"During the solidification process in casting, hot tearing may occur. It is a severe defect that normally involves the formation of a macroscopic tear, which generates cracks either on the surface or inside the casting. Over the past decades, many strategies have been developed to evaluate the hot tearing tendency. Unfortunately, most of the tests can only provide qualitative information. Therefore, a reliable and quantitative test to evaluate hot tearing in aluminum alloys is highly desirable. To address this issue, WPI and CANMET MTL (both members of the Light Metal Alliance) jointly developed a quantitative hot tearing test and established a specific methodology. Using a constrained rod mold, the hot tearing formation can be quantitatively studied by measuring the contraction force, time and temperature during solidification for a restrained casting or linear contraction, time and temperature for a relaxed casting. This study investigated cast aluminum alloys A380.1 and A390 and wrought aluminum alloys 6061 and 7075. The results show that wrought aluminum alloys have a much stronger hot tearing tendency than cast aluminum alloys based on a quantitative analysis. Also, the study involves the effects of adding strontium and oxides respectively into the cast aluminum alloy A380.1. Compared with the pure A380.1 alloy, the introduction of strontium decreases the hot tearing tendency, while the inclusion of oxide greatly increases the hot tearing. The information obtained through these tests provides a database of hot tearing phenomenon and establishes a new hot tearing index."

Universiẗat, Diss., 2001--Stuttgart.

Major advances have been observed during the last two decades in the field of seismic engineering with further refinements of performance-based seismic design philosophies and the subsequent definition of corresponding compliance criteria. Following the globally recognized expectation and ideal aim to provide a modern society with high (seismic) performance structures able to sustain a design level earthquake with limited or negligible damage, alternative solutions have been developed for high-performance, seismic resisting systems. In the last two decades, an alternative approach in seismic design has been introduced for precast concrete buildings in seismic regions with the introduction of “dry” jointed ductile systems also called “hybrid” systems based on unbonded post-tensioned rocking connections. As a result structural systems with high seismic performance capabilities can be implemented, with the unique capability to undergo inelastic displacement similar to their traditional monolithic counterparts, while limiting the damage to the structural system and assuring full re-centring capabilities (negligible residual or permanent deformations). The continuous and rapid development of jointed ductile connections for seismic resisting systems has resulted in the validation of a wide range of alternative arrangements, encompassed under the general umbrella of “hybrid” systems. This research provides a comprehensive experimental and analytical investigations of 2- and 3-Dimensional, 2/3 scaled, exterior beam-column joints subjected both uni and bi-directional (four clove) quasic-static loading protocols into the behaviour, modelling, design and feasibility of new arrangements for “dry” jointed ductile systems for use in regions of high seismicity. In order to further emphasize the enhanced performance of these systems, a comparison with the experimental response and observed damage of 2-D and 3-D monolithic beam-column benchmark specimens is presented. However, after a lot of attention given to the behaviour of the skeleton structure, more recently the focus of research in Earthquake Engineering has concentrated on the behaviour of the floor system within the overall 3D behaviour of the building and the effects of beam elongation. The effects of beam elongation in precast frame systems have been demonstrated to be a potential source of un-expected damage, unless adequate detailing is provided in order to account for displacement incompatibilities between the lateral resisting systems and the floor. Two contributions to beam elongation are typically recognized: a) the material contribution due to the cumulative residual strain within the steel, and b) the geometrical contribution due to the presence of a neutral axis and actual depth of the beam. Regarding jointed ductile connections with re-centering characteristics, the extent of beam elongation is significantly reduced, being limited to solely the geometrical contribution. Furthermore, such effects could be minimized when a reduced depth of the beam is adopted due to the use of internal prestressing or external post-tensioning. However, damage to precast floor systems, resulting from a geometric elongation of the beam, has yet to be addressed in detail. In order to emphasize the enhanced performance in controlling and minimizing the damage of the structural elements via the use of the proposed advanced hybrid solutions, this research presents via experimental and analytical validation of two alternative and innovative solutions to reduce the damage to the floor using 2 and 3-Dimensional, 2/3 scaled, exterior beam-column joints. The first approach consists of using standard precast rocking/dissipative frame connections (herein referred to as “gapping”) in combination with an articulated or “jointed” floor. This system uses mechanical devices to connect the floor and the lateral beams which can accommodate the displacement incompatibilities in the connection. The second approach to reduce the floor damage investigates the implementation of a “non-gapping” connection, also called non-tearing-floor connection, using a top hinge at the beam-column interface, while still relying on more traditional floor-to-frame connections (i.e. topping and continuous starter bars). Additionally, further refinements and constructability issues for the non gapping connection are investigated under the experimental and analytical validation of a major 2-Dimensional, 2/3 scaled, two-story one-bay frame using non-tearing floor connections. Based on the non-tearing floor connections, a series of parametric analysis for beam-column joints and frames are carried out. Furthermore, the analysis and design of two prototype frames using different solutions is presented. The frames are subjected to cyclic adaptive pushover and inelastic time history analysis in order to investigate analytically the response characteristics of hybrid frames using non-tearing connections, as well as how the beam growth affects the frame response under earthquake loading. Computational models for hybrid PRESSS frames and a conventional reinforced concrete frames are developed and compared with the ones using non-tearing connections.

The aim of this thesis is to propose a more general understanding of the influence of the thickness of the plate and of the microstructural and mechanical properties of the material on the resistance to ductile tearing in thin metallic plates. The objective is to attempt unifying different observations made in the literature together with the results of a new extensive experimental campaign. The final goal is to develop predictive simulation tools with a micromechanics-based foundation. In order to reach this objective, a detailed experimental campaign has been performed concerning the fracture behavior of the aluminium alloy 6082, complemented by experiments on a stainless steel A316L and on a set of 14 other materials. In a first modelling effort, we propose very simple closed-form models in order to separate the different contributions to the total work of fracture in thin plates: the work of necking and the work of damage and material separation. The respective contributions are compared and an unique explanation of the different behaviors observed experimentally is proposed. In a second modelling step, we develop a full 3D numerical tool based on cohesive elements for simulating crack propagation in thin ductile plates. Three different methods are proposed to calibrate the parameters of the model in order to reproduce the experimental data and to extrapolate the results to other material properties or geometric conditions. Finally, the parameters of the cohesive zone model are justified using micromechanics-based arguments. / Le but de cette thèse est de proposer un modèle général à base micromécanique permettant de comprendre l’influence de l’épaisseur de la tôle ainsi que de la microstructure et des propriétés mécaniques du matériau sur la résistance à la rupture ductile de plaques minces métalliques. L’objectif est d’essayer d’unifier les différentes observations de la littérature ainsi que les résultats d’une nouvelle campagne expérimentale afin d’aboutir au développement d’outils numériques prédictifs. Pour atteindre cet objectif, nous avons réalisé une campagne d’essais concernant le comportement à la rupture de différents matériaux. Cette campagne a été menée en profondeur sur l’alliage d’aluminium 6082 et de manière moins approfondie sur un acier inoxydable A316L ainsi que sur 14 autres matériaux. Dans un premier temps, nous présentons une série de modèles semi-analytiques simples dont le but est de séparer les différentes contributions au travail de rupture total dans les tôles minces : le travail de striction et le travail d’endommagement du matériau. Ces deux contributions sont ensuite comparées et nous proposons une explication qui reprend les différents comportements observés expérimentalement. Dans un deuxième temps, nous développons un outil numérique 3D complet destiné à simuler la propagation de fissures dans les tôles minces ductiles et qui utilise des éléments cohésifs. Trois méthodes différentes sont proposées pour calibrer les paramètres du modèle de manière à reproduire les données expérimentales et à permettre l’extrapolation des résultats à d’autres matériaux ou d’autres épaisseurs de tôles. Finalement, les paramètres du modèle de zone cohésive sont justifiés grâce à des arguments à fondement micromécanique.

No description available.

Published Article / Rapid and reliable methods for component development and economic manufacturing layout are today crucial factors for the application of press tooling techniques in mass production of automotive industry components. The use of Finite Element Analysis (FEA) based forming simulation can provide a more detailed insight into the real behaviour of a structure. An LS-DYNA finite element model was developed to analyse the material behaviour during the piercing process of a drainage hole for a shock absorber seat. The simulation is intended to simulate tearing that occurs during the manufacturing stage. Once the current punch produces the observed tearing we can modify the punch to eliminate the problem.

Traditional seismic design philosophy for reinforced concrete seismic frame structures localises damage and inelastic deformation to regions of significant plasticity within the beam (plastic hinge zones) during a severe earthquake event. Collapse prevention of the frame is applied through capacity design methods, requiring the maximum expected flexural strength of the beam plastic hinges to be reliably assessed in order to design for, and ensure, the predominantly elastic flexural response of the columns in the frame. Previous experimental and numerical investigations have shown that significant and detrimental damage to the frame and floor system occurs due to the formation and elongation of ductile beam plastic hinges; requiring extensive post-earthquake repair or demolition with likely loss of function of the building. This poses significant economic consequences to occupiers of the building, as the time required to reinstate the integrity of the structural and non-structural building components is often lengthy. More importantly, it has been highlighted that the interaction between elongating ductile plastic hinges and the accompanying floor system enhances the flexural strength of the beam hinges, altering the distribution of forces in the seismic frame compared to that assumed during capacity design. Research has shown that the consideration of frame-floor interaction in current New Zealand design codes significantly underestimates the flexural strength enhancement of beam plastic hinges, threatening the hierarchy of strength and collapse prevention mechanisms employed in capacity design. Recent research has introduced change in the design philosophy of precast concrete seismic frames. Rather than designing for localised damage in the frame, unique Non-tearing (of the floor) connection details have been developed which provide a gap or slot between the end of the beam and column face and force connection rotation to occur about a shallow hinge located at the top of the beam, thereby avoiding the formation of plastic hinges and associated beam elongation effects altogether. Research investigations have shown that Non-tearing connections successfully minimise damage to the structural frame and floor, while providing seismic energy dissipation characteristics at least comparable to that of traditional reinforced concrete connections. In this research, the mechanics of different non-tearing connection arrangements were investigated and original theory introduced for the aspects of connection behaviour which diverged from fundamental reinforced concrete design. A variety of precast concrete non-tearing connection details were developed, with the design focus placed on economic and construction efficiency in order to encourage the rapid implementation of non-tearing connection technology into New Zealand construction industry. The performance of the developed connection details were explored and assessed experimentally and analytically. A two bay precast concrete frame with precast floor system was tested under a demanding reversed cyclic, quasi-static loading protocol using displacement control. The seismic response of the non-tearing connection details employed in the test frame successfully minimised damage to the frame and floor systems. Only minor repair of one primary crack at each connection between the floor diaphragm and supporting beam would be required after a design level earthquake. Issues encountered with buckling of the longitudinal reinforcement in the bottom of the beam reduced the connection performance at high levels of drift. However, detailing measures were successfully employed in successive tests which improved the drift capacity of the connections. Detailing improvements to enhance the seismic response of the developed non-tearing connections were recommended based observations from the frame test. Numerical analysis of the non-tearing connection details was performed using simple rotational and compound spring models, with the key features of the experimental response captured with excellent accuracy. The analytical models were constructed using engineering theory, rather than by calibration with experimental observations. The modelling assumptions and principles adopted in the analysis have been presented for use in design offices or future research programmes when designing and analysing seismic frames using non-tearing connections. This research successfully contributed to the development and progression of non-tearing frame technology. With further research and the refinement of construction details, non-tearing floor connections exhibit impressive potential for providing superior seismic safety, performance and efficiency in precast concrete seismic frame buildings.

No description available.

In highly conductive plasmas described by the ideal magnetohydrodynamics (MHD), magnetic field lines are frozen-in to the plasma. The contrary process takes place when the localized non-ideal and diffusive effects allow the field lines to break and reform, and therefore, called "magnetic reconnection" process. Magnetic reconnection is well recognized as an important plasma process capable of converting enormous amounts of stored magnetic energy to both thermal energy and bulk acceleration of the plasma. Single-fluid MHD model of this process can not explain the rate of magnetic reconnection observed in the space and laboratory plasmas, but the two-fluid model has raised the promises of explaining the magnetic reconnection satisfactorily. This thesis by employing the two-fluid MHD model of the magnetic reconnection studies theoretically this process.

No description available.