Warm forming of aluminum alloys of has shown promising results for increasing the formability of aluminum alloy sheet. Warm forming is a term that is generally used to describe a sheet metal forming process, where part or all of the blank is formed at an elevated temperature of less than one half of the material’s melting temperature. The focus of this work is to study the effects of warm forming on Novelis X926 clad aluminum brazing sheet. Warm forming of clad aluminum brazing sheet, which is commonly used in automotive heat exchangers has not been studied. This work can be split into three main goals: i) to characterize the material behavior and develop a constitutive model, ii) to experimentally determine the effects of warm forming on deep drawing; and, iii) to create and validate a finite element model for warm forming of Novelis X926. For an accurate warm forming material model to be created, a temperature and rate dependant hardening law as well as an anisotropic yield function are required. Uniaxial isothermal tensile tests were performed on 0.5mm thick Novelis X926at 25°C (room temperature), 100°C, 150°C, 200°C, and 250°C. At each temperature, tests were performed with various strain rates between 7.0 E -4 /sec and 7.0 E -2 /sec to determine the strain rate sensitivity. Tensile tests were also performed at 0° (longitudinal), 45° (diagonal), and 90° (transverse) with respect to the material rolling direction in order to assess the anisotropy of the material. It was found that increasing forming temperature increases elongation to failure by 200%, decreases flow stress by 35%, and increases strain rate sensitivity. Barlat’s Yield 2000 yield function (Barlat et al., 2003a) and the Bergström work hardening law (van den Boogaard and Huétink , 2006) were found to accurately method model the material behavior. Warm deep drawing of 101.6 mm (4”) diameter cylindrical cups was performed using specially designed tooling with heated dies and a cooled punch. Deep drawing was performed on 228.6 mm (9“) and 203.2 mm (8”) diameter blanks of 0.5 mm thick Novelis X926. Deep drawing was performed with die temperatures ranging from 25°C to 300°C with a cooled punch. Teflon sheet and Dasco Cast 1200 lubricants were used in experiments. Different punch velocities were also investigated. 228.6 mm diameter blanks, which could not be drawn successfully at room temperature, were drawn successfully using 200°C dies. Increasing the die temperature further to 250°C and 300°C provided additional improvement in formability and reduced tooling loads. Increasing the punch velocity, increases the punch load when forming at elevated temperatures, reflecting the strong material rate sensitivity at elevated temperatures. A coupled thermal mechanical finite element model was developed using the Bergström hardening rule and the Yield 2000 yield surface using LS-DYNA. The model was found to accurately predict punch force for warm deep drawing using Teflon sheet as a lubricant. Results for Dasco Cast 1200 were not as accurate, due to the difficulties in modeling the lubricant’s behavior. Finite element simulations demonstrated that warm forming can be used to reduce thinning at critical locations, compared to parts formed at room temperature.

応用力学論文集v.15

The effect of coupled thermo-poroelastic behavior on hydraulic fracture propagation is of much interest in geothermal- and petroleum-related geomechanics problems such as wellbore stability and hydraulic fracturing as pore pressure and temperature variations can significantly induce rock deformation, fracture initiation, and propagation. In this dissertation, a two-dimensional (2D) boundary element method (BEM) was developed to simulate the fully coupled thermo-poroelastic fracture propagation process. The influence of pore pressure and temperature changes on the fracture propagation length and path, as well as on stress and pore pressure distribution near wellbores and fractures, was considered in isotropic and homogeneous rock formations. The BEM used in this work consists of the displacement discontinuity (DD) method and the fictitious stress (FS) method. Also, a combined FS-DD numerical model was implemented for the hydraulically or thermally-induced fractures in the vicinity of a wellbore. The linear elastic fracture mechanics (LEFM) theory was adopted to numerically model within the framework of poroelasticity and thermo-poroelasticity theory. For high accuracy of crack tip modeling, a special displacement discontinuity tip element was developed and extended to capture the pore pressure and temperature influence at the tip. For poroelastic fracture propagation, a steadily propagating crack driven by fluid pressure was modeled to find the effect of pore pressure on crack path under the two limiting poroelastic conditions (undrained and drained). The results indicate that the pore pressure diffusion has no influence on the crack growth under the undrained condition because the crack propagation velocity is too fast for the diffusion effect to take place. On the other hand, its influence on the crack path under the drained condition with its low propagation velocity has significance because it induces a change in principal stress direction, resulting in an alteration of fracture orientation. For the thermal fracturing, when the rock around a wellbore and a main fracture is cooled by injecting cold water in a hot reservoir, the rapid decrease in temperature gives rise to thermal stress, which causes a crack to initiate and propagate into the rock matrix. The single and multiple fracture propagation caused by transient cooling in both thermoelastic and poro-thermoelastic rock were numerically modeled. The results of this study indicate that the thermal stresses induced by cooling may exceed the in-situ stress in the reservoir, creating secondary fractures perpendicular to main fracture. Furthermore, the faster cooling rate produces longer crack extension of the secondary thermal fractures. This implies that the faster cooling induces a higher tensile stress zone around the fracture, which tends to produce larger driving forces to make the secondary fractures penetrate deeper into the geothermal reservoir.

The single crystal superalloy PWA 1484 is used in hot section turbine blade applications due to its performance at high temperatures. In practice, the turbine blades are often coated in order to protect them from environmental degradation. However, under repeated cyclic loading, the coating may serve as a site for crack initiation in the blades. Fundamental out-of-phase (OP) thermo-mechanical fatigue (TMF) studies, primarily using uncoated solid cylindrical test samples, have previously examined both crack initiation and propagation in PWA 1484. In this work, mechanical strain-controlled OP TMF tests were performed on coated and uncoated specimens of a hollow cylindrical geometry in order to study the effects of both geometry and coating on the TMF crack initiation behavior. To accomplish this, it was necessary to create and analyze a modified gripping mechanism due to the unique geometry of the test samples, and as predicted by hand calculations and finite element analysis, these modifications proved to be successful. The TMF test results for the uncoated material were compared to those from previous studies under the same testing conditions, and it was found that the differences in geometry had a minimal impact on fatigue life. Comparisons of the results for the coated and uncoated material suggested that the coating may have offered a slight improvement in life, although insufficient results were available to determine whether these differences were statistically significant. Damage mechanisms resulting from different test conditions were also observed through microscopy on failed specimens.

At a time when global environmental change is posing a growing challenge to the world’s economy and creating uncertainties to livelihood of its inhabitants, Coal thermal power plants are under pressure to meet stringent environmental regulations into achieving worldwide set millennial goals for mitigating the effect of emission gases on the atmosphere. Owing to its abundance, it is unlikely to see the use of coal completely missing from the global energy mix within the next hundred years to come. While innovative emission reduction technologies are evolving for the better, trendy technological solutions which require reintegration of these coal plants with alternative greener fuels are growing at the moment. Among these solutions, the following paper investigates possible means for repowering a coal steam power plant with indirect solar heating solutions to boost its annual outputs. Two widely deployable solar thermal technologies, parabolic trough and Central tower receiver systems, are introduced at different locations in the steam plant to heat working fluid thereby enhancing the thermodynamic quality of steam being generated. Potential annual energy output was estimated using commercially available TRNSYS software upon mass and heat balance to every component of solar and steam plant. The annual energy outputs are weighed against their plant erecting and running costs to evaluate the economic vitality of the proposed repowering options. The results show that parabolic trough heating method could serve as the most cost effective method generating electricity at competitive prices than solar only powered SEGS plants. While cost may be acceptable in the unit of energy sense, the scale of implementation has been proven to be technically limited. / Kriel Power Plant

No description available.

The objective of this study was to determine how Al2O3 nanoparticle concentration affected the specific heat, heat of fusion, melting point, thermal diffusivity and thermal conductivity of Alumina-Nitrate nanofluids. Al2O3 nanoparticles were dispersed in a eutectic of sodium nitrate and potassium nitrate (60:40 for mole fraction) to create nanofluids using a hot plate evaporation method and an air dryer method. The nominal Al2O3 (alumina) mass fraction was between 0 and 2%, and was determined as the ratio of the mass of Al2O3 nanoparticles to the total mass of the nanofluid. After the preparation of the nanofluids, Neutron Activation Analysis (NAA) was used to measure the actual Al2O3 mass fraction in the nanofluids. The specific heat, heat of fusion, and melting point were measured with a Modulated Differential Scanning Calorimeter (MDSC). The thermal diffusivity and thermal conductivity were measured with Laser Flash Analysis (LFA). The MDSC results showed that the addition of Al2O3 nanoparticles enhanced the specific heat of the nanofluids synthesize from both methods. There was a parabolic relation between the specific heat and the Al2O3 mass fraction for the nanofluids synthesized from the hot plate evaporation method, with a maximum 31% enhancement at 0.78% Al2O3 mass fraction. The nanofluids synthesized from the air dryer method also resulted in enhanced specific heats which were higher at the same Al2O3 mass fraction than those of the nanofluids synthesized from the hot plate evaporation method. It was not determined why this enhancement occurred. The results also showed that the introduction of Al2O3 nanoparticles had no significant effect on the heat of fusion and melting point of the nanofluids synthesized from either method. The LFA results showed that adding Al2O3 nanoparticles decreased the thermal diffusivity and the thermal conductivity of the nitrate eutectic.

The thermo-oxidative degradation of unstabilized polyamide 6 (PA-6) was investigated by a number of novel techniques in an attempt to achieve a better understanding of the mechanisms involved in the oxidative degradation of polymers. Particular attention was given to the influence of end groups on PA-6 oxidation by studying samples that terminated predominantly in carboxylic, amine or methyl end groups. The changes occurring in the oxidative stabilities and mechanisms of PA-6 as a result of altering the end groups of PA-6 were investigated by a technique termed CL-DSC, which simultaneously measures the chemiluminescence (CL) and heat flow (DSC) from a sample. When amine end groups were abundant in the PA-6 sample a chemically induced electron exchange luminescence (CIEEL) mechanism could occur directly and the CL intensity was proportional to the heat flow curve of the DSC. However, when amine end groups were absent it was the first derivative of the CL intensity that was proportional to the heat flow curve because the CIEEL mechanism could not operate until an easily oxidisable luminescent oxidation product was formed. Due to the dramatic effect end groups have on the oxidation mechanisms of PA-6 it was hypothesized that end groups could be sites analogous to the impurities in polyolefins that lead to heterogeneous oxidation. To test this hypothesis, CL Imaging was used to map the occurrence and extent of oxidation across samples of PA-6 to display the influence end groups have on the homogeneous or heterogeneous nature of PA-6 oxidation. Sequences of FTIES spectra collected at specified time intervals during the in situ oxidation of PA-6 samples terminating in the different end groups were turned into oxidation product profiles. The differences between spectra related to significant points on the oxidation profiles were compared in an attempt to elucidate the chemical or physical changes occurring in the samples during oxidation. To identify the species involved in the mechanistically different oxidation processes resulting from the different end groups, methods for the MALDI-TOF analysis of non-oxidized and oxidized PA- 6 samples were developed via trial and error. It was only possible to detect the occurrence of degradation products by MALDI-TOF MS after considerable oxidation as measured by chemiluminescence, by which time the species were the result of a number of oxidative processes. Therefore, identification of the species formed was not possible.

Bonded copper interconnects were created using thermo-compression bonding and the dicing yield was used as an indication of the bond quality. SEM images indicated that the Cu was plastically deformed. Our experimental and modeling results indicate that the effective contact area is directly proportional to the applied load. Furthermore, for first time, results have been obtained that indicate that the dicing yield is proportional to the measured bond strength, and the bond strength is proportional to the effective contact area. It is also shown that films with rougher surfaces (and corresponding lower effective bonding areas) have lower bond strengths and dicing yields. A quantitative model for the relationship between measured surface roughness and the corresponding dicing yield has been developed. An appropriate surface-roughness data acquisition methodology has also been developed. The maximum possible applied load and the minimum possible surface roughness are required to obtain the maximum effective contact area, and hence to achieve optimum yields (both mechanically and electrically). / Singapore-MIT Alliance (SMA)

The large number of vehicles operating on the roads cause high emissions and consequently a negative effect on the environment. When developing and optimizing internal combustion engines, certain requirements must be considered, which are environmental regulations, reduced fuel consumption and increased specific power. In order to meet these demands, an increase of the engine combustion pressure will occur usually accompanied with a temperature increase. During start-up and shut-down of an engine, it is subjected to cyclic thermo-mechanical fatigue (TMF) loads. The turbo manifold and exhaust manifolds connected to the engine is also subjected to these thermo-mechanical fatigue loads and thereby exposed to alternating tensile and compression loads. As these TMF loads will increase in the near future due to the development and optimization of internal combustion engines, it is important to understand the limitations of the material for these loads. In collaboration with Scania CV AB in Södertälje, this thesis covers the investigation of influence of nitrocarburizing (NC) on TMF properties of three ductile irons (DCI) labelled HiSi, SiMo51 and SiMo1000 intended to be used for components in the exhaust system. Nitrocarburizing is a thermo-chemical process where nitrogen and carbon diffuses from the process medium into the surface zone of a ferrous metal. The purpose of the NC is to increase the wear properties in contact areas between different parts. The oxidation with and without nitrocarburizing are studied both after isothermal and stress free oxidation tests at 780 °C and after TMF loads with combined cyclic variation of mechanical and thermal loads. In addition, the properties such as hardness, defects, porosity, microstructure, composition of both the materials and of the oxide layer have been investigated. For SiMo1000+NC cracks formed during nitrocarburizing were positioned parallel to the surface edge in the diffusion zone and consequently an increased diffusion of nitrogen into the material, i.e. deeper diffusion depth. SiMo1000+NC showed highest hardness, highest compressive residual stresses and thickest oxide layer. SiMo1000 showed highest resistance against oxidation due to the protective aluminum oxide layer. Oxide crack initiations after thermo-mechanical tests with a protective silicon oxide layer around the cracks for HiSi and SiMo51 and a protective aluminum oxide layer around the cracks for SiMo1000. In materials with nitrocarburizing, these protective layers of either silicon oxide or aluminum oxide were more distributed into the material. In SiMo1000+NC, crack initiations were not oxidized.