Auswahl der wissenschaftlichen Literatur zum Thema „Friction rate-and-state“

AbstractAdhesion is one of essences with respect to rubber friction because the magnitude of the friction force is closely related to the magnitude of adhesion on a real contact area. However, the real contact area during sliding depends on the state and history of the contact surface. Therefore, the friction force occasionally exhibits rate-, state-, and pressure dependency. In this study, to rationally describe friction and simulate boundary value problems, a rate-, state-, and pressure-dependent friction model based on the elastoplastic theory was formulated. First, the evolution law for the friction coefficient was prescribed. Next, a nonlinear sliding surface (frictional criterion) was adopted, and several other evolution laws for internal state variables were prescribed. Subsequently, the typical response characteristics of the proposed friction model were demonstrated, and its validity was verified by comparing the obtained results with those of experiments conducted considering the contact surface between a rough rubber hemisphere and smooth acrylic plate.

In the present study, the rate- and state-dependent friction model [Hashiguchi and Ozaki, 2008] is implemented in the dynamic finite element method. The typical rate- and state-dependent frictional contact problems, which are consisted by elastic and rigid bodies having simple shapes, are then analyzed by the present method. The validity of the present method for the microscopic sliding and stick-slip instability is examined under various dynamic characteristics of the system, such as contact load, elastic stiffness, driving velocity and frictional properties. It is shown that the present method can solve simultaneously not only rate- and state-dependent frictional behavior on the contact boundary but also coupling effects with internal deformations, whereas it cannot predicted by the conventional finite element analysis with the Coulomb’s friction law.

The frictional forces between the concrete slab and base has been combined with the movements of the horizontal slab that have been induced by variations of the moisture and temperature in concrete slabs. The frictional drag that acts on the slab bottom as a result of base friction is in an opposite horizontal slab displacement direction, and resist movements of the horizontal slab. A condition of smoother interface provides lower resistance to slab movement. On the other hand, rough interfaces are beneficial in the reduction of the load-related stresses. As bonding degree between slab and foundation affects the friction that has been mobilized at interface, a realistic evaluation of friction of the interface is required for the rational designs of the concrete pavement. In this work, push-off test has been performed. Based upon results of the friction tests, the friction characteristics of concrete and soil have been researched. The parameters that influence the maximal displacement and friction coefficient are (interface state, rate of movement) for friction and (rate of movement, interface condition) for the displacements, respectively. Finally, once the applied force reaches a stable state, the frictional force increases dramatically. The most important influence on this force is the interface state, which is accompanied by movement rate. The change of the interface from a smooth to a rough surface increases the overall coefficient of friction.

Les glissements de terrain déclenchés par des séismes sont une source de risque majeure. C'est un phénomène ubiquiste, et ses conséquences s'étendent des coupures des voies de communication aux tsunamis. Afin de comprendre les processus qui entraînent leur instabilité, nous nous sommes demandé : peut-on modéliser les glissements sur faille préexistante et leur déclenchement sismique à l'aide d'une loi de friction rate-and-state dérivée d'expériences de laboratoire ? Dans un premier temps, nous avons identifié les rôles des propriétés du glissement (friction, épaisseur de la masse sédimentaire...) et de l'onde incidente (fréquence, durée et amplitude) au travers de simulations numériques par la méthode des éléments spectraux, et d'analyses théoriques. En suivant la variable d'état de la loi rate-and-state, il nous est possible de déterminer l'état de stabilité d'un glissement considéré dans le cas d'ondes incidentes mono-fréquentielles. Dans un second temps, nous avons appliqué ces résultats théoriques au cas de la pente sous-marine de l'aéroport de Nice. À l'aide des précédentes études et de récentes carottes sédimentaires forées aux abords de la cicatrice d'arrachement du glissement de 1979 (campagne IFREMER MaRoLyS-PenFeld), et de tests de friction au laboratoire de l'Université La Sapienza à Rome, nous avons contraint les paramètres géomécaniques du glissement dans nos simulations. Ces valeurs, associées à l'utilisation de fonctions de Green empiriques pour les ondes incidentes, nous ont permis d'analyser la stabilité de la pente sous-marine niçoise sous différents scénarios de déclenchement sismique, en particulier des séismes de magnitude 6.5 à différentes distances épicentrales
Coseismic landslides contribute to casualties and economic losses during earthquakes.This phenomena is ubiquitous, and its consequences range from cutting off road portions to tsunamis. In order to understand the processes that lead to their instability, we asked ourselves: can we model landslides on pre-existing faults and their seismic triggering using a friction law derived from laboratory experiments, the rate-and-state law. In a first step, we identified the roles of the properties of the landslide (friction, thickness of these dimentary mass, etc.) and the incident wave (frequency, duration, and amplitude) through numerical simulations using the spectral element method and theoretical analyses. By following the state variable of the rate-and-state law, we can determine the stability state of a landslide considered in the case of single-frequency incident waves. In a second step, we applied these theoretical results to the case of the underwater slope of Nice airport. Using previous studies and recent sediment cores drilled near the 1979 landslide tear scar (IFREMER MaRoLyS-PenFeld oceanographic campaign), and rate-and-state laboratory tests at University La Sapienza in Rome, we constrained the geomechanical parameters of the landslide in our simulations. These values, combined with the use of empirical Green's functions for incident waves, allowed us to analyze the stability of the Nice submarine slope under different seismic triggering scenarios, in particular earthquakes of magnitude 6.5 with different epicentral distance

It is of great interest to isolate the fundamental physical mechanism controlling observed statistical properties of seismicity patterns. We present four numerical studies investigating the e ciency of uid related mechanisms and the role of fault zone heterogeneity in producing observed earthquake complexities. The 3-D models of the continuous class are governed by rate- and state-dependent friction and, depending on the problem, by elasto-hydraulic interactions or heterogeneous frictional properties on the 2-D fault plane. First, for certain ranges of hydraulically relevant parameters dilatant processes are shown to stabilize accelerating slip instabilities on a uid in ltrated fault, leading to nonuniform spatio-temporal slip evolution. The second model demonstrates the ability of heterogeneous pore pressure conditions in an undrained environment to produce complex slip pattern, where unstable sliding corresponds to regions with low degrees of overpressurization. In the third study we focus on the role of complex fault zone structure, parameterized by heterogeneous distributions of the rate and state slip weakening distance. The approach is shown to be a powerful and consistent method to generate seismicity patterns with properties similar to those of natural seismicity. Due to the e ciency of this parameterization we use it in the fourth study to investigate fault zones at di erent evolutionary stages and associated seismic response types. Using heterogeneous, correlated maps of the slip weakening distance we explore systematically the e ect of the range of size scales, correlation lengths and a statistical parameter related to roughness, on seismic response characteristics. In summary, we observe an increase in e ciency from the rst to the last study to generate synthetic seismicity with realistic statistical properties, suggesting that the range of size scales is the most fundamental parameter in explaining complex earthquake related phenomena. In the last part we analyze the generated synthetic seismicity catalogs with respect to their overall source scaling behavior. We nd that the general scaling trends of source properties of the simulated slip maps are in very good agreement with observations reported in the literature. We also show that the catalog of source models provides a useful resource on physically self-consistent scenario earthquakes for groundmotion simulations. We make use of this resource calculating waveforms and shake intensity maps for a suite of example events.
Institute of Geophysics, ETH Zurich. This work was sponsored by EC-Project RELIEF (EVG1-CT-2002-00069).
Unpublished
open

I terremoti indotti dalle attività antropiche costituiscono una discriminante per il successo delle attività industriali quali l’iniezione di acque reflue, l’estrazione di petrolio e gas naturale, lo sfruttamento di energia geotermica e lo stoccaggio geologico di anidride carbonica. Negli ultimi anni, i basalti hanno catturato l’attenzione dell’industria del settore energetico e della comunità scientifica, a causa della loro vasta diffusione nella litosfera oceanica e della loro capacità di mineralizzare la CO2 “trasformandola in roccia” (New York Times 9/2/2015). Tale proprietà, consente di fatto di sottrarre a lungo termine l’anidride carbonica presente in atmosfera, contribuendo pertanto alla riduzione locale delle emissioni di CO2 di origine antropogenica. Comprendere le proprietà di attrito, meccaniche e idrologiche di faglie e fratture in basalto ha assunto pertanto un'importanza fondamentale, per le dirette implicazioni riguardanti l'enucleazione dei terremoti, la loro propagazione e l'arresto in ambienti geologici dominati dai basalti. Per meglio comprendere le proprietà meccaniche di faglie e fratture in basalto, e in particolare la fase di enucleazione dei terremoti, esperimenti di attrito sono stati realizzati mediante l’apparato biassiale BRAVA e l’apparato di tipo rotativo SHIVA, entrambi installati presso l’Istituto Nazionale di Geofisica e Vulcanologia (INGV, Roma). Invece, per caratterizzare le proprietà di trasporto delle carote di basalto e delle faglie sperimentali, la trasmissività idraulica è stata misurata mediante il permeametro, prima e dopo gli esperimenti di attrito su SHIVA. Sono stati trattati tre principali argomenti seguendo un approccio sperimentale per caratterizzare: 1) le proprietà di resistenza di attrito, stabilità e di healing delle faglie sperimentali in basalto (ovvero, faglie polverizzate e superfici di faglia) in condizioni di umidità atmosferica e in condizioni bagnate, integrando i dati meccanici con quelli microstrutturali (Capitolo 2); 2) le instabilità dell’attrito ed i processi di carbonatazione delle superfici di faglia sperimentali aventi diversi gradi di alterazione, cagionati dall’iniezione di fluidi ricchi in H2O, CO2, misture H2O-CO2, e Argon (Capitolo 3); 3) le variazioni delle proprietà idromeccaniche delle superfici di faglia sperimentali e la loro influenza sul loro comportamento durante l’iniezione di acqua in pressione (Capitolo 4). Per quanto concerne i cilindri cavi descritti nel capitolo 4, l’analisi accurata dello stato di sforzo negli esperimenti di tipo rotativo, ha richiesto lo sviluppo di un modello basato sui dati sperimentali che tenesse conto della geometria cilindrica dei campioni montati su SHIVA, la quale modifica il modo in cui la pressione di fluido influisce sullo sforzo normale efficace agente sulla faglia (Appendice 1). Tutti i test sono stati realizzati a temperatura ambiente, che può emulare le condizioni di temperatura di un sito energetico a bassa entalpia. In questa tesi, complessivamente si osservano valori del coefficiente di attrito statico intorno a μ ~ 0.6 – 0.8, a diverse condizioni che spaziano dall’umidità atmosferica a quelle sovra-idrostatiche, indipendentemente dello stato di alterazione dei basalti e della composizione chimica del fluido iniettato durante gli esperimenti a breve termine (< 60 min). Pertanto, le faglie in basalto sono considerate “forti”, e gli elevati tassi di healing testimoniano la loro abilità di riguadagnare la resistenza al taglio durante il periodo intersismico. Secondariamente, metto in evidenza come la struttura delle faglie controlli le proprietà di rate and state e la stabilità delle stesse: mentre le polveri sono più propense ad enucleare terremoti (ovvero possiedono un comportamento di indebolimento con l’aumento di velocità: velocity weakening) quando, a seguito di processi cataclastici con riduzione della granulometria, la deformazione diventa localizzata lungo zone di deformazione ben sviluppate, al contrario, le superfici di faglia passano a un comportamento di incremento dell’attrito con l’aumentare della velocità (velocity strengthening), a seguito di processi di dilatanza che accompagnano la produzione di detrito durante lo scivolamento. Infine, si è osservato che i cambiamenti nelle proprietà idromeccaniche durante la pressurizzazione di fluido possono diventare dominanti rispetto agli effetti prodotti dai cambiamenti di attrito di secondo ordine predetti dalle leggi di rate. A tale riguardo, ho rilevato un più pronunciato indebolimento idromeccanico, laddove la trasmissività idraulica della faglia è minore. Questa osservazione fornisce un efficace meccanismo per l’indebolimento delle faglie e in ultima istanza, portare all’enucleazione di terremoti anche nelle porzioni faglie in basalto caratterizzate da un comportamento “velocity strengthening”.
Earthquakes induced by anthropic activities are a major concern for the success of the industrial operations associated with in-situ underground wastewater injection, oil and gas withdrawals, geothermal energy exploitation, and geological carbon sequestration. Over the last few decades, basalt rocks have drawn heightened attention from the geo-energy industry and the scientific community because of their widespread occurrence in the oceanic lithosphere and their efficiency to act as carbon sinks, thus contributing to locally reduce the CO2 anthropogenic emissions. Given the direct implications for earthquake nucleation, propagation, and arrest in basaltic-dominated environments, understanding the frictional, mechanical, and transport properties of basalts-bearing faults and fractures has become of paramount importance. To gain better insights on the mechanical behavior of basalt-hosted faults, notably the earthquake nucleation phase, friction experiments were performed using the biaxial deformation machine BRAVA and the rotary-shear apparatus SHIVA, both installed at the National Institute of Geophysics and Volcanology (INGV, Rome), Italy. Whereas, to characterize the transport properties of basalt cores and simulated faults, hydraulic transmissivity was measured on the permeameter and before and after friction tests on SHIVA. Three main scientific topics were addressed using an experimental approach: 1) the frictional strength, stability, and healing properties of basalt-built experimental faults (i.e., simulated gouge and bare rock surfaces) under room-dry and wet conditions, by integrating the mechanical data with fault microstructures (Chapter 2); 2) the frictional instabilities and carbonation processes of simulated initially bare rock surfaces with different degree of alteration, triggered by injection of pressurized H2O, pure CO2 , CO2 - rich water, and Argon (Chapter 3); 3) the hydromechanical properties changes of simulated initially bare rock surfaces and their influence on the fault slip behavior during water pressurization (Chapter 4). The accurate stress paths analysis from rotary-shear tests involving hollow bare rock surfaces in Ch.4 required the development of an experimentally derived model accounting for the cylindrical geometry of SHIVA samples, that modifies the fluid pressure contribution on the effective normal stress acting on the laboratory fault, (Appendix 1). All the tests were performed at ambient temperature, which may mimic the temperature conditions in low enthalpy geo-energy sites in basalts. In this dissertation, overall, I demonstrate that the static friction coefficient of basalts is in the range of μ ~ 0.6 – 0.8, at conditions ranging from room-dry to supra-hydrostatic, regardless of the alteration state of basalts and the fluid chemistry during short-term laboratory experiments (< 60 min). Therefore, basalts are inherently frictionally strong and the high healing rates testify their ability to regain shear strength during the interseismic period. Secondly, I show that fault microstructure controls their frictional stability: while simulated gouge are more prone to host earthquake nucleation (i.e., velocity weakening behavior) when deformation becomes localized along well-developed shear zones formed in response to cataclasis and grain size reduction, bare rock surfaces show the opposite behavior, transitioning to velocity strengthening behavior promoted by dilatancy processes coupled with gouge production during shearing. Finally, I illustrate that changes in coupled hydromechanical properties during fluid pressurization can dominate over the effects of second-order frictional changes predicted by the rate-and state-friction laws. In this regard, I observed that hydromechanical weakening effects become more pronounced the lower the fault transmissivity. This evidence provides an effective mechanism for inducing fault weakening and ultimately, to bring about earthquake slip also in velocity-strengthening basalt fault patches.

This chapter discusses dynamical solvent effects on the rate constants for chemical reactions in solution. The effect is described by stochastic dynamics, where the influence of the solvent on the reaction dynamics is included by describing the motion along the reaction coordinate as Brownian motion. Two theoretical approaches are discussed: Kramers theory with a constant time-independent solvent friction coefficient and Grote–Hynes theory, a generalization of Kramers theory, based on the generalized Langevin equation with a time-dependent solvent friction coefficient. The expressions for the rate constants have the same form as in transition-state theory, but are multiplied by transmission coefficients that incorporate the dynamical solvent effect. In the limit of fast motion along the reaction coordinate, the solvent molecules can be considered as “frozen,” and the predictions of the Grote–Hynes theory can differ from the Kramers theory by several orders of magnitude.

AbstractMechanical surface hardening processes have long been of interest to science and technology. Today, surface modification technologies have reached a new level. One of them is friction stir processing that refines the grain structure of the material to a submicrocrystalline state. Previously, the severe plastic deformation occurring during processing was mainly described from the standpoint of temperature and deformation, because the process is primarily thermomechanical. Modeling of friction stir welding and processing predicted well the heat generation in a quasi-liquid medium. However, the friction stir process takes place in the solid phase, and therefore the mass transfer issues remained unresolved. The present work develops the concept of adhesive-cohesive mass transfer during which the rotating tool entrains the material due to adhesion, builds up a transfer layer due to cohesion, and then leaves it behind. Thus, the transfer layer thickness is a clear criterion for the mass transfer effectiveness. Here we investigate the effect of the load on the transfer layer and analyze it from the viewpoint of the friction coefficient and heat generation. It is shown that the transfer layer thickness increases with increasing load, reaches a maximum, and then decreases. In so doing, the average moment on the tool and the temperature constantly grow, while the friction coefficient decreases. This means that the mass transfer cannot be fully described in terms of temperature and strain. The given load dependence of the transfer layer thickness is explained by an increase in the cohesion forces with increasing load, and then by a decrease in cohesion due to material overheating. The maximum transfer layer thickness is equal to the feed to rotation rate ratio and is observed at the axial load that causes a stress close to the yield point of the material. Additional plasticization of the material resulting from the acoustoplastic effect induced by ultrasonic treatment slightly reduces the transfer layer thickness, but has almost no effect on the moment, friction coefficient, and temperature. The surface roughness of the processed material is found to have a similar load dependence.

Abstract Earthquakes can be triggered by fluid injection into underground formations. Fluid injection can cause large changes in the underground volume that exert stresses on nearby preexisting faults, leading to seismic activity. Assuming an increase in underground development activities in the future, our understanding of the mechanism underlying induced seismicity must be improved, and methods must be developed to properly assess the risk of seismic events. The objective of this study is to develop a seismicity prediction model that calculates the magnitude and timing of triggered earthquakes or seismic events occurring during various subsurface fluid injection activities. We developed an injection-induced seismicity analysis model that predicts the dynamic earthquake nucleation caused by changes in stress and pore pressure that occur during various subsurface activities. The governing equations consisting of the dynamic motion of the poroelastic spring-slider system, rate and state friction law, and pore pressure diffusion equation were solved using the embedded semi-implicit Runge–Kutta method. The seismicity analysis model was also incorporated into the finite element method model, considering the variations in the stresses and pore pressures in the formation. A field case study was also conducted to compare the model results with typical microseismicity responses observed from hydraulic fracturing treatments in shale fields. Contrary to the popular understanding derived from Amonton's law, the dynamic friction model revealed that a large normal stress on the fault increases the seismic risk. A larger normal stress accumulates a large amount of elastic energy until it slips owing to fluid injection, nucleating large seismic waves. The poroelastic spring-slider model estimated reasonable seismic magnitudes for hydraulic fracturing treatment but overestimated the time required to trigger a seismic event under field conditions. To improve the analysis results, the poroelastic spring-slider model was coupled with a linear elastic FEM that considered the complex interplay of stress changes from hydraulic fracturing and the associated pore pressure variation in the formation. Compared with the field data, the coupled simulation model estimated a reasonable timing for the induced seismic events when the increasing pore pressure during hydraulic fracturing penetrated deep into the formation. These findings suggest the existence of permeable natural fractures in the formation, which intensify early frictional sliding during treatment. The seismicity prediction model presented in this study simulates the magnitude and timing of seismic nucleation, helping to manage and mitigate the environmental impacts of induced seismicity during various subsurface development activities, such as oil and gas extraction, hydraulic fracturing, geothermal, and carbon dioxide sequestration. Moreover, the case study results imply that the time-series of seismic events predicted by the model can be used to understand the possible fracture geometry and extent of fluid invasion for field applications.

ABSTRACT The Coulomb failure stress (CFS) criterion based on the Mohr-Coulomb friction theory is commonly used to explain physical mechanisms governing injection-induced seismicity. While the CFS criterion can evaluate the onset of fault reactivation caused by fluid perturbations, it cannot tell the kinematic process of fault failure (seismic or aseismic slip). An alternative model for simulating time-dependent earthquake cycles is the rate-and-state friction model, which provides evolving friction depending on slip rate and slip history. To explore the dynamics and stability of fault slip associated with fluid injection, we extend a spring-slider system with a rate-and-state dependent friction law by incorporating the evolving fluid pressure and poroelastic stress. Simulations of continuous constant fluid mass injection rate scenarios using the developed model suggest that injection-induced fault slip behavior is controlled by fault orientation, diffusivity and poroelasticity, and injection rate. Compared to the CFS criterion, our model can offer a temporal component to fault friction and provide new insights on slip, slip rate and trajectory in phase space. We also investigate the relation between cumulative slip displacement versus cumulative injection volume, as well as the event recurrence interval. Our proposed approach has the potential application for evaluating the reactivation of pre-existing faults embedded in the reservoir associated with fluid pressure operations in the field practice. INTRODUCTION It has been well established that fluid injection into subsurface reservoirs can induce earthquakes (Healy et al., 1968; Raleigh et al., 1976). One of the most frequently discussed scientific issues is the physical mechanism that causes earthquakes to nucleate in response to fluid injection. The Coulomb failure stress criterion based on the Mohr-Coulomb friction theory is commonly used to explain the mechanism of induced seismicity (Ge & Saar, 2022). The Coulomb failure stress (CFS) and the change in Coulomb failure stress (ΔCFS) can be expressed as (Equation) (Equation) where μ is the friction coefficient that is assumed to be constant above, τ, σn and P refer to shear stress, normal stress (positive for compression), and pore pressure acting on the fault, respectively. Δτ, Δσn and ΔP denote the changes of shear stress, normal stress and pore pressure, respectively.

The friction and wear properties of nanocrystalline cobalt (nc Co) with a grain size of 20±5 nm and a hardness of 503±13 HV were studied using a pin-on-disc tribometer. Tests performed under unlubricated sliding conditions in ambient air showed that large tribolayer area covered the nc Co’s wear track. The oxygen concentration of the tribolayer was higher than that formed on contact surfaces of microcrystalline cobalt (mc Co) with a grain size of 16±3 μm and a hardness of 299±8 HV tested under the same conditions, due to the higher tendency of nc Co for oxidation. Higher rate of oxidational wear in nc Co resulted in higher initial surface damage in this material compared to the mc Co. Once the tribolayer was formed on top of the contact surfaces, a steady-state wear regime prevailed, reducing the coefficient of friction (COF) and the wear rate in this sample.

Abstract Injecting fluid into subsurface strata has the potential to cause earthquakes by altering pore pressure and subsurface stress. To assess the seismic hazard associated with subsurface flow processes, it is necessary to understand the underlying mechanics of fluid-induced fault reactivation. In this study, we conduct a coupled hydro-mechanical modeling of fluid injection to a strike-slip fault with rate-and-state friction. We account for the fluid flow across and along the fault, as well as the hydromechanical properties of faults in the normal and tangential directions. We model the injection-induced slip of a strike-slip fault, and the simulation results indicate that there are two primary factors that affect injection-induced seismicity. The first factor is that the initiation of rupture is directly related to the diffusion of pore pressure in the near field where there is high shear stress and a large reduction in fault strength due to the significant pressure change. The second factor is that the transfer of shear stress from the nucleation zone promotes the advancement of the slip front to the near- and far field. Our results are quite conservative since the model chose pf as the relevant pressure when calculating the effective normal stress and the shear stress has a slight effect on the pressure variation. Finally, the sensitivity analysis indicates that greater tangential permeability values delay the onset of fault rupture and diminish the likelihood of fault reactivation. Higher stiffness induces fault slip earlier but reduces its magnitude.

Superhydrophobic surfaces are surfaces with fluid contact angles larger than 150°. Superhydrophobicity can be achieved by chemically modifying the surface or introducing texturing which increases the real or effective area of the surface. In this work we focus on the latter approach. If the texturing leads to a Cassie non-wetting state, the surface can also exhibit drag reduction characteristics. Thus, for the same energy input drag reducing surfaces lead to higher flow velocities or, conversely, in order to achieve the same flow rates and velocities, drag reducing surfaces require less energy input. In order to optimize the surface topography of the superhydrophobic surface, a stratified two-phase model of flow between flat plates was developed to simulate the friction reduction characteristics of the surface as a function of varying ‘fluid to gas’ ratios. The Stokes flow equation was used to derive velocity profiles with appropriate slip/no-slip conditions within the flow. Non-dimensional formulations were used to optimize the liquid flow rate as a function of the gas layer thickness. Based on these formulations, a pressure drop reduction of 72% is achieved when the air layer height to the total channel height is 7%. The results of the theoretical model were also compared against experimental measurements of microfluidic channels with different substrate surface topographies. Two different types of silicon substrates were used: one with flat plane topography and one with a micropillar array. The substrates were irreversibly bonded to PDMS (poly-dimethylsiloxane; Dow Corning) microfluidic channel replicas and were silanized to further enhance hydrophobicity. Flow was induced using a constant pressure source and the flow rate was measured using a high-precision scale. As expected, the experimental results deviated somewhat from the expected theoretical model due to the presence of the micropillar obstruction in the air layer. It was also observed that there was a certain ‘pillar-to-channel height’ ratio that minimized the pressure drop for a given flow rate.

The tribological behaviours of conventional nickel with a grain size of 20±5 μm and nanocrystalline (nc) nickel with a grain size of 15±3 nm were compared. A pin-on-disc tribometer was employed for the friction and wear measurements under unlubricated conditions in ambient air with ∼35% relative humidity. As the grain size was decreased to the nanometer size, a reduction of about 18% was observed in the peak value of coefficient of friction (COF), but the steady-state COF remained almost unchanged. Also it was shown that the wear rate of nc nickel was about 82% lower than that of conventional nickel at 2 N load. This behaviour is mainly attributed to considerable reduction in plastic deformation and microplowing due to increased hardness.

Stability control systems on the market today, while effective, operate without full information on the vehicle states and road friction properties. This paper presents a vehicle control scheme that takes into account vehicle state information on sideslip angle and yaw rate, as well as road coefficient of friction, to keep the vehicle within a safe region of the state space. The controller limits state growth outside of the safe area to a sliding surface defined by the distance to the closest operating point in the safe region. Experimental results validate a simple version of the controller on a low friction surface. The controller successfully stabilizes the vehicle using steer-by-wire as a control input.

Current Friction Stir Welding (FSW) process modeling research is concerned with the detailed analysis of local effects such as material flow, heat generation, etc. These detailed thermo-mechanical models are typically solved using finite element or finite difference schemes and require substantial computational effort to determine temperature, forces, etc. at a single point in time. Dynamic models describing the total forces acting on the tool throughout the entire welding process are required for the design of feedback control strategies and improved process planning and analysis. In this paper, empirical models relating the process parameters (i.e., plunge depth, traverse rate, and rotation speed) to the process variables (i.e., axial, traverse, and lateral forces) are developed to understand their dynamic relationship. First, the steady-state relationship between the process parameters and variables is constructed, and the relative importance of each process parameter on each process variable is determined. Next, the dynamic process response characteristics are determined using Recursive Least-Squares. The results indicate that the steady-state relationship between the process parameters and variables is well characterized by a nonlinear power relationship, and the dynamic responses are well characterized by low-order linear equations. Experiments are conducted that validate the developed FSW dynamic models.