Friction Data Research Articles

R1234yf and R1234ze(E) as environmentally friendly replacements of R134a: Assessing flow boiling on an experimental basis

R134a is ongoing a phase-down process in several countries and R1234yf can be a viable low GWP alternative together with R1234ze(E). Since new experimental data are always requested by researchers and industry to increase the global database, to assess the existing correlations, and to expand the knowledge about the refrigerant applications, in this paper more than 100 new heat transfer coefficient and frictional pressure drop data are presented during R1234yf flow boiling inside a 4 mm inner diameter smooth copper tube. Three saturation temperatures 10 °C, 15 °C and 20 °C were investigated and critically discussed, for each temperature the heat flux ranged from 15 to 30 kW m−2 and the mass velocity from 300 to 600 kg m−2 s−1. Furthermore, some literature correlations were implemented and carefully examined and their deviations are reported. Finally, the R1234yf heat transfer performance is compared against R1234ze(E) and R134a, highlighting the peculiarities of each alternative molecule. R1234ze(E) outperforms the other refrigerants at high vapor qualities but both R1234yf and R1234ze(E) can be viable low GWP substitutes to R134a, if the new design is able to take advantage of the qualities of each refrigerant.

RSFit3000: A MATLAB GUI-based program for determining rate and state frictional parameters from experimental data

AbstractWe present a MATLAB graphical user interface (GUI) software package for analyzing rate and state friction experiments. Called RSFit3000, the software allows users to easily determine frictional parameters by fitting velocity-step and slide-hold-slide events using the aging- and slip-law forms for state variable evolution. RSFit3000 includes features for removing strain hardening or weakening trends from the data, and provides options for using two state variables, applying a weighting function, and treating stiffness as a fitting parameter. Completed fits are conveniently saved in MATLAB structure arrays that contain fitted parameter values with their error intervals, and all information required to reproduce a given fit. The GUI makes the program simple to use, as all fitting tasks are completed via interaction with the interface. Here we describe how to use the software, and illustrate its flexibility and utility by analyzing two sets of synthetic friction data, as well as some previously published experimental data. Although descriptions of rate and state friction fitting routines have been published in the past, RSFit3000 marks the first time a software package for analyzing friction experiments has been described in the literature.

The optical microscopic observation of frictional interface between high strength gels and a glass ball by ball-on-disk method

The polymer gels with the properties of high strength and low friction show promise as the ideal materials for a living body-like soft robot joint. To date, the relationship between contact area and friction behavior of hydrogels has been revealed from the observation of flat frictional surface. Here, we designed the microscopic observation system for the friction surface of soft materials under deformation condition. A specially arranged ball-on-disk measuring part was combined with inverted microscope to observe the frictional interface during the friction. Both of transparent glass ball and moving glass stage were employed in this instrument and the microscope camera was thereby able to take the moving picture of frictional interface imaged by the refractive index differences between a soda-lime glass ball, transparent hydrogels and air. By using this customized measuring instrument, when water was not rich on the friction surface, the moving pictures of the frictional interface between glass ball and transparent polymer gels were observed while measuring the frictional force. The visualized frictional contact area due to the differences of refractive index and the frictional measurement data indicated that the local deformation by adhesion between gels and counter sample cause high friction resistant as a static friction coefficient.

Wear Prediction of UHMWPE Cup Against Commercially Pure Titanium Hip Implant With The Nonlinear Load and Contact Area Wear Equation

Hip implants are prostheses designed for severe hip arthritis patients. One of its components, acetabular cup is mostly made from ultra-high molecular weight polyethylene (UHMWPE). This polymer has a wear problem. Its debris would cause osteolysis followed by implant failures. Numerical studies of this problem were commonly used a linear contact pressure wear equation. Recently it was found not suitable to model the polymer wear in hip implants. This study used a nonlinear equation to predict the UHMWPE volumetric wear. It states a nonlinear relation between wear, load and contact area. The computation model was a biaxial rocking motion hip simulator assembly with UHMWPE acetabular cup sliding on commercially pure titanium femoral head. Multidirectional pin on disc experiments conducted to obtain wear factor and coefficient of friction data for the computations. Commercial finite element software calculated the other parameters. Predicted volumetric wear has a good agreement with the experimental data. Lowest numerical error was found 0.53 %.

Influence of laser processing conditions for texturing on ultra-high-molecular-weight-polyethylene (UHMWPE) surface

Surface modification of Ultra-High Molecular Weight Polyethylene (UHMWPE) for improving tribological performance is difficult due to its high melt viscosity. Therefore, the potential of laser processing to produce textures on the surface of this material in specific dimensions was evaluated in this study. The influence of several conditions and laser processing parameters was determined. The laser texturing at room temperature, freezing temperature, and producing distilled water layer on UHMWPE disks was not helpful to produce the textures in required dimensions. The textures in specific dimensions were produced at 2 overscan times, 25 mm/s scanning speed and 30 MHz laser intensity using aluminum foil. The coefficient of friction (COF) versus time data under different concentrations of Bovine Serum Albumin (BSA) was recorded for textured and un-textured specimens. The wear performance in term of wear track depth was also compared for textured and un-textured specimens using a confocal microscope.

Memory Distance for Interfacial Chemical Bond-Induced Friction at the Nanoscale.

Macroscale rate and state friction (RSF) laws include a memory distance, Dc, which is considered to be the distance required for a population of frictional contacts to renew itself via slip, counteracting the effects of aging in slow or static contact. This concept connects static friction and kinetic friction. Here, we use atomic force microscopy to study interfacial chemical bond-induced kinetic friction and the memory distance at the nanoscale for single silica-silica nanocontacts. We observe a logarithmic trend of decreasing friction with sliding velocity (i.e., velocity-weakening) at low velocities and a transition to increasing friction with velocity at higher velocities (i.e., velocity-strengthening). We propose a physically based kinetic model for the nanoscale memory effect, the "activation-passivation loop" model, which accounts for the activation and passivation of chemical reaction sites and the formation of new chemical bonds from dangling bonds during sliding. In the model, we define the memory distance to be the average sliding distance that accrues before an activated reaction site becomes passivated. Results from numerical simulations based on this model match experimental friction data well in the velocity-weakening regime and show that Dc is sensitive to the surface chemistry, and nearly independent of sliding velocity. The simulations also show values of Dc that are consistent with those obtained from the experiments. We propose a semiquantitative physical explanation of the observed logarithmic velocity-weakening behavior based on the conservation of the number of interfacial bonds during sliding. We also extract from the experimental data physically reasonable values of the energy barriers to the activation of reaction sites. Our results provide one possible physical mechanism for the nanoscale memory distance.

Comparison of Wear with Friction Data in Chemical Mechanical Planarization

Optimization of Chemical Mechanical Planarization (CMP) on integrated circuits requires an accurate model of material removal. The current model of polishing is two-body abrasion of particles trapped at the asperity-substrate junction, however, this is not supported by friction measurements. A modified friction model is applied to data collected from a pin-on-disk experiment preformed on a fused silica wafer with various polymer polishing pads, flooded with a silica nanoparticle slurry. Results indicate that the friction arises predominantly in the swept region preceding the asperity contact. We will add to this measurements of the volume of the ware-track, indicating the rate of material removal. Our aim is to develop an Archard style relationship between friction and the material removal rate that is independent of the pad material. The long-term goal of this investigation is to aid in the optimization of CMP for structures of decreasing size.

The relationship between fault zone structure and frictional heterogeneity, insight from faults in the High Zagros

Frictional heterogeneities within fault zones depend strongly on lithology as well as shear fabric and strain localization structures. Here we investigate the impact of fault rock composition and shear fabric on friction constitutive properties for mature High Zagros (Iran) fault rocks. We present field observations along with results from friction experiments on intact and powdered fault rock from carbonate, quartzofeldspathic and schist. We measured frictional strength and rate/state dependence over normal stresses from 25 to 100 MPa and shear slip velocities from 3 to 300 μm/s. The sliding friction coefficients of intact fault rocks are lower than their powdered equivalents. The foliated quartzofeldspathic and lensoidal carbonate rocks from the Main Zagros Reverse Fault (MZRF) exhibit velocity strengthening friction and stable sliding. In contrast, the cataclasite carbonate rocks from the younger, Main Recent Fault (MRF) exhibit potentially unstable, velocity-weakening frictional behavior, consistent with regional seismicity. Our results suggest that solution seams, cleavage processes, and development of stylolite and calcite veins in the carbonate fault rocks of the MZRF continually rejuvenate and rework shear structures, which leads to dominantly aseismic shear. In contrast, younger fault rocks of the MRF form highly localized shear zones and shear fabric that result in velocity-weakening frictional behavior and seismic slip. Our field observations and laboratory data provide insights for using friction data and fault zone structure to build predictive models of the mode of fault slip in zones of tectonic collision.

Fusing wind-tunnel measurements and CFD data using constrained gappy proper orthogonal decomposition

In this article gappy proper orthogonal decomposition (POD) is used to fuse wind-tunnel measurements and computational fluid dynamics (CFD) data to provide a consistent and more comprehensive output of greater utility. The technique is used to fuse a very limited and ‘gappy’ set of wind-tunnel surface pressure measurements with computational surface pressure and friction data. By integrating the resulting fused surface data over a complete aerodynamic surface, the corresponding aerodynamic coefficients are obtained. A comparison between these integrated aerodynamic coefficients and the wind-tunnel balance measurements show a better agreement than that obtained between CFD and the wind-tunnel experiment. In addition, the original gappy POD algorithm is modified and extended to provide an accurate and consistent match between the integrated coefficients evaluated from the fused data set and the wind-tunnel balance measurements, thus producing a set of data distributed over the entire aerodynamic surface that conforms to experiment. This is achieved by considering the wind-tunnel balance measurements as equality constraints in the gappy POD least-squares problem formulation. The technique is demonstrated for steady flow around an RAE 2822 airfoil and an industrial wing-body aircraft configuration. Also, an important feature of the methodology that is essential for its use in an industrial environment, namely regularization, is discussed.

A closer look at the locked-wheel pavement friction data in the ltpp database for selected states

AbstractPavement friction data from the Long-Term Pavement Performance (LTPP) database were treated and analyzed in order to examine its quality, patterns, and potential use. Data from a number of ...

Testing for low-speed skid resistance of road pavements

Low-speed skid resistance tests provide useful information not available from the normal high-speed skid resistance testing of road pavements. There are known limitations of the two commonly used measuring devices of low-speed skid resistance, i.e. British Pendulum Tester (BPT) and Dynamic Friction Tester (DFT). This study develops a Walking Friction Tester (WFT) to offer a practical alternative to measure low-speed pavement skid resistance. This paper compares WFT with BPT and DFT measurements in the following three aspects: correlation relationship, measurement variability, and speed comparison of field tests. Tests were also performed to examine the variability of WFT test results with respect to walking speed and water film thickness, respectively. The study showed that there was a good correlation coefficient of 0.80 between BPT and WFT measured data, and 0.70 between DFT and WFT friction coefficients. WFT measured data were found to have smaller variability than those of BPT and DFT. In field tests performed over a length of pavement section, WFT generated continuous friction data in much shorter times compared with the spot measurement mode of either BPT or DFT. The results also showed that WFT tests were independent of changes in walking speeds of operators and variations of water film thickness applied.

Experimental investigation of condensation pressure drop of R134a in smooth and grooved multiport flat tubes of automotive heat exchanger

Condensation frictional pressure drop was obtained in multiport micro channel smooth and grooved flat tubes using R134a. The study consists of two smooth and one grooved tube. Refrigerant R134a is studied over a mass flux 490–1600 kg/m2s, heat flux 5.5–19 kW/m2 and saturation temperature 51–68 °C. Results reveal that frictional pressure drop increases with an increment of mass flux, vapor quality and decreases with the increase of saturation temperature. The effect of heat flux on frictional pressure drop was negligible. Grooved tube shows the lowest pressure drop and surprisingly highest heat transfer coefficient. The frictional pressure drop data was compared with famous correlations. Zhang and Webb predicted 86% of the data with in ±30%.

수평 원관 내 공기-오일 이상유동 양식과 마찰 압력강하에 관한 실험적 연구

This study examines the air-oil two-phase flow regime and frictional pressure drop in a horizontal circular pipe with a diameter of 40 mm. The pressure drop fluctuation characteristics for different air and oil superficial velocities are investigated. Observed flow regimes and frictional pressure drop data are compared with previous flow regime maps and frictional pressure drop correlations, respectively.

Stable and unstable friction in fretting contacts

Abstract Designing contacts susceptible to fretting is a challenging task due to uncertainties related to friction. For example, coefficient of friction has shown to vary as a function of load cycles and so-called non-Coulomb friction can exist during individual load cycles. Concepts of stable and unstable friction are presented in this manuscript. Based on experiments, no fretting is to be expected if the utilization of friction is kept below unstable friction threshold. If contact is subjected to tangential load above this threshold, reciprocating slippage, fretting, is to be expected even if the contact was initially in stick. Experimental evidence for existence of such threshold is presented in form of friction data, slip data and fretting scars.

Effect of Coating Thickness on Wear Performance of Inconel 625 coating

INCONEL® nickel-chromium alloy 625 is widely used for its high strength, excellent corrosion resistance and creep resistance on prolonged exposure to aggressive environment. However, despite its properties and industrial application, its wear resistance is not entirely satisfactory. The article investigates and presents the tribological performance of Inconel 625 coating using reciprocating scratch test. High velocity oxyfuel (HVOF) method was used to deposit Inconel 625 film onto 304 stainless steel surfaces. The frictional behavior and wear data were studied in order to observe the effect of coating thickness on wear failure. The results revealed that coatings of higher thickness showed lower coefficient of friction, better adhesion and good wear resistance in comparison to the low coating thickness.

Improving cutting performance of carbide twist drill combined internal cooling and micro-groove textures in high-speed drilling Ti6Al4V

High-speed cutting technology has become the main direction of development of machinery manufacturing industry; however, the excessive temperature and sharp tool wear have greatly restricted the improvement of drilling speed. To reduce the cutting temperature and alleviate the severe friction and wear of the tool/chip and tool/workpiece interface, micro-groove textures were fabricated on the rake face and flank face of the carbide internal cooling twist drills by an Nd:YAG laser. Thus, two developed cutting tools were fabricated: (1) the internal cooling twist drills with textured rake-face (TF) and (2) the internal cooling twist drills with textured flank-face (TB). High-speed drilling experiments were carried out on Ti6Al4V using these developed tools, and the resulting friction and wear data were compared with those from the conventional one (NT). Results obtained in this work demonstrated the feasibility of fabricating micro-groove textures on internal cooling tool to improve the high-speed cutting performance. Moreover, the mechanisms for the effects of micro-textures fabricated on rake face and flank face were different. The micro-groove textures fabricated on the rake face can significantly increase the degree of chip curling, reduce the contact length at the tool/chip interface, and improve the chip breaking ability; while, the internal cooling abilities were significantly enhanced by the micro-textures fabricated on the flank face; thus, the effects on decreasing the drilling force and improving the anti-adhesive and abrasive wear properties were achieved.

Contributions of lunate cells and wax crystals to the surface anisotropy of Nepenthes slippery zone.

Nepenthes slippery zone presents surface anisotropy depending on its specialized structures. Herein, via macro–micro–nano scaled experiments, we analysed the contributions of lunate cells and wax crystals to this anisotropy. Macroscopic climbing of insects showed large displacements (triple body length within 3 s) and high velocities (6.16–20.47 mm s−1) in the inverted-fixed (towards digestive zone) slippery zone, but failed to climb forward in the normal-fixed (towards peristome) one. Friction force of insect claws sliding across inverted-fixed lunate cells was about 2.4 times of that sliding across the normal-fixed ones, whereas showed unobvious differences (1.06–1.11 times) between the inverted- and normal-fixed wax crystals. Innovative results from atomic force microscope scanning and microstructure examination demonstrated the upper layer of wax crystals causes the cantilever tip to generate rather small differences in friction data (1.92–2.72%), and the beneath layer provides slightly higher differences (4.96–7.91%). The study confirms the anisotropic configuration of lunate cells produces most of the anisotropy, whereas both surface topography and structural features of the wax crystals generate a slight contribution. These results are helpful for understanding the surface anisotropy of Nepenthes slippery zone, and guide the design of bioinspired surface with anisotropic properties.

Luminescent Oil Film Flow Tagging Skin Friction Meter Applied to FAITH Hill

The luminescent oil film flow tagging skin friction meter is presented and used to collect time-averaged spatially distributed skin friction data on the surface of NASA’s FAITH model. The technique estimates skin friction by monitoring the evolution of a luminescent oil film evolving under the influence of skin friction, pressure gradients, and gravity. The thickness distribution of the oil film, , and the velocity of the surface of the oil film, , are measured experimentally, and the dynamic viscosity of the oil, , is taken from the oil manufacturer or is determined through calibration. Then, skin friction is estimated as . The thickness distribution of the oil film is measured by ratioing the intensity of the luminescent signal from the fluorescent oil with the intensity of the incident light that is scattered from the surface of the model. The resulting quotient is independent of the intensity of the incident light and directly proportional to the thickness of the oil film. The velocity of the surface of the oil film is measured by tagging and following photochromic molecules in the oil film. The new luminescent oil film flow tagging skin friction data compare well with fringe image skin friction data. Also, an experimental study is conducted to determine the extent to which it is appropriate to neglect the effects of a pressure gradients and gravity when determining skin friction from the evolution of an oil film. Results are presented supporting that this approximation is appropriate for a sufficiently thin film of oil or for a sufficiently high skin friction magnitude.

Gain and Scan Rate Dependence of Friction at the Nanoscale Measured by Lateral Force Microscopy

Lateral force microscopy (LFM) is used to examine friction at the nanoscale. Although the friction at the tip-sample contact measured by LFM can provide extensive information beyond the friction force itself, such as the crystallographic orientation, the presence of defects, etc., there have been many contradictory reports regarding friction coefficients, nanoscale friction laws, etc. obtained by this technique. Here we investigate the effect of scan rate and gain on the LFM frictional force measurements. We show that the ratio of friction measured on SiO2 to friction measured on graphene can vary from about 1 to approximately 9.2 depending on the combination of these parameters. We discuss how to optimize the associated scan parameters to obtain reliable friction data at the nanoscale.

Field Investigation of Relationship between Pavement Surface Texture and Friction

Proper tire–pavement interaction is essential for the safety of motorists. Pavement surface texture is a major contributing factor to tire–pavement friction. This study performed a series of statistical analyses of field-measured friction and texture data to find the texture–friction correlation. Three test sections with different pavement types were selected within the state of Texas. Data were collected at three locations in the right wheel path and three locations in the center of the lane for each test section. To measure the texture data, the researchers used the circular track meter (CTM) and a prototype measurement device developed in-house and consisting of a line laser scanner (LLS). Friction measurements were obtained with the dynamic friction tester (DFT) and Grip-Tester. The mean profile depth (MPD) was calculated by using the measured texture data. The relationship between the MPD values and the friction numbers obtained from the Grip-Tester and DFT was investigated at speeds of 50 and 70 km/h (31.1 and 43.5 mph). The repeatability and reliability of both the developed LLS prototype and the Grip-Tester were also evaluated, as well as the effect of test speed on friction measurement. The results indicated a strong positive correlation between the texture and friction data. In addition, the developed LLS prototype was able to scan the pavement surface texture more reliably and precisely than the CTM in terms of vertical and horizontal resolution. The Grip-Tester showed promising results compared with the DFT with regards to the friction measurement.