Fracture of aluminum naval structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007."June 2007."Includes bibliographical references (leaves 238-272).Structural catastrophic failure of naval vessels due to extreme loads such as underwater or air explosion, high velocity impact (torpedoes), or hydrodynamic loads (high speed vessels) is primarily caused by fracture. Traditionally, naval structures have been designed to resist yielding, buckling and fatigue, but not fracture. Consequently, adequate methods and procedures to design ships against fracture have not been developed. The rapidly increasing application of lightweight materials, such as aluminum alloys, in the shipbuilding industry requires fundamental understanding of mechanisms and mechanics of fracture that govern naval stiffened panels. Therefore, a comprehensive tool consisting of application of advanced fracture models, material calibration, and validation through component testing is provided that will increase the survivability envelope and speed up the development process of new vessels. Cracking is a major cause of structural degradation, which is a primary source of costly repair work on metal structures. This thesis studies the structural response of various stiffened plates and compares them with unstiffened plates represented by compact tension (CT) specimens.(cont.) An extensive experimental program is presented that includes coupon testing and small and intermediate scale tests on naval aluminum structures including a variety of monolithic T-type extruded and flatbar welded specimens. Representative naval designs are selected and subjected to quasi-static loading and a number of key parameters, such as geometry, loading rate and structural configuration are evaluated with respect to fracture. Numerical modeling and analyses of ductile fracture initiation and propagation on a pre-cracked geometry using a commercial finite element code (ABAQUS), taking into account the behavior of simple uncracked material, has been performed showing a very good agreement with small and intermediate scale tests. Two major contributions of this thesis are the mapping of crack patterns in stiffened plates and the development of a methodology which enables ship designers to evaluate critical areas within a structure with respect to crack initiation, propagation, optimum material usage, and computational cost.by Konstantinos P. Galanis.Ph.D

Finite element analysis of localised rolling to reduce residual stress and distortion

Fusion welding processes cause residual stress due to the uneven heat distribution produced by the moving welding torch. These residual stresses are characterised by a large tensile component in the welding direction. Due to the self-equilibrated nature of the residual stress, compressive ones are present in the far field next to the weld seam, which can cause different kind of distortion such as bending or buckling. Welding residual stress can be responsible of premature failure of the components, such as stress crack corrosion, buckling, and reduction of fatigue life. Localised rolling is a stress engineering technique that can be used to reduce the residual stress and distortion caused by welding. It induces plastic strain in the rolling direction, counteracting the plastic strain produced during welding. In this thesis three techniques were investigated, pre-weld rolling, post-weld rolling, and in situ rolling. These techniques have been seldom studied in the past, particularly pre-weld rolling; consequently the mechanisms are poorly understood. Finite element models allow stress and strain development during both welding and rolling processes to be better understood, providing an improved understanding of the mechanisms involved and aiding process development. A literature survey was done to find the state of the art of the computational welding mechanics simulations, stress management, and the residual stress measurement techniques, as well as the knowledge gaps such as, the thermal losses through the backing-bar in the thermal simulation, the frictional interaction in the rolling process, and the material properties of the steel used in the models. In the literature not many models that investigate the management of welding residual stress were found. After this, the general considerations and assumptions for the welding thermal mechanical models presented in this thesis were discussed. The effect of different backing-bar conditions, as well as different material properties where investigated. Both influenced the residual stress profile to varying degrees. In particular, temperature dependent heat loss to the backing-bar was necessary to capture the improved heat loss near the weld. The distortion predicted by the model was investigated to determine whether it was due to bending or buckling phenomena. Lastly, the temperature distribution and residual stress predictions were validated against thermocouple and neutron diffraction measurements conducted by Coules et al. [1–3]. Pre-weld rolling was the first of the three rolling methods considered, in which rolling is applied to the plates before performing GMA butt-welds. The principle behind this technique consisted in inducing tensile residual stress in the weld region before welding; therefore, it is similar to mechanically tensioning the weld, which can significantly reduce the residual stress and distortion. However, there was no significant change in the tensile residual stresses. On the other hand, it was possible to achieve a small reduction in the distortion, when the plates were rolled on the opposite surface to the weld; rolling in this way induced distortion in the opposite direction to the distortion induced by welding, reducing the magnitude of the latter. These results were compared with experiments conducted by Coules et al. [1,4]. A subsequent investigation combined pre-weld rolling with post-weld heating. With this additional process the residual stress and distortion were significantly reduced, and flatter residual stress profile was achieved. The post-weld rolling and in situ rolling techniques were discussed afterwards. In the post-weld rolling models, rolling was applied after the weldment was cooled to room temperature. In in situ rolling the roller was applied on top of the weld bead at some distance behind the torch, while it was still hot. The principle behind these techniques consisted in applying positive plastic strain to the weld bead region by a roller, counteracting the negative plastic strains produced in the welding process. Two roller profiles were investigated, namely, grooved, and double flat rollers. The post-weld rolling on top of the weld bead models, which used the grooved roller, showed good agreement against experimental results, producing a large reduction of the residual stress and distortion. Some discrepancies were present when the weld toes were rolled with the dual flat roller. The former roller was more efficient for reducing residual stress and distortion. The influence of different friction coefficients (between the roller and weldment, and between the backing-bar and the weldment), were investigated. It showed significant dependency on the residual stress distribution when high rolling loads were used. The frictional interaction constrained the contact area inducing more compressive stress in the core of the weld bead; therefore it produced more tensile residual stress in the surface of the weldment. Additionally, the influence of rolling parameters on the through-thickness residual stress variation was investigated. Low loads only influence the residual stress near the surface, while high loads affected the material through the entire thickness. When the dual flat roller was used to roll next to the weld bead, significant compressive residual stress was induce in the weld bead; however, the residual stress reduction was very sensitive to the contact of the roller to the weld toes; therefore, when rolling a weld bead that varies in shape along the weld, the residual stress reduction is not uniform and varies along the length. On the other hand, the in situ rolling did not produced significant residual stress or distortion reduction in all the cases analysed. The rolling occurred when the material was still hot and the residual stress was subsequently formed as the material cooled to room temperature. Numerical modelling was a very useful tool for understanding the development of stress and plastic strain during the welding and rolling processes

Friction stir welding parameters influencing the fracture resistance of an Al 5083 alloy welded joint

Фрикционо заваривање мешањем представља релативно нов, савремен поступак заваривања великог броја материјала, као што су легуре алуминијума, бакра, титанијума, магнезијума итд. Јединствена особина овог поступка је да се одвија у чврстом стању, без појаве топљења. У овој дисертацији, испитивана је Al-Mg легура 5083, коју одликује добра комбинација чврстоће, жилавости и отпорности на корозију. Током фрикционог заваривања мешањем, специјално дизајниран алат, који се ротира, продире у материјал, управо у линији спајања две плоче које се заварују. На контактној површини ослобађа се топлота која омекшава материјал, олакшава кретање алата уз истовремено мешање материјала. Овако заврени спојеви имају читав низ предности у односу на класично заварене спојеве – укупна потрошња енергије далеко је мања, нема појаве течних фаза, чврстоћа споја често буде већа него код основног материјала и, коначно, нема штетних утицаја на природну околину. Постоје, наравно, и недостаци ове технологије, пре свега повезани са дужином заварених спојева која зависи од димензија машине на којој се поступак изводи. Током овог поступка заваривања, у зони заварених спојева јављају се јасно дефинисане зоне утицаја топлоте, као и код поступка класичног заваривања. Међутим, код фрикционог заваривања мешањем, појављује се и зона термо-механичког утицаја под симултаног дејства топлоте и пластичне деформације материјала. У овој дисертацији, испитиван је утицај процесних параметара фрикционог заваривања на чврстоћу заварених спојева. Испитиван је утицај (i) ротационе брзине заваривања (у опсегу 500 до 800 обртаја у минути), (ii) утицај транслационе брзине (75-150 mm/min) и, (iii) утицај нападног угла алата (1о-4о). Сви заварени спојеви испитивани су на отпорност према ударној жилавости. Нумеричком обрадом експерименталних резултата, одређена је ударна жилавост заварених спојева као и брзина и енергија лома. Поред механичких испитивања, извршена су опсежна микроструктурна испитивања применом оптичког и скенинг електронског микроскопа (SEM). Ова испитивања омогућила су бољи увид у механизам и кинетику дуктилног лома заварених спојева...Friction stir welded is relatively new- solid-states joining process for welded several material such as aluminum, copper, titanium and magnesium. Also FSW technique is preformed in solid state without melting hence avoiding hot cracking. In this research selected aluminum 5083 alloy, it is widely used in applications in which the combination of strength and low weight is attractive. In friction stir welding (FSW) pin connected to a shoulder in rotated and slowly plunged into the joint line between two pieces of plats. When the shoulder tools rotation and contact the material surface, it generated friction heating between the welding tool and the material of the work pieces. This heat causes the latter soften without reaching the melting point and allows traversing of tool along the welding. Friction stir welding presents several benefit for joining of various alloys, specially of aluminum alloy one of the significant advantage of FSW is the heat inputs are small relative to fusion welding techniques and due to the low temperature of the process, material such as Al, Cu, Mg alloys that cannot be welded by fusion processes are easily weld by FSW. On the other hand, FSW has some drawback is often slower traverse rate then some fusion welding and exit hole left when tool is withdrawn. Friction stir welding process generates three distinct microstructural zones that result from the welding process as following: nugget zone also known as the dynamically recrystallized zone (DRZ) where the tool piece pin passes into this zone and by experience, it has high deformation and high heat, generally consists of fine equated grains due to recrystallisation, the thermo mechanically affected zone (TMAZ) and the heat affected zone (HAZ), all zones together are called welding zone. After welded aluminum alloy tested specimens alloy by charpy impact test to evaluate absorbed energy caused the fracture material and toughness of material. Also obtained high resolution images by macro-photographs and by scanning electron microscope (SEM) to evaluate type of surface fracture and detected fracture and micro void in material then analysis material by energy dispersive x-ray spectroscopy (EDX) to shown distribution elements of chemical compound in aluminum alloy after heating and cooling precipitation. Finally, selection the optimized FSW parameters for welded aluminum 5083 alloy, it achieved higher fracture resistance in welded zone of alloy..

Latest Hydroforming Technology of Metallic Tubes and Sheets

This Special Issue and Book, ‘Latest Hydroforming Technology of Metallic Tubes and Sheets’, includes 16 papers, which cover the state of the art of forming technologies in the relevant topics in the field. The technologies and methodologies presented in these papers will be very helpful for scientists, engineers, and technicians in product development or forming technology innovation related to tube hydroforming processes

Engineering Principles

Over the last decade, there has been substantial development of welding technologies for joining advanced alloys and composites demanded by the evolving global manufacturing sector. The evolution of these welding technologies has been substantial and finds numerous applications in engineering industries. It is driven by our desire to reverse the impact of climate change and fuel consumption in several vital sectors. This book reviews the most recent developments in welding. It is organized into three sections: “Principles of Welding and Joining Technology,” “Microstructural Evolution and Residual Stress,” and “Applications of Welding and Joining.” Chapters address such topics as stresses in welding, tribology, thin-film metallurgical manufacturing processes, and mechanical manufacturing processes, as well as recent advances in welding and novel applications of these technologies for joining different materials such as titanium, aluminum, and magnesium alloys, ceramics, and plastics

Thermal modelling of gas metal arc welding using finite element analysis

Thesis (M.E.Sc.) -- University of Adelaide, Dept. of Mechanical Engineering, 199

Ship collision and grounding performances

PhD ThesisThis present thesis investigates the accidental load of ship collision and grounding performances. To achieve this objective the thesis is composed of several main tasks. The main tasks comprise the rupture prediction, validation of material failure, ship grounding analysis and ship collision analysis. To predict material rupture, FLD material failure was used and validated with available experimental and FEA data. The FLD was extended to established material failure scaling laws which consider onset failure at plane strain in relation to mesh sizes. This was accomplished by running mesh convergence studies at different mesh sizes and at different FLD0. The linear material damage evolution is adopted in this case until the convergence results were satisfied. The material damage was used for all of further analysis in ship collision and grounding and employed mild steel and high tensile steel material properties. The ship grounding structure damage was investigated by deploying conical rocks at different locations of the ship's double bottom structure. The analysis focused on vertical penetration and horizontal penetration which contributed to significant damage to the structure. The ship collision analysis was investigated in various types of structures arrangement and diverse ship striking scenarios to penetrate struck ship and collide rigid wall. Furthermore, the prediction of ship collision and grounding were extended by using simplified approaches that were capable to predict ship collision to rigid wall, rigid body striking ship collided with deformable struck ship and deformable collision of striking and struck ship. Finally, this substantial amount of research work achieved the objectives of the study when the results of accidental load were validated and correlate well with experimental, empirical and FEA simulations at more than a satisfactory level