Browsing by Author "Moura, M. F. S. F. de"
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- Adhesively-bonded repair proposal for wood members damaged by horizontal shear using carbon-epoxy patchesPublication . Campilho, Raul; Moura, M. F. S. F. de; Ramantani, Dimitra A.; Morais, J. J. L.; Barreto, A. M. J. P.; Domingues, J. J. M. S.In this work, a repair technique with adhesively bonded carbon-epoxy patches is proposed for wood members damaged by horizontal shear and under bending loads. This damage is characterized by horizontal crack growth near the neutral plane of the wood beam, normally originating from checks and shakes. The repair consists of adhesively bonded carbon-epoxy patches on the vertical side faces of the beam at the cracked region to block sliding between the beam arms. An experimental and numerical parametric analysis was performed on the patch length. The numerical analysis used the finite element method (FEM) and cohesive zone models (CZMs), with an inverse modelling technique for the characterization of the adhesive layer. Trapezoidal cohesive laws in each pure mode were used to account for the ductility of the adhesive used. To fully reproduce the tests, horizontal damage propagation within the wood beam was also simulated. A good correlation with the experiments was found. Regarding the effectiveness of the repair, for the conditions selected for this work, a full strength recovery was achieved for the bigger value of patch length tested.
- Buckling strength of adhesively-bonded single and double-strap repairs on carbon-epoxy structuresPublication . Campilho, Raul; Moura, M. F. S. F. de; Ramantani, Dimitra A.; Morais, J. J. L.; Domingues, J. J. M. S.This work reports on an experimental and finite element method (FEM) parametric study of adhesively-bonded single and double-strap repairs on carbon-epoxy structures under buckling unrestrained compression. The influence of the overlap length and patch thickness was evaluated. This loading gains a particular significance from the additional characteristic mechanisms of structures under compression, such as fibres microbuckling, for buckling restrained structures, or global buckling of the assembly, if no transverse restriction exists. The FEM analysis is based on the use of cohesive elements including mixed-mode criteria to simulate a cohesive fracture of the adhesive layer. Trapezoidal laws in pure modes I and II were used to account for the ductility of most structural adhesives. These laws were estimated for the adhesive used from double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively, using an inverse technique. The pure mode III cohesive law was equalled to the pure mode II one. Compression failure in the laminates was predicted using a stress-based criterion. The accurate FEM predictions open a good prospect for the reduction of the extensive experimentation in the design of carbon-epoxy repairs. Design principles were also established for these repairs under buckling.
- Experimental and numerical evaluation of composite repairs on wood beams damaged by cross-grainingPublication . Campilho, Raul; Moura, M. F. S. F. de; Barreto, A. M. J. P.; Morais, J. J. L.; Domingues, J. J. M. S.An experimental and Finite Element study was performed on the bending behaviour of wood beams of the Pinus Pinaster species repaired with adhesively-bonded carbon–epoxy patches, after sustaining damage by cross-grain failure. This damage is characterized by crack growth at a small angle to the beams longitudinal axis, due to misalignment between the wood fibres and the beam axis. Cross-grain failure can occur in large-scale in a wood member when trees that have grown spirally or with a pronounced taper are cut for lumber. Three patch lengths were tested. The simulations include the possibility of cohesive fracture of the adhesive layer, failure within the wood beam in two propagation planes and patch interlaminar failure, by the use of cohesive zone modelling. The respective cohesive properties were estimated either by an inverse method or from the literature. The comparison with the tests allowed the validation of the proposed methodology, opening a good perspective for the reduction of costs in the design stages of these repairs due to extensive experimentation.
- Interlaminar fracture characterization of a carbon-epoxy composite in pure mode IIPublication . Campilho, Raul; Moura, M. F. S. F. de; Pinto, A. M. G.; Ramantani, Dimitra A.The interlaminar fracture toughness in pure mode II (GIIc) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization test. The value of GIIc was extracted by a new data reduction scheme avoiding the crack length measurement, named Compliance-Based Beam Method (CBBM). This method eliminates the crack measurement errors, which can be non-negligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the Fracture Process Zone (FPZ) effects. A numerical study using the Finite Element Method (FEM) and a triangular cohesive damage model, implemented within interface finite elements and based on the indirect use of Fracture Mechanics, was performed to evaluate the suitability of the CBBM to obtain GIIc. This was performed comparing the input values of GIIc in the numerical models with the ones resulting from the application of the CBBM to the numerical load-displacement (P-) curve. In this numerical study, the Compliance Calibration Method (CCM) was also used to extract GIIc, for comparison purposes.
- Mode III interlaminar fracture of carbon/epoxy laminates using the edge crack torsion (ECT) testPublication . Morais, A. B. de; Pereira, A. B.; Moura, M. F. S. F. de; Magalhães, A. G.The mode III interlaminar fracture of carbon/epoxy laminates was evaluated with the edge crack torsion (ECT) test. Three-dimensional finite element analyses were performed in order to select two specimen geometries and an experimental data reduction scheme. Test results showed considerable non-linearity before the maximum load point and a significant R-curve effect. These features prevented an accurate definition of the initiation point. Nevertheless, analyses of non-linearity zones showed two likely initiation points corresponding to GIIIc values between 850 and 1100 J/m2 for both specimen geometries. Although any of these values is realistic, the range is too broad, thus showing the limitations of the ECT test and the need for further research.
- Modelação da furação de laminados por elementos finitosPublication . Durão, Luís M.; Gonçalves, Daniel J. S.; Moura, M. F. S. F. de; Marques, A. T.O recurso ao Método de Elementos Finitos (MEF) para a simulação e estudo da maquinagem tem vindo a merecer um interesse crescente. Neste trabalho comparam-se os resultados experimentais obtidos usando diferentes brocas helicoidais com uma simulação numérica da furação usando elementos sólidos tridimensionais do software ABAQUS® Explicit. Os resultados obtidos mostram a aplicabilidade do modelo, nomeadamente no que respeita à variação do principal parâmetro de corte no processo de furação – o avanço – e da geometria da ferramenta. O desenvolvimento das forças axiais durante a furação está directamente relacionado com a selecção dos parâmetros de corte e geometria de ferramenta e contribui para a existência de danos na placa. Assim, a minimização destas forças permite reduzir o risco de delaminação. Os resultados relativos à força para o início da delaminação são comparados com modelos analíticos já conhecidos. Os resultados numéricos apresentam uma concordância apreciável com os obtidos experimentalmente.
- Numerical evaluation of three-dimensional scarf repairs in carbon-epoxy structuresPublication . Pinto, A. M. G.; Campilho, Raul; Moura, M. F. S. F. de; Mendes, Isabel R.The widespread employment of carbon-epoxy laminates in high responsibility and severely loaded applications introduces an issue regarding their handling after damage. Repair of these structures should be evaluated, instead of their disposal, for cost saving and ecological purposes. Under this perspective, the availability of efficient repair methods is essential to restore the strength of the structure. The development and validation of accurate predictive tools for the repairs behaviour are also extremely important, allowing the reduction of costs and time associated to extensive test programmes. Comparing with strap repairs, scarf repairs have the advantages of a higher efficiency and the absence of aerodynamic disturbance. This work reports on a numerical study of the tensile behaviour of three-dimensional scarf repairs in carbon-epoxy structures, using a ductile adhesive (Araldite® 2015). The finite elements analysis was performed in ABAQUS® and Cohesive Zone Modelling was used for the simulation of damage onset and growth in the adhesive layer. Trapezoidal cohesive laws in each pure mode were used to account for the ductility of the specific adhesive mentioned. A parametric study was performed on the repair width and scarf angle. The use of over-laminating plies covering the repaired region at the outer or both repair surfaces was also tested as an attempt to increase the repairs efficiency. The obtained results allowed the proposal of design principles for repairing composite structures.
- Numerical prediction of delamination onset in carbon/epoxy composites drillingPublication . Durão, Luís M.; Moura, M. F. S. F. de; Marques, A. T.Despite the fact that their physical properties make them an attractive family of materials, composites machining can cause several damage modes such as delamination, fibre pull-out, thermal degradation, and others. Minimization of axial thrust force during drilling reduces the probability of delamination onset, as it has been demonstrated by analytical models based on linear elastic fracture mechanics (LEFM). A finite element model considering solid elements of the ABAQUS® software library and interface elements including a cohesive damage model was developed in order to simulate thrust forces and delamination onset during drilling. Thrust force results for delamination onset are compared with existing analytical models.
- Single-lap joints of similar and dissimilar adherends bonded with an acrylic adhesivePublication . Pinto, A. M. G.; Magalhães, A. G.; Campilho, Raul; Moura, M. F. S. F. de; Baptista, A. P. M.In this study, the tensile strength of single-lap joints (SLJs) between similar and dissimilar adherends bonded with an acrylic adhesive was evaluated experimentally and numerically. The adherend materials included polyethylene (PE), polypropylene (PP), carbon-epoxy (CFRP), and glass-polyester (GFRP) composites. The following adherend combinations were tested: PE/PE, PE/PP, PE/CFRP, PE/GFRP, PP/PP, CFRP/CFRP, and GFRP/GFRP. One of the objectives of this work was to assess the influence of the adherends stiffness on the strength of the joints since it significantly affects the peel stresses magnitude in the adhesive layer. The experimental results were also used to validate a new mixed-mode cohesive damage model developed to simulate the adhesive layer. Thus, the experimental results were compared with numerical simulations performed in ABAQUS®, including a developed mixed-mode (I+II) cohesive damage model, based on the indirect use of fracture mechanics and implemented within interface finite elements. The cohesive laws present a trapezoidal shape with an increasing stress plateau, to reproduce the behaviour of the ductile adhesive used. A good agreement was found between the experimental and numerical results.
- A straightforward method to obtain the cohesive laws of bonded joints under mode I loadingPublication . Moura, M. F. S. F. de; Gonçalves, J. P. M.; Magalhães, A. G.A simple procedure to measure the cohesive laws of bonded joints under mode I loading using the double cantilever beam test is proposed. The method only requires recording the applied load–displacement data and measuring the crack opening displacement at its tip in the course of the experimental test. The strain energy release rate is obtained by a procedure involving the Timoshenko beam theory, the specimen’s compliance and the crack equivalent concept. Following the proposed approach the influence of the fracture process zone is taken into account which is fundamental for an accurate estimation of the failure process details. The cohesive law is obtained by differentiation of the strain energy release rate as a function of the crack opening displacement. The model was validated numerically considering three representative cohesive laws. Numerical simulations using finite element analysis including cohesive zone modeling were performed. The good agreement between the inputted and resulting laws for all the cases considered validates the model. An experimental confirmation was also performed by comparing the numerical and experimental load–displacement curves. The numerical load–displacement curves were obtained by adjusting typical cohesive laws to the ones measured experimentally following the proposed approach and using finite element analysis including cohesive zone modeling. Once again, good agreement was obtained in the comparisons thus demonstrating the good performance of the proposed methodology.