Although we are surrounded by hundreds of surfaces we can still distinguish them from each other simply by touch. The tactile information, interpreted by our brain and given by our fingers, is precise, but to put words to the sensation is very difficult — is it smooth, sticky or harsh? We do not only perceive surfaces differently, we also describe them in our own way. Luckily the forces and deformations that the skin are exposed to when sliding over a surface is ruled by laws of nature. This thesis investigates the contact between finger and surface and how it is affected by, for example, material properties, surface texturing or changes in climate. By measuring contact area, friction coefficient, and adhesion, using different materials and under different conditions, conclusions could be drawn. Also, a model for the contact between a finger and a sinus­oidal surface was developed, which could be used to estimate contact area, deformation and resulting friction coefficient. Results showed how differences in for example material, surface topography and environ­ment affect the interaction between finger and surface, and what consequences it has. If the objective is to change the feel of a surface or to alter the grip, this thesis could work as a support. Paper A investigates the area and friction between finger and glass surface under different conditions. Paper B presents a model for the contact area and deformation for a finger in contact with a sinusoidal surface. Paper C is a validation of the contact area model. Here it was used on new surfaces and compared with new finger friction measurements. Paper D mainly examines whether the adhesion or stickiness of different surfaces is distinguishable by a test panel and how this affects the perceived pleasantness of the surface. Paper E relates to the adhesion and friction for a bioskin probe and skin. Tests were made to evaluate how an artificial probe can be used to evaluate the tactile properties of a surface. / Även om vi omges av hundratals olika ytor kan vi fortfarande skilja dem åt med hjälp av känseln. Den taktila informationen från fingertopparna som tolkas av hjärnan är precis, men att sätta ord på hur ytan känns är väldigt svårt. Len, sträv eller grov? Vi upplever inte bara ytorna olika utan beskriver dem också på olika sätt. Krafterna och deformationerna som huden utsätts för när den glider över en yta styrs dock av naturlagar. Denna avhandling utreder kontakten mellan fingertopp och yta och hur den påverkas av exempelvis materialval, ytstruktur eller förändringar i klimat. Genom mäta kontaktarea, friktionskoefficient och adhesion för olika material i varierande omgivning kunde slutsatser dras. En modell för kontakten mellan fingertopp och sinusformad yta togs fram vilken kunde användas till att uppskatta kontaktarea, deformation och resulterande friktionskoefficient. Resultaten visade hur skillnader i exempelvis material, yttopografi och omgivning påverkar kontakten mellan finger och yta och vilka kon­sekvenser detta får. Om målet är att förändra känslan eller friktionen för en yta kan denna avhandling fungera som stöd. Artikel A undersöker kontakten och friktionen mellan fingertopp och glasyta för olika förhållanden. Artikel B presenterar en modell för arean och deformationen som sker för fingertopp och sinusformad yta i kontakt. Artikel C är en validering av modellen. Här användes den för nya ytor och jämfördes med nya mätningar av fingerfriktion. Artikel D undersöker i huvudsak huruvida en testpanel kan särskilja adhesionen för olika ytor och hur detta påverkar hur den känns. Artikel E arbetar vidare med adhesion och undersöker och hur en testkropp av artificiell hud kan användas för adhesionsmätningar av en yta. Detta för att med relativt enkla mätningar kunna uppskatta ytans taktila egenskaper. / <p>QC 20160504</p>

Thesis advisor: Fabio Ghironi / This dissertation consists of three essays on macroeconomics with frictions and uncertainty shocks. The first essay is "Collateral Constrained Workers' Unemployment". Financial market and labor market are closely interconnected each other in the sense that unemployed workers have difficulty not only in borrowing new loan but also in repaying outstanding loan. In addition, if unemployment entails loss from default and no new loan, credit constrained workers will accept lower wage to avoid the loss from losing job. In this paper, we try to investigate the role of the interaction between financial market and labor market over the business cycle. To do that, we assume credit constrained workers can borrow against their houses and repay outstanding loans only when they are employed. We also introduce labor search and matching framework into our model to consider unemployment and wage bargaining process explicitly. With this setup, we find that adverse housing preference shock leads to substantial negative impact on labor market by reducing the benefit from maintaining job. As a result, high unemployment significantly amplifies the business cycle by reducing supply of loan and increasing default. This result would be helpful to understand recent "Great Recession" which was originated from the collapse of housing market and accompanied by high unemployment and default rate. The second essay is "International Financial Business Cycles". Recent international macroeconomics literature on global imbalances explains the U.S. persistent current account deficit and emerging countries' surplus, i.e., the U.S. is the borrower. Little research has been done on the banking-sector level, where U.S. banks are lenders to banks in emerging countries. We build a two country framework where banks are explicitly modeled to investigate how lending in the banking sector can affect the international macroeconomy during the recent crisis. In steady state, banks in the developing country borrows from the U.S. banks. When the borrowers in the U.S. pay back less than contractually agreed and damage the balance sheet of the U.S. banks, with the presence of bank capital requirement constraint, U.S. banks raise lending rates and decrease the loans made to U.S. borrowers as well as banks in the developing country. The results are a sharp increase in the lending spread, a reduction in output and a depreciation in the real exchange rate of the developing country. They are the experience of many emerging Asian markets following the U.S. financial crisis starting in late 2007. Another feature of our model captures an empirical fact, documented by Devereux and Yetman (2010), that across different economies, countries with lower financial rating can suffer more when the lending country deleverages. The third essay is "Uncertainty, Collateral Constrained Borrowers, and Business Cycle". Standard RBC model fails to generate the co-movement of key macro variables under uncertainty shock because precautionary saving motive decreases consumption but increases investment and labor. To fill this gap, we build a DSGE model with collateral constrained borrowers who can borrow against housing and capital. In the model with modest risk aversion, we can generate the desired co-movement of key macro variables under uncertainty shock and the co-movement comes from the collateral constraint channel through drop in housing price. Under uncertainty shock, highly indebted borrowers sell collaterals to avoid uncertainty in future consumption. As a result, housing price goes down and it makes credit crunch to borrowers through collateral constraint channel. The negative effect of uncertainty shock is strengthened in the economy with higher indebted borrowers. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Economics.

This thesis explores two aspects of modeling the behavior of joint friction in structures. The first aspect deals with the accurate and efficient simulation of a simple system that incorporates the LuGre friction law. Energy transfer and dissipation in a structural joint model is the second topic of this thesis. It is hypothesized that friction could serve to pump energy from one frequency to higher frequencies where it might be dissipated more quickly. Motivation for this study stems from the need to have accurate models of high-precision space structures. Because friction at connecting joints plays a major role in the damping capacity of the structure, a good understanding of this mechanism is necessary to predict the vibratory response and enhance the energy dissipation of the structure. Simulation results of a dynamic system with LuGre friction show that the system is relatively well-conditioned when the slip velocity is small, and ill-conditioned for large slip velocities. Furthermore, the most efficient numerical method to simulate this system is determined to be an implicit integration scheme. To study the energy transfer and dissipation, two models of a jointed structure with friction are considered. Results from the steady-state forced responses of the two structural systems indicate that friction converted low frequency, single harmonic excitation to multi-harmonic response through internal resonances. However, differences in energy dissipation results between the models show that the response of a frictional system is highly sensitive to system parameters and friction laws. Conclusions and suggestions for future research are also discussed.

This Master Thesis was performed in terms of robotic three dimensional friction stir welding with T-butt joint. Friction stir welding (FSW) is a solid state welding method that achieves the weld temperature by friction of a rotating non-consumable tool with the workpiece. Science and technology fast developing requires for higher seam quality and more complex welding joint geometry like 3D welds. In order to acquire high productivity, capacity and flexibility with acceptable cost, robotic FSW solution have been proposed. Instead of the standard FSW machine, using a robot to perform complicated welds such as, three-dimensional. In this report, a solution for weld a 3D T-butt joint, which located in an aluminium cylinder with 1.5 mm thickness using a robot, was developed. Moreover, two new paths were investigated in order to avoid the use of two welds to perform this type of joint. The paths were tested on 2D and on 3D (with a 5050 curvature radius) geometries. Both paths had good results. What is more, the parameter developing methods of FSW process, which is composed of necessary parameter setting, positional compensation was introduced. Specially,the study demonstrates how complicate geometry can be welded using a robot. Also,it shows that TWT temperature control is able to acquire high quality 3D welds. In addition, an analysis of the 2D welding and 3D welding was performed, which exposed that, keeping exactly the same welding conditions, higher lateral forces on the tool were found during 3D welding. Basis on the special case in this paper, when the tool goes like "climbing" the sample, the suffering force of tool decreasing with increasing the height(Z position); nevertheless, when the tool goes like "downhill", the suffering force of tool decreasing with decreasing the height (Z position). What is more, in 2D weld, increasing the downforce (Fz) results increasing the lateral forces which can be Fx and/or Fy. Finally, the future works suggestions were presented in terms of (1) performing the new paths into a real cylinder, (2) performing tensile test on the paths and comparing it with conventional path which weld twice, (3) researching how the downforce (Fz) influence the Fx and Fy during welding of different 3D geometries, (4) how the cooling rate of backing bar influence the seam quality when it is use the same welding parameters and (5) the effect of performing welds in the same welding temperature achieved with different combination of the tool rotational speed and downforce on the material properties

Numerous engineering applications have been proposed to exploit the load-carrying and non-contact nature of noncoalescing and nonwetting systems. One such application is a lab-on-a-chip , or LOC, in which liquid samples would be delivered from point-to-point by sliding over a film of air without requiring either the large driving forces required to pump liquid through a microchannel or liquid-solid contact that could lead to sample-to-sample contamination. Due to the axisymmetry of the flow fields in both the lubricating gas and droplet associated with a stationary nonwetting droplet, such a situation has a vanishing coefficient of static friction. However, once motion is imparted, droplet deformation requires that a force be applied to sustain such motion. The program of research in this dissertation focuses on investigating the lubrication force between a drop of silicone oil and a moving unwetted substrate due to the presence of a gas lubricating film driven by a rotating disk. The frictional (or lubrication) force was measured using an optical-lever technique as a function of: (1) linear velocity of the moving solid; (2) relative displacement of the drop toward the solid; (3) drop volume; and (4) viscosity. The data reveal an increase in magnitude of the measured force with either increasing relative squeezing of the drop against the glass or increasing speed of the rotating disk. Contrary to initial expectations, no pattern could be isolated regarding drop volume or viscosity of the oil. The experimental data collected will serve to validate numerical work as further models are developed.

Belt drives are used in numerous applications to transmit power between various machine elements. One limitation of the use of belt drives is the poor convergence of complex models which did not make them applicable for manufacturing use in industry. A source of convergence failure is the sharp changes in the solution. It is believed that the inclusion of an Elastic/Perfectly-Plastic (EPP) friction law into the belt/pulley contact mechanics can yield mathematical models with enhanced accuracy. This new friction model more accurately captures the true behavior of an elastic belt that exhibits microslip prior to fully-developed slip than previous regularized friction models. The Elastic/Perfectly-Plastic friction model was applied to a two-pulley flat belt system, and the equations of motions were derived using Hamilton's Principle. The results from the analytical model were compared to results from a finite element model. It was found that, unlike Coulomb's Law, the solutions with the EPP model had no slope discontinuities in the normal force. The elimination of these slope discontinuities could potentially help alleviate convergence issues for more complex models. It was also found that if the EPP spring stiffness is too small, then the belt cannot undergo the prescribed tension change. If it is too large, then the EPP model approaches Coulomb's Law and sharp changes appear. The Elastic/Perfectly-Plastic friction model was also applied to a v-belt model. It was found that the solutions and convergence properties with the EPP friction model were similar to the solutions with the Coulomb friction model. When compared to Coulomb's Law, the range of possible high tensions for a given low tension was reduced slightly for the EPP friction. Convergence fails due to sharp changes of the inclination angle and the sliding angle. Because the sharp changes occur when the belt exits the pulley, the EPP friction model cannot smooth the slope discontinuities.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains x, 97 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 69-76).

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Includes bibliographical references (p. 227-233).

In recent decades, significant requirements of changes in composition of brake friction materials in order for faster and more reliable transportation as well as their environmental friendly characterizations attracted attentions. However, the relation between performance and formulation/composition is not clear since friction processes are accompanied by many complex problems such as instability in the coefficient of friction, noise, vibration and wear. Creep-groan is a low frequency vibration which could originate in different part of the system (vehicle). The resulting resonant vibration in the passenger compartment causes discomfort and often leads to complaints of customers and related increase of warranty costs covered by manufacturers. In spite of relatively large amount of publications addressing the creep-groan phenomena, there is not an universal solution addressing the engineering aspects of brake/vehicle design. In addition, Relevance of wear occurring in brake materials increased particularly with relation to the released chemicals and corresponding health and environmental hazards. It is well known that humidity can considerably modify the adhesion of rubbing counterfaces by creating menisci and increasing the contact area. The chemistry, morphology and phase composition of the friction layers (third body) generated on the friction surfaces could play a determining role when amounts of adsorbed water on brake surfaces is concerned. The friction layer is typically a complex mix of numerous materials and, as a rule, contains the agglomerated or sintered nanoparticles. Hence, quantum effects could further modify the adsorption of water. This work addresses the impact of humidity on wear and creep groan of two commercial brake material types: the so called i) "non-asbestos organic" (NAO) and the ii) "semi-metallic" (SM) brake materials rubbed against pearlitic gray cast iron rotors typically used in the passenger vehicles. The friction and wear tests were performed with the Universal Mechanical Tester (UMT) manufactured by Bruker and the wear surfaces/mechanisms were studied by using of scanning electron microscopy, energy dispersive X-ray microanalysis, and optical topography methods. The applied wear testing conditions were designed as a series of particularly designed drag tests and were performed at several different relative humidity levels ranging between 50% and 80%. The major findings confirmed the considerable effect of humidity on wear of brake materials. Both pad types wore noticeably less at increased humidity. This was ascribed to a better capacity to form a protective friction layer. The complex wear mechanisms including abrasive, adhesive, fatigue, and corrosion wear were observed on both material types, irrespectively of humidity levels, and they dependent on the chemistry and phase composition of the friction layer. Humidity also influenced the level of friction.

Este trabalho apresenta o desenvolvimento de um tribômetro para a realização de ensaios com materiais de fricção utilizados em freios veiculares, capaz de servir como uma ferramenta adicional para pesquisa e desenvolvimento de novos produtos. São apresentadas todas as etapas de desenvolvimento da máquina, desde suas premissas básicas, projeto mecânico e de automação e procedimento de calibração até os testes finais para comprovação das suas potencialidades. Construiu-se um equipamento de pequeno porte (dimensões reduzidas), a fim de permitir ensaios rápidos e de baixo custo. O sistema atuador e de rotação foi dimensionado a partir de faixas de velocidade e pressão de contato que ocorrem em sistemas de freio comerciais. O tribômetro permite a troca rápida dos corpos de prova além do ajuste do raio de deslizamento do par de atrito. Além disso, um mecanismo de atuação próprio permite a realização de ensaios sem a influência do tipo de sistema de freio empregado. O sistema de automação, composto por hardware para aquisição de dados e programa para gerar interface gráfica, gerencia todos os processos envolvidos na máquina, além de tornar a sua operacionalidade simples e racional. Um conjunto composto por resistência aquecedora, soprador de ar, relé e termopares é responsável por controlar a temperatura do disco, separando esta variável dos demais parâmetros de operação (pressão e velocidade de escorregamento) durante os ensaios. O procedimento de calibração adotado mostra que o equipamento projetado fornece medições confiáveis da força normal, do torque de frenagem e, conseqüentemente, do coeficiente de atrito. Testes de avaliação do sistema atuador apontam que a força medida sofre a influência da rugosidade do disco, da baixa capacidade do reservatório de ar do sistema pneumático, de efeitos de histerese mecânica e da dilatação térmica e de ruídos eletromagnéticos. Os resultados dos ensaios de frenagem empregados comprovam que a máquina projetada é capaz de separar a variável temperatura do disco dos demais parâmetros de operação e, desta forma, pode servir de ferramenta adicional na compreensão do comportamento do atrito (e do desgaste em um momento posterior) frente às diferentes condições de operação contribuindo, de certa forma, para o aumento do conhecimento dos fenômenos tribológicos. / This work presents the development of a tribometer for friction tests with materials used in vehicular brakes, able to serving as an additional tool for research and development of new products. Every stage of the project are presented since its basic premises, mechanical and automation design, the calibration procedure until the final tests to prove its potential. It was constructed an equipment with small dimensions to enable rapid and low cost testing. The actuator and rotation system has been designed to operate in the range of velocity and contact pressures that occur in commercial brake systems. The tribometer allows the rapid exchange of specimen beyond the adjustment of the wear track radius. Furthermore, a special mechanism of action was designed to allow testing without the influence of the type of brake system employed. The automation system, composed of a data acquisition hardware and software to generate graphical interface, manages all processes involved in the machine, and makes its operation simple and rational. A set composed of heating resistance, air blower, relay and thermocouples is responsible for controlling the temperature of the disk, separating it from other variable operating conditions (pressure and slip velocity) during tests. The calibration procedure performed show that the equipment designed provides reliable measurements of normal force, braking torque and friction coefficient. The tests for evaluating the system actuator suggest that the force measurement has influence of the disk surface roughness, of low air capacity in the tank, of thermal dilation and mechanic hysteresis effects and of electromagnetic noise. The results of braking tests performed show that the designed machine is capable of separating the variable disk temperature of the other operation parameters and thus can serve as an additional tool in understanding the friction behavior (and wear a moment later) under a set of operating conditions. It can contribute, in some way, to enlarge the knowledge with respect to tribologics phenomenons.