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Barbosa A.Q.,IDMEC | Da Silva L.F.M.,University of Porto | Ochsner A.,University of Technology Malaysia
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications | Year: 2014

The inclusion of particles (micro or nano) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, the use of rubber particles being one of the most common processes. In the present study, natural micro particles of cork were used with the objective to increase the ductility of a brittle epoxy adhesive. The idea is for the cork particles to act like a crack stopper. The influence of the amount of cork particles was studied. Particles of cork ranging in size from 125 to 250 mm were mixed in the epoxy adhesive Araldite 2020 from Huntsman. The amount of cork in the adhesive was varied between 0.5% and 5% in weight. This evaluation was made using tensile tests and it was evident that the failure strain was related to the amount of cork particles in the resin. The results concerning the single lap joints and the glass transition temperature confirm the increased ductility obtained in the tensile tests. © IMechE 2013.


Lopes J.,University of Lisbon | Freitas M.,University of Lisbon | Stefaniak D.,German Aerospace Center | Camanho P.P.,IDMEC
Frattura ed Integrita Strutturale | Year: 2015

Bolted joints are the most common solution for joining composite components in aerospace structures. Critical structures such as wing to fuselage joints, or flight control surface fittings use bolted joining techniques. Recent research concluded that higher bearing strengths in composite bolted joints can be achieved by a CFRP/ Titanium hybrid lay-up in the vicinity of the bolted joint. The high costs of titanium motivate a similar research with the more cost competitive austenitic steel. An experimental program was performed in order to compare the apparent inter-laminar shear stress (ILSS) of a CFRP reference beam with the ILSS of hybrid CFRP/Steel beams utilizing different surface treatments in the metallic ply. The apparent ILSS was determined by short beam test, a three-point bending test. Finite element models using cohesive elements in the CFRP/Steel interface were built to simulate the short beam test in the reference beam and in the highest interlaminar shear stress hybrid beam. The main parameters for a FEM simulation of inter laminar shear are the cohesive elements damage model and appropriate value for the critical energy release rate. The results show that hybrid CFRP/Steel have a maximum ILSS very similar to the ILSS of the reference beam. Hybrid CFRP/Steel is a competitive solution when compared with the reference beam ILSS. FEM models were able to predict the maximum ILSS in each type of beam. © 2014, Gruppo Italiano Frattura. All rights reserved.


Martinez X.,International Center for Numerical Methods in Engineering | Martinez X.,Polytechnic University of Catalonia | Oller S.,International Center for Numerical Methods in Engineering | Oller S.,Polytechnic University of Catalonia | And 6 more authors.
International Journal of Fatigue | Year: 2015

This paper presents a plastic-damage formulation and a new isotropic hardening law, based on the Barcelona plastic damage model initially proposed by Lubliner et al. (1989) [1], which is capable of predicting steel failure due to Ultra Low Cycle Fatigue (ULCF). This failure mechanism is obtained when the material is subjected to cyclic loads and breaks after applying a very low number of cycles, usually less than hundreds. The failure is driven by the plastic response of the material, and it is often predicted based on the plastic strains applied to it. The model proposed in this work has been formulated with the objective of predicting accurately the plastic behavior of the material, as well as its failure due to ULCF. This is achieved taking into account the fracture energy dissipated during the whole loading process. This approach allows the simulation of ULCF when it takes place due to regular cyclic loads or non-regular cyclic loads, as it is the case of seismic loads. Several simulations are conducted in order to show the capabilities of the formulation to reproduce the mechanical response of steel when it is subjected to regular and non-regular cyclic loads. The formulation is validated comparing the numerical results with several experimental tests made on X52 steel specimens. The agreement between the numerical and experimental results asses the validity of the proposed model to predict the plastic behavior of steel and its failure due to Ultra Low Cycle Fatigue. © 2014 Elsevier Ltd.


Carmona Benitez R.B.,Anahuac University of North Mexico | Carmona Paredes R.B.,National Autonomous University of Mexico | Lodewijks G.,Technical University of Delft | Nabais J.L.,IDMEC
Expert Systems with Applications | Year: 2013

This paper presents a modification of the Grey Model (GM) to forecast routes passenger demand growth in the air transportation industry. Forecast methods like Holt-Winters, autoreg ressive models, exponen- tia smoothing, neural network, fuzzy logic, GM model calculate very high airlines routes pax growth. For this reason, a modification has been done to the GM model to damp trend calculations as time grows. The simulation results show that the modified GM model reduces the model exponential estimations grow. It allows the GM model to forecast reasonable routes passenger demand for long lead-times forecasts. It makes this model an option to calculate airlines routes pax flow when few data points are available. The United States domestic air transport market data are used to compare the performance of the GM model wit hthe proposed model. © 2013 Elsevier Ltd. All rights reserved.


Barbosa A.Q.,IDMEC | Da Silva L.F.M.,University of Porto | Ochsner A.,University of Technology Malaysia | Abenojar J.,Charles III University of Madrid | Del Real J.C.,Comillas Pontifical University
Ciencia e Tecnologia dos Materiais | Year: 2013

The inclusion of particles (nano or micro) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, such as rubber particles. In the present study, natural micro particles of cork were used with the objective to increase the mechanical properties of a brittle epoxy adhesive. The cork particles act like as a crack stopper leading to higher displacement of the specimen. The influence of the inclusion of cork particle was studied. Particles of cork ranging from 125 to 250 μm were mixed in the epoxy adhesive Araldite 2020 from Huntsman. This evaluation was made using tensile and impact tests and it was evident that mechanical properties were related to amount of cork particles in the resin, considering a uniform particle distribution. © 2013 Sociedade Portuguesa de Materiais (SPM).

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