The effect of thermo-mechanical processing by deformation and annealing on the grain boundary configuration of commercially pure Ni-200 is reported in this thesis. Ni-200 is unalloyed, thus avoiding the complex effects associated with alloying elements on the formation and development of different types of grain boundaries. One step strain-recovery with strain levels in the range of 3% to 7.5% (with 1.5% intervals) and annealing temperatures in the range of 800ºC to 1000ºC (with 100ºC intervals) were used in processing. The effects of parameters such as strain level, annealing temperature, annealing time and grain growth on grain boundary configurations were studied. Using Orientation Image Microscopy (OIM) it was found that the Fsp (fraction of special grain boundaries) value of strained samples annealed in the range of 800ºC to 1000ºC began to increase after a critical length of time, after which the Fsp value increased quickly and becoming a maximum in 2~4 minutes. The length of the critical annealing time for the increase of Fsp was shorter in the material with the higher levels of strain at a constant annealing temperature. Also the critical annealing time was shorter when annealed at higher temperatures under a fixed level of strain. The Fsp value increased to 80% from an as received value of about 30% in the samples with varying strain levels. However, the Fsp values only increased from 30% to 45% in the material without strain. Due to grain boundary migration, the Fsp values increased with grain size and became a maximum during the heat treatment of the strained material. In the material without strain however even when grain growth occurred, limited improvement in Fsp values occurred showing that contribution of strain is very important to the formation of special boundaries. By varying the strain levels, annealing temperatures and times, material with high Fsp values in a wide range of grain size can be obtained. Under the present processing conditions used however, multi-cycle was not helpful to the improvement of Fsp. TEM observations indicated dislocation tangles occurred near the grain boundary of the 1x6% strained samples. These dislocation tangles decreased with time at 800˚C and were reduced considerably after 20 minutes. Thermodynamic and kinetic models were used in the calculations of twin density-grain size relationships. The results indicated that the contribution of strain is equivalent to the increase of grain boundary energy, which provided an extra driving force and improved probability of twin embryo formation.

No description available.

No description available.

No description available.

The effect of thermo-mechanical processing by deformation and annealing on the grain boundary configuration of commercially pure Ni-200 is reported in this thesis. Ni-200 is unalloyed, thus avoiding the complex effects associated with alloying elements on the formation and development of different types of grain boundaries. One step strain-recovery with strain levels in the range of 3% to 7.5% (with 1.5% intervals) and annealing temperatures in the range of 800ºC to 1000ºC (with 100ºC intervals) were used in processing. The effects of parameters such as strain level, annealing temperature, annealing time and grain growth on grain boundary configurations were studied. Using Orientation Image Microscopy (OIM) it was found that the Fsp (fraction of special grain boundaries) value of strained samples annealed in the range of 800ºC to 1000ºC began to increase after a critical length of time, after which the Fsp value increased quickly and becoming a maximum in 2~4 minutes. The length of the critical annealing time for the increase of Fsp was shorter in the material with the higher levels of strain at a constant annealing temperature. Also the critical annealing time was shorter when annealed at higher temperatures under a fixed level of strain. The Fsp value increased to 80% from an as received value of about 30% in the samples with varying strain levels. However, the Fsp values only increased from 30% to 45% in the material without strain. Due to grain boundary migration, the Fsp values increased with grain size and became a maximum during the heat treatment of the strained material. In the material without strain however even when grain growth occurred, limited improvement in Fsp values occurred showing that contribution of strain is very important to the formation of special boundaries. By varying the strain levels, annealing temperatures and times, material with high Fsp values in a wide range of grain size can be obtained. Under the present processing conditions used however, multi-cycle was not helpful to the improvement of Fsp. TEM observations indicated dislocation tangles occurred near the grain boundary of the 1x6% strained samples. These dislocation tangles decreased with time at 800˚C and were reduced considerably after 20 minutes. Thermodynamic and kinetic models were used in the calculations of twin density-grain size relationships. The results indicated that the contribution of strain is equivalent to the increase of grain boundary energy, which provided an extra driving force and improved probability of twin embryo formation.

This thesis details an investigation into the properties and applications of shape memory alloy (SMA) composites. SMA-composites are a new material which have the possibility of having a large impact on what the structures as we know today, are constructed with. SMA-composites are adaptive materials which can be used to control the shape and frequencies of vibration of a structure. In order to determine the effectiveness of such a material, research into the functional properties of SMAs and SMA-composites was conducted. As an initial step, the transformation behaviour of constrained SMAs was investigated in order to obtain a better understanding into the recovery stress generation of these wires when embedded into a composite material. It is known that the transformation is based on two types of martensite within the alloy; self accommodating and preferentially oriented martensite. The amounts of each type and how they vary with differing pre-strain were determined through DSC measurements and an explanation for why preferentially oriented martensite is not observed during DSC testing was made. The next step was to investigate the effectiveness of embedding SMA wires into composites and the thermomechanical properties of the SMA wires and the SMA-composites were determined. This was completed using a specially designed tensile testing machine which was capable of having the whole specimen immersed into an oil bath and heated and cooled repeatedly. The stress-strain, strain-temperature, stress-temperature, resistance-strain and cyclic properties of various wires were obtained, giving a better understanding of the behaviour of SMA wires under different test conditions. NiTiCu SMA wires were embedded into kevlar composite materials and the recovery stress generation (stress-temperature), stress-strain, and strain-temperature behaviour was determined. If SMA-composites are to be used as new materials for structural applications, verification that the embedment of pre-strained SMA wires into the material doesn't adversely affect the impact behaviour needs to be carried out. SMA-composite specimens with varying volume fractions of superelastic SMA wires, pre-strain and position through the thickness were made up for impact damage characterisation. These specimens were impacted at three different energy levels. The results showed that by embedding SMA wires into composite materials there is a reasonably low damage accumulation after impact. There is also no adverse impact effect on the structure compared with structures without wires as well as structures with other types of wires such as steel and martensitic SMA wires. The SMA-composites showed good damping and energy absorption capabilities. A novel application of SMA-composites is their use as a SMA patch in order to repair damage in existing cracked metallic structures. An analytical study and finite element modelling showing the closure stresses obtainable for use as patches was made.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Dr. Richard W. Neu; Committee Member: Dr. David L. McDowell; Committee Member: Dr. W. Steven Johnson.

Ulm, Univ. Diss., 2006.

Today, there is a great focus on downsizing the engines, this means that the engines are made smaller in size but retain the same power. This in combination with the drive to increase the power of the engines has led to the engine components being exposed to high thermal loads. Today’s engines also use very high cylinder pressure. The high thermal loads in combination with the high cylinder pressure have led to that the engine components are often very close to their material limits, so close that damage is common. This places high requirements on the materials, which makes the material selection a critical part of the engineering process.The main focus in this thesis work has been to develop and investigate a FEM model that can be used to quickly evaluate materials in an exhaust manifold that is exposed to thermo-mechanical fatigue (TMF). The model was then used to verify a material selection made for an existing exhaust manifold. One of AVL’s own software programs has also been evaluated, to see if it is a viable alternative to ABAQUS when preforming TMF simulations.The material selection made in this master thesis had the restriction that the exhaust manifold should not fail due to low cycle fatigue (LCF) when exposed to TMF. The goal has been to minimize the mass of the exhaust manifold by selecting a strong material with low density. The reason for this is because today there is a big focus on energy efficient cars with low emission levels. The simplest way to achieve this is to minimize the mass of the vehicle.The simulations conducted in this work has been performed in two different software’s, ABAQUS and AVL Fire M. In AVL Fire M flow simulations and steady-state heat transfer simulations have been performed. In ABAQUS, steady-state and transient heat transfer simulations and stress-strain simulations have been performed.The material selection process showed that Inconel 601 is the most suitable material for an exhaust manifold exposed to TMF. The simulations using Inconel 601 showed that this material will not fail due to LCF.The FEM model that was developed in this thesis was a lot faster compared to the existing TMF model used at AVL.CPU time for the existing model: 14 days 13 hours 14 minutes and 30 seconds (Core time).CPU time for the model developed in this thesis: 1 day 6 hours 37 minutes and 49 seconds (Core time).Two alternative models have been proposed for TMF simulations, one that uses the model developed in this thesis and one that is a combination of the existing model and the model developed in this work.

Inscrite dans le cadre du projet européen Coke Oven Operating Limits, cette thèse porte sur la modélisation thermomécanique d'un piédroit de cokerie. Le piédroit est une maçonnerie alvéolaire, chauffée par des gaz à haute température (supérieure à 1200°C). Pendant la cuisson du charbon dans les fours à coke, celui-ci se pyrolyse en coke provoquant une poussée sur les panneresses du piédroit. Ce projet a pour objectif de déterminer la pression maximale supportée par ces structures. Afin de répondre à cette problématique, un nouveau modèle thermomécanique de piédroit a été développé. Ce travail prend en compte à la fois le comportement non-linéaire de la maçonnerie, mais également les interactions avec l'environnement extérieur. La modélisation de la structure maçonnée est basée sur une approche macroscopique où les briques et le mortier sont remplacés par un matériau homogène équivalent, et ce pour différents états de joints. La non-linéarité du comportement est reproduite grâce à un critère d'ouverture qui permet de passer d'un état de joint à un autre. Les propriétés homogénéisées sont identifiées selon une approche énergétique couplée à un algorithme d'identification inverse. Plusieurs simulations numériques d’essais issus de la littérature ont permis de valider cette approche. Les paramètres régissant le comportement mécanique et thermique des matériaux sont déterminés expérimentalement ainsi que la tenue de l'interface brique/mortier. Les conditions aux limites du modèle sont établies à l'aide d'une instrumentation thermomécanique sur site industriel. Les simulations thermomécaniques du piédroit permettent de localiser des phénomènes de dégradation observés dans les faits. / This study lies within the framework of European project called Coke Oven Operating Limits. This thesis deals with the thermomechanical modelling of a coke oven heating wall. The heating wall is an alveolar masonry, heated thanks to gas at high temperature (superior to 1200°C). During coking time in coke ovens, there is a pyrolysis of coal to coke which implies a coke swelling pressure on chamber wall. The aim of this project is to determine the maximal lateral pressure allowed by these structures. In order to answer to this problem, a new thermomechanical model of heating wall was built. This work takes into account both non-linear masonry behaviour and interactions between the heating wall and its components. Structure modelling is based on a macroscopic approach where bricks and joints are replaced by a homogeneous equivalent medium for different joint states. The non-linearity behaviour is then reproduced thanks to a transition criterion which allows to go from one state to another one. Effective properties are determined with an energetic approach and an algorithm of inverse identification. Several numerical simulations were performed and compared with experimental tests extracted from literature to validate this approach. Material thermomechanical properties were identified experimentally, likewise masonry brick/mortar interface behaviour. Boundary conditions and loads were established from thermomechanical instrumentation of an in situ heating wall. Thermomechanical simulations of the heating wall allow to locate damages in good agreement with plants observations.