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The floating-element (FE) principle, introduced nearly a century ago, remains one of the most versatile direct wall shear stress measurement methods. Yet, its intrinsic sources of systematic error, associated with the flow-exposed gap, off-axis load sensitivity, and calibration, have thus far limited its widespread application. In combination with the lack of standard designs and testing procedures, measurement reliability still hinges heavily on individual judgement and expertise. This paper presents a framework to curb these limitations, whereby the design and operation of a FE balance are leveraged by an analytical model that attempts to capture the behaviour and predict the relative contribution of the systematic sources of error. The design is based on a parallel-shift linkage and a zero-displacement force-feedback system. The FE has a surface area of 200 × 200 mm, and measurement sensitivity is adjustable depending on the surface condition and the Reynolds number. It is thus suitable for application in a wide range of low-speed, boundary-layer wind tunnels, small or large scale. Measurements of the skin friction coefficient over a smooth wall show a remarkable agreement with oil-film interferometry, especially for Re θ > 1.3 × 10 4 . The discrepancy relative to the empirical Coles–Fernholz relation ( κ = 0.39 and C = 4.352 ) is within 0.5 %, and the level of uncertainty is below 1 % for a confidence interval of 95 %. [ABSTRACT FROM AUTHOR]

Two stress regime‐dependent creep constitutive equations, that is, two‐regime Norton (2RN) model and Sin‐hyperbolic (Sinh) model, are adopted to numerically investigate the influence of constitutive equation and friction on the measurement of steady‐state creep strain rate based on five types small specimens. Results show that the influence of constitutive equation and friction depends strongly on the type of small specimen. A measurement error of more than 19 times may occur if the 2RN model is used, while the use of the Sinh model is able to generate a consistently lower measurement error. Of all small specimens, the cantilever beam testing (CaBT) is revealed to exhibit the best comprehensive performance in terms of measurement error and response to friction. To validate numerical findings, creep tests on the CaBT specimen of P91 steel are conducted at 600°C. Finally, recommendations on the choice of small specimen for obtaining creep strain rate are made. Highlights: Sinh model is better in describing stress regime‐dependent creep behavior.Influence of friction is strongly dependent on the type of small specimen.Small cantilever beam testing has the best comprehensive performance.Attention should be paid to selecting the constitutive equation and specimen type. [ABSTRACT FROM AUTHOR]

The complex dynamics of tribofilm formation on boundary-lubricated steel surfaces were investigated in real time by combining in situ measurements of the temporal variation of the coefficient of friction and contact voltage. Sliding experiments were performed with various blends consisting of base oil, zinc dialkyl dithiophosphate (ZDDP) additive, and two different dispersants at an elevated oil temperature for a wide range of normal load and fixed sliding speed. The evolution of the transient and steady-state coefficient of friction, contact voltage, and critical sliding distance (time) for stable tribofilm formation were used to evaluate the tribological performance of the tribofilms. The blend composition affected the load dependence of the critical sliding distance for stable tribofilm formation. Tribofilm friction was influenced by competing effects between the additive and the dispersants. Among various formulations examined, the tribofilm with the best friction characteristics was found to be the blend consisting of base oil, a small amount of ZDDP, and a bis-succinimide dispersant treated with ethylene carbonate. The results of this study demonstrate the effectiveness of the present experimental approach to track the formation and removal of protective tribofilms under boundary lubrication conditions in real time. [ABSTRACT FROM AUTHOR]

Ultraprecision positioning to better than nanometer in accuracy is of a great interest in various applications. The paper reports on recent pre-rolling experimental measurements of the moment friction during the phase of pre-rolling, i.e., at the start and at the finish. To discuss the case properly, the experiments are set for pure rolling without any external effect nor any slippage at the contact patch. The pendulum was designed to secure pure pre-rolling without slippage. Beside the fact that the experiments have shown extremely low values of rolling coefficients in the order of (× 10–7), it is possible to establish the appearance of real hysteresis curves showing the dependance of the friction moment on the displacement. The sensitivity of the measurement of rolling friction moments is in the order of 10–10 Nm, the measurement error does not exceed 10% with 125 nm minimum displacement of the ball peel adhesion density evaluated as 2.2 × 10–4 J/m2. [ABSTRACT FROM AUTHOR]

In ultra-precision equipment, nanoscale rough surface contact widely exists, and the adhesion between interfaces has become a critical component in researching the friction phenomenon. However, adhesion is not considered in traditional friction models. In this study, the adhesion model between fractal contact surfaces is established by proposing the critical truncated area of adhesion, a c u t ′ , and a new friction model considering adhesion is established by combining with the traditional model. The present model shows that adhesion increases as the fractal dimension (D) increases or the surface roughness coefficient (G) decreases when contact occurs between nanoscale rough surfaces. The friction coefficient calculated based on the new model is larger than that of the traditional model, especially when G is small. In addition, a static friction force measurement system was built by using the reticle and vacuum chuck in the lithography system. The experimental system measured the maximum static friction between the chuck and the reticle under different normal loads, and the experimental results show that the new model proposed in this paper is more accurate in characterizing the static friction of nanoscale rough surfaces. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the repeatability of locked-wheel skid trailer (LWST) and sideway-force coefficient routine investigation machine (SCRIM) measurements and investigate the influences of test speed and test temperature on the friction measurements. This study selected 14 test sections from the NCAT Test Track with different surface texture and friction characteristics. The LWST and SCRIM tests were conducted at different test speeds and test times (or temperatures) on the two consecutive days. The repeatability analysis indicates that both LWST skid number (SN) and SCRIM reading (SR) measurements were most repeatable at a test speed of 50 mph and tangent section. The SN measured in the late afternoon and SR measured at noon were more repeatable than those measured at other times. This study recommended an acceptable precision of friction measurement be within 2.5 SN or 3 SR units. In addition, this study concluded that the SN and SR had a good linear correlation. The statistical regression analysis demonstrates that test speed, air temperature, pavement mean profile depth, and pavement type were significant variables affecting asphalt pavement friction. The developed regression models for SN and SR were helpful to correct the friction measurements to a reference speed or temperature. [ABSTRACT FROM AUTHOR]

Objective: The mechanical condition and tactile evaluation of skin are essential for the development of skin care products. Most of the existing commercial instruments and studies aim to evaluate the skin surface by pressing it for hardness or by using imaging sensors, but there have been few instrumental measurements employing rubbing motion. Here, we have developed a sensor specialized for tactile sensation and the contact phenomenon during skin rubbing. Methods: The developed sensor has three features: It can measure body parts including cheeks and arms, automate the rubbing motion of the probe and measure vibration and friction simultaneously. It is hand‐held, with metal probes that rub the skin surface while rotating under a motor drive; it has an accelerometer and a force sensor beneath the probe measuring vibration and friction forces. To evaluate the validity of the sensor's measurements, artificial skin models were measured using the developed sensor and commercially available sensors and the results were compared. The relationship between the sensor output, surface roughness measurement and sensory evaluation was also investigated. Additionally, we evaluated the inter‐rater reliability when measuring actual skin. Results: The measurements of five artificial skin models with different surface shapes showed a high correlation (r = 0.99) between the vibration intensity values evaluated by the developed sensor and those measured by a tri‐axial acceleration sensor attached to a fingernail. The correlation coefficient between the vibration intensity values and surface roughness was r = 0.91, and the correlation with the sensory evaluation score of roughness was r = 0.99. The friction coefficients measured by the developed sensor and the force plate had r = 0.93, based on measurements of five artificial skin models with different friction conditions. The inter‐rater correlation coefficients between the three participants of the developed sensor were as high as 0.92 and 0.94 for the vibration and friction measurements respectively. Conclusion: The vibration intensities and friction coefficients from the sensor were highly correlated with those of the conventional sensor. The inter‐rater reliability was also high. The developed sensor can be useful for tactile evaluation in skin‐care product development. [ABSTRACT FROM AUTHOR]

If road friction coefficients can be measured in real time, the performance of advanced driver-assistance systems (ADASs), such as anti-lock braking systems (ABS) and automatic braking systems, can be improved. Generally, ADASs use information obtained from wheel speed sensors and acceleration sensors. However, it is difficult to accurately measure the road friction coefficients using these sensors. Therefore, many studies have been conducted to measure road friction coefficients using strains at the inside surface of a tire tread, which are the only places that ground road surfaces. However, sensors installed on the inside surface of a tire tread, where significant deformation occurs locally because of protrusions on road surfaces, can easily be peeled off or damaged. Therefore, this study investigated a method for measuring road friction coefficients using strains on the sidewalls of the tires. If the sidewalls of tires are used instead, stable measurements can be expected because the sidewalls of tires are harder to deform locally compared with the inside surface of a tire tread. First, using an experimental device that simulates the complete slip state of a tire, we measured the relationship between the triaxial direction loads acting on the ground surface of the tire and the strain induced on the sidewall of the tire. Subsequently, we established experimental formulas that express the relationship and devised methods for measuring the road friction coefficient. It was confirmed that the friction coefficients of multiple friction surfaces, even under triaxial direction loads, could be accurately measured using the proposed method. In conclusion, the proposed method was confirmed to be able to measure the load acting on a tire and friction coefficient of the tire grounding surface at low speeds and under full-slip conditions. • We proposed a measurement method for road friction coefficient by using strains on a tire sidewall. • The strain on the sidewall is confirmed to vary almost linearly concerning three-axis loads acting on a tire. • Three-axis loads and road friction coefficient can be obtained from sidewall-strain at three different tire rotation angles. [ABSTRACT FROM AUTHOR]

[Display omitted] Contact angle and sliding angle measurements are widely used to characterize superhydrophobic surfaces because of the simplicity and accessibility of the technique. We hypothesize that dynamic friction measurements, with increasing pre-loads, between a water drop and a superhydrophobic surface is more accurate because this technique is less influenced by local surface inhomogeneities and temporal surface changes. A water drop, held by a ring probe which is connected to a dual-axis force sensor, is sheared against a superhydrophobic surface while maintaining a constant preload. From this force-based technique, static and kinetic friction forces measurements are used to characterize the wetting properties of the superhydrophobic surfaces. Furthermore, by applying increased pre-loads to the water drop while shearing, the critical load at which the drop transitions from the Cassie-Baxter to Wenzel state is also measured. The force-based technique predicts sliding angles with reduced standard deviations (between 56 and 64%) compared to conventional optical-based measurements. Kinetic friction force measurements show a higher accuracy (between 35 and 80%) compared to static friction force measurements in characterizing the wetting properties of superhydrophobic surfaces. The critical loads for the Cassie-Baxter to Wenzel state transition allows for stability characterization between seemingly similar superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

Detecting mechanical fatigue of metallic components is always a challenge in industries. In this work, we proposed to monitor the low-cycle fatigue of a 6061 aluminum alloy based on internal friction (IF) measurement, which is realized by a quantitative electromechanical impedance (Q-EMI) method using a small piezoelectric wafer bonded on the specimen. Large stress amplitude (230 MPa) was employed in the testing; thus, the fatigue life can always be below 105 cycles. It was found that except for the initial testing stage, the IF always increases steadily with the increasing fatigue cycles. Before the fatigue failure, the IF can reach 2.5–3.4 times the initial value, which is thought to be caused by the micro-cracks forming and growing. In comparison, the resonance frequency of the specimen just drops by about 2% compared with the initial value. Finally, a fatigue criterion based on IF measurement is suggested for aluminum alloys and it can also be extended to other metals. [ABSTRACT FROM AUTHOR]

A new friction measurement system was developed to investigate piston ring friction in detail based on the floating liner method. Unlike the conventional floating liner experiments, an extended piston was adopted for the conventional crank-piston system to minimize friction from a piston lateral motion. The interaction between piston skirt and liner caused by piston thrust force can be excluded with the new measurement system. Therefore, a parametric study for the only piston ring friction can be possible. In this study, the experiments were conducted under both motoring and firing conditions. The oil temperature was changed to observe the effect of the oil viscosity on the piston ring friction. The friction data showed good agreement with the Stribeck curve for the engine speed, in-cylinder pressure, and oil temperature changes. [ABSTRACT FROM AUTHOR]

The fuel tank of the advanced heavy launch vehicle is made of 2219 aluminum alloy with a thickness of 18 mm. Friction stir welding (FSW) technology is used to achieve the welding of the fuel tank. The increase of thickness directly affects welding temperature field, which ultimately affects the welding quality. At present, the temperature distribution law of FSW thick 2219 aluminum alloy is still blank, which is a bottleneck to ensure high welding quality. In this paper, a temperature field measurement and analyses system based on K-type thermocouple and LabVIEW for friction stir welding is developed first. Then experiments are carried out and the temperature data in welding and thickness directions are measured and the temperature law is analyzed. The comparison of temperature distribution of FSW thick and thin 2219 aluminum alloy is made, and the influence of welding thickness on the temperature field is studied. Based on temperature field, weld strength and microstructure, the influence of different process parameters on welding quality is explored. The research results lay a foundation for optimization of the welding process parameters and realization of constant temperature control of FSW. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drops characterization of air and CO2 through smooth and rough micro tubes are investigated experimentally. Nine smooth stainless steel micro tubes with inner diameters of 50, 80, 100, 250, 300, 400, 800, 900, and 950 μm, and six rough micro tubes with inner surface roughness are tested in the present study to characterize the friction factors, the pressure drops, the heat transfer Nusselt numbers, and evaluating the surface temperature on the wall of the micro tubes. It was observed that heat transfer Nusselt numbers on smooth micro tubes fitted well with the conventional correlations of the Gnielinski' (Gnielinski in Int J Chem Eng 16:359–368, 1976) correlation, the Dittus-Boelter' (Dittus and Boelter in Univ Cali Berkeley Publ Eng 2(13):443–461, 1930) correlation, and the Petukhov' (Petukhov and Kirillov in Teploeenergetica 4(4):63–68, 1958) correlation. Moreover, the test results of the friction factors were in a very good agreement of the Gnielinski' (Gnielinski in Int J Chem Eng 16:359–368, 1976) correlation, and Blasius' (Blasius in Forschg. Arb. Ing. -Wes, pp 131–137, 1913) equation in both laminar and turbulent regions, except for the micro tubes in the range of 50~100 μm. However, for the rough micro tubes, the test results scattered above the standard predictions of the Filonenko' (Filonenko in All Union Thermotechnical Institute, 1948) correlation. The test results showed that the heat transfer enhancement of the fabricated smooth surface micro tubes is better than that of the structural rough surface micro tubes. It was also observed that the transition regime occurred at Re of approximately 1,500~2,000 due to the increase in surface roughness. [ABSTRACT FROM AUTHOR]

Pavement skid resistance measurement is a fundamental component of roadway management and maintenance. Most traditional approaches rely on manual operations or heavy devices, which lead to a labor‐intensive, inefficient, and vulnerable testing environment. Precise laser scanning technology lays a solid foundation for effective and continuous pavement friction measurement. This paper proposed an automated pavement friction estimation model using 3D point cloud data and a deep neural network. The fine‐grained texture data of over 800 pavement sections with various anti‐skidding abilities were collected. The impact of the multi‐scale textures on pavement friction was separated and analyzed via two‐dimensional wavelet decomposition. A multi‐input fusion network with deep aggregation modules was designed to fuse the features of sub‐images generated by wavelet decomposition. The results show that the average prediction error is 0.0935, outperforming most state‐of‐the‐art models. The impact of different texture scales on friction estimation is then revealed. The proposed method provides a new tool for effective and large‐scale pavement friction evaluation. [ABSTRACT FROM AUTHOR]

To characterize liquid-solid friction using molecular dynamics simulations, Bocquet and Barrat (BB) [Phys. Rev. E 49, 3079–3092 (1994)] proposed to use the plateau value of a Green-Kubo (GK) integral of the friction force. The BB method is delicate to apply in finite-size simulations, where the GK integral vanishes at long times. Here, we derive an expression for the GK integral in finite-size systems, based on a Langevin description of a coarse-grained system effectively involving a certain thickness of liquid close to the wall. Fitting this expression to GK integrals obtained from simulations of a liquid slab provides the friction coefficient and the effective thickness of the coarse-grained system. We show that the coarse-grained system for a Lennard-Jones fluid between flat and smooth solid surfaces is 2–3 molecules thick, which provides a criterion for measuring the friction coefficient independently of confinement. As compared to nonequilibrium simulations, the new approach is more accurate and removes some ambiguities of nonequilibrium measurements. Overall, we hope that this new method can be used to characterize efficiently liquid-solid friction in a variety of systems of interest, e.g., for nanofluidic applications. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of materials can be described by a phenomenological relationship that binds strain to stress, by the complex modulus function: M (ω) , which represents the frequency response of the medium in which a transverse mechanical wave is propagated. From the experimental measurements of the internal friction obtained when varying the frequency of a transverse mechanical wave, the parameters that characterize the complex module are determined. The internal friction or loss tangent is bound to the dissipation of the specific mechanical energy. The non-equilibrium thermodynamics theory leads to a general description of irreversible phenomena such as relaxation and viscosity that can coexist in a material. Through the state variables introduced by Ciancio and Kluitenberg, and applying the fractional calculation due to a particular memory mechanism, a model of a viscoanelastic medium is obtained in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Mechanical products are becoming more diversified with the continuous development of precision processing and materials technologies. The friction force generated by the O-ring seal in a hydraulic cylinder was once considered redundant. However, its utilization has recently been proposed. The hardness of the O-ring and the inner diameter of its groove directly affect the normal pressure between the O-ring and the inner wall of the cylinder, thereby affecting the friction behavior. In order to explore this friction behavior, a strain-based friction force measurement system is developed in this study, and the steady-state and dynamic friction values under different working conditions are studied and discussed in depth. This research on the friction behavior in the cylinder provides a theoretical basis for more convenient design and utilization of the friction force generated between the O-ring and the inner wall of the cylinder. [ABSTRACT FROM AUTHOR]

Double-plane grinding plays an important role in the processing of high-precision flat parts. A double-side autonomous grinding (DSAG) method was proposed and investigated to improve the uniformity of the abrasive grains of the grinding disc on the surface of the workpiece by releasing the constraints on the workpiece. In DSAG, the main driving force of workpiece movement comes from the friction force generated by the relative movement of the upper and lower grinding pads. Experiment of dynamic friction coefficient measurement between workpiece and grinding plates is carried out to take into account the differences of dynamic friction factor under different speed and force conditions. A mathematical model of the relative motion trajectory between the particles on the upper and lower plate and the workpiece is established based on kinematics and dynamics theories. Then, the effects of the linkage mode of the lower plate, grinding pressure, and speed ratio of the upper and lower plates on the frequency and direction uniformity of the grinding trajectory of the abrasive particles on the workpiece surface are simulated and analyzed. Aiming at the trajectory uniformity, the grinding parameters are optimized through orthogonal experiment and verified on the independently developed experimental platform. The flatness and roughness of the workpiece are detected. The results show that the DSAG performs well in obtaining the better roughness and flatness of the workpiece surface than that of planetary grinding. [ABSTRACT FROM AUTHOR]

We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials. [ABSTRACT FROM AUTHOR]

The paper reveal some aspects and proposals for improving the statistical learn of the measured friction coefficient values, research on the effect of climatic parameters(humidity, temperature of the contact surface) on the friction coefficient. [ABSTRACT FROM AUTHOR]

This paper presents a general method to extract a surface pressure field from a skin friction field in complex flows as an inverse problem, focusing on its application to global luminescent oil-film (GLOF) skin friction measurements. The main technical aspects of this method are described, including the basic equation relating surface pressure to skin friction, variational method, numerical algorithm, and approximate method with the constant boundary enstrophy flux (BEF). The proposed method is evaluated through simulations in the Falkner–Skan flow and the flow over a 70°-delta wing to investigate the effects of the Lagrange multiplier, downsampling rate, noise level, and the value of a constant source term in the approximate method. Further, the approximate method is applied to the skin friction fields obtained by GLOF measurements in the flow over a 65°-delta wing and the square junction flow to obtain the normalized surface pressure fields. The proposed method provides a useful tool to obtain the high-resolution fields of both surface pressure and skin friction by GLOF measurements in complex flows (particularly at low speeds). The normalized surface pressure field extracted from the skin friction field in the square junction flow, where skin friction lines are superposed. [ABSTRACT FROM AUTHOR]

This study aims to minimize the friction forces in order to obtain the best feel and minimum effort in the command and control cable part of the shift systems. For this purpose, lubricant, amount of lubricant and application method were investigated. Theoretical and experimental approaches are discussed by examining the studies on this subject. In the presented study, friction measurements were taken in the laboratory environment by applying different amounts of lubrication on the wire exposed to friction, and the amount of lubrication required for minimum effort was determined experimentally. It was observed that the friction force was decreased about 60% with oil running compared to dry running. Studies show that friction forces increase in excessive lubrication as well as in cases where there is no lubrication and little lubrication. The optimum amount of oil was obtained to be 0.1 g. It has developed guidelines for determining the friction factor for those who design shift cable and control cable used in different areas. [ABSTRACT FROM AUTHOR]

Concrete pavements comprise a considerable portion of pavements that are currently in use. This paper presents the results from a laboratory investigation on the effect of temperature and surface texture on the skid resistance of concrete pavements. Briquettes representative of concrete pavements were created and textured in the laboratory. Readings of skid resistance on laboratory specimens were obtained using a portable British pendulum tester. Water and liquid hand soaps were used as lubricants in an attempt to separate the adhesion and hysteresis components of friction. In order to simulate the wear and ageing of concrete pavements, several cycles of polishing were applied to the briquettes. Tests were conducted at five different temperatures. The results showed that there is a correlation between the magnitude of friction components and temperature. A normalisation procedure for friction measurements based on temperature was developed, which can be used to normalise measurements to a standard temperature of 293·15 K. [ABSTRACT FROM AUTHOR]

Mechanical testers have commonly been used to measure the frictional properties of socks. However, the friction values may be susceptible to the level of stretchiness of tested fabrics or human variability. Thus, the aim of this study was to propose a novel method that enables friction measurement of socks in a sock-wearing condition with less human variability effects. Five socks with different frictional properties were chosen. Three experimental ramp tests were performed with an artificial structure shaped like the foot-ankle complex (last) and a ramp tester to quantify the static coefficient of friction (COF) at the foot against sock, at the sock against an insole, and the foot wearing socks against the insole, respectively. The angle where the last slipped while the ramp surface was gradually inclined was used to compute the static COF values for each sock. The reliability was 0.99, and COF values ranged from 0.271 to 0.861 at the foot-sock interface, 0.342 to 0.639 at the sock-insole interface, and 0.310 to 0.614 in the third test. Socks with different frictional properties were successfully distinguished each other. Thus, the suggested protocol could be a reliable option for measuring the static COF values in the tension similar with it found in a sock-waring condition with reduced effects of human variability. [ABSTRACT FROM AUTHOR]

• Measuring terrain surface roughness. • Static tyre characterisation tests on different surfaces. • Tyre characteristics for different tread wear conditions. • Static tyre vs. rolling tyre testing. Tyre characterisation is not a trivial or inexpensive exercise thus it is important to obtain representative measurements during tyre characterisation tests. Different test methods exist and vary from laboratory tests to outdoor tests on different surfaces. Each of the test surfaces have different surface roughness and will result in different tyre characteristics. This study compares friction coefficient measurements on dry non-deformable surfaces for laboratory test surfaces and outdoor test tracks on the same agricultural tyre with large lugs at two inflation pressures and three tread wear conditions. The influence of surface roughness on friction coefficient is investigated. The macrotexture and microtexture of multiple surfaces are measured and compared. The importance of measuring the microtexture of the outdoor test surface is noted. Static/Non-rolling tyre tests in a laboratory are compared to Static/Non-rolling tyre tests as well as Dynamic/Rolling tyres tests at an outdoor test facility. Excellent correlation is found between the Static vs. Dynamic and laboratory vs. outdoor tests results when the laboratory tests are conducted on a surface representing the outdoor surface. [ABSTRACT FROM AUTHOR]

• An automated system is developed for ultrasonic fatigue and internal friction testing. • Fatigue damage can be effective monitored using system damping. • Internal frictions of damaged copper specimens are over 3 times of fresh specimens. • Yield stresses of damaged specimens are obviously lower than the fresh specimens. • A practical damage variable based on internal friction is proposed. Fatigue damage in metals and alloys would reduce their strength and should be identified as early as possible. However, fatigue damage is usually difficult to detect even using advanced nondestructive testing method. Here we developed an automated damping monitoring system for ultrasonic fatigue testing based on a quantitative electromechanical impedance method. During fatigue testing, the damping and resonance frequency of the Piezo actuator/horn/specimen system is measured regularly using an impedance analyzer. When the system damping suddenly increases, it implies that fatigue damage may be generated in the specimen. Separate impedance measurements on the copper specimen in a vacuum chamber show that the internal frictions of damaged specimens are much larger than the fresh specimens. Subsequent X-Ray micro-CT scanning shows that microcracks up to 0.8 mm in length were generated in those fatigued specimens with increased internal frictions. Further tensile testing shows that compared to the fresh specimen, specimens with fatigue damage exhibits reduced tensile strength and elongation at break. Furthermore, the larger the internal friction, the smaller the remaining yield strength and a nearly linear relationship between them holds. Finally, a damage variable D Q is defined based on internal friction which can act as a measure of fatigue damage in metals. [ABSTRACT FROM AUTHOR]

Health monitoring of robot manipulators is crucial for their long-term reliable operation. Since the feedback data provided by the controller of robot manipulators are most often from the actuator's side, health monitoring of rotate-vector (RV) reducers housed on the output of actuators is challenging. To avoid the increased cost and complexity of adding sensors to robot manipulators, this paper investigates the long-term health monitoring of RV reducers, using only feedback data from the controller. A generic model for RV reducers is first developed to characterize their nonlinear behaviours and performance degradation. The model is then extended to robot manipulators to analyse their dynamic response under the time-varying excitation generated by healthy and faulty RV reducers. Next, a new motor-torque-based residual together with an adaptive threshold is proposed for the health monitoring of RV reducers, taking payload, measurement noise and friction disturbance into account. The proposed threshold depends only on the variance of measurement noise, and is therefore easy to implement in practice. Extensive simulations and experiments are conducted on two robot manipulators to validate the proposed model, residual and threshold. The results show that the proposed model is able to account for the effects of faults and nonlinear behaviours of RV reducers on the motor performance. The proposed residual with an adaptive threshold is sensitive to faults in RV reducers and robust to friction disturbance and time-varying payload, and it outperforms conventional momentum-based residuals. • RV reducers generic model with nonlinear behaviour & performance degradation • Manipulator dynamics including excitations caused by normal & faulty RV reducers • Torque-based residual w. adaptive threshold for RV reducers health monitoring [ABSTRACT FROM AUTHOR]

Friction on snow surface, especially in relation to winter sports, is difficult to measure directly because of its complexity and variability. In this work, a wearable snow friction measurement device for cross-country skiing was designed and constructed to calculate the friction force and monitor movement signals through the logical placement of three-component sensors and connection parts on skis. Both theoretical simulation and experiments have been conducted, which proved that the deformation of cross-country skis outputs few abnormal force signals during applying large load. Through the static validation in the laboratory, the output accuracy and stability of the device were confirmed. To verify the availability of the device, measurements on a skiing treadmill were conducted, and the results showed that the device can work well and distinguish the different friction behaviors of various waxes. More important, through the ski field test, the device was able to monitor the technical actions and evaluate the frictional coefficient of different ski waxes on snow surface comprehensively, which provides a scientific reference for athletes and coaches in routine training. [ABSTRACT FROM AUTHOR]

Geophysical flows, characterized by diverse particle-size mixtures, pose challenges for hazard risk assessment due to their anomalous fluidity and rheological behavior. This study investigates the rheological behavior of dry granular mixture flow in a flume setup using particle image velocimetry and triaxial force measurement. Key findings include a power-law relationship between the inertia value and the internal friction coefficient, affirming the applicability of the monodisperse friction model in mixed particle-size scenarios. A strong positive correlation is identified between the effective basal friction coefficient and internal friction coefficient, suggesting an influence of internal frictional properties on basal friction for granular flows on a flat surface. Additionally, positive correlations are observed between normalized stress fluctuation and both internal friction coefficient and inertia number, linking basal stress fluctuations to variations in internal frictional characteristics. These insights enhance our understanding of granular material dynamics, holding potential implications for geological disaster risk assessment. [Display omitted] • Experimental verification the μ (I) rheological law of dry granular mixtures. • The relationship between the effective basal friction coefficient and internal friction coefficient is analyzed. • The internal friction between particles is emphasized as a controlling factor for basal stress fluctuation. [ABSTRACT FROM AUTHOR]

High static friction between functional surfaces poses a serious challenge for industrial valves, particularly for those used in the advanced ultra-supercritical power generation industry that requires raising the steam temperature to 760 °C. In this context, we established the static friction measurement methodology by evaluating the effects of temperature, loading time, and number of static cycles on the evolution of the static coefficient of friction up to 800 °C. This methodology was applied on a nickel-based solid lubricant coating that appears an attractive candidate to withstand very harsh environments. The tribological behavior is explained in terms of a competition of three mechanisms governing the static friction processes: micro-welding or sintering of asperities, creep of asperities, and formation of lubricious oxides. • Establishment of the static friction measurement methodology to simulate valve applications. • First time to systematically report the evolution of static COF with number of static test cycles, time and temperature. • Three main mechanisms governing the static friction processes. [ABSTRACT FROM AUTHOR]

[Display omitted] Popular deep eutectic solvents (DESs) typically lack amphiphilic molecules and ions and therefore do not have the useful self-assembled nanostructures prevalent in many ionic liquids. We hypothesise that nanostructure in DESs can be induced via an amphiphilic hydrogen bond donor (HBD), and that nanostructure becomes better defined with HBD chain length. The structure of DESs formed from choline chloride mixed with either butyric acid (ChCl/BuOOH) or hexanoic acid (ChCl/HeOOH) in a 1:4 M ratio were studied using atomic force microscopy (AFM) imaging, force curves, and friction measurements combined with bulk rheology. DESs formed with both the C 4 and C 6 acids are nanostructured. As the length of the acid group is increased from C 4 to C 6 , AFM images reveal the nanostructure becomes larger and better defined due to the longer acid chain, and AFM force curves show the interfacial nanostructure extends further from the surface. Self-assembled nanostructure in these systems is a consequence of choline cations, chloride anions, and acid alcohol groups clustering together due to electrostatic attractions and hydrogen bonding to form polar domains. Acid alkyl chains are solvophobically excluded from the polar domains and aggregate into apolar domains. [ABSTRACT FROM AUTHOR]

Triboscopy focuses on the analysis of the temporal evolution of a tribological system, combining local and time-resolved information, most commonly the evolution of friction. In this work, this technique is applied on measurements, which were carried out with a custom-built ultra-high vacuum tribometer in ball-on-disc configuration. Based on these experiments, an extended classification to distinguish different triboscopic features is suggested, depending on the persistence in both track position and time: Uniform, Global, Local, and Sporadic. Further, a filter technique for quantifying triboscopic data regarding this classification is introduced. The new and improved triboscopic techniques are applied to various dry friction measurements of hydrogen-free carbon coatings under varying humidity and pressure. The resulting specific triboscopic features are correlated to wear phenomena, such as counter body coating abrasion, inhomogeneities in the wear track, non-uniform track wear, stick-slip and debris in the contact area, demonstrating the increased analysis and monitoring capabilities when compared to conventional friction curves and wear track images. [ABSTRACT FROM AUTHOR]

This study examines surface roughness measurements via piezo acoustic disks and appropriate signal processing. Surface roughness is one characteristic of surface texture that can have various irregularities inherent to manufacturing methods. The surface roughness parameters and corresponding surface profiles are acquired by a stylus profilometer. Simultaneously, elastic waves propagated along metal surfaces caused by the friction of a diamond tip are obtained in the form of raw sound via piezo acoustic disks. Frequency spectrum analysis showed apparent correlations between the traditionally obtained measurement parameters and the piezo acoustic measurement data. Thus, it is concluded that acoustic friction measurement shows promising results as a novel measurement method for the surface roughness states of certain materials. [ABSTRACT FROM AUTHOR]

Pavement texture is an important characteristic affecting user safety and satisfaction. Despite the large number of studies on pavement texture, there is still a need to further explore the models relating texture to tyre-pavement interaction. In this study, 29 cores were extracted from 15 different pavement surfaces and tested in the laboratory by Ohio department of transportation. The data set includes high density texture scans, modified mean texture depth (MMTD) values estimated using a modified sand patch test, and friction measurements acquired using dry and wet British pendulum (BP) testes. The texture scans were characterised using the mean profile depth (MPD) and wavelets energy. The correlation between MPD values and the coefficients of friction (COF) estimated using BP tests were not as strong as the correlation between the wavelet energies and the wet and dry COFs. Statistically significant models were derived using the wavelet energies to predict the MMTD (R2 = 0.94) as well as the dry and wet COF (R2Dry = 0.49/R2Wet = 0.60). The principle component analysis was used to decorrelate the wavelet energies and overcome the multicollinearity, while the least absolute shrinkage and selection operator was used to reduce the number of variables in the model. [ABSTRACT FROM AUTHOR]

Objective: Drawing parallels from rheotribology can be used to develop a robust instrumental protocol for non‐subjective characterization, product development and design of topical dosage forms with desired sensory attributes. However, instrumental characterization of cosmetic products can be influenced by the measurement protocol, thixotropy, flow anomalies like shear banding or wall slip and nature of the film formed on the skin surface. In this study, we evaluated the influence of above parameters on the instrumental sensory evaluation of 12 topical formulations of different galenic forms. Methods: Oscillatory strain sweep measurements (SAOS and LAOS) were performed to investigate the influence of frequency and wall slip on the material parameters. The textural attributes at different consumer touchpoints were evaluated by accounting time‐dependent simulation of viscoelastic flow. Further, the influence of film thickness and sample drying on the tactile properties of the topical formulations were studied on a non‐biological skin model using a sliding probe tribometer. Results: The study shows that the flow properties of the semi‐solid formulations depend on the timescale of the problem. A few formulations exhibited wall slip to varying degrees in the linear viscoelastic regime where the behaviour was found not to be characteristic of a particular topical dosage form. The material functions obtained from the Lissajous plots suggest that the non‐linear flow behaviour of different galenic forms is least influenced by the boundary conditions imposed by the measurement geometry. The results were statistically analysed using principal component analysis where the attributes used for discriminating skin creams during pick up and rub out are found to be closely associated with non‐linear rheology. The friction coefficient exhibited speed dependence where it formed different parametric group with rheological data depending on the lubrication regime. Conclusion: The study highlights that correlations are possible amongst rheological, tribological and instrumental textural analysis data, which can act an impetus for the development of models to predict attributes that drive perception at different consumer touchpoints. However, the choice of instrumental settings, anomalies associated with rheological measurements and friction dependence on a number of parameters can influence the model prediction. [ABSTRACT FROM AUTHOR]

Accurate tyre models are important to ensure valid and reliable simulation of vehicle behaviour. To this purpose, the Magic Formula (MF) became the de facto standard for vehicle dynamics simulations, despite requiring many empirically derived coefficients. This paper shows how the accuracy of a simple physical-based brush-type tyre model can be enhanced to simulate tyre behaviour that closely matches MF results. To do so, the real, highly complex rubber friction characteristics are incorporated into the brush model. The friction characteristics were obtained from friction measurements with a tread rubber block. The developed model is validated against experimental tyre data obtained on a flat-track test rig and the corresponding MF model. Results show that the inclusion of friction characteristics allow accurate simulation of longitudinal and lateral slip conditions over a wide range of normal loads with the simple brush model. At extreme loading scenarios, the simulation accuracy deteriorates because of the significant influence of highly nonlinear deformation behaviour of the tyre that is not accounted for in the brush model formulation. [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to investigate the microstructural anisotropy of Ti-6Al-4V samples fabricated by selective laser melting. Design/methodology/approach: Specimens are fabricated through a Renishaw AM400 selective laser melting machine. Three microstructures (as-built, 850°C annealed and 1,050°C annealed) and two building orientations, parallel (PA) and perpendicular (PE) to the building platform, are considered. Starting from in-depth microscopic observations and comprehensive electron backscattered diffraction imaging, the study addresses non-conventional techniques such as internal friction and electrical resistivity measurements to assess the anisotropy of the fabricated parts. Findings: Microscope observations highlight a fine texture with columnar grains parallel to the building direction in the as-built and 850°C annealed samples. Besides, coarse grains characterized the 1,050°C annealed specimens. Internal friction measurements pointed out the presence of internal stress while storage modulus analyses appear sensitive to texture. Electrical resistivity is resulted to be dependent on grain orientation. Originality/value: The work uses some novel characterization techniques to study the anisotropy and internal stresses of Ti-6Al-4V samples processed by selective laser melting. Mechanical spectroscopy results suitable in this kind of study, as it mimics the operating conditions of the material. [ABSTRACT FROM AUTHOR]

Continuous friction measurement equipment (CFME) has been demonstrated as an appropriate method to acquire friction around the critical slip ratio for the Pavement Friction Management (PFM) programme. Through a comprehensive field data collection on testing sites with different preventive maintenance (PM) treatments, this study assessed the influences of various characteristics on the CFME measurements. Field friction data were collected by a Grip Tester, one type of CFME, and the corresponding texture profiles were acquired by a high speed texture profiler. For each testing site, friction data was measured under nine operational conditions with three water film depths and three testing speeds. An adaptive signal processing technique, Hilbert-Huang Transformation (HHT), was applied to extract amplitude and instantaneous frequency of profiles as the texture parameters. Subsequently, the HHT texture parameters, along with pavement conditions and testing operational characteristics, were deployed as the influencing factors for the development of statistical friction model. The results reveal that the CFME measurements are most sensitive to the change of water film depth and pavement surface texture properties. This study is anticipated to gain a better understanding of the characteristics of CFME measurements and implement CMFE to support PFM programme effectively. [ABSTRACT FROM AUTHOR]

To study the friction of cell monolayers avoiding damage due to stress concentration, cells can be cultured on fibrin gels, which have a structure and viscoelasticity similar to that of the extracellular matrix. In the present research, we studied different gel compositions and surface coatings in order to identify the best conditions to measure friction in vitro. We examined the adhesion and growth behavior of mesothelial cell line MET-5A on fibrin gels with different fibrinogen concentrations (15, 20, and 25 mg/mL) and with different adhesion coatings (5 μg/mL fibronectin, 10 μg/mL fibronectin, or 10 μg/mL fibronectin + 10 μg/mL collagen). We also investigated whether different substrates influenced the coefficient of friction and the ability of cells to stick to the gel during sliding. Finally, we studied the degradation rates of gels with and without cells. All substrates tested provided a suitable environment for the adherence and proliferation of mesothelial cells, and friction measurements did not cause significant cell damage or detachment. However, in gels with a lower fibrinogen concentration, cell viability was higher and cell detachment after friction measurement was lower. Fibrinolysis was negligible in all the substrates tested. [ABSTRACT FROM AUTHOR]

Ungrooved stone mastic asphalt (SMA) provides an alternate to grooved dense graded asphalt (DGA) as a runway surface. To be verified as suitable in this application, SMA must provide similar or better asphalt surface performance, compared to DGA, and must achieve regulated aircraft skid resistance requirements. A pilot runway resurfacing project provided the basis for this laboratory and field comparison of ungrooved SMA to grooved DGA as a runway surface. Laboratory performance testing demonstrated adequate and comparable deformation, fatigue and moisture resistance of SMA, while the quality assurance records indicated acceptable asphalt production. The construction records indicated that SMA had a lower air void content in the compacted layer, which can only provide a more durable surface. Most importantly, the aircraft skid resistance of SMA exceeded that of grooved DGA, based on surface macro-texture and wet friction measurements. It was concluded that SMA performed equal to or better than DGA, particularly with regard to surface texture and wet friction and it is recommended that airports consider ungrooved SMA as a runway surface in the future. The demonstration project should also be monitored in the future, to verify the long-term performance of SMA, compared to the DGA. [ABSTRACT FROM AUTHOR]

The aim of the work was to identify the physical parameters relevant to different perceptual attributes by comparing a range of commercial skin creams with various compositions. We investigated the possibility of predicting sensory properties using rheological and tribological methods. Rheological evaluations of the skin creams were carried out using continuous shear, creep, creep recovery, rheodestruction and recovery measurements, oscillatory strain sweep measurements [small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS)], and frequency sweep measurements. Friction measurements were performed on the nonbiological skin model to investigate how surface properties are influenced by the application of different topical formulations. Furthermore, the formulations were characterized by a broad range of instrumental texture measurements. In vivo sensory analysis based on the spectrum descriptive analysis method was performed to discriminate the skin creams during pickup, rub out, and after feel. Using principal component analysis meaningful correlation relating rheotribological properties and sensory attributes during the entire process of product application was carried out. Rheological parameters deduced from the nonlinear regime were found to be important parameters affecting the frictional response of skin creams. In addition, friction data were correlated with slipperiness and stickiness—subjective attributes used for the sensory evaluation of after feel. Furthermore, a number of key textural parameters and sensorial data showed good correlation with results obtained from linear and nonlinear rheological measurement, indicating rheological analysis can be sufficiently used as a precise and valid tool for sensorial mapping of topical formulations. Our study further suggests that objective evaluation based on the flow curve, oscillatory strain sweep (SAOS and LAOS), and friction measurements can be used for sensorial screening of large number of prototype formulations, which otherwise may be time consuming and costly using a sensory panel. [ABSTRACT FROM AUTHOR]

[Display omitted] • We utilized sodium alginate and dopamine copolymers as a functional platform for grafting SBMA to obtain a surface coating. • The swelling and nanomechanical properties of brushes were investigated based on the AFM technique. • The adhesion and friction coefficient of the pDASA-SBMA brush coatings can be reduced by 80 % and 91 %, respectively. Clinical applications of interventional catheters suffer from poor lubricity and nonspecific protein adhesion, which can cause severe damage to soft tissues and even life-threatening injuries. With the proposed strategy in the grafting of sulfobetaine methacrylate (SBMA) zwitterionic polymer brushes using dopamine (DA) and sodium alginate (SA) as an intermediate layer, the pDASA-SBMA brush coatings with a controlled increase in density and thickness were prepared by tuning the grafting time. Based on the atomic force microscopy (AFM) technique, we systematically investigate the effect of variations in structural parameters on the swelling and nanomechanical properties of brushes. In particular, the dependence of the lateral deformability of brush chains on structural parameters and the viscoelastic relaxation behavior of the brush chains to recover their equilibrium conformation after deformation are elucidated, both of which have an impact on friction measurements. The experimental results show that the coating adhesion and friction coefficient in pure water decrease with the increase in grafting time, which is caused by the increase in hydration lubrication capacity of dense and thick brushes, as well as the weakening of lateral deformability. With the grafting of 24 h, the adhesion and friction coefficient of the coating can be reduced by 80 % and 91 %, respectively, compared to the unmodified surfaces. With the significant friction reduction and anti-adhesion effects, the proposed surface coating technique can find versatile applications in high-performance interventional catheters. [ABSTRACT FROM AUTHOR]

We performed high-frequency (0.4–1.7 MHz) measurements of the internal friction (IF) on 14 bulk metallic glasses (MGs). It is found that 12 of these MGs display relaxation IF peaks at temperatures ≈ 400–500 K, which are weakly affected by heat treatment within the amorphous state. The corresponding relaxation time is about 0.3 μ s. This fast relaxation is reported for the first time in the literature. The apparent activation enthalpy for 4 MGs is determined. It is shown that if measured at a low frequency of 1 Hz, these IF peaks would appear near room temperature or by 30–50 K below it. Thus, the IF peaks found in the present investigation are direct analogues of low-temperature/low-frequency β ′ - and/or γ -relaxations described in the literature. It is argued that the origin of these IF peaks is related to the relaxation of structural defects similar to dumbbell interstitials, which are frozen in solid glass from the liquid state upon melt quenching. These defects can be considered as elastic dipoles and their re-orientation in the applied stress provides anelastic deformation and corresponding IF peak. After correction for temperature dependence of the shear modulus, the true activation enthalpy (0.62–1.08 eV) and activation entropy (4 ≤Δ S ∕ k B ≤15) for defect re-orientation are determined. • High-frequency (0.4–1.7 MHz) internal friction (IF) measurements are performed. • IF peak located at 400–500 K is found in 12 metallic glasses. • The relaxation time corresponding to this peak is about 0.3 μ s. • Corresponding activation enthalpy and activation entropy are determined. • The origin of the IF peak is supposed to be due to defect re-orientation. [ABSTRACT FROM AUTHOR]

An analytical EHL model suitable for highly loaded point contacts operating in mixed and boundary regimes of lubrication is presented. The results are compared and validated with the experimental measurements of friction coefficient from an MTM tribometer. Measurements are carried out upon a PAO40 oil at medium to high temperatures. In general, a good agreement in trend and magnitude over a wide range of operating conditions is observed. Both the experimental findings and the model predictions show an inverse relationship between friction and contact load in mixed and boundary regimes of lubrication, an effect not hitherto reported in literature. [ABSTRACT FROM AUTHOR]

This work investigates the mechanisms for friction reduction in textured journal bearings. The measured friction of laser-etched connecting-rod big-end shells was compared with a non-textured reference under engine-like conditions. Experiments covered several working conditions, and the friction and lubricant film temperatures were measured for three texture configurations. The origin of the verified friction reduction was explored using a numerical model that simulates the hydrodynamic lubrication and global thermal effects. The simulation results accurately matched the friction measurements, revealing that the microscopic mechanisms of reduction in lubricant shear stress and effective viscosity due to texture-induced cavitation act simultaneously to reduce the friction coefficient for the textured shells. The findings have practical applications for the optimal design of textured hydrodynamic bearings. • Friction in textured journal bearings was experimental and numerically investigated. • A journal bearing model with thermal effects was validated using experimental data. • Textures reduced the lubricant shear stress and viscosity and lowered friction. • The shear area reduction mechanism for textured hydrodynamic bearings was confirmed. [ABSTRACT FROM AUTHOR]

Accurate friction-slip modelling is particularly important for locomotive traction and rail damage studies. This paper compares friction-slip measurement results from a hand operated (HO) tribometer and a twin disc tribomachine on AS60 rail. The results were used to calibrate traction coefficient-slip curves using the Modified Fastsim contact model. The differences between the two friction measuring instruments including scale, speed, and rail cleaning effect influenced the friction coefficient and slip value at which the friction peak occurred. The proposed equalisation technique was able to match the HO tribometer traction coefficient-slip curve with the tribomachine curve while preserving most of the Modified Fastsim contact parameters. • Friction-slip behaviour is characterised for locomotive traction and rail damage. • Friction measurements using tribomachine and hand-operated tribometer are compared. • Tribomachine and hand-operated tribometer results implemented in Modified Fastsim. [ABSTRACT FROM AUTHOR]

Real contact area is a challenging phenomenon in tribology, where despite established theoretical models, detailed experimental studies are still lacking. This work is focused on the evolution of the real contact area of multi-asperity steel contacts during sliding, by using sub-micron characterisation of entire multi-asperity contact area and simultaneous measurement of the friction force. Steel-sapphire contact was monitored by using an in-situ optical system in real time during 5 mm of unidirectional sliding in surface roughness range R a = 0.1–1.0 µm and normal loads up to the nominal contact pressure equal to steel yield strength (Y). The results showed that the initial real contact area caused by static loading increased during sliding. At low nominal contact pressure of 0.3 Y , the increase in real contact area was the largest: for R a = 0.1 µm it went from 9.5% of the nominal area under static conditions to 15.0% after sliding, a 56% relative increase, and for R a = 1.0 µm it went from 5.0% to 11.5% after sliding, a 106% increase. The increase in real contact area was smaller at higher nominal contact pressures of 0.6 Y and 1.0 Y. At 1.0 Y for R a = 0.1 µm the real contact area went from 18.0% to 21.8% after sliding and for R a = 1.0 µm it went from 9.4% to 12.9% after sliding. The coefficient of friction increased with roughness at the lowest nominal contact pressure 0.3 Y (for R a = 0.1 µm it was 0.24 and for R a = 1.0 µm it was 0.29) but became independent of roughness at higher contact pressures (on average 0.18). • In-situ measurement of real contact area evolution during 5 mm of sliding. • Whole multi-asperity steel contacts observed using optical method. • Real contact area increased due to sliding from the initial static conditions. • The largest increase from static to sliding conditions was at the lowest load. [ABSTRACT FROM AUTHOR]