Hollow section members are widely used in industrial applications for the design of many machine and structural components. These components are often fabricated at lower cost by welding rather than by casting or forging. For instance, in agricultural machinery, the hollow tubes are typically connected together through welding to form T-joints. Such T-joint connections are also employed in other engineering applications such as construction machinery, offshore structures, bridges, and vehicle frames. In this dissertation, the behaviour of tubular T-joint connections, in particular square hollow section (SHS)-to-SHS T-joints, subjected to static and cyclic loads is studied both experimentally and numerically. The techniques used for the fabrication of the T-joint connections can affect their strengths to different degrees. With modern advances in manufacturing technologies, there are many alternatives for the fabrication of the T-joint connections. For instance, in recent years, the use of the laser beam has become increasingly common in industrial applications. From a manufacturing point of view, the T-joint connections can be fabricated by using traditional mechanical cutting or laser cutting techniques. Currently, for the fabrication of the T-joint connections, the straight edge of one tube is cut using mechanical tools (e.g., flame cutting) and then welded to the body of the other tube. A major contribution of this research work is investigating the feasibility of using laser cutting to produce welded square hollow-section T-joints with similar or higher fatigue strengths than their conventional mechanical cut counterparts. For this purpose, a total of 21 full-scale T-joint samples, typical of those found in the agricultural machinery, are included for the study. Finite Element (FE) models of the T-joints manufactured with the different cutting techniques are also developed and the FE results are verified with the experiments. The results of the numerical and experimental study on the full-scale T-joint samples show that the fatigue strength of the samples that are manufactured with laser cutting is higher than those fabricated with conventional mechanical cutting. From a structural analysis view point, despite of the wide use of tubular T-joint connections as efficient load carrying members, a practical but yet simple and accurate approach for their design and analysis is not available. For this purpose, engineers must often prepare relatively complicated and time consuming FE models made up of shell or solid elements. This is because unlike solid-section members, when hollow section members are subjected to general loadings, they may experience severe deformations of their cross-sections that results in stress concentrations in the connections vicinity. One of the objectives/contributions of this research work is the better understanding of the behaviour of SHS-to-SHS T-joint connections under in-plane bending (IPB) and out-of-plane bending (OPB) loading conditions. Through a detailed Finite Element Analysis (FEA) using shell and solid elements, the stiffness and stress distribution at the connection of the tubular T-joints are obtained for different loading conditions. It is observed that at a short distance away from the connection of the T-joints, the structure behaves similar to beams when subjected to loadings. The beam like stresses cease to be valid only in the vicinity of the connection. Therefore, several parameters are defined to recognize the joints stress concentrations and the bending stiffness reduction. These parameters permit the accurate modelling of the tubes and the T-connection by simple beam elements with certain modifications. The models consisting of beam elements are significantly easier to prepare and analyze. Through several numerical examples, it is shown that the modified beam models provide accurately all important information of the structural analysis (i.e. the stresses, displacements, reaction forces, and the natural frequencies) at substantially reduced computational effort in comparison with the complicated Finite Element (FE) models built of shell or solid elements. Another contribution of this research work is the FE modelling of the weld geometry and its effect on the stresses at the vicinity of the connection. The results of the FE modelling are verified through a detailed experimental study. For the experimental study, two test fixtures with hydraulic actuators capable of applying both static and cyclic loadings are designed and used. Strain gauges are installed at several locations on full-scale T-joint samples to validate the developed FE models. It is shown that the membrane stresses which occur at the mid-surface of the tubes remain similar regardless of the weld geometry. The weld geometry only affects the bending stresses. It is also shown that this effect on bending stresses is highly localized and disappears at a distance of about half of the weld thickness away from the weld-toe. To reduce the stress concentrations at the T-joint, plate reinforcements are used in a number of different arrangements and dimensions to increase the load carrying capacity of the connection.

Hollow section members are widely used in industrial applications for the design of many machine and structural components. These components are often fabricated at lower cost by welding rather than by casting or forging. For instance, in agricultural machinery, the hollow tubes are typically connected together through welding to form T-joints. Such T-joint connections are also employed in other engineering applications such as construction machinery, offshore structures, bridges, and vehicle frames. In this dissertation, the behaviour of tubular T-joint connections, in particular square hollow section (SHS)-to-SHS T-joints, subjected to static and cyclic loads is studied both experimentally and numerically. The techniques used for the fabrication of the T-joint connections can affect their strengths to different degrees. With modern advances in manufacturing technologies, there are many alternatives for the fabrication of the T-joint connections. For instance, in recent years, the use of the laser beam has become increasingly common in industrial applications. From a manufacturing point of view, the T-joint connections can be fabricated by using traditional mechanical cutting or laser cutting techniques. Currently, for the fabrication of the T-joint connections, the straight edge of one tube is cut using mechanical tools (e.g., flame cutting) and then welded to the body of the other tube. A major contribution of this research work is investigating the feasibility of using laser cutting to produce welded square hollow-section T-joints with similar or higher fatigue strengths than their conventional mechanical cut counterparts. For this purpose, a total of 21 full-scale T-joint samples, typical of those found in the agricultural machinery, are included for the study. Finite Element (FE) models of the T-joints manufactured with the different cutting techniques are also developed and the FE results are verified with the experiments. The results of the numerical and experimental study on the full-scale T-joint samples show that the fatigue strength of the samples that are manufactured with laser cutting is higher than those fabricated with conventional mechanical cutting. From a structural analysis view point, despite of the wide use of tubular T-joint connections as efficient load carrying members, a practical but yet simple and accurate approach for their design and analysis is not available. For this purpose, engineers must often prepare relatively complicated and time consuming FE models made up of shell or solid elements. This is because unlike solid-section members, when hollow section members are subjected to general loadings, they may experience severe deformations of their cross-sections that results in stress concentrations in the connections vicinity. One of the objectives/contributions of this research work is the better understanding of the behaviour of SHS-to-SHS T-joint connections under in-plane bending (IPB) and out-of-plane bending (OPB) loading conditions. Through a detailed Finite Element Analysis (FEA) using shell and solid elements, the stiffness and stress distribution at the connection of the tubular T-joints are obtained for different loading conditions. It is observed that at a short distance away from the connection of the T-joints, the structure behaves similar to beams when subjected to loadings. The beam like stresses cease to be valid only in the vicinity of the connection. Therefore, several parameters are defined to recognize the joints stress concentrations and the bending stiffness reduction. These parameters permit the accurate modelling of the tubes and the T-connection by simple beam elements with certain modifications. The models consisting of beam elements are significantly easier to prepare and analyze. Through several numerical examples, it is shown that the modified beam models provide accurately all important information of the structural analysis (i.e. the stresses, displacements, reaction forces, and the natural frequencies) at substantially reduced computational effort in comparison with the complicated Finite Element (FE) models built of shell or solid elements. Another contribution of this research work is the FE modelling of the weld geometry and its effect on the stresses at the vicinity of the connection. The results of the FE modelling are verified through a detailed experimental study. For the experimental study, two test fixtures with hydraulic actuators capable of applying both static and cyclic loadings are designed and used. Strain gauges are installed at several locations on full-scale T-joint samples to validate the developed FE models. It is shown that the membrane stresses which occur at the mid-surface of the tubes remain similar regardless of the weld geometry. The weld geometry only affects the bending stresses. It is also shown that this effect on bending stresses is highly localized and disappears at a distance of about half of the weld thickness away from the weld-toe. To reduce the stress concentrations at the T-joint, plate reinforcements are used in a number of different arrangements and dimensions to increase the load carrying capacity of the connection.

L'optimisation des structures aéronautiques fabriquées en composite a mené EADS-IW à développer une technique d’assemblage par cloutage qui a pour objectif la fabrication à coût réduit de sous ensembles structuraux avec un fort niveau d'intégration. L'objectif de ce travail de thèse est d'étudier le comportement mécanique des assemblages cloutés et de proposer des modélisations associées. Dans le cadre d'une démarche multi-niveau, le comportement des liaisons clou/résine et clou stratifié a d'abord été étudié. Des campagnes expérimentales ont montré l'influence du diamètre du clou et de la profondeur d'enfoncement sur la tenue en arrachement ainsi que des similitudes entre le comportement du clou noyé dans de la résine et implanté dans le stratifié. Une modélisation capable d'estimer la tenue en arrachement d'un clou a été développée. Au niveau éprouvettes technologiques, des éprouvettes cloutées représentatives de structures aéronautiques de type « L » ou « T » ont été testé en sollicitations statiques montrant l'influence des paramètres de conception et les possibles avantages du cloutage. Finalement, des modélisations basées sur les études élémentaires ont permis de simuler le comportement de ces éprouvettes cloutées retrouvé en essai et notamment de prédire leur tenue sous sollicitations différentes validant ainsi la démarche multi-niveau.

This study demonstrates an experimental investigation into the efficacy of a novel surface pre-treatment technique to improve the toughness and energy dissipation of composite CFRP T-joints. This novel technique optimizes CO2 laser irradiations to remove surface contaminations and modify the surface morphology of CFRP T-joint adherents. Pull-off tests were performed on T-joints that experienced peel-ply (PP) treatment and to those that were ablated with 10% (LC) and 30% (LA) laser power respectively. A further developed alternative pattern between LA and LC surface pre-treatment was examined. Two different quasi-isotropic stacking sequences have been studied by having surface fibers aligned in 0° and 45° direction. A series of surface roughness analysis, optical microscopy, SEM, CT scan and pictorial findings have been carried out to characterize the surface morphologies and failure modes prior to and after the failure. The patterning technique promoted non-local damage mechanisms which resulted in large improvements in the toughness and energy dissipation as compared to the other pre-treatment techniques. Up to ~12 times higher energy dissipation compared to peel-ply pre-treated T-joint were achieved with patterned T-joint structures that are stacked with a 0° surface fiber direction.

The purpose of the proposed research is to evaluate the mechanical response of stitched T-joints under tension, bending, and combined tensile-flexure loading. The use of fiber-reinforced polymer matrix composites has increased in primary load-bearing structures due to their many attributes, such as their high strength and stiffness-to-weight ratio, and tailorability. Composite T-joints are often used in aerospace, marine, and wind turbine structures to provide load connectivity between orthogonal components, such as stiffeners to skins. However, one of the main drawbacks of polymer matrix composites is their low interlaminar strength, which can lead to delamination when subjected to out-of-plane loads. Techniques such as braiding, knitting, stitching, tufting, and z-pinning have been used to reinforce T-joints in the through-thickness direction. Most research has been focused on the tensile or bending behavior of T-joints, although these joints are often subjected to a combination of tensile and bending loads in service. A few experimental and analytical studies have been conducted on the mechanical response under combined tensile-flexure loading conditions, but no studies have been conducted on stitched T-joints. In this study, mechanical tests of 3D stitched and unstitched T-joints under tension, bending, and combined tensile-flexure were conducted, and the ultimate load, displacement, and absorbed energy were obtained. The average displacement at total failure under tension, bending, and combined tensile-flexure loading conditions for the stitched specimens were found to be 34%, 51%, and 24% greater, respectively, when compared to their unstitched counterparts. Similarly, the average absorbed energy for stitched specimens is 58%, 82%, and 51% greater under tension, bending, and combined tensile-flexure loading conditions. The failure surfaces of stitched and unstitched T-joints were analyzed using an optical microscope, and areas of interest, such as resin-rich regions, stitches, and different damage types, were identified. Furthermore, the skin-flange interface fracture surface of the combined loading T-joint specimens were analyzed using a scanning electron microscope. Significant differences in the fracture surface indicated varying degrees of mixed-mode loading conditions within a specimen for all specimen types. A numerical analysis of a stitched double cantilever beam specimen was conducted to evaluate smeared cohesive laws to represent stitched regions. Overall, stitching results in improved damage tolerance in T-joints subjected to various loading conditions.

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