Percorrer por autor "Costa, J. D."
A mostrar 1 - 10 de 12
Resultados por página
Opções de ordenação
- Analysis of fatigue crack propagation in laser sintering metalPublication . Borrego, L. F. P.; Antunes, F. V.; Ferreira, J. A. M.; Costa, J. D.; Capela, C.Laser sintering metal has recently been used in the manufacture of components for different applications like aerospace or medicine. The approach to engineering design based on the cracks propagation assumption applying the concepts of linear elastic fracture mechanics (LEFM) is commonly used for aerospace engineering. However, fatigue crack propagation is linked to irreversible and non-linear mechanisms at the crack tip, therefore LEFM parameters can be successfully replaced by non-linear crack parameters, namely the plastic CTOD. A model linking da/dN with plastic CTOD is proposed here to characterize fatigue crack propagation. A comparison is made with other materials showing that for the same plastic CTOD the laser sintering material has a relatively large crack growth rate.
- Cyclic plastic behaviour of 7075 aluminium alloyPublication . Nogueira, F.; Cunha, J.; Mateus, A.; Malça, C.; Costa, J. D.; Branco, R.This paper aims at studying the cyclic plastic behaviour of the 7075-T651 aluminium alloy under fully-reversed strain-controlled conditions. Tests are conducted under strain-control mode, at room temperature, in a conventional servo-hydraulic machine, from smooth samples, using the single step method, with strain amplitudes (∆ε/2) in the range ±0.5 to ±2.75%. This material has exhibited a mixed behaviour, i.e. cyclic strain-hardens at higher strain amplitudes (∆ε/2/>1.1%) and cyclic strain-softens at lower strain amplitudes (∆ε/2<1.1%). A linear relationship between the degree of cyclic strain-hardening and the strain amplitude has been established for higher strain amplitudes. Fatigue-ductility and fatigue-strength properties agree with those found in the open literature for the same loading conditions.
- Dynamic Mechanical Analysis of Hybrid Fibre/Glass Microspheres CompositesPublication . Ferreira, J. A. M.; Capela, Carlos; Costa, J. D.This article presents the results obtained in a current study of the viscous properties on hybrid short fibre/hollow glass microspheres composites fabricated with epoxy binder. The effect of the filler volume fraction and of the fibre reinforcement on the dynamic stiffness modulus, damping coefficient and glass transition temperature was studied. These properties were determined using dynamic mechanical analysis (DMA) in three points bending mode. The specimens were cut from plates produced by vacuum resin transfer moulding with microspheres weight contents up to 17%. Net resin exhibits storage modulus significantly higher than the 2% in weight of microspheres foam, while negligible effect was observed on the maximum loss modulus, maximum damping coefficient and glass transition temperature. The increase in filler volume fraction tends to decrease significantly storage and loss modulus at stable regions and the maximum damping coefficient, while glass transition temperature is only marginally affected. The addition of low contents of short fibre increases significantly storage modulus, particularly for carbon fibre, while maximum loss modulus does not exhibit a well‐defined tendency. Important reduction in the maximum damping coefficient was observed by the addition of both fibre reinforcements.
- Effect of the foam core density on the bending response on sandwich compositesPublication . Capela, C.; Ferreira, J. A. M.; Costa, J. D.This paper presents the results of a current study of sandwich panels manufactured by using homogeneous and multilayer core foams with the purpose of improving specific flexural stiffness modulus. In the present study, the core foams were produced by using Verre ScotchitTM-K20 hollow microspheres manufactured by 3M and the selected binder resin was epoxy 520 with hardener 523. The skin was a 2 mm thick carbon/epoxy laminate. The ARAMIS technique was used as an alternative technique to obtain accurate displacement fields. It was concluded that the multilayered panels with different loads of microspheres, putting higher percentage of microsphere in the center and lower in the outer layers, have also higher resistance and stiffness than the panels with homogeneous microsphere percentage cores. It was observed that both properties have a tendency to increase when the displacement rate increases from 0.5 mm/min to 10 mm/min for all architectures. Experimental stiffness agrees well with analytical model predictions. © 2013 The Korean Fiber Society and Springer Science+Business Media Dordrecht.
- Fatigue behavior of Ti6Al4V alloy components manufactured by selective laser melting subjected to hot isostatic pressing and residual stress reliefPublication . Jesus, J. S.; Borrego, L. P.; Ferreira, J. A. M.; Costa, J. D.; Capela, C.Fatigue behavior (Rε = −1) of HIPed and stress relieved Ti6Al4V alloy specimens produced by selective laser melting (SLM) was analyzed and compared resulting that the hot isostatic pressing (HIP) process caused a microstructural transformation decreasing the hardness and monotonic properties leading to cyclic softening that not allowed fatigue strength to increase. A bilinear behavior in the elastic strain–fatigue life curve was observed due to the decrease of the Young's modulus during the cyclic elastoplastic tests, consequence of subgrains formation. The Smith–Watson–Topper and total strain energy density models adjusted by the bilinear behavior showed a good concordance between predicted and experimental fatigue lives in notched samples.
- Fatigue behaviour in hybrid hollow microspheres/fibre reinforced compositesPublication . Ferreira, J. A. M.; Salviano, K.; Costa, J. D.; Capela, C.This article presents the results of a current study concerning the influence of the addition of short fibres on the fatigue behaviour of syntactic foams. The material was obtained by vacuum-assisted resin transfer moulding adding hollow glass microspheres to an epoxy resin acting as binding matrix. Specimens with microsphere contents up to 50% and fibre reinforcement up to 1.2% in volume were tested at three-point bending at room temperature. Foams show significantly lower static and fatigue strength than an epoxy matrix. A significant decrease in the absolute strength with filler increase was observed, and even specific strength decreases for low filler contents and is nearly constant for the higher filler contents. Fatigue strength also decreases with the increase in filler content. The addition of glass fibre reinforcement produces only a slight improvement in flexure strength, while the addition of carbon fibres promotes an important improvement; a hybrid composite containing 0.9% carbon fibre is about 30% stronger than unreinforced foams. An improvement in fatigue strength more than 30% was obtained by the addition of small percentages of glass or carbon fibre.
- Fatigue crack growth behaviour in Ti6Al4V alloy specimens produced by selective laser meltingPublication . Jesus, J. S.; Borrego, L. P.; Ferreira, J. A. M.; Costa, J. D.; Capela, C.The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio R = 0 was detected and for R = 0.4 no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in β phase contouring the martensitic α phase.
- Fatigue crack growth under corrosive environments of Ti-6Al-4V specimens produced by SLMPublication . Jesus, J. S.; Borrego, L. P.; Ferreira, J. A. M.; Costa, J. D.; Capela, CarlosAdditive manufactured parts made of Ti-6Al-4V alloy are increasingly used in medical devices and in the aeronautical industry, because of its high strength, low weight and excellent biocompatibility. Most of these components work under environmentally assisted cyclic loading, i.e. under corrosion-fatigue. Anisotropic microstructure of additive manufactured materials significantly influences the propagation of cracks, particularly under corrosion-fatigue. This paper presents the results of the failure mode and fatigue crack propagation study in titanium Ti-6Al-4V specimens produced by selective laser melting (SLM), under corrosive ambient. Three environment solutions were used: artificial saliva, Ringer's solution and 3.5%wt NaCl solution. Tests were performed using standard 6 mm thick compact specimens (CT) tested at R = 0.05 with frequencies of 1 and 10 Hz. The main objective was to study the effect of the corrosion potential on da/dN-ΔK curves and on the fatigue failure mechanisms. It was observed a very important accelerating effect on the crack initiation and fatigue crack propagation for tests under corrosion ambient, particularly for 3.5%wt NaCl solution, for which fatigue crack growth is 3.3 times higher in comparison with inert ambient tests.
- Multiaxial fatigue behaviour of maraging steel produced by selective laser meltingPublication . Branco, R.; Costa, J. D.; Martins Ferreira, J. A.; Capela, C.; Antunes, F. V.; Macek, W.This paper studies the multiaxial fatigue behaviour of maraging steel samples produced by selective laser melting. Hollow cylindrical specimens with transverse circular holes are subjected to different in-phase bending-torsion loading scenarios. Fatigue crack initiation sites and fatigue crack angles are predicted from the first principal stress field. Fatigue lifetime is computed using a straightforward approach, based on a one-parameter damage law, developed via uniaxial low-cycle fatigue tests. The cyclic plasticity at the notch-controlled process zone is accounted for by combining the equivalent strain energy density concept and the theory of critical distances within a linear-elastic framework. Regardless of the multiaxial loading scenario, experimental observations and predicted lives are very well correlated.
- Notch fatigue analysis and crack initiation life estimation of maraging steel fabricated by laser beam powder bed fusion under multiaxial loadingPublication . Branco, R.; Prates, P. A.; Costa, J. D.; Ferreira, J. A. Martins; Capela, C.; Berto, F.This paper deals with the notch fatigue behaviour and crack initiation life estimation in maraging steel fabricated by laser beam powder bed fusion under multiaxial loading. Tests are conducted in tubular geometries with lateral holes considering different normal stress to shear stress ratios and multiaxial loading levels. The cyclic stress–strain response at the notch-controlled process zone is simulated numerically using two alternative approaches: a generalised isotropic plasticity model with mixed isotropic-kinematic hardening, and a linear-elastic model. Both approaches demonstrated to be suitable for predicting the crack initiation sites, the directions of crack growth, and the fatigue life. Fatigue life was calculated from a SWT-based model combined with the Theory of Critical Distances. Elastic-plastic predictions led to smaller errors but slightly shifted to the non-conservative side.