Frictional Work Research Articles

A microstructure-based wear model for grid-to-rod fretting of clad nuclear fuel rods

Coolant flow-induced, grid-to-rod-fretting (GTRF) can occur in pressurized water nuclear reactor cores leading to cladding wear-through, fuel material leakage, and plant shutdowns. The goal of this effort was to derive a microstructure-based interpretation of the fretting wear factor (Wf) for use in an existing, multi-stage GTRF wear model. Physically, the magnitude of Wf (expressed in units of mm3/N-m) reflects the effectiveness of converting frictional work into material removal during fretting between Zircaloy-4 clad nuclear fuel rods and their supporting grids of a similar or dissimilar material. A tribosystem analysis of this complex situation not only involves mechanical influences on cladding wear, but also the effects of temperature and tribo-corrosion. A previously published GTRF wear model underlies this work. It assumes that the cladding wear rate can change over time and takes place in stages depending on the evolving nature of the materials in the interface. A materials science-based derivation of wear factor introduces four concepts: a wear process volume (WPV) from which debris particles originate, a population of wear debris embryos (DEms) of a critical size within the WPV, primary debris origins from the subsurface of the cladding alloy, and secondary debris detachment from third-body layers. The probability of nucleating loose particles from DEms and the coalescence of cracks in highly-worked Zircaloy-4 to form primary debris is estimated from wear test data obtained with a high-temperature, pressurized-water GTRF wear simulator at Oak Ridge National Laboratory (USA). The limitations and challenges for quantifying certain microstructure-based parameters that comprise the wear factor derivation are discussed.

Effects of moisture content during densification of biomass pellets, focusing on polysaccharide substances

In this study, we pelletized four different pure polysaccharides represented cellulose - Avicel, hemicelluloses - locus bean gum mannan and beech xylan and other polysaccharides - apple pectin, and three woods - pine, spruce and beech. All were pelletized at 100° in a single pellet press unit with different level of moisture content from 0 to 15%. The maximal friction force and work required for compression and friction was analyzed together with the pellet density and hardness. The results showed that xylan pellets completely changed in color at 10% moisture content, and this also occurred to some extent with pectin pellets. The color of both Avicel and locus bean gum pellets were not affected at all. During compression, the results showed that water does not affect compression up to 5 kN, while above 5 kN water decreases the energy need for densification of Avicel, locus bean gum and woods. Above 5 kN the energy needs for compressing xylan and pectin increases with increased moisture content. The hardest pellets were produced from Avicel, while locus bean gum produced the weakest pellets. The study concludes that there is a significant difference in how water affects the two hemicelluloses, glucomannan and xylan, during densification.

Thermally assisted lubricity and negative work tails in sliding friction

We discuss and qualify a previously unnoticed connection between two different phenomena in the physics of nanoscale friction, general in nature and also met in experiments including sliding emu- lations in optical lattices, and protein force spectroscopy. The first is thermolubricity, designating the condition in which a dry nanosized slider can at sufficiently high temperature and low velocity exhibit very small viscous friction f ~ v despite strong corrugations that would commonly imply hard mechanical stick-slip f ~ log(v). The second, apparently unrelated phenomenon present in externally forced nanosystems, is the occurrence of negative work tails ("free lunches") in the work probabilty distribution, tails whose presence is necessary to fulfil the celebrated Jarzynski equality of non-equilibrium statistical mechanics. Here we prove analytically and demonstrate numerically in the prototypical classical overdamped one-dimensional point slider (Prandtl-Tomlinson) model that the presence or absence of thermolubricity is exactly equivalent to satisfaction or violation of the Jarzynski equality. The divide between the two regimes, satisfaction of Jarzynski with ther- molubricity, and violation of both, simply coincides with the total frictional work per cycle falling below or above kT respectively. This concept can, with due caution, be extended to more complex sliders, thus inviting crosscheck experiments, such as searching for free lunches in cold ion sliding as well as in forced protein unwinding, and alternatively checking for a thermolubric regime in dragged colloid monolayers. As an important byproduct, we derive a parameter-free formula expressing the linear velocity coefficient of frictional dissipated power in the thermolubric viscous regime, correcting previous empirically parametrized expressions.

Tribological Investigations of Silicon Nitride Lubricated by Ionic Liquid Aqueous Solutions

Water-lubricated ceramic components are promising material combinations for tribological applications because they are wear resistant and can exhibit low coefficients of friction under different loads. The aim of the work was to show in model tribological experiments with silicon nitride that the lubricating properties of water can be improved by adding water-soluble ionic liquids (ILs). The geometry used and the experimental parameters were selected to approximately simulate a piston ring–cylinder liner contact. We found that the addition of 1% IL tributyl methyl phosphonium acetate [P4441][OAc] leads to a reduction of the maximum friction work by 58% compared to pure water lubrication and even by 96% compared to oil-lubricated steel. It was shown that suitable running-in conditions and sufficient cooling by the lubricant at an early stage are important prerequisites for achieving low friction and wear values. In addition, the chemical interaction of water and IL with silicon nitride strongly affects the tribological behavior. As increasing demands on sustainability (green tribology) become more important due to environmental aspects, this work provides important insights for the development of novel water-based lubricant systems.

Hot on the trail: Coseismic heating on a localized structure along the Muddy Mountain fault, Nevada

Recent advances in the use of thermal proxies to identify past earthquake slip in exhumed faults have created an extraordinary opportunity to map the geometry of past ruptures in unprecedented detail. This approach can reveal along-strike differences in the structure and speed of earthquake slip. Here, we present organic thermal maturity data collected along the Muddy Mountain thrust in Nevada to investigate where it was within the fault that earthquake slip occurred and whether this is consistent along strike. We observe large changes in thermal maturity, which represent temperature-rise variations along the fault. Modeling of thermal maturity measurements yield peak temperatures of 760–1090 °C where the principal slip zone (PSZ) is narrowest (0.8 mm). Nearby where the PSZ is thicker (1.5 mm) these temperatures did not exceed 500 °C. Based upon estimates of temperature rise within a submillimeter PSZ, mean frictional work during earthquake slip was 6.3–7.1 MJm−2. These results show that thickness of the active slipping zone can vary at small spatial scales and development of a continuously narrow (<1 mm) slip layer is not required for earthquake propagation. Therefore, even for an earthquake with spatially uniform slip, we may observe large variations in heating and the effectiveness of thermally-activated dynamic-weakening mechanisms.

Research on vehicle starting control based on reflux power condition

Based on a detailed introduction of the working principle and starting process of a new power-cycling continuously variable transmission (PCVT), a starting dynamic model of the system is established. According to its special structure and the characteristics of power reflux, pure-throttle and first-gear-clutch starting controls are proposed. Pure-throttle starting control of PCVT can realize accurate throttle control in stages, by using start time and jerk as control objectives and combining them with driver starting intentions. The clutch-starting control takes the jerk, friction work, and starting time as control objectives. Furthermore, its control strategy of the clutch-oil pressure change rate is formulated in stages, combined with driver starting intentions. To achieve the optimal starting performance, an optimization method is proposed, based on the analytic hierarchy process and genetic algorithm. Simulation results show that the two strategies can effectively guarantee the quality of vehicle starting with consideration of driver intentions. After optimization, the starting jerk is effectively reduced by 36.6% in the pure-throttle starting mode and 34.2% in the first-gear-clutch starting mode.

Evaluation of the Tribotechnical Characteristics of Therma-Barrier Sealing Coatings under Critical Loads

This article deals with the initial stage of a comprehensive multifactor study of the wear resistance of thermal-barrier sealing coatings in the turbine of a gas turbine engine (GTE) under critical loads without taking into account the effect of structural phase transformations that can occur in coating materials during engine start-up and the subsequent long exposure of the GTE parts to high temperatures of about 1150°C. In compliance with the requirements of the GOST 30 480–97 state standard, the process of wear intrinsic to the full-scale tribological conjugation was simulated to obtain frictional-wear characteristics such as the friction coefficient, wear rate, and friction work under preset conditions that were used for further analysis of the wear process and acquisition of the initial data for field trials. The results of the above tribotechnical tests allow us to draw some tentative conclusions on the effect of alloying the nickel-based sealing coatings by yttrium-containing alloying compositions on their behavior during the tribological interaction and propose coating compositions that can be recommended for the subsequent stages of the study of the durability of sealing coatings under high-temperature conditions accompanied by volumetric changes in products and coatings as a result of structural phase transformations in multicomponent coating compositions.

Numerical simulation of high-speed joining of sheet metal structures

Abstract The increasing trend of multi-material and space-frame design in automotive car body constructions consolidates the need for mechanical joining technologies with one-sided accessibility. The high-speed joining process (also called “RIVTAC®” or “Impact”) is an innovative and flexible technology. In this process, a tack having a profiled shank and an ogival shaped tip is pushed into the joining partners with a speed of 20 to 40 m/s without any pre-punching. The plastic and friction work is converted into heat, which causes an abrupt rise of temperature in the joining zone. This improves the flowability of the material which leads to filling the annular grooves existing on the shank of the tack. Simultaneously, a force-fit is created between the sheet metal and the tack. Furthermore, the effects of inertia are used so that the thin-walled structures can be joined without any additional tools (e.g. a die). The joining process causes the deformation of the structure, which must be taken into account with regard to tolerances. The numerical simulation is a powerful tool to predict the deformation and the joinability of a complex structure as well as the mechanical properties of the connection. The paper is about the detailed process- and load-simulation of high-speed joining as well as the required experimental data basis, such as temperature and strain rate dependent flow curves. Furthermore, the friction parameters are very important for the physical consideration of the force-fit connection. Coupled structural-thermal simulations of an axisymmetric 2D-model are performed using explicit and implicit codes.

Modification of tribological tester t-01m allowing testing in conditions of lubrication

Researchers often want to develop their experiments in terms of wide range of testing conditions and results possible to acquire. One way to achieve this is to adapt existing testing equipment to new requirements. This paper proposes a method for extending the capabilities of the T-01M tester with a pressure lubrication system of a pin on disc contact, its construction and adjustments. Tribological tests carried out with the presence of a lubricant showed the correct functioning of the modified tester. We obtained several variables for the selected friction pair and friction parameters in the lubricated joint: friction force, friction coefficient, mass wear, friction work, wear intensity and specific wear. Verification tests confirmed suitability of the tester modification, adapting it to friction and wear tests in a lubricated contact. The expanded stand can be used (in the future) for tribological tests of lubricated frictional pairs to measure friction, wear, friction pairs temperature and to compare used lubricants.

The influence of detachment strength on the evolving deformational energy budget of physical accretionary prisms

Abstract. Tracking the evolution of the deformational energy budget within accretionary systems provides insight into the driving mechanisms that control fault development. To quantify the impact of these mechanisms on overall system efficiency, we estimate energy budget components as the first thrust fault pair develops in dry-sand accretion experiments. We track energy budget components in experiments that include and exclude a basal layer of glass beads in order to investigate the influence of detachment strength on work partitioning. We use the measurements of normal force exerted on the backwall to estimate external work, and measurements of strain observed on the sides of the sand packs to estimate the internal work, frictional work and work against gravity done within increments of each experiment. Thrust fault development reduces the incremental external work and incremental internal work, and increases the incremental frictional work and incremental gravitational work. The faults that develop within higher-friction detachment experiments produce greater frictional work than the faults in experiments with glass bead detachments because the slip distribution along the detachments remains the same, while the effective friction coefficient of the detachment differs between the experiments. The imbalance of the cumulative work budget suggests that additional deformational processes that are not fully captured in our measurements of the energy budget, such as acoustic energy, consume work within the deforming wedge.

A non-linear friction work formulation for the analysis of self-excited vibrations

Even though much research has been devoted to understand friction-induced vibrations, its root cause is not yet fully understood. Reliable prediction of friction-induced unstable vibrations such as in brake squeal or hip squeak remains a challenge because of nonlinearities involved and because the complex eigenvalue analysis (CEA) widely used in industry is linear. The energy fed back into the system by friction has been shown to be useful for analysis of measurements and numerical simulations. In numerical simulations, the linearised method of feed-in energy, calculated purely based on friction work has provided some insights into the physical mechanism for instabilities. However, the dynamics due to friction-induced instabilities is highly nonlinear and damping may offset some or all of the excess friction energy provided to the system. By using a nonlinear 2-DOF dry friction oscillator, a nonlinear friction work formulation is proposed to demonstrate that in combination with viscous damping the energy budget provides an improved analysis capability over linearised friction work. The results highlight the potential of nonlinear friction work as a reliable tool to study friction-induced instabilities to gain deeper physical insights into squeal triggering mechanisms and to better understand the over- and under-predictive character inherent to linear methods.

The effects of cylinder deactivation on the thermal behaviour and fuel economy of a three-cylinder direct injection spark ignition gasoline engine

The changes in thermal state, emissions and fuel economy of a 1.0-L, three-cylinder direct injection spark ignition engine when a cylinder is deactivated have been explored experimentally. Cylinder deactivation improved engine fuel economy by up to 15% at light engine loads by reducing pumping work, raising indicated thermal efficiency and raising combustion efficiency. Penalties included an increase in NOx emissions and small increases in rubbing friction and gas work losses of the deactivated cylinder. The cyclic pressure variation in the deactivated cylinder falls rapidly after deactivation through blow-by and heat transfer losses. After around seven cycles, the motoring loss is ~2 J/cycle. Engine structural temperatures settle within an 8- to 13-s interval after a switch between two- and three-cylinder operation. Engine heat rejection to coolant is reduced by ~13% by deactivating a cylinder, extending coolant warm-up time to thermostat-opening by 102 s.

Simulation of the behavior of aluminum alloys welded in Friction Stir Welding FSW

The friction stir welding FSW is a topical technique to date, either for experimental or numerical research topics. Based on the fact that the majority of the heat generated, comes from the contact between the workpiece and the shoulder, and from the work of friction done by the pin. Thus, it is crucial to approach the reality, by simulating the most of the conditions, related to the FSW process. The development of a numerical model allows us, not only to simulate the FSW welding process, but also to conduct a parametric study and vary the welding conditions as desired. Therefore, a numerical study is conducted on the friction stir welding of AA 5083H111 work piece, in order to perceive the optimal settings. A three dimensional model designed by ANSYS APDL was used. Before proceeding with this, a preliminary experimental study was carried out on FSW welded AA 6082 –T6 specimens and base material, using tensile and fatigue tests. The numerical results obtained, essentially longitudinal residual stresses, are compared to other studies, experimental and numerical.

Biomarker thermal maturity experiments at earthquake slip rates

Evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred, as well as some attributes of the earthquake such as energy output. Biomarker thermal maturity can elucidate structures within a fault zone that were hotter than surrounding rock, and therefore likely to have hosted earthquakes. We performed a series of friction experiments on thin gouge layers of Woodford shale at slip rates of 1 m/s. Our results show that biomarker thermal maturity can clearly delineate zones of slip that are as thin as 150 μm. We establish that the kinetics of methylphenanthrene reaction previously determined at longer durations and lower temperatures are applicable at seismic slip rates and timescales. Temperature estimates show that once the slipping zone reached ∼500 °C, biomarker reaction was significant, as predicted by the reaction kinetics. Our results demonstrate that temperature rise in fault zones is a function of both the work density and power density during the earthquake. Biomarker reactions, along with other Arrhenius-style reactions such as those that lead to dynamic shear weakening, are both a function of slip speed and frictional work.

Kinematics and Force Analysis of Flexible Screw Mechanism for a Worm Robot

AbstractThis paper presents a new type of flexible screw mechanism (FSM), which is composed of a nut, flexible axle, and roller. It can be used in a worm robot to achieve flexible peristaltic motion, as well as curvilinear motion and deformation. This type of FSM uses a roller to decrease the friction. We investigated the transmission principle and the kinematic characteristics of this FSM, established the model of the velocity, acceleration of the roller, characterized the feed motion characteristics of the flexible shaft, and achieved an analytical solution of the flexible shaft's velocity. Furthermore, by considering the position of the pure rolling section of the roller, the spin slide model is proposed based on Hertz theory. To investigate the friction loss between the roller and the flexible axle, we established a friction work model of the entire FSM system. Finally, the motion characteristics of the FSM are evaluated through experiments.

Influence of humidity on the endurance of silver-plated electrical contacts subjected to fretting wear

The use of connectors in electrical devices for automotive has significantly increased during the last decades. These connectors need to keep a low and stable electrical contact resistance (ECR) otherwise disconnects may occur, inducing critical failures. Close to the engine, these connectors are subjected to vibrations inducing fretting in the contact (i.e. wear damage induced by small oscillating sliding).This phenomenon induces surface wear and the formation of oxide debris (third body) which, being trapped within the interface, can drastically increase the electrical contact resistance.The aim of this study is to investigate the effects of the relative humidity (RH) on the fretting wear rate and the Electrical Contact Resistance (ECR) of a silver plated electrical contact. The analyses show that an increase of RH tends to increase the ECR fretting endurance Nc related to the failure condition (ΔR > ΔRth = 4 mΩ). However, a discontinuous evolution is observed. Below a RH threshold, RH = 50%, the endurance increase is rather slow and can be related to a decrease of the friction work at the interface. Above RH = 50%, the high humidity conditions modify the rheological properties of the debris layer. The wear rate is becoming smaller and a larger wear volume is required to reach the ECR failure. This induces a fast linear increase of Nc. Whatever the wear processes, the investigation confirms that ECR failure is reached when the silver concentration in the inner part of the contact is becoming lower than [Ag]th = 5%.

New strategy for the Puluj-type experiment

The frictional mechanical work is calculated as a function of the rotational direction of the calorimeter in the Puluj-type experimental setup (a calorimeter with water inside). Based on this calculus, a new experimental strategy is proposed for demonstrating the equivalence of heat and mechanical work, which reduces the number of physical quantities that need to be measured during the experiment.

Scratch Modeling of Paint Coated Sheet Metal for Multi-Stage Deep Drawing Process

The scratch phenomenon of paint coated sheet metal in multi-stage deep drawing has been investigated. The tensile tests at a quasi-static strain rate of sheet metals coated with two different types of paint were conducted along the rolling, diagonal, and transverse directions of the sheet metal in order to consider strain hardening and anisotropy. It has been found that the types of paint are negligible for plasticity behavior. Finite element simulations of the multi-stage deep drawing process was performed using the obtained material properties and the simulation results were compared with the actual testing. Through the simulation results, three approaches, which include the contact pressure, the accumulated slip distance, and the accumulated friction work were implemented to investigate the scratch phenomenon and they were compared for the scratched and non-scratched regions. It was found that the accumulated slip distance at the scratched region was larger than that of the non-scratched region with a good correlation from experimental observation, whereas either the contact pressure or accumulated friction work approach did not predict the experiments. Therefore, it is concluded that the accumulated slip distance is a good method to identify the scratch on the paint coated sheet metal during deep drawing process.

THE RENAISSANCE OF FRICTION: FROM EMPIRISM TO PHYSICS – AT THE NANOSCALE

Friction and its science hold more than a record. One is longevity: from its very origins, mankind has had to reckon with it. Still today, reducing (or increasing) sliding friction remain technological and practical objectives of enormous importance — one can read for example that no less than 5% of all energy produced daily degrades into wasted frictional work. Another record is that despite the involvement of great scientist like Leonardo, who already five centuries ago gave friction its first scientific bases, there is still today no proper theoretical formulation of friction. Theorists like us mostly limit themselves to what P.W.Anderson jokingly defined in a different context “the indignity of numerical simulations”. However, progress in science does not take place because it is necessary, but because it is possible. In the last decades, new mesoscopic and nanoscopic experimental techniques opened new windows on frictional phenomena at the atomic and molecular level. Jump-started by the necessity and by the challenge to understand some of that data, theory and simulation progress is moving on along some lines which I will briefly describe.

Simulation, Experimental Testing and Optimization of Starting and Shifting Control Strategies of DCT Wet Dual Clutches with Respect to Sliding Friction

For starting and shifting processes of dual-clutch transmission vehicle, the combined starting control strategy for dual clutches is proposed to reduce temperature rise of clutch and the dynamics simulation model of starting and shifting is built. For the two different downshift modes during inertia phase, the work and power of sliding friction are comparatively analysed. The results can provide boundary conditions of heat source for thermal equilibrium analysis under two typical starting conditions. During the benchmark test, the extreme starting condition under full load and large gradient and the three typical shifting conditions are set. Then, the sliding friction test is executed. The transmitted torque of wet dual clutches and the speed of driving and driven plates of dual clutches are measured to calculate work and power of sliding friction, and the corresponding temperature rise of wet dual clutches is obtained. Furthermore, the heat generation and heat dissipation conditions under different sliding friction modes are analysed. The corresponding influences on clutch control and temperature rise of cooling oil are also discussed. Based on the simulation and test results, the thermal equilibrium of clutches is analysed. The evaluation indexes of sliding friction work are calculated, and then the starting and shift control strategies are optimized.