A aplicação de aços de alta resistência está fortemente difundida na indústria mecânica, principalmente em aplicações onde a redução de peso é importante, como por exemplo, a indústria de implementos rodoviários e maquinários agrícolas. Esta dissertação visa analisar a influência de diferentes aportes térmicos no comportamento microestrutural e mecânico de juntas soldadas através do processo MAG de modo a obter juntas com propriedades otimizadas. Para este estudo foram utilizadas chapas metálicas com espessura de 3 mm do aço de alta resistência e baixa liga USI LNE700. Os parâmetros nominais de energia de soldagem foram variados de acordo com os valores descritos pelo fornecedor SSAB. O processo de soldagem foi realizado com o auxílio de um sistema robotizado para manter a homogeneidade ao longo da junta, distância e posicionamento correto no comprimento total da junta soldada. Os ensaios para verificação do comportamento microestrutural e mecânico foram realizados com auxílio de microscopia ótica, perfis de microdureza e ensaios de tração. Buscaram-se utilizar aportes térmicos reduzidos para minimizar os efeitos de revenimento da martensita presente no metal de base, um dos principais responsáveis pela redução das propriedades mecânicas da junta soldada. Os resultados não mostraram significativa variações na microestrutura e propriedades de tração do material, porém o preenchimento da junta soldada formada apresenta um comportamento diretamente proporcional à energia de soldagem. / The application of high strength steels is strongly diffused in the engineering industry, especially in applications where weight reduction is important, such as the industry of agricultural machines and trailers industry. This investigation aims to analyze the influence of different heat inputs on microstructural and mechanical behavior of joint welded by GMAW in order to obtain joints with optimized properties. Sheet metal with a thickness of 3 mm high strength low alloy steel LNE700 (supplier Usiminas) were used. The welding energy was varied around the nominal value informed by the steel supplier SSAB for this study. The welding process was made using robotic system to maintain homogeneity along the joint, right distance and position during the total weld joint length. The microstructural and mechanical behaviors were performed with the optical microscope, microhardness profile and traction test. We attempted to use lower heat inputs to minimize the effects of tempering of martensite present in the base metal, a major contributor to the reduction of the mechanical properties of the welded joint. The results did not show significant microestructural and tensile properties variation, however, when the welding energy is increased, the welded joint penetration formed was increased as well.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering; and, (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Includes bibliographical references. / by John Emmanuel Agapakis. / M.S.

Este trabalho tem como objetivo investigar o comportamento pós-pico de pilares com concretos de alta resistência confinados com e sem adição de fibras metálicas, submetidos à flexo-compressão. Para tanto, realizou-se inicialmente uma extensa revisão bibliográfica sobre o tema, na qual, elencaram-se os principais fatores que influenciam o comportamento pós-pico desses elementos estruturais. Em seguida, desenvolveram-se estudos paramétricos com o intuito de estabelecer os modos de influência de cada fator. Com base nesses estudos preliminares, um programa experimental, dividido em duas fases, foi proposto. Na primeira fase, ensaiaram-se vinte e seis pilares de concreto armado e 14 pilares de concreto simples. Três fatores foram estudados: a taxa de adição de fibras metálicas, a taxa de armadura transversal e a resistência à compressão do concreto. Os pilares possuíam seções transversais quadradas com dimensões 15 cm x 15 cm e altura de 50 cm e foram ensaiados à compressão centrada com controle de deslocamento. Observou-se que os três fatores analisados influenciam diretamente a ductilidade desses elementos estruturais e que, por meio de adição de fibras metálicas, é possível garantir índices de ductilidade aceitáveis para os pilares com concreto de alta resistência. Outrossim, analisando-se os diagramas força vs. deformação dos pilares, observou-se que para o primeiro pico de força, toda a seção resiste a esforços de compressão e que a adição de fibras melhora o trabalho conjunto entre o cobrimento e o núcleo do pilar. Finalizando esta fase, propuseram-se modificações para o modelo para concreto confinado de Cusson e Paultre (1995), de modo que, permitisse ao mesmo modelar o comportamento do concreto confinado com e sem adição de fibras metálicas. Na segunda fase do programa experimental, quinze pilares de concreto armado foram ensaiados a compressão excêntrica. Essa fase teve como objetivo investigar a influência da excentricidade, da taxa de armadura transversal e da taxa de adição de fibras metálicas no comportamento pós-pico desses elementos estruturais. Os pilares, nessa fase, possuíam seção transversal quadrada com dimensões de 15 cm x 15 cm e altura de 170 cm. Para realização dos ensaios foi confeccionado um par de rótulas unidirecionais, as quais apresentaram excelente desempenho, conseguindo transferir integralmente o momento externo aplicado ao pilar. Observou-se que, tanto a flambagem das barras da armadura longitudinal, quanto a perda de massa de concreto nesses pilares ocorrem de modo bem mais crítico que nos pilares submetidos à compressão centrada. Constatou-se, ainda, que quando os valores dos três fatores analisados são elevados, a ductilidade desses elementos estruturais é melhorada. Finalmente, foi observado que o efeito da flexão faz com que as tensões de confinamento se distribuam de modo diferenciado dentro da seção transversal dos pilares; contudo, observou-se que a tensão de confinamento na região comprimida da seção transversal pouco é modificada. / This thesis aims to investigate the ductility of high strength concrete columns, with confinement and/or with steel fibre addition, under combined axial load and bending. For this, an extensive survey was performed, and the main factors, which influence the column ductility, were pointed out. Then, parametric studies were done, aiming to establish the mode of influence of each factor. Based on these preliminaries studies, an experimental investigation, divided in two steps, was proposed. In the first part, twenty-six reinforced concrete columns and fourteen concrete columns were tested and three factors were analysed: the ratio of fibre addition, the transversal reinforcement ratio and the concrete compressive strength. The columns have 15 cm x 15 cm square cross section and were 50 cm high. They were tested under concentric load with displacement control. It was observed that all factors interfere in the columns\' ductility, and that, by adding fibre in the concrete, is possible to provide acceptable ductility index to high strength concrete columns. The load vs. column strain diagrams were studied and it was verified that for the first load peak, all the column cross section resists the axial load. Besides that, it was observed that the fibre addition improves the work together between the cover and the column core. Finally, several modifications to the concrete confinement model of Cusson and Paultre (1995) were proposed to allow this model to analyse the compressive behaviour of the confined concrete with or/not fibre addition. In the second part of the experimental program, fifteen reinforced concrete columns were tested under eccentric load. The effects of the load eccentricity, the ratio of steel fibre addition and the transversal reinforcement ratio on the column ductility were investigated. The columns have 15 cm x 15 cm square cross section and were 170 cm high. A pair of unidirectional hinges was design, which presented exceptional performance, transferring integrally the external applied moment to the columns. A most critical model of longitudinal reinforcement buckling and mass lost was observed. In addition, it was verified that when the values of all factors were increased, the column ductility was improved. Finally, a different distribution mode of the confinement pressure on the column cross section was observed, when combined axial load and bending is applied. Nevertheless, the confinement pressure on the compressive region of the column cross section is not modified.

Friction Bit Joining (FBJ) is a new technology that can be used to join dissimilar materials together. This ability makes it a good candidate for creating light weight structures for the automotive industry by combining lightweight materials such as aluminum to stronger materials like advanced high-strength steels. The automotive industry and many other industries have great interest in reducing structure weight to increase fuel efficiency. The purpose of this research is to make FBJ of GADP 1180 to AA 7085-T76 a production ready process by (1) better understanding the effects of process parameters, bit design and tool design on joint strength and reliability especially as they relate to different joint configurations; (2) determining if consecutive FBJ joints on a part will be additive in strength; (3) improving surface finish for better coating adhesion so that joints can be made to withstand extended corrosion testing; and (4) determining the failure modes and fatigue life of joint components at high and low load amplitudes. No universal parameter set for optimizing peak load for T-peel, cross tension, and lap-shear tension configurations were found. Due to the extreme load conditions of T-peel and the smaller margin of safety it is better to optimize for T-peel. However, strength and reliability were still improved across the board. Cutting features and tapered shanks were found to not always be necessary. Removing cutting features from the bit design increased peak weld cycle loads, but a stiffer machine can overcome this. Consecutive FBJ joints on a part are mostly additive in nature. When the weakest joint fails, its load is distributed to the remaining joints and will limit the peak load of the whole part. If all joints are "good" then the peak load will be approximately additive. Most of the stress is localized on the side of the bit opposite of the pulling direction. Failure modes in lap-shear tend to change from weld nugget pullouts in single weld specimens to aluminum material failures in multi-weld specimens. This is because of the added stiffness that additional material and welds provide to resist coupons bending and creating a peeling action. Surface finish was improved by development of a floating carbide cutting system which cut aluminum flash as it was generated around the head of the bit. A new internal drive design provided the ability to drive bits flush with the aluminum top layer if desired with minimal reductions in strength. Flush bits provided benefits in safety, cosmetics, and coating adhesion.

Built on the recent successes in understanding the crystallographic mechanism for short fatigue crack (SFC) growth across a grain boundary (GB) and developing an experimental method to quantify the GB resistance against short crack growth, a microstructure-based model was developed in this study to simulate the growth behaviors of SFCs in 3-D, by taking into account both the driving force and resistance along at each point along the crack front in an alloy. It was found that the GB resistance was a Weibull function of the minimum twist angle of crack deflection at the boundary in AA2024-T3 Al alloys. In the digital microstructure used in the model, the resistance at each GB that the short crack interacted with could be calculated, as long as the orientations of grains and the crack were known. In the model, an influence function accounting for the overlapping effect of the resistance from the neighboring grain boundaries was proposed, allowing for calculation of the total resistance distribution along the crack front. In order to overcome the time consuming problem for the existing equations to derive the distribution of stress intensity factor along the crack front under cyclic loading, an analytical equation was proposed to quantify the stress intensity factor distribution along an irregular shape planar crack. By introducing two shape-dependent factors, the fractured area and the perimeter of the crack front, the newly proposed equation could readily and accurately derive the stress intensity factor distribution along the crack front that had large curvatures and singularities. Finally, a microscopic-scale Paris’ equation was proposed that took into account both the driving force, i.e., stress intensity factor range, and the total resistance to calculate the growth rate at each point along crack front. The model developed in this work was able to incorporate microstructure, such as grain size and shape, and texture into simulation of SFC growth in 3-D. It was capable of simulating all the anomalous growth behaviors of SFCs, such as the marked scatters in growth rate measurement, retardation and arrest at grain boundaries, and crack plane deflection at grain boundaries, etc. The model was used to simulate the growth behaviors of SFCs initiated from prefractured constituent particles in order to interpret the multi-site fatigue crack initiation observed in AA2024-T351 Al alloys. Three types of SFCs were observed initiating from these particles, namely, type-I non-propagating cracks; type-II cracks which were arrested soon after propagating into the matrix; and type-III propagating cracks. To quantitatively study the 3-D effects of particle geometry and micro-texture on the growth behaviors of micro-cracks in these particles, rectangular micro-notches with different dimensions were fabricated using focused ion beam in the selected grains on the T-S planes in AA2024-T351 Al alloys, to mimic the pre-fractured particles in these alloys. Knowing the notch dimensions or particle shape, grain orientation and GB geometry, the simulated crack growth behaviors were consistent with the experimental observations, and the model was able to verify that the three types of cracks evolved from these particles were mainly associated with the thickness and width of the pre-fractured particles, though the particle geometry and grain orientation could also affect the behaviors of fatigue crack initiation at the particles. When the widths of the particles were less than 15 μm, like in most high strength Al alloys, the simulated results confirmed that the crack type was only associated with the particle thickness, consistent with the experimental results in AA2024-T351 alloys with a strong rolling texture. The lives for the SFCs to reach 0.5 mm in length were quantified with the model in the AA2024 alloy, revealing that there was a bimodal distribution in the life spectrum calculated, with the longer life peak being related to larger twist angles of crack deflection at the first GB the cracks encountered and the shorter life peak being associated with small twist angles (< 5°) at the first GB. The model further demonstrated the influence of grain structure on SFC growth by considering two different grain structures with the same initial short crack, namely, a layered grain structure with only the primary GBs perpendicular to the surface and the layered grains with both primary and secondary GBs. Depending on their positions and geometry, the secondary GBs could still exert a strong retarding effect on SFC growth on surface. The model was validated by matching to the growth rate measured on surface of a SFC in an AA8090 Al-Li alloy. Good consistency was achieved between the simulated and experimentally measured growth rates when both the primary and secondary GBs were considered in the model. The model developed in this study exhibits its potential applications to optimizing the microstructure and texture in alloys to enhance their fatigue resistance against fatigue crack growth, and to satisfactory life prediction of engineering alloys.

"June 2002" Includes bibliographical references (leaves 349-374)

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

"June 2002" / Includes bibliographical references (leaves 349-374) / xxxix, 416 leaves : ill., plates ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Civil and Environmental Engineering, 2002

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.

Located on one of the highly active seismic fault systems in the world, the building stock in Turkey is mainly composed of reinforced concrete frames with 4-5 stories. Due to design and construction deficiencies resulting from the use of unqualified personnel and insufficient supervision, many of these buildings lack lateral stiffness, ductility and strength. For many structures, there is a need to alleviate these deficiencies by means of some rehabilitation techniques prior to earthquakes. One approach also used very widely in Turkey is to fill some of the frame bays by cast-in-place R/C panels. The procedure appears to be very practical at first glance. It also appears to be very economical as far as the production of the panels is concerned. However, the production phase is slow, dirty, destructive and disruptive to occupants. Moreover, it requires relatively skilled personnel and special equipment. Therefore, the real life experience shows that the actual cost in practice is much higher when all other hidden costs are taken into account. The aim of this experimental study is to explore the potential of using infill walls made of custom shaped and high strength concrete blocks as a simpler and more practical alternative to cast-in-place R/C panels to increase the lateral load bearing capacity of frame structures. The effectiveness of FRCM (Fiber Reinforced Cementitous Matrix) system on damaged structures is also investigated in this study.