Browsing by Author "Silva, L.F.M. da"
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- Adhesive thickness effects of a ductile adhesive by optical measurement techniquesPublication . Campilho, Raul; Moura, D.C.; Banea, Mariana D.; Silva, L.F.M. daAdhesive bonding is an excellent alternative to traditional joining techniques such as welding, mechanical fastening or riveting. However, there are many factors that have to be accounted for during joint design to accurately predict the joint strength. One of these is the adhesive layer thickness (tA). Most of the results are for epoxy structural adhesives, tailored to perform best with small values of tA, and these show that the lap joint strength decreases with increase of tA (the optimum joint strength is usually obtained with tA values between 0.1 and 0.2 mm). Recently, polyurethane adhesives were made available in the market, designed to perform with larger tA values, and whose fracture behaviour is still not studied. In this work, the effect of tA on the tensile fracture toughness (View the MathML source) of a bonded joint is studied, considering a novel high strength and ductile polyurethane adhesive for the automotive industry. This work consists on the fracture characterization of the bond by a conventional and the J-integral techniques, which accurately account for root rotation effects. An optical measurement method is used for the evaluation of crack tip opening (δn) and adherends rotation at the crack tip (θo) during the test, supported by a Matlab® sub-routine for the automated extraction of these parameters. As output of this work, fracture data is provided in traction for the selected adhesive, enabling the subsequent strength prediction of bonded joints.
- Comparative Failure Assessment of Single and Double Lap Joints with Varying Adhesive SystemsPublication . Nunes, S.L.S.; Campilho, Raul; Silva, F.J.G. da; Sousa, C.C.R.G. de; Fernandes, T.A.B.; Bane, M.D.; Silva, L.F.M. daJoining by adhesively bonded methods is being increasingly used in the design of mechanical structures, because of the significant benefits of this technique compared to the traditional ones. Commercial adhesives range from strong and brittle (e.g., Araldite® AV138) to less strong and ductile (e.g., Araldite® 2015). A new family of polyurethane adhesives combines high strength and ductility (e.g., Sikaforce® 7888). This work compares the tensile performance of the three aforementioned adhesive systems in single and double lap aluminum joints with varying values of overlap length (LO). A cohesive zone modeling (CZM) numerical analysis was carried out to analyze through-thickness normal and shear stresses in the adhesive layer, to study the CZM damage variable and to predict the joint strength. A comparison was also performed with simple analytical models. The stress and damage variable analyses helped in understanding the differences between adhesives in which pertains to the failure process and joint strength. The proposed work also enabled concluding that the CZM predictions are typically accurate, and which family of adhesives is more suited for each joint configuration, with clear advantage for the Sikaforce® 7888. As a result of this work, design guidelines were proposed for adhesively bonded joints.
- Damage analysis of composite–aluminium adhesively-bonded single-lap jointsPublication . Ribeiro, T.E.A.; Campilho, Raul; Silva, L.F.M. da; Goglio, L.Adhesive bonding is used to fabricate structures of complex shape that could not be manufactured in one piece. In many high performance structures, joints between composite materials with other light metals such as aluminium are required for structural optimization. This work numerically and experimentally addresses adhesive joints between aluminium and carbon–epoxy composites, considering different adhesives and value of overlap length (LO). Numerically, the Finite Element Method (FEM) coupled with cohesive zone models (CZM) is used to perform a detailed stress analysis that enables the comparison between different planes in the joint and LO values. A damage analysis during damage and crack growth is also considered to fully describe the failure process. The use of the proposed numerical technique enables predicting the joint strength and creating a simple and rapid methodology that can be used in the design of hybrid structures, e.g. in the selection of the joint geometry and adhesive. The joints’ strength and failure modes were highly dependent on the adhesive, and this behaviour was successfully modelled numerically. Using a brittle adhesive resulted in a negligible maximum load (Pm) improvement with LO. The joints bonded with the ductile adhesive showed a nearly linear improvement of Pm with LO.
- Effect of the size reduction on the bulk tensile and double cantilever beam specimens used in cohesive zone modelsPublication . Costa, M.; Viana, G.; Canto, C.; Silva, L.F.M. da; Bane, M.D.; Chaves, F.; Campilho, Raul; Fernandes, A.A.Cohesive zone elements used in finite element analysis are a reliable way to design and predict the behaviour of the joint. The characterisation of the traction separation law used in these models is done using tensile and fracture tests, and the parameters of such laws depend on humidity and temperature. Water diffusion tests are therefore necessary, which are dependent on specimen geometry, meaning a bigger specimen takes longer to fully saturate. To solve this problem and increase the efficiency of the ageing process, smaller tensile bulk and double cantilever beam (DCB) specimens are necessary. Another advantage of smaller DCB specimens is that they can be tested in smaller high-temperature chambers, where normal DCB specimens do not fit. Smaller geometries of the bulk tensile and DCB tests are analysed, and a proposed geometry for each test is shown to produce very satisfactory results, validating the use of these specimens.
- Experimental and numerical failure analysis of aluminium/composite single-L jointsPublication . Domingues, N.R.E.; Campilho, Raul; Carbas, R.J.C.; Silva, L.F.M. daAdhesive bonding is frequently used to manufacture complex-shaped structures. Fibre-reinforced composite materials are extensively used in many industries, such as boat building, automotive and aeronautical. Although the manufacturing methods reduce the connections to the minimum, these are still necessary due to component size and design, technological and logistical limitations. Frequently, the combination of composites with metals such as aluminium or titanium brings design benefits. This work aims to study, by experimentation and cohesive zone modelling (CZM), single-L adhesive joints between aluminium components and carbon-epoxy composites under a peel loading, considering different joint configurations and adhesives of distinct ductility. The addressed geometric parameter is the L-part thickness (tP2). The numerical analysis included the stress distributions, damage evolution, strength and failure modes. The experimental tests validated the numerical results and provided design guidelines for single-L joints. It was shown that the L-part geometry and adhesive type highly influence the joints׳ strength.
- Geometry and adhesive optimization of single-lap adhesive joints under impactPublication . Peres, L.M.C.; Arnaud, M.F.T.D.; Silva, A.F.M.V.; Campilho, R.D.S.G.; Machado, J.J.M.; Marques, E.A.S.; Reis, M.Q dos; Silva, L.F.M. daThe use of composite adhesive joints increased in the last decades through structural applications, comprising the aeronautical and automotive industries. Contrary to the static loading case, in many real situations, adhesive joints are subjected to impact loads, such as in the event of vehicle crashes. Despite this fact, numerical modelling of this loading type is seldom addressed in the literature. This work evaluates the effect of the overlap length (LO) and adhesive type on the strength of composite single-lap joints (SLJ), when impact loaded, through experimental tests and cohesive zone models (CZM). Two different types of adhesives were tested (Araldite® AV138 and Sikaforce® 7752), keeping constant the composite adherends with unidirectional lay-up. The joints were subjected to a drop test and validated through the numerical model, by the analysis of stresses and damage, predicting the joints’ strength for different geometries and adhesives. It was concluded that the increase of LO increases the joint strength, especially in those with a more flexible adhesive (Sikaforce® 7752), since this type of adhesive prevents significant stress concentrations, and being a ductile adhesive, provides the ability to absorb peak stresses. The impact CZM was able to predict the impact joints’ strength with good accuracy.
- Strength and damage growth in composite bonded joints with defectsPublication . Ribeiro, F.M.F.; Campilho, Raul; Carbas, R.J.C.; Silva, L.F.M. daThe use of adhesive joints is increasing in various industrial applications because of their advantages such as weight reduction, reduction of stress concentrations and ease of manufacture. However, one of the limitations of adhesive joints is the difficulty in predicting the joint strength due to the presence of defects in the adhesive. This paper presents an experimental and numerical study of single-lap joints (SLJ) with defects centred in the adhesive layer for different overlap lengths (LO) and adhesives. The numerical analysis by cohesive zone models (CZM) included the analysis of the peel (σy) and shear (τxy) stress distributions in the adhesive layer, the CZM damage variable study and the strength prediction. The joints’ behaviour was accurately characterized by CZM and showed a distinct behaviour as a function of the defect size, depending on the adhesive.