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Bond in a reinforced concrete (RC) structure is the interaction force that transfers force between the steel and concrete. It influences the structural performance and serviceability of a structure under both static and cyclic loading. Corrosion of reinforcing steel in RC structures is the primary reason behind bond loss in RC elements. A loss of bond in concrete results in a decrease in the serviceability strength and eventually causes a brittle and sudden failure. Structures, such as bridges, are vulnerable to corrosion and at the same time are subjected to repeated loading rather than static loading. Nevertheless, little experimental or analytical studies that address the problem of corroded steel-concrete bond under repeated loading exist. This study was aimed at increasing the understanding of the behaviour of bond between corroded reinforcing steel bars and concrete for structures subjected to repeated loading. In addition, the effect of fibre reinforced polymers (FRP) as a rehabilitation method was assessed. Fibre reinforced polymers is considered to be a state-of-the art rehabilitation material due to its advantages, such as high strength, light weight and ease of handling and application. Forty-seven anchorage-beam specimens were cast and tested. The specimens’ dimensions were 152 x 254 x 2000 mm reinforced with two 20M bars. The steel reinforcement in a specimen was unbonded except for 250 mm from each end. This bonded length was selected to ensure a bond failure. The corrosion was induced using an accelerated corrosion process. The parameters investigated were the corrosion level (0, 5 and 9% measured mass loss), whether the specimen was wrapped in the anchorage zone with a U-shaped carbon fibre reinforced polymer (CFRP) sheets or not, and the load range applied. The minimum load applied was 10% of the static bond capacity of the specimen. The maximum load was varied to give the desired range of fatigue lives (103 to 106 cycles). The test frequency for all repeated tests was 1.5 Hz. Results showed that the repeated loading either pushed the bottom concrete cover away from the steel bar by wedge action for unwrapped beams or cracked and crushed the CFRP confined bottom concrete cover for wrapped beams. The concrete damage caused the bond stress to undergo a gradual redistribution, moving the peak bond stress from the loaded end towards the free end, resulting in failure of the specimens by fatigue of bond. Corrosion levels of 5% and 9% decreased the fatigue bond strength on average by 19%. The rate of slip of the steel bar increased as the corrosion level increased. CFRP sheets changed the mechanism by which the concrete resist the bond forces by engaging the bottom cover. This in turn increased the fatigue bond strength at all corrosion levels on average by 31% compared to unwrapped specimens. Based on the test results and observations, a hypothesis of the mechanics of bond under repeated loading was postulated and a fatigue slip-growth analysis (similar to the fracture mechanics crack growth approach) was proposed to calculate the fatigue life of a specimen that fail in bond. The proposed analysis was in reasonable agreement with the experimental results.

Deterioration of reinforced concrete structures due to reinforcement corrosion is a serious problem that faces concrete infrastructure worldwide. Effect of the rebar corrosion in the shear span on the structural behaviour is not fully addressed in the published literature. This study examined the effects of corrosion of the longitudinal reinforcement in the shear span on the structural behaviour of RC beams and the effectiveness of three rehabilitation schemes on the structural performance of such beams. The experimental program consisted of testing fifteen medium-scale reinforced concrete beams (150mm wide x 350mm deep x 2400mm long) under static load. Test variables included: span to depth ratio, the degree of corrosion and the anchorage end condition and repair schemes. Two span to depth (a/d) ratios were considered: a/d=3.4 with one-point loading and a/d=2.4 with two-point loading. Two anchorage end-conditions were used: bonded or un-bonded reinforcement in the an-chorage zone. Four degrees of corrosion were chosen to simulate minor (2.5% to 5% mass loss), medium (7.5% mass loss), and severe (15% mass loss) degrees of corrosion. Corrosion was induced in the longitudinal reinforcement in the shear-span using accelerated corrosion techniques based on Faradays’ law. Three different repair scenarios were applied. The first scenario included removing the deteriorated concrete, cleaning the corroded steel and patching with a new self-compacting concrete. The second scenario included U-wrapping the beams cross-section using Glass fiber reinforced cement-based composite (GFRCM), and Carbon fiber reinforced cement-based composite (CFRCM) without removing the deteriorated concrete. The third scenario included patch repair and confinement by wrapping with GFRCM or CFRCM. Following corrosion and repair, all specimens were loaded statically to failure. Test results showed no major effect of shear-span corrosion on the flexural behaviour for the beams with end anchorage whereas a noticeable effect on the flexural behaviour was observed for beams with no end anchorage regions. The corrosion degree and the shear span to depth ratio affected the mode of failure for the specimens with no end anchorages. The type of repair significantly affected the overall behaviour of the corroded specimens. An analytical model was proposed and used to predict the load-deflection response of the tested specimens. The program calculated the mid-span deflection for a given load as an integration of the deflection of a series of elements, with the deflection being based on the elongation of the steel reinforcement in each element. A modified bond stress-slip model was incorporated into the calculations to account for the change in bond strength caused by the corrosion and/or confinement that are provided by repairs. The predicted results were in reasonable agreement with the experimental results.

Bond in a reinforced concrete (RC) structure is the interaction force that transfers force between the steel and concrete. It influences the structural performance and serviceability of a structure under both static and cyclic loading. Corrosion of reinforcing steel in RC structures is the primary reason behind bond loss in RC elements. A loss of bond in concrete results in a decrease in the serviceability strength and eventually causes a brittle and sudden failure. Structures, such as bridges, are vulnerable to corrosion and at the same time are subjected to repeated loading rather than static loading. Nevertheless, little experimental or analytical studies that address the problem of corroded steel-concrete bond under repeated loading exist. This study was aimed at increasing the understanding of the behaviour of bond between corroded reinforcing steel bars and concrete for structures subjected to repeated loading. In addition, the effect of fibre reinforced polymers (FRP) as a rehabilitation method was assessed. Fibre reinforced polymers is considered to be a state-of-the art rehabilitation material due to its advantages, such as high strength, light weight and ease of handling and application. Forty-seven anchorage-beam specimens were cast and tested. The specimens’ dimensions were 152 x 254 x 2000 mm reinforced with two 20M bars. The steel reinforcement in a specimen was unbonded except for 250 mm from each end. This bonded length was selected to ensure a bond failure. The corrosion was induced using an accelerated corrosion process. The parameters investigated were the corrosion level (0, 5 and 9% measured mass loss), whether the specimen was wrapped in the anchorage zone with a U-shaped carbon fibre reinforced polymer (CFRP) sheets or not, and the load range applied. The minimum load applied was 10% of the static bond capacity of the specimen. The maximum load was varied to give the desired range of fatigue lives (103 to 106 cycles). The test frequency for all repeated tests was 1.5 Hz. Results showed that the repeated loading either pushed the bottom concrete cover away from the steel bar by wedge action for unwrapped beams or cracked and crushed the CFRP confined bottom concrete cover for wrapped beams. The concrete damage caused the bond stress to undergo a gradual redistribution, moving the peak bond stress from the loaded end towards the free end, resulting in failure of the specimens by fatigue of bond. Corrosion levels of 5% and 9% decreased the fatigue bond strength on average by 19%. The rate of slip of the steel bar increased as the corrosion level increased. CFRP sheets changed the mechanism by which the concrete resist the bond forces by engaging the bottom cover. This in turn increased the fatigue bond strength at all corrosion levels on average by 31% compared to unwrapped specimens. Based on the test results and observations, a hypothesis of the mechanics of bond under repeated loading was postulated and a fatigue slip-growth analysis (similar to the fracture mechanics crack growth approach) was proposed to calculate the fatigue life of a specimen that fail in bond. The proposed analysis was in reasonable agreement with the experimental results.

Previous studies show that the Piezoelectric-Driven Stick-Slip (PDSS) actuator is a promising device in many micropositioning and micromanipulation applications, where positioning with a long range and a high resolution is required. However, research in this area is still in its early stage and many issues remain to be addressed. One key issue is the representation of the dynamic displacement of the end-effector. It is known that such factors as the dynamics of piezoelectric actuator (PEA) and the presliding friction involved can significantly contribute to the displacement dynamics. Although this has been widely accepted, specific quantitative relationship between the aforementioned factors and the displacement dynamics has rarely been defined. The aim of this research is to develop a model to represent the displacement of the end-effecter of the PDSS actuators, in which both the presliding friction and the PEA dynamics are addressed. <p>In order to represent the presliding friction, the models reported in literatures, including Dahl model [Olsson, et al., 1998], Reset Integrator model [Haessig and Friedland 1991], LuGre model [Canudas de Wit et al., 1995] and Elastoplastic model [Dupont et al., 2002] were reviewed and examined; and the LuGre model was chosen to be used because of its efficiency and simple formulation. On the other hand, a linear second order dynamic system model was employed to represent the combination of a PEA and its driven mechanism. On the basis of the pre-sliding friction model and the linearized PEA dynamics model, a model representative of the end-effector displacement of the PDSS actuator model was developed. <p>In order to validate experimentally the developed PDSS model, a displacement measuring and data acquisition experiment system was established and a prototype was developed based on dSPACE and Simulink. On the prototyped actuator, two experiments were designed and conducted to identify the parameters involved in the model. One experiment is for the determination of the parameters of the second order system for the dynamics of the combination of a PEA and its driven mechanism; and other one is for the determination of the parameters of the chosen friction model. The identified parameters were then employed in the developed PDSS model to simulate the displacements and the results were compared with the experimental results that were obtained under the same operating conditions as the simulation. The comparison suggests that the model developed in this study is promising for the end-effector displacement of the PDSS actuator.

Piezoelectric actuators (PEA) hold the most promise for precision positioning applications due to their capability of producing extremely small displacements down to 10 pm (1 pm = 10-12 m) as well as their high stiffness and force output. The piezoelectric-driven stick-slip (PDSS) actuator, working on the friction-inertia concept, has the capacity of accomplishing an unlimited range of motion. It also holds the promises of simple configuration and low cost. On the other hand, the PDSS actuator has a relatively low efficiency and low loading capability, which greatly limits its applications. The purpose of this research is to improve the performance of the PDSS actuators by employing specially-designed working surfaces.<p> The working surfaces, referred as anisotropic friction (AF) surfaces in this study, can provide different friction forces depending on the direction of relative motion of the two surfaces, and are used in this research to accomplish the aforementioned purpose. To fabricate such surfaces, two nanostructure technologies are employed: hot filament chemical vapour deposition (HFCVD) and ion beam etching (IBE). The HFCVD is used to deposit diamond on silicon substrates; and the IBE is used to etch the diamond crystalloid with a certain angle with respect to the coating surface to obtain an unsymmetrical-triangle microstructure. <p> A PDSS actuator prototype containing the AF surfaces was developed in this study to verify the function of the AF surfaces and characterize the performance of PDSS actuators. The designed surfaces were mounted on the prototype; and the improvement in performance was characterized by conducting a set of experiments with both the normal isotropic friction (IF) surfaces and the AF surfaces, respectively. The results illustrate that the PDSS actuator with the AF surface has a higher efficiency and improved loading capability compared to the one with the IF surfaces.<p> A model was also developed to represent the displacement of the novel PDSS actuator. The dynamics of the PEA and the platform were approximated by using a second order dynamic system. The pre-sliding friction behaviour involved was investigated by modifying the LuGre friction model, in which six parameters (Note that three parameters are used in the LuGre model) were employed to represent the anisotropic friction. By combining the PEA mechanism model, the modified friction model, and the dynamics of end-effector, a model for the PDSS actuator with the AF surface was developed. The model with the identified parameters was simulated in MATLAB Simulink and the simulation results obtained were compared to the experimental results to verify the model. The comparison suggests that the model developed in this study is promising to represent the displacement of the novel PDSS actuators with AF surfaces.

<p>This thesis focuses on the fluctuation in the switching current $I_s$ of superconducting Al nanowires. We discovered that the maximum current which nanowires can support is limited by a single phase slip at low temperature. </p><p>Al superconducting nanowires less than 10 nm wide were fabricated based on a MBE grown InP ridge template in an edge-on geometry. The method utilizes a special substrate featuring a high standing 8nm-wide InP ridge. A thin layer of Al was evaporated on the substrate and Al on the ridge formed nanowires.</p><p>The fluctuation effects starts to dominate in the nanowire due to reduced energy barrier. One of such effects is the phase slip. The phase slip is a topological event, during which the superconducting phase between two superconducting electrodes changes by $2\pi$. The phase slip broadens the normal-superconducting transition. Part of the nanowire becomes normal during the phase slip and forms a normal core. The normal core generates heat and causes the premature switching in superconducting nanowires.</p><p></p><p>The nanowire becomes superconducting below the critical temperature $T_c$. The superconducting-normal transition was studied in the thesis. The transition of nanowires with superconducting leads qualitatively fits the thermally activated phase slip (TAPS) theory. On the other hand, the transition of the nanowires with normal leads showed a resistive tail due to the inverse-proximity effect.</p><p>The nanowire switches from the superconducting state to the normal state as the current is increased. Ideally, the maximum current is set by a pair-breaking mechanism, by which the kinetic energy of quasi-particles exceeds the bonding energy of Cooper pairs. This is called the critical current, $I_c$. In practice, the measured maximum current, called the switching current $I_s$, cannot reach $I_c$ because of the phase slip.</p><p>$I_s$ shows stochasticity due to the phase slip. For the nanowires with superconducting leads, the average $I_s$ approximately follows but falls below $I_c$. The fluctuation in $I_s$ shows non-monotonic behavior, in contrast to other studies. The fluctuation first increases and then decreases rapidly with increasing temperature. The fluctuation behavior is consistent with a scenario where the switch is triggered by a single phase slip at low temperature while by multiple phase slips at higher temperature. Thermal activation of phase slips appears dominant at most temperatures. However, in the thinnest nanowire, the saturation of the fluctuation at low temperature indicates that the phase slips by macroscopic quantum tunneling.</p><p>The superconducting nanowires with normal leads were also studied. One of the distinctive properties of our nanowire (the critical field of 1D nanowire is 10 times larger than that of a 2D superconducting film) allowed us to study the same nanowire with different leads (superconducting or normal). Both the average $I_s$ and the fluctuation in $I_s$ differed qualitatively depending on whether the leads were superconducting or normal. The temperature dependence of the average $I_s$ followed the $I_c$ of the Josephson junction instead of the phenomenological pair-breaking $I_c$. The difference was found to depend on both the temperature (close to $T_c$ or 0) and the length (shorter or longer than the charge imbalance length). Our study also showed that nonlinear current-voltage (IV) curves were observed due to the inverse-proximity effect.</p> / Dissertation

Efficiency is one of the key issues in numerical simulation of large-scale problems with complex 3-D geometry. Traditional domain based methods, such as finite element methods, may not be suitable for these problems due to, for example, the complexity of mesh generation. The Boundary Element Method (BEM), based on boundary integral formulations (BIE), offers one possible solution to this issue by discretizing only the surface of the domain. However, to date, successful applications of the BEM are mostly limited to linear and continuum problems. The challenges in the extension of the BEM to nonlinear problems or problems with non-continuum boundary conditions (BC) include, but are not limited to, the lack of appropriate BIE and the difficulties in the treatment of the volume integrals that result from the nonlinear terms. In this thesis work, new approaches and techniques based on the BEM have been developed for 3-D nonlinear problems and Stokes problems with slip BC. For nonlinear problems, a major difficulty in applying the BEM is the treatment of the volume integrals in the BIE. An efficient approach, based on the precorrected-FFT technique, is developed to evaluate the volume integrals. In this approach, the 3-D uniform grid constructed initially to accelerate surface integration is used as the baseline mesh to evaluate volume integrals. The cubes enclosing part of the boundary are partitioned using surface panels. No volume discretization of the interior cubes is necessary. This grid is also used to accelerate volume integration. Based on this approach, accelerated BEM solvers for non-homogeneous and nonlinear problems are developed and tested. Good agreement is achieved between simulation results and analytical results. Qualitative comparison is made with current approaches. Stokes problems with slip BC are of particular importance in micro gas flows such as those encountered in MEMS devices. An efficient approach based on the BEM combined with the precorrected-FFT technique has been proposed and various techniques have been developed to solve these problems. As the applications of the developed method, drag forces on oscillating objects immersed in an unbounded slip flow are calculated and validated with either analytic solutions or experimental results.

This thesis describes the effect of dislocation slip on superplastic deformation of AZ31 magnesium alloy. Through two different routes of ECAE (equal channel angular extrusion), two types of specimens having the same grain size but different texture were obtained. One is favorable for basal slip and the other is not. Under the same condition of deformation, the strain rate sensitivity and contribution of grain boundary sliding to total elongation in these two different specimens are almost the same. As for elongation, not much difference was found. The present results demonstrate that the relationship between dislocation slip and grain boundary sliding in superplastic AZ31 magnesium alloy is non-obvious.

This work is initiated from an observation of the rolling-sliding kinematic behavior observed in the motion of the knee joint. We use the slip ratio as a parameter to analyze the rolling-sliding properties of a mechanism, and also propose a method to design new mechanisms with specific slip ratio. In this research, we first verify the many definitions of the slip ratio, then modify a best definition to suit various rolling-sliding motions. Most importantly, we propose two types of rolling-sliding mechanism design. By changing the parameters of the mechanism, we can adjust its slip ratio curve to be close to a desired curve. In addition, when the idea of adjustable link length is used in the design of the mechanism, exact slip ratio curve as specified can be generated by the use of some cams.