Global ETD Search

51	Strengthening Mechanisms in Microtruss Metals Ng, Evelyn 18 December 2012 (has links) Microtrusses are hybrid materials composed of a three-dimensional array of struts capable of efficiently transmitting an externally applied load. The strut connectivity of microtrusses enables them to behave in a stretch-dominated fashion, allowing higher specific strength and stiffness values to be reached than conventional metal foams. While much attention has been given to the optimization of microtruss architectures, little attention has been given to the strengthening mechanisms inside the materials that make up this architecture. This thesis examines strengthening mechanisms in aluminum alloy and copper alloy microtruss systems with and without a reinforcing structural coating. C11000 microtrusses were stretch-bend fabricated for the first time; varying internal truss angles were selected in order to study the accumulating effects of plastic deformation and it was found that the mechanical performance was significantly enhanced in the presence of work hardening with the peak strength increasing by a factor of three. The C11000 microtrusses could also be significantly reinforced with sleeves of electrodeposited nanocrystalline Ni-53wt%Fe. It was found that the strength increase from work hardening and electrodeposition were additive over the range of structures considered. The AA2024 system allowed the contribution of work hardening, precipitation hardening, and hard anodizing to be considered as interacting strengthening mechanisms. Because of the lower formability of AA2024 compared to C11000, several different perforation geometries in the starting sheet were considered in order to more effectively distribute the plastic strain during stretch-bend fabrication. A T8 condition was selected over a T6 condition because it was shown that the plastic deformation induced during the final step was sufficient to enhance precipitation kinetics allowing higher strengths to be reached, while at the same time eliminating one annealing treatment. When hard anodizing treatments were conducted on O-temper and T8 temper AA2024 truss cores, the strength increase was different for different architectures, but was nearly the same for the two parent material tempers. Finally, the question of how much microtruss strengthening can be obtained for a given amount of parent metal strengthening was addressed by examining the interaction of material and geometric parameters in a model system. microtruss strengthening copper C11000 aluminum AA2024 cellular metal mechanical properties compressive strength densification energy modulus work hardening precipitation hardening heat treatment nanocrystalline coating aluminum oxide coating coating anodizing electrodeposition strengthening mechanisms geometry pyramidal compression buckling architecture truss uniaxial compression perforation Ramberg-Osgood Holloman nickel-iron annealing relative density periodic cellular hybrid composite microhardness stretch bend peak strength scanning electron microscopy stress strain bending stretching failure yielding column critical buckling strength critical buckling stress yield strength forming 0794
52	Strengthening Mechanisms in Microtruss Metals Ng, Evelyn 18 December 2012 (has links) Microtrusses are hybrid materials composed of a three-dimensional array of struts capable of efficiently transmitting an externally applied load. The strut connectivity of microtrusses enables them to behave in a stretch-dominated fashion, allowing higher specific strength and stiffness values to be reached than conventional metal foams. While much attention has been given to the optimization of microtruss architectures, little attention has been given to the strengthening mechanisms inside the materials that make up this architecture. This thesis examines strengthening mechanisms in aluminum alloy and copper alloy microtruss systems with and without a reinforcing structural coating. C11000 microtrusses were stretch-bend fabricated for the first time; varying internal truss angles were selected in order to study the accumulating effects of plastic deformation and it was found that the mechanical performance was significantly enhanced in the presence of work hardening with the peak strength increasing by a factor of three. The C11000 microtrusses could also be significantly reinforced with sleeves of electrodeposited nanocrystalline Ni-53wt%Fe. It was found that the strength increase from work hardening and electrodeposition were additive over the range of structures considered. The AA2024 system allowed the contribution of work hardening, precipitation hardening, and hard anodizing to be considered as interacting strengthening mechanisms. Because of the lower formability of AA2024 compared to C11000, several different perforation geometries in the starting sheet were considered in order to more effectively distribute the plastic strain during stretch-bend fabrication. A T8 condition was selected over a T6 condition because it was shown that the plastic deformation induced during the final step was sufficient to enhance precipitation kinetics allowing higher strengths to be reached, while at the same time eliminating one annealing treatment. When hard anodizing treatments were conducted on O-temper and T8 temper AA2024 truss cores, the strength increase was different for different architectures, but was nearly the same for the two parent material tempers. Finally, the question of how much microtruss strengthening can be obtained for a given amount of parent metal strengthening was addressed by examining the interaction of material and geometric parameters in a model system. microtruss strengthening copper C11000 aluminum AA2024 cellular metal mechanical properties compressive strength densification energy modulus work hardening precipitation hardening heat treatment nanocrystalline coating aluminum oxide coating coating anodizing electrodeposition strengthening mechanisms geometry pyramidal compression buckling architecture truss uniaxial compression perforation Ramberg-Osgood Holloman nickel-iron annealing relative density periodic cellular hybrid composite microhardness stretch bend peak strength scanning electron microscopy stress strain bending stretching failure yielding column critical buckling strength critical buckling stress yield strength forming 0794
53	Rehabilitation of Exterior RC Beam-Column Joints using Web-Bonded FRP Sheets Mahini, Seyed Saeid Unknown Date (has links) In a Reinforced Concrete (RC) building subjected to lateral loads such as earthquake and wind pressure, the beam to column joints constitute one of the critical regions, especially the exterior ones, and they must be designed and detailed to dissipate large amounts of energy without a significant loss of, strength, stiffness and ductility. This would be achieved when the beam-column joints are designed in such a way that the plastic hinges form at a distance away from the column face and the joint region remain elastic. In existing frames, an easy and practical way to implement this behaviour following the accepted design philosophy of the strong-column weak-beam concept is the use a Fibre Reinforced Plastic (FRP) retrofitting system. In the case of damaged buildings, this can be achieved through a FRP repairing system. In the experimental part of this study, seven scaled down exterior subassemblies were tested under monotonic or cyclic loads. All specimens were designed following the strong-column weak-beam principal. The three categories selected for this investigation included the FRP-repaired and FRP-retrofitted specimens under monotonic loads and FRP-retrofitted specimen under cyclic loads. All repairing/retrofitting was performed using a new technique called a web-bonded FRP system, which was developed for the first time in the current study. On the basis of test results, it was concluded that the FRP repairing/retrofitting system can restore/upgrade the integrity of the joint, keeping/upgrading its strength, stiffness and ductility, and shifting the plastic hinges from the column face toward the beam in such a way that the joint remains elastic. In the analytical part of this study, a closed-form solution was developed in order to predict the physical behaviour of the repaired/retrofitted specimens. Firstly, an analytical model was developed to calculate the ultimate moment capacity of the web-bonded FRP sections considering two failure modes, FRP rupture and tension failure, followed by an extended formulation for estimating the beam-tip displacement. Based on the analytical model and the extended formulation, failure mechanisms of the test specimens were implemented into a computer program to facilitate the calculations. All seven subassemblies were analysed using this program, and the results were found to be in good agreement with those obtained from experimental study. Design curves were also developed to be used by practicing engineers. In the numerical part of this study, all specimens were analysed by a nonlinear finite element method using ANSYS software. Numerical analysis was performed for three purposes: to calculate the first yield load of the specimens in order to manage the tests; to investigate the ability of the web-bonded FRP system to relocate the plastic hinge from the column face toward the beam; and to calibrate and confirm the results obtained from the experiments. It was concluded that numerical analysis using ANSYS could be considered as a practical tool in the design of the web-bonded FRP beam-column joints. Reinforced concrete Reinforcement Bar strong-column weak-beam principal Exterior Beam-Column Joints Core Small scale modelling Experimental Test set-up Control Plain specimen Rehabilitation Retrofitting retrofitted Repair Repaired Repairing Strengthening Controlling Relocating Plastic hinge Pre-cracking Cracking Crushing Post-cracking Crack Cracked Penetration Damaged Pre-cracked Web-Bonded Wrapping De-bonding Fibre Reinforced Plastic FRP Sheets Composite Confinement Monotonic loads Cyclic loading Loads Displacement control Load control Regime Phase Loading sequence Procedure Available ductility factor Curvature ductility factor Ultimate First yield Flexural yielding Strength Moment Stiffness Failure mechanism Modes Sudden Brittle failure Measured beam-tip displacement Deflection Numerical analysis ANSYS Solid65 Solid45 Link8 Non-linear Finite Element (FE) Convergence criteria Analytical Closed from FRP rupture tension failure Compression failure Minimum thickness Maximum thickness Compressive Tensile Steel Ratio Design charts Effective Moment of Inertia Compatibility Strain Stress

Page generated in 0.0518 seconds