COFs of lenses worn on left eyes was measured, while lenses worn on right eyes were imaged in parallel by AFM in tapping mode. Root mean square (RMS) surface roughness was calculated for all lenses to determine the effect of wear on surface topography. Results: Both static and kinetic COFs measured on unworn delefilcon A silicone hydrogel lenses were greater than on nesofilcon A traditional hydrogel lenses. Static COF on nesofilcon A increased significantly after wear, while kinetic COF trended higher but did not change significantly. Similarly, static COF on delefilcon A also increased significantly after wear, and kinetic COF trended higher but did not change significantly, both remaining greater than on worn nesofilcon A. Parallel AFM analysis demonstrated that nesofilcon A lenses are smoother than are delefilcon A out of the package. Both lenses attracted deposits during wear, but the nesofilcon A surface was less altered by on-eye wear than was the delefilcon A surface. Conclusion: A system to rapidly measure static and kinetic COFs was successfully developed. Static and kinetic COFs measured on delefilcon A were greater than on nesofilcon A lenses. More deposits and greater surface roughness were observed after wear on delefilcon A relative to nesofilcon A. Parallel AFM images of worn and unworn lenses were not predictive of measured COFs, but increased roughness visible by AFM was consistent with observed increases in COF, although not all increases were statistically significant.

Dummy headforms used for impact testing have changed little over the years, and frictional characteristics are thought not to represent the human head accurately. The frictional interface between the helmet and head is an essential factor affecting impact response. However, few studies have evaluated the coefficient of friction (COF) between the human head and helmet surface. This study’s objectives were to quantify the human head’s static and dynamic COF and evaluate the effect of biological sex and hair properties. Seventy-four participants slid their heads along a piece of helmet foam backed by a fixed load cell at varying normal force levels. As normal force increased, static and dynamic human head COF decreased following power–law curves. At 80 N, the static COF is 0.32 (95% CI 0.30–0.34), and the dynamic friction coefficient is 0.27 (95% CI 0.26–0.28). Biological sex and hair properties were determined not to affect human head COF. The COFs between the head and helmet surface should be used to develop more biofidelic head impact testing methods, define boundary conditions for computer simulations, and aid decision-making for helmet designs.

In the experiment, we observed such a phenomenon: the alternating normal force changes the vibration state of a friction system. A single-degree-of-freedom mathematical model was used in this paper to discuss the effects of a constant and alternating normal force on the stick–slip vibration characteristics for different dynamic and static friction coefficients. Under the condition that the applied constant normal force continues to increase, the vibration amplitude of the system, the amplitude of the limit cycle, and the adhesion time of the system increase. When the difference between the dynamic and static friction coefficients (DSFCs) is small, the system has a complete and clear limit cycle. When the dynamic friction coefficient is reduced, the difference between DSFCs increases, and the limit cycle of the system is deformed. The friction system has more abundant dynamic vibration characteristics under an alternating normal force than a constant normal force. The vibration state of the system presents a single-cycle stick–slip vibration when the alternating normal force excites the multi-order harmonic response of the friction system, and the excitation frequency of the alternating normal force is the same as the main response frequency of the system with the highest energy or the low-order even-order main frequency. In contrast, the system exhibits various vibration modes when the excitation frequency of the alternating normal force is dissimilar to the main frequency of the system's highest energy response or is consistent with the odd-order main frequency. In addition, increasing the difference between DSFCs or using very high excitation frequencies and excitation amplitudes increases the likelihood of the system entering a chaotic vibration state.

A method for estimating the coefficient of friction at high temperatures up to 220°C in the thermal chamber of a bursting machine has been developed. It is shown that the coefficient of kinetic friction with a change in temperature from 25°C to 220°C varies from 0.04 to 0.1. In the developed method, the coefficient of static friction and the coefficient of kinetic friction are determined. The coefficient of static friction at a temperature of 25°C to 220°C varies from 0.06 to 0.13.

The paper presents the results of coefficient of static and kinetic friction depending on the load. During the study, the sample in the form of a pin with expanded graphite, mounted in a holder, was forcibly pressed the Fn to the steel countersample. The device on which the tests were carried out research allows sliding friction in reciprocating motion. It has been found that there is a noticeable difference between the coefficient of static friction and kinetic for both fixed and different pressures. In the field of applied pressure, there were no significant their impact on the coefficient of friction; applied force was not sufficiently high which may have contributed to this state. The study had a distinctive character.

The study of Leymus chinensis seed cleaning has been hindered by the lack of accurate discrete-element contact parameters for seed–straw interactions, thereby limiting, to some extent, the optimization of cleaning equipment. To address this issue, the present study analyzed a mixture of L. chinensis seeds and straw, and determined their fundamental physical and contact parameters via laboratory experiments. The Hertz–Mindlin (no slip) discrete element simulation model was employed to calibrate the parameters of the seed–straw mixture. A Plackett–Burman test was used to identify key factors significantly affecting the repose angle, including the seed–seed static friction coefficient and the seed–straw static and dynamic friction coefficients. These factors’ optimal ranges were further refined using steepest ascent experiments. A Box–Behnken design was used to optimize contact parameters, resulting in the following values: a seed–seed static friction coefficient of 0.709, a seed–straw static friction coefficient of 0.281, and a seed–straw dynamic friction coefficient of 0.085. Validation experiments demonstrated an error of less than 2.14%, confirming the reliability of the calibrated parameters. This study offers a theoretical foundation for discrete element simulations in L. chinensis seed cleaning applications.

Justification of the soil dem-model parameters for predicting the plow body resistance forces during plowing

This paper aims to improve the accuracy of the finite element analysis for the preform shape transformation according to the difference in the type of carbon fiber and friction force. The carbon fiber type (Toray T700, Zoltek), the fiber direction (0°/90°, ± 45°), and the frictional force were measured, which occurred between the fiber and the mold and between the fiber and the fiber. A comparative experiment was conducted through actual preform production by designating it as a variable for element analysis. The analysis was conducted according to the difference between the static friction coefficient and the dynamic friction coefficient in the molding analysis. Within the range of the Coulomb equation, the shear angle changing was compared with the actual preform shape to show similar results.

The coefficient of dynamic friction is often the controlling factor for solids conveying, pressure generation, and thermal decomposition of a resin in the feed section of a single‐screw plasticating extruder. The coefficients of friction are, however, very poorly understood, and the interpretation of the measurements are complicated by the dissipation of frictional energy at the sliding interface. A new instrument was recently built to help understand dynamic friction, and a numerical technique was developed to estimate the interface temperture. Coefficients of dynamic friction for several polyethylene resins are presented in this paper as a function of the surface temperature, pressure, and velocity. The relationship of the data with respect to the extrusion process is discussed.

Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution

ABSTRACTA FREE from DAMage friction connection is a connection where at the level of the lower beam flange is realized a friction damper by means of angles and plates pre‐stressed with high strength bolts. In these connections, the bending moment transferred to the column through the connection can be easily calibrated controlling the friction coefficient of the material employed to realize the friction damper and the value of the preload applied by means of high strength bolts. Therefore, in order to guarantee a correct behaviour of the friction connection both under Serviceability and Ultimate limit states (SLS, ULS), it is necessary to know with a sufficient level of accuracy the value of the static and dynamic friction coefficients and the value of the bolts' preloading.The aim of this paper is to provide a characterization of the friction coefficient of some materials appropriate for the application in the FREEDAM connections. To this scope, the results of an experimental programme consisting of 51 specimens, developed following the guidelines provided by EN 1090‐2 and EN 15129 (anti‐seismic devices), are presented. The experimental work is mainly devoted to the evaluation of the response of three different materials, applied as a coating on steel plates by means of thermal spray, investigating the influence of the bolt preloading level and of the washer's typology on their response. In particular, the possible influence of the pressure applied to the analysed interfaces is considered carrying out tests at variable values of the pre‐loading (between 40% and 100% of the standard pre‐loading value), and the influence of the washers' configuration is evaluated performing tests with flat washers and different configurations of series and parallel of Belleville disc springs. The influence of the washer's typology is considered in order to assess the effectiveness of the Belleville disc springs in the reduction of the bolts' loosening during a cyclic test.The main results of the experimental activity are represented in terms of static and kinetic friction coefficients evaluating also the bolt's force degradation, the development of heat during the test and the degradation of the effective damping. Some of the tests are realized with the aim to determine the random variability of the friction coefficient of the analysed interfaces and to provide values of the friction coefficient to be used in design for ULS and SLS.

A critical component in the wet tree development of deep water fields is the umbilical riser. Failure of the umbilical can result in partial or total loss of production, with resulting severe economic consequences. In many recent deep water projects, steel tubes for control and injection lines have become the preferred solution over traditional thermoplastic hose based designs. The fatigue design of these steel tube umbilicals (STUs) with accurate modeling of the steel tube interaction friction stresses is critical for any development using STUs. Simplified conservative approaches where the nonlinearities of the tube friction are ignored have frequently been used for STU fatigue design. This paper presents the theoretical basis for an enhanced non-linear time domain fatigue model for deepwater STUs. In developing this model, the authors have proposed an analytical fatigue design approach that accounts for the non-linearity of tube friction using slip theory combined with time-domain global and local analytical methods. A worked example is presented, where an ExxonMobil West of Africa field development STU has been analyzed using both the enhanced nonlinear time domain fatigue model and an alternative simplified model. The enhanced non-linear time domain fatigue model, has been used to evaluate the influence on fatigue life of several parameters, including friction, water depth and STU cross-section geometry, some of which become increasingly important in deeper water. The merits of the enhanced nonlinear fatigue model over the simplified model are critically evaluated, as are the lessons learned on the importance of critical design parameters and analysis assumptions affecting STU fatigue life. Conclusions are also presented on the benefit of further enhancement of the analytical approaches commonly used to design for umbilical fatigue. STU Fatigue Models In dynamic floating production steel tube umbilical risers, cyclic stresses occur in the metal tubes within the umbilical due to the wave-induced dynamic response of the umbilical and the motion of the floating vessel. These dynamic stresses are a combination of the axial and bending cyclic stress variations in tubes, induced by the global motions of the umbilical, combined with the effect of friction between tubes and surrounding structure in the umbilical cross section. Cross-section models and fatigue analysis techniques used to design these tubes frequently make simplified assumptions about how to calculate and combine cyclic stresses in these tubes to determine fatigue life. Such assumptions typically include :regular wave analysis of global riser response to fatigue scatter diagram seastates compared to alternative irregular wave time or frequency domain approachesnon-phase-consistent calculation of tube fatigue stresses under global umbilical bending and tensionfatigue calculations based on simplified tube friction/slip or no-friction modelapproximate cycle counting based on a regular wave response and single dominant frequency instead of an alternative which captures actual seastate frequency content and multiple amplitude cycles, for example a rainflow counting irregular wave approachin-plane fatigue analysis based on analysis of umbilical response to global motions in the same planeprediction of friction stresses using static friction coefficient rather than separate static and dynamic friction coefficients.

Making use of appliance MXD-02, Labthink, China, coefficient of friction at rest with sliding tendency (static coefficient of friction) and coefficient of friction at sliding (dynamic coefficient of friction) for fabrics made of 100% wool as well as woolen fabrics with admixtures of polyamide and polyester were determined. The influence of the magnitude of normal pressure on the coefficients of friction was studied. Experiments taking into consideration the following different directions of relative fabric movement were performed: warp by warp threads, and warp by weft. An analysis of the obtained results was made.

This study investigated the concrete-to-concrete friction behavior under dynamic cyclic loading at different loading rates, vertical loads, and surface roughness. The present work answers essential questions about the dynamic behavior of concrete-to-concrete friction since most of the available literature deals with static or quasi-static loading conditions. To this end, an experimental program was devised by casting 96 concrete blocks. A total of 48 dynamic push–pull tests were performed on each pair of blocks (mobile top block and fixed bottom block). Test variables included three types of surface roughness, four different loading rates, and two normal stresses. Performance measures included the static and dynamic friction forces coefficients of static and kinetic friction in addition to effective stiffness and effective damping. Moreover, the test results showed that the static and kinetic friction coefficients, effective stiffness, and effective damping decrease with increasing loading rates. Moreover, increasing the normal stress increases the friction force, thus increasing the effective stiffness and reducing the effective damping surface for all surface roughness types. The effects of test variables on the hysteresis behavior were also investigated.

<div class="htmlview paragraph">This paper investigates the coefficients of static and kinetic friction between the hardwood flooring of a used semi-trailer and the bottom surfaces of both an old and a new hardwood pallet, with varying cargo weights, at varying locations in the trailer. A total of 22 tests were performed with the pallets moving parallel to the longitudinal axis of the trailer and perpendicular to this axis.</div> <div class="htmlview paragraph">The results show that the variations in load did not statistically affect the friction coefficients. The older pallet had statistically smaller static and kinetic friction coefficients (s=0.32, k=0.26) than that of the newer pallet (s=0.35, k=0.29). The static friction coefficients were statistically dependent upon whether the test took place at the front (0.36), middle (0.34) or rear (0.32) of the semi-trailer. Kinetic coefficients behaved similarly: front (0.29), middle (0.27) or rear (0.26).</div>