An improved knowledge of wood material behavior in hot-pressing environments can provide the benefit of an increased understanding of material properties during the manufacture of wood-based composites as well as insight into the development of new processes and products which manipulate the viscoelastic nature of wood. Two specific areas where additional knowledge is needed are: the high temperature equilibrium moisture content (EMC) behavior and the moisture dependent softening behavior. EMC data was collected and desorption isotherms were generated for mature and juvenile wood of aspen, loblolly pine and yellow-poplar at 50 and 160°C. High temperature EMC behavior was found to be distinct from that at lower temperatures, and considerable differences between the isotherms for juvenile and mature wood were detected. Substantial thermal degradation was observed during desorption at 160 °C and found to be strongly influenced by relative humidity. The thermal softening behavior of wood was evaluated using dielectric thermal analysis (DETA) at moisture levels from 0 to 20 percent. Coincident in situ relaxations of hemicellulose and amorphous cellulose in the range of 20 to 200 °C were observed and found to exhibit the characteristics of a secondary (glass) transition. The moisture dependence of this transition was characterized, and differences in the observed Tg were detected between juvenile and mature wood. Time-temperature superposition was also shown to be applicable to the wood-water system. / Ph. D.

La projection à froid, aussi appelée cold spray, est considérée comme un nouveau membre de la famille de laprojection thermique depuis une trentaine d'années maintenant. Cette thèse propose d'étudier le comportement endéformation des particules et du substrat et de mettre en avant les liaisons formées dans le revêtement par deuxapproches complémentaires, expérimentale et de simulation.Une méthode innovante pour observer directement la surface fracturée des particules déposées après décollementdu substrat a été testée avec succès. Par ce moyen, la surface de contact entre particule et substrat sousdifférentes conditions a été analysée.Concernant les résultats expérimentaux, une nouvelle théorie a été proposée pour expliquer le mécanisme deliaison interfaciale d'un revêtement dur de Ni sur substrat mou d'Al reposant sur l'effet de martelage répété desparticules, sur l'effet de pression du gaz principal et sur l'effet de préchauffage du substrat. La transformation dumécanisme de liaison revêtement/substrat au cours de la construction du dépôt a été mise en évidence en passantdu verrouillage mécanique à une combinaison d'une liaison mécanique et d'une liaison métallurgique, puis à laformation d'instabilités sous forme d'un mélange tourbillonnaire à l'interface. Plus de zones de liaisonsmétallurgiques sont générées sous forte pression, une plus grande déformation plastique apparaît grâce latempérature de préchauffage, et une adhérence plus forte au sein des dépôts est capable de se produire en dépitde la présence d'un film d'oxyde épais sur la surface du substrat. / Cold spraying, also called cold gas dynamic spraying, is a new coating technology which has been developed duringthe past three decade. In this study, a comprehensive investigation on particle deformation behavior and bondingbehavior between particle and substrate was conducted by experiment and numerical method.This thesis aims at presenting an innovative method to directly observe the fractured contact surface between thecold sprayed particle and substrate. By this means, the particle/substrate fractured contact surfaces were analyzedfor different conditions.Based on the experimental results, a new theory was proposed to explain the interfacial bonding mechanism of hardNi coating onto soft Al substrate. It is assumed that the particle peening effect is essential for the formation ofdiscontinuous metallurgical bonding. The dominant coating/substrate bonding mechanism is responsible of thetransformation during the coating build-up process of the initial mechanical interlocking to a combination ofmechanical interlocking and metallurgical bonding therefore of the formation of interfacial instabilities. The highcontact pressure is the relevant factor determining the particle/substrate metallurgical bonding. More metallurgicalbonding areas were generated due to strengthen peening effect of the subsequently deposited particles with higherpropelling gas pressure. Finally, stronger adhesion is able to occur despite the presence of a thick oxide film on thesubstrate surface by the preheating of the substrate. Higher temperatures help the materials to undergoes astronger plastic deformation that disrupts the oxide films. That leads to initiate an intimate contact between particleand substrate.

The temperature rise at the interface of two sliding bodies has significant bearing on the friction and wear characteristics of the bodies. The friction heat generated at the interface can be viewed as "loss of exergy" of the system, which also leads to accelerated wear in the form of oxidation, corrosion, and scuffing. This has a direct impact on the performance of the components or the machinery. If the sliding interface is also conducting electric current then the physics at the interface becomes complicated. The presence of electrical current leads to Joule heat generation at the interface along with other effects like electromotive, electroplasticity, stress relaxation and creep. The interface of an electrical contact, either stationary or dynamic, is a complex environment as several different physical phenomena can occur simultaneously at different scales of observations. The main motivation for this work stems from the need to provide means for accurate determination or prediction of the critical contact parameters viz., temperature and contact resistance. Understanding the behavior of electrical contacts both static and dynamic under various operating conditions can provide new insights into the behavior of the interface. This dissertation covers three major topics: (1) temperature rise at the interface of sliding bodies, (2) study on static electrical contacts, and (3) study of factors influencing behavior of sliding electrical contacts under high current densities. A model for determining the steady-state temperature distribution at the interface of two sliding bodies, with arbitrary initial temperatures and subjected to Coulomb and/or Joule heating, is developed. The model applies the technique of least squares regression to apply the condition of temperature continuity at every point in the domain. The results of the analysis are presented as a function of non-dimensional parameters of Peclet number, thermal conductivity ratio and ellipticity ratio. This model is first of its kind and enables the prediction of full temperature field. The analysis can be applied to a macro-scale contact, ignoring surface roughness, between two bodies and also to contact between two asperities. This analysis also yields an analytical expression for determining the heat partition between two bodies, if the Jaeger's hypothesis of equating average temperatures of both the bodies is being implemented. In general for design purposes one is interested in either the maximum or the average temperature rise at the interface of two sliding bodies. Jaeger had presented simple equations, based on matching the average temperatures of both bodies, for square and band shaped contact geometries. Engineers since then have been using those equations for determining the interface temperature for circular and elliptical shaped contact geometries. Curve fit equations for determining the maximum and the average interface temperature for circular and elliptical contact with semi-ellipsoidal form of heat distribution are presented. These curve fit equations are also applicable for the case when both the bodies have dissimilar initial bulk temperatures. The equations are presented in terms of non-dimensional parameters and hence can easily be applied to any practical scenario. The knowledge of electrical contact resistance between two bodies is important in ascertaining the Joule heat generation at the interface. The prediction of the contact resistance thus becomes important in predicting the performance of the contact or the machinery where the contact exists. The existing models for predicting ECR suffer from the drawback of ambiguity of the definition of input parameters as they depend on the sampling resolution of the measuring device. A multi-scale ECR model which decomposes the surface into its component frequencies, thus capturing the multi scale nature of rough surfaces, is developed to predict the electrical contact resistance. This model, based on the JS multi-scale contact model, overcomes the sensitivity to sampling resolution inherent in many asperity based models in the literature. The multi-scale ECR model also offers orders of magnitude of savings in computation time when compared to deterministic contact models. The model predictions are compared with the experimental observations over a wide range of loads and surface roughness of the specimens, and it is observed that the model predictions are within 50% of the experimental observations. The effect of current cycling through static electrical contact is presented. It is observed that, the voltage drop across the contact initially increases with current until a certain critical voltage is increased. Beyond this critical point any increase in the current causes essentially no increase in steady-state contact voltage. This critical voltage is referred to as "saturation voltage." The saturation voltage for Al 6061 interface is found to be in the range of 160 - 190 mV and that for Cu 110 interface is in the range of 100 - 130 mV. The effect of load and surface roughness on voltage saturation is also demonstrated experimentally. An explanation based on the softening of the interface, due to temperature rise, is proposed rather than more widely referred hypothesis of recrystallization. The phenomenon of voltage saturation is also demonstrated in sliding electrical contacts. The behavior of sliding interfaces of aluminum-copper (Al-Cu) and aluminum-aluminum (Al-Al) are analyzed under high current densities. Experimental results are presented that demonstrate the influence of load, speed, current and surface roughness on coefficient of friction, contact voltage, contact resistance, interface temperature and wear rate. The experimental results reveal that thermal softening of the interface is the primary reason for accelerated wear under the test conditions. The results from the experiments presents an opportunity to form constitutive equations which could be used to predict the performance of the contact based on input parameters. The fusion of the findings of this dissertation provide methodologies along with experimental tools and findings to model, study and interpret the behavior of electrical contacts.

No description available.

Cost-effective machining of hardened steel (>60 HRC) components such as a large wind turbine bearing poses a significant challenge. This thesis investigates a new laser tempering based hybrid turning approach to machine hardened AISI 52100 steel parts more efficiently and cost effectively. The approach consists of a two step process involving laser tempering of the hardened workpiece surface followed by conventional machining at higher material removal rates using lower cost ceramic tooling to efficiently cut the laser tempered material. The specific objectives of this work are to: (a) study the characteristics of laser tempering of hyper-eutectoid 52100 hardened steel, (b) model the laser tempering process to determine the resulting hardness, and (c) conduct machining experiments to evaluate the performance of the laser tempering based hybrid turning process in terms of forces, tools wear and surface finish. First, the microstructure alterations and phase content in the surface and subsurface layers are analyzed using metallography and x-ray diffraction (XRD) respectively. Laser tempering produces distinct regions consisting of - a tempered white layer and a dark layer- in the heat affected subsurface region of the workpiece. The depth of the tempered region is dependent on the laser scanning conditions. Larger overlap of laser scans and smaller scan speeds produce a thicker tempered region. Furthermore, the tempered region is composed of ferrite and martensite and weak traces of retained austenite (~ 1 %). Second, a laser tempering model consisting of a three dimensional analytical model to predict the temperature field generated by laser scanning of 52100 hardened steel and a phase change based hardness model to predict the hardness of the tempered region are developed. The thermal model is used to evaluate the temperature field induced in the subsurface region due to the thermal cycles produced by the laser scanning step. The computed temperature histories are then fed to the phase change model to predict the surface and subsurface hardness. The laser tempering model is used to select the laser scanning conditions that yield the desired hardness reduction at the maximum depth. This model is verified through laser scanning experiments wherein the hardness changes are compared with model predictions. The model is shown to yield predictions that are within 20 % of the measured hardness of the tempered region. Using the laser scanning parameters determined from the laser tempering model, cutting experiments using Cubic Boron Nitride (CBN) tools and low cost alumina ceramic tools are conducted to compare the performance of laser tempering based hybrid turning with the conventional hard turning process. The machining experiments demonstrate the possibility of higher material removal rates, lower cutting forces, improved tool wear behavior, and consequently improved tool life in the laser tempering based process. In addition, the laser tempered based hybrid turning process produce is shown to yield lower peak-to-valley surface roughness height than the conventional hard turning process. Furthermore, it is found that lower cost ceramic tools can be used in place of CBN tools without compromising the material removal rate.

<p>Within the framework of presented PhD, friction stir spot welding (FSSW) of<br />multiple ultrathin sheets of AA 5754 &ndash; H111 (AlMg3) alloy 0.3 mm thick, was<br />studied. The influence of tool geometry and process parameters such as rotational<br />speed and axial load have been analysed using numerous techniques. It has been<br />understood that during the welding at low rotational speeds weld zone undergoes<br />strain hardening, while at high rotational speeds weld zone undergoes thermal<br />softening. It was observed that during FSSW at low rotational speeds a complex<br />layer at weld interface is present, which causes delamination when welded samples<br />are subjected to load.</p> / <p>У оквиру ове докторске дисертације испитивано је тачкасто заваривање трењем са мешањем ултратанких лимова дебљине 0.3 mm од легуре АА 5754 &ndash; H111 (AlMg3). Утицај геометрије алата и параметара као што су угаона брзина и аксијално оптерећење су детаљно анализирани уз помоћ бројних техника. Установљено је да приликом заваривања ниским угаоним брзинама долази до деформационог ојачавања, док на високим угаоним брзинама долази до термичког омекшавања зоне завара. Код узорка завареног са најмањим бројем обртаја долази до формирања комплексног слоја на међуконтактној површини који изазива деламинацију приликом испитивања механичких особина.</p> / <p>U okviru ove doktorske disertacije ispitivano je tačkasto zavarivanje trenjem sa mešanjem ultratankih limova debljine 0.3 mm od legure AA 5754 &ndash; H111 (AlMg3). Uticaj geometrije alata i parametara kao što su ugaona brzina i aksijalno opterećenje su detaljno analizirani uz pomoć brojnih tehnika. Ustanovljeno je da prilikom zavarivanja niskim ugaonim brzinama dolazi do deformacionog ojačavanja, dok na visokim ugaonim brzinama dolazi do termičkog omekšavanja zone zavara. Kod uzorka zavarenog sa najmanjim brojem obrtaja dolazi do formiranja kompleksnog sloja na međukontaktnoj površini koji izaziva delaminaciju prilikom ispitivanja mehaničkih osobina.</p>