Abstract
Joints make them attractive in industries such as aeronautics, automotive and civilengineering. When compared to mechanically fastened joints, adhesive joints have theadvantages of having fewer sources of stress concentrations, more uniformdistribution of load, and better fatigue properties. Structures that are adhesivelybonded must be designed in such a way so that certain types of loading should beavoided or at least minimized (cleavage). The adhesive should be mainly loaded inshear if optimum strength is required. Adhesives are generally stronger incompression than in tension, and this should be taken into account in the design byusing appropriate pressure depended yield-failure criteria.The scope of this thesis is the experimental study and numerical analysis of balancedand unbalanced joints with the aim of deeper investigation of the phenomenon, as inliterature, large differences between the experimental results and theoreticalpredictions of the existing bending moment coefficient ...
Joints make them attractive in industries such as aeronautics, automotive and civilengineering. When compared to mechanically fastened joints, adhesive joints have theadvantages of having fewer sources of stress concentrations, more uniformdistribution of load, and better fatigue properties. Structures that are adhesivelybonded must be designed in such a way so that certain types of loading should beavoided or at least minimized (cleavage). The adhesive should be mainly loaded inshear if optimum strength is required. Adhesives are generally stronger incompression than in tension, and this should be taken into account in the design byusing appropriate pressure depended yield-failure criteria.The scope of this thesis is the experimental study and numerical analysis of balancedand unbalanced joints with the aim of deeper investigation of the phenomenon, as inliterature, large differences between the experimental results and theoreticalpredictions of the existing bending moment coefficient, especially after the erosion ofthe joint material, which is a random event whose scope we cannot predefined,especially when the adherends have different mechanical properties. The configuration of the Single Lap Joint will be used in the current work, as it issimple to manufacture and is the basis for most joint configurations. Single lap jointswere manufactured using Boron/Epoxy and Aluminum 2024-T3 as adherends andFM94 as adhesive. All joints were manufactured in Hellenic Airspace Industry S.A.(H.A.I S.A.), according to ASTM D-1002. A total of 224 specimens were tested tofailure. One of the aims of the current investigation was to access the effect on theoverlap length on the strength and failure of the single lap joint. Although simple tomanufacture, the single lap joint is quite complex to analyze. The eccentricity of theload path causes the joint to rotate under load. The materials (aluminum 2024-T3 andFM94) can also deform plastically under load. Poisson’s ratio effects will give rise totransverse stresses in both the adhesive and adherends. As a consequence, a threedimensionalstress state develops throughout the joint. The behavior of bothadherends and adhesives in the strength to the joints so both adhesive and adherendsmust be modeled accurately.Joints with four different overlap lengths were manufactured, ranging from 13mm to40mm (20 and 30mm). The free length was kept constant at 63.5mm for all the jointconfigurations. Another parameter affecting the strength of joints is the glue linethickness, which kept constant at 0.04mm. Most specimens remain in a salt spraychamber for 72, 166, and 236 hours, respectively at 35oC with 5% NaCl, in order toinvestigate the effect of a controlled corrosive environment in the strength and failureof the specimens. Once the stress and strain states have been defined by usingappropriate modeling procedures and criteria, then a failure analysis will be needed.The failure criteria must be applicable in a variety of cases and basic materialproperties (from tensile and shear tests) should be used for the prediction process. Thefailure criterion should be set in such a way that the prediction could be made withoutknowing a-priori the critical variable causing failure. Once the strength of joints canbe predicted and the fracture processes can be understood, optimization of joints canbe performed without the need for costly and time-consuming experiments, leading tomore efficient designs. However, it is believed that the experimental validation andtesting will still be needed, as it is an integral part of the design process. It is theamount of testing that will be reduced, speeding up the optimization processes and reducing the cost.The increased application of adhesive joints was accompanied by the development ofmathematical models to analyze the behavior of those joints. Both analytical andnumerical models have been developed. In adhesive bonding, the aim is to transfer theload smoothly from one adherend to the other, minimizing the peak shear stresses andpeel stresses in the adhesive layer.Increasing the overlap length reduces the maximum peel, and shear stresses and theaverage shear stresses in the middle of the overlap region. Furthermore, the strengthto the joints should increase as the glue line thickness decreases. For that purpose four(4) different glue line thickness (0.04, 0.1, 1 and 2mm) was used, while the overlaplength remains constant at 13 and 40mm, respectively.
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