This bachelor thesis has been carried out at GKN Aerospace. GKN is a member of European Space Agency, designing and manufacturing rocket-nozzles for the Ariane rockets. The manufacturing process entails many welds. Weld-simulations have been made to investigate stresses and plastic strains on simplified geometries. Plastic strains have been evaluated parallel and normal to the weld for plate geometries of shell-elements with rectangular cross-section and sandwich-cross-section, using the FEM-program MSC.marc. Results shows that plate width and length have negligible effect on the plastic strains when one weld is made. A comparison between a sandwich-sector cone and a sandwich plate was made, to investigate how plastic strains and stresses were affected of geometry. Plastic strains and stresses parallel the weld are the same. Plastic strains and stresses normal the weld are affected by changing geometry. Studies on differences in stresses between solid and shell elements propose use of solid elements near the weld region, if stresses are of interest.

T-joints are common in beam-column connections of steel frames, vierendeel girders and at mid-span of N-trusses. Strengthening the members of these structures increases the demand on the joints, which may require joint strengthening. This thesis examines different strengthening techniques of T-joints of RHS members. In Phase I, the effectiveness of through-wall steel bolts is examined. This is accomplished by controlling the web outward buckling of the chord under the brace axial load. The study examined the effect of the number and pattern of bolts, as well as the web height-to-wall thickness (h/t) ratio of the chord, on strengthening effectiveness. Rectangular 203x76x(3.09, 4.5, and 5.92) mm chord members were tested. The 8 mm diameter steel bolts varied from a single bolt to 15 bolts of various distributions. The joint strength increased by 3.1%, 6.2%, and 29% for chords with (h/t) of 34, 45, and 65, respectively. The number and distribution of bolts had little effect on their effectiveness. In Phase II, similar T-joint specimens were strengthened using adhesively bonded GFRP plates, 9.5 mm thick, of different configurations, and 2 mm thick high-modulus CFRP plates of equivalent stiffness. It was shown that strength gain increases significantly, from 9% to 38%, as (h/t) ratio of the HSS chord increases from 34 to 65. In thin-walled HSS (h/t = 65), retrofitting provided significant gains in strength but not in ductility. In thick-walled HSS (h/t = 34), retrofitting provided little strength gain, but enhanced ductility, especially with properly bonded plates extending on the brace. Generally, plates fractured under local bending or delaminated within plate layers while bond was fully intact. In Phase III, selected configurations of the two retrofitting methods were used in additional T-joints with chord (h/t) ratio of 65, to study their effectiveness in presence of axial compression load in the chord. Two sustained load levels were induced in the chord, representing 45% and 80% of its full axial capacity. The transverse brace load was then gradually increased to failure. The through-wall steel bolts increased the joint capacity by 13% to 25%, depending on the chord’s axial load level, while the bonded GFRP plate increased the capacity by 38 to 46%. / Thesis (Master, Civil Engineering) -- Queen's University, 2012-09-28 12:40:44.479

An investigation was undertaken to better understand the corrosion behaviour of dissimilar welded Cu-lean AA7003 and AA7108 extrusions. The major variables under study were the heat-treated condition (as-welded T6 vs. as-welded T6+Paint Bake (PB)), extrusion alloy Cu composition (AA7003 vs. AA7108), weld filler composition (ER4043 vs. ER5356), weld joint geometry (lap-joint vs. T-joint), and weld direction with respect to extrusion direction (parallel (═) vs. perpendicular (┴)). The corrosion behaviour of the various weld configurations under investigation was observed using an ASTM standard practice for modified salt spray testing (ASTM G85-A2), a GM worldwide engineering standard for cyclic corrosion testing (GMW-14872), and potentiodynamic polarization measurements. The effect of exposure to GMW-14872 on the tensile-shear behaviour of the various weld configurations under study was also investigated using a custom tensile jig. Examination post exposure to ASMT G85-A2 revealed the presence of differing pitting corrosion morphologies between AA7003 and AA7108. Due to increased Cu-content, AA7003 displayed deep pitting corrosion which penetrated the entirety of the dynamically recrystallized top surface layer and reached the fine-grained interior. Shallow pitting of the recrystallized surface layer was observed on AA7108, with very few penetration sites that reached the underlying fine-grained interior. No difference in corrosion behaviour was observed between the heat affected zone (HAZ) and unaffected base alloy of welded AA7003 and AA7108, also consistent with potentiodynamic polarization results. However, the HAZ displayed dual corrosion bands separated by a thin band of unattacked alloy; a result of distinct local microstructural changes induced by thermal cycling from welding. Tensile-shear testing revealed four types of observed fracture modes: shear across the weld throat, fracture along the AA7xxx/ER5356 interface, fracture along the AA6063/ER5356 interface and fracture in the HAZ of AA7xxx. Little to no corrosion was observed on weld configurations exposed to GMW-14872, resulting in no differences in the tensile-shear behaviour of exposed and unexposed weld configurations. Large variations observed in the tensile-shear results were a result of numerous weld defects. / Thesis / Master of Applied Science (MASc)

Stationary shoulder friction stir welding (SSFSW) is a recently developed variant of conventional friction stir welding (FSW). Recent studies have shown that SSFSW can join high strength aluminum alloys with improved mechanical strength and reduced distortion as a result of a narrower and more uniform thermal profile. However, a lack of understanding on the residual stress development in the SSFSW process makes it difficult to assess the structural integrity and delays a widespread application of this technique to industry. This dissertation reports the first systematic investigation into the development of residual stress induced by the SSFSW process and comparison between SSFSW and FSW techniques. Welding residual stresses were experimentally assessed with both the contour method and neutron diffraction. The weld microstructure and hardness distributions were characterized and used to understand the formation of residual stresses during the welding process. The results have shown that for both FSW and SSFSW processes, the residual stresses distribute in the form of ‘M’ shaped profile while the magnitude and size of tensile residual stress zone were effectively reduced (by 25%) in the SSFSW process, even when input welding power was identical. Other improvements seen in the SSFSW process include a reduction in the heat affected zone width, an increase in the minimum hardness and a more uniform through-thickness microstructure and hardness. The dominating welding process parameter affecting the welding residual stress was travel speed as compared to rotation speed and tool downforce. With a 90 degree shaped shoulder, SSFSW has been shown to produce defect-free T-sections by dual fillet welds. For these components, an asymmetrical distribution of microstructure, hardness and residual stresses were found as a consequence of the thermal effects induced by second weld on the first weld. The material softening caused by the first weld provides the potential of utilizing a lower heat input on the subsequent pass so as to optimize the welding parameters.

This work involves investigation of the kinetics data of a joint formation during aluminum alloy brazing. Data was generated by several groups of experiments conducted under conditions of a controlled oxygen level of the background brazing atmosphere. Generated data are examined to identify the phases of the joint formation and the time frame of its evolution. Specifically, the triple line kinetics data are analyzed to verify whether a power law between (1) the triple line of the molten metal preceding joint formation and (2) the formation time can be established for each formation phase. In addition, both initial and residual clad thicknesses on brazing sheets are studied to check phenomenologically an impact of silicon diffusion on joint formation. Formation shapes are also inspected in order to study if a 2-D configuration of joint formation is present. The kinetics data from different sets of experiments under adverse atmosphere conditions are compared to understand the impact of oxygen level on joint formation. This study is not necessarily aimed at building a mathematical model for T-Joint formation during brazing process, but intends to understand possible influential parameters on the development of the formation. KEYWORDS: Aluminum Brazing, Kinetics, T-Joint, Background Atmosphere, Capillary Flow.

Wind turbines are classic examples of structures where their operating lifetime is controlled by the fatigue properties of the material. This is exacerbated by the 2D nature of the composite materials used in blade construction which are typically fabrics in a variety of formats (e.g. non crimp fabrics, uniweave, woven). The formation of internal detailed shapes within the blade, allowing features such as spars, shear webs and other connections, inevitably requires these 2D material configurations to be formed into 3D shapes. This introduces positions within the structure where load transfer occurs across regions with no fibre reinforcement. These weak areas become natural positions for the initiation of damage that can occur well before fatigue damage would be expected in the basic material subject to simple in-plane loading. The aim of this study is to modify and improve the blade structure in order to extend its working life and minimize geometry related fatigue issues. To achieve this goal T-sections have been manufactured as representative element of the blade's spar. T-sections have been made of carbon or glass fabric infused with epoxy resin using a vacuum-assisted resin-transfer moulding technique. The structure has been modified with different toughening techniques to increase its interlaminar fracture resistance (toughness) and hence delay or stop crack propagation. Methods such as the use of veil layers, tufting and 3D weaving techniques have been employed to improve the interlaminar fracture toughness of the T-joint. The changing parameters in samples are, the addition of the veil layer to the composite structure, veil material, tufting stitches and use of different 3D fibre weaving architectures in the fabrication of the composite T-joint. For T-joint testing, there was no standardised specimen shapes and no standard for specimen dimensions; as well as no test fixture designs or test procedures. Consequently, it was required to design a test rig and develop a test procedure for tensile and fatigue tests of T-joints. An additional investigation was performed to establish test specimen geometry suitable for testing in available Instron machines. Manufactured specimens were quasi-static and fatigue tested. Test results were compared and showed that 3D woven and polyester veil T-joints had the best performance among modified structures. However, it has been found that these structural modifications are performing differently in quasi-static and fatigue loading. The 3D woven four layer to layer inter wave sample that showed the best result in a quasi-static test was not the one with the best fatigue results but it was amongst the ones with the highest performance. SEM and optical microscopy were used to investigate fractured specimens in an attempt to establish the mechanisms involved in the fracture process of the T-joint. Finally, based on test and investigations results it has been concluded that the 3D weaving was the most effective modification to improve the static and fatigue properties of the T-joint. The T-joint modified with polyester veil showed the second best performance in both static and fatigue tests but the addition of the polyamide caused had negative effects on these properties.

In this thesis, the application of composite materials for marine structures and specifically naval vessels has been explored by investigating its damage criticality. The use of composite materials for Mine Counter Measure Vessels (MCMVs) was desirable, especially for producing material characteristics, such as light weight, corrosion resistance, design flexibility due to its anisotropic nature and most importantly stealth capability. The T-Joint structure, as the primary connection between the hull and bulkhead forms the focus of this research. The aim of the research was to determine the methodology to predict the damage criticality of the T-Joint under a pull-off tensile loading using FE (Finite Element) based fracture mechanics theory. The outcome of the research was that the Finite Element (FE) simulations were used in conjunction with fracture mechanics theory to determine the failure mechanism of the T-Joint in the presence of disbonds in the critical loca tion. It enables certain pre-emptive strengthening mechanisms or other preventive solutions to be made since the T-Joint responses can be predicted precisely. This knowledge contributes to the damage tolerance design methodology for ship structures, particularly in the T-Joint design. The results comparison between the VCCT (Virtual Crack Closure Technique) analysis and the experiment results showed that the VCCT is a dependable analytical method to predict the T-Joint failure mechanisms. It was capable of accurately determining the crack initiation and final fracture load. The maximum difference between the VCCT analysis with the experiment results was approximately 25% for the T-Joint with a horizontal disbond. However, the application of the CTE (Crack Tip Element) method for the T-Joint displayed a huge discrepancy compared with the results (fracture toughness) obtained using the VCCT method, because the current T-Joint structure geometry did not meet the Classical Laminate Plate Theory (CLPT) criteria. The minimum fracture toughness difference for both analytical methods was approximately 50%. However, it also has been tested that when the T-Joint structure geometry satisfied the CLPT criteria, the maximum fracture toughness discrepancy between both analytical methods was only approximately 10%. It was later discovered from the Griffith energy principle that the fracture toughness differences between both analytical methods were due to the material compliance difference as both analytical methods used different T-Joint structures.

When joining steel to aluminium there is a reaction between iron and aluminium which results in the formation of brittle intermetallic compounds (IMC). These compounds are usually the reason for the poor mechanical strength of the dissimilar metallic joints. The research on dissimilar metal joining is vast but is mainly focused on the automotive industry and therefore, the material in use is very thin, usually less than 1 mm. For materials with thicker sections the present solution is a transition joint made by explosion welding which permits joining of steel to aluminium and avoids the formation of IMCs. However, this solution brings additional costs and extra processing time to join the materials. The main goals of this project are to understand the mechanism of formation of the IMCs, control the formation of the IMCs, and understand their effects on the mechanical properties of the dissimilar Fe-Al joints during laser welding. Laser welding permits accurate and precise control of the welding thermal cycle and thereby the underpinning mechanism of IMC formation can be easily understood along with the factors which control the strength of the joints. The further goal of this project is to find an appropriate interlayer to restrict the Fe-Al reaction. The first stage of the work was focused on the formation and growth of the Fe-Al IMCs during laser welding. The understanding of how the processing conditions affect the IMC growth provides an opportunity to act and avoid its formation and thereby, to optimize the strength of the dissimilar metal joints. The results showed that even with a negligible amount of energy it was not possible to prevent the IMC formation which was composed of both Fe2Al5 and FeAl3 phases. The IMC growth increases exponentially with the applied specific point energy. However, for higher power densities the growth is more accentuated. The strength of the Fe-Al lap-joints was found to be not only dependent on the IMC layer thickness but also on the bonding area. In order to obtain sound joints it is necessary to achieve a balance between these two factors. The thermal model developed for the laser welding process in this joint configuration showed that for the same level of energy it is more efficient to use higher power densities than longer interaction iv times. Even though a thicker IMC layer is formed under this condition due to higher temperature there is also more melting of aluminium which creates a larger bonding area between the steel and the aluminium. The joint strength is thus improved ... [cont.].

With increasing climate change the automotive industry is facing increasing demands regarding emissions and environmental impact. To lower emissions and environmental impact the automotive industry strives to increase the efficiency of vehicles by for example reducing the weight. This can be achieved by the implementation of lightweight products made of composite materials where different materials must be joined. A key technology when producing lightweight products is adhesive joining. In an effort to expand the implementations of structural adhesives Volvo Buses wants to increase their knowledge about adhesive joining techniques. This thesis is done in collaboration with Volvo Buses and aims to increase the knowledge about numerical simulations of adhesively bonded joints. A numerical model of an adhesively bonded T-joint is presented where the adhesive layer is modelled using the Cohesive Zone Model. The experimental extraction of cohesive laws for adhesives is discussed and implemented as bi-linear traction-separation laws. Experiments of the T-joint for two different load cases are performed and compared to the results of the numerical simulations. The experimental results shows a similar force-displacement response as for the results of the numerical simulations. Although there were deviations in the maximum applied load and for one load case there were deviations in the behavior after the main load drop. The deviations between numerical and experimental results are believed to be due to inaccurate material properties for the adhesive, the use of insufficient bi-linear cohesive laws, the occurrence of a combination of adhesive and cohesive fractures during the experiments and dissimilar effective bonding surface areas in the numerical model and the physical specimens.

Articulated haulers are fundamental equipment to transport material. The load carrying structure on a hauler consists mainly of welded frames. During welding of the frames high residual stress will be introduced. These stresses may have a significant impact on the fatigue life of the frames. This is the reason for having good knowledge of the weld residual stresses. The finite element method was used to calculate the residual stress distributions in a butt weld and a T-join weld. Simulation of the welding process with thermal and mechanical analysis was prepared by means of welding GUI implemented in LS-PrePost. The welding simulation is a computer intensive operation with high CPU time. That is why it is important to investigate which process factors that have the largest impact on welding simulation results. The aim of this thesis is to investigate the correlation between designed models in FEA software with published results of weld residual stress measurements and conclude which parameters should be mainly taken into consideration.