Percorrer por autor "Capela, C."
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- 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.
- Analysis of the initial delamination size on the mode I interlaminar fracture of carbon/epoxy compositesPublication . Reis, P.N.B.; Ferreira, J.A.M.; Antunes, F.V.; Costa, J.D.M.; Capela, C.; Capela, Carlos;This paper describes an experimental study of the initial delamination length effects on the mode I fracture toughness using DCB tests. Delamination failure was also predicted using exponential cohesive model and a quite good agreement with experimental results was obtained. Numerical predictions showed a significant influence of initial delamination length on the force-displacement curves.
- Assessment of the fatigue life on functional hybrid laser sintering steel componentsPublication . Ferreira, J.A.M.; Santos, L.M.S.; Silva, J. da; Costa, J.M.; Capela, C.The construction of hybrid parts: comprised of two different materials or obtained by two distinct technological processes is one of the main advantages of laser sintering metal. Various important aspects strongly affect the mechanical properties of sintering metal components: porosity, surface roughness, scan speed, layer thickness, and residual stresses. A major drawback is the occurrence of pores originating from initial powder contaminations, evaporation or local voids after powder-layer deposition, once these pores can act as stress concentrators leading to failure, especially under fatigue loading. The purpose of present work was to study the effect scan speed on the porosity and mechanical properties. Also the performance of two different material parts was studied. The sintering laser parts were manufactured in maraging steel AISI 18Ni300, while the substrates of hybrid specimens were produced alternatively in two materials: the steel for hot work tools AISI H13 and the stainless steel AISI 420. The results showed that a very high scan speed (400 or 600 mm/s) causes the appearance of high porosity percentages and consequent drastic reduction of tensile strength and stiffness. Tensile properties of sintered specimens and two different material parts was similar. However, the fatigue strength of two different material parts tends to decrease, for long lives, when compared with single sintered specimens. © 2015 The Authors.
- Assessment of the fatigue performance of heat-treated addictive manufactured TiAl6V4 specimensPublication . Borrego, L.P.; Jesus, J. de; Ferreira, J.A.M.; Costa, J.D.M.; Capela, C.; Capela, C.Titanium Ti6Al4V alloy has excellent mechanical properties and corrosion resistance combined with low specific weight, and is commonly used in biomedical applications, automotive and aerospace components, involving fatigue loadings. Current work studies the fatigue behavior under strain amplitude control of titanium TiAl6V4 specimens, intending to characterize fatigue strength from low to high life range after different heat treatments. Fatigue tests were carried out at room temperature, using round dog bone specimens produced by selective laser melting (SLM), where laser powder deposition occurred in layers perpendicular to the loading direction. Two batches of specimens were tested: one subjected to a stress relieve treatment and a second one treated by the hot isostatic pressing process (HIP). The material was characterized in terms of the tensile mechanical properties, cycle curve, Basquin and Coffin equations. Additional analysis of the hardness and scanning electron microscopy was carried out to complement the discussion of the results. The obtained dada showed that the stress relieved specimens exhibits significantly cyclic softening, increasing with applied strain, while HIP specimens show a practically stable cyclic behavior in relation to the monotonic curve. Material response for both treatments is well fitted by Basquin and Coffin-Manson formulations. The transition life was 187 reversals and 326 reversals, for stress relieved and HIP specimens, respectively. Fatigue life for a given strain is governed by the strain value, independently of the post manufacturing heat treatment.
- Effect of artificial saliva on the fatigue and wear response of TiAl6V4 specimens produced by SLMPublication . Jesus, J. de; Borrego, L. P.; Vilhena, L.; Ferreira, J. A. M.; Ramalho, A.; Capela, C.Additive manufactured (AM) parts made in TiAl6V4 alloy are increasingly used in medical prostheses and dental implants, because of its high strength, low weight and excellent biocompatibility. These components work under environmentally assisted cyclic loading, i.e. under corrosion-fatigue, and/or subject to wear conditions. Fatigue performance of additive manufactured alloys is significantly influenced by the porosities, residual stresses, which can reduce its strength when compared with traditional materials. This paper presents the results of a fatigue crack propagation study in titanium TiAl6V4 specimens produced by selective laser melting (SLM) under artificial saliva ambient. Tests were performed using standard 6 mm thick compact specimens (CT) tested at R=0.05 and with frequencies of 1 and 10 Hz. The main objective of the current research work was studying the corrosion effect on the fatigue crack propagation of Selective Laser Melting (SLM) specimens, and to compare the tribocorrosion behaviour of two different specimens: one produced by SLM and the other by the conventional/traditional method. The study concluded that: AM Titanium Ti6Al4V alloy exhibits a moderate effect of saliva ambient on fatigue crack nucleation and on fatigue crack propagation, the wear rate coefficients for SLM and conventional manufactured specimens is of the same order, and the mechanism of abrasive wear is mainly with grooves aligned with the direction of sliding.
- Effect of bead characteristics on the fatigue life of shot peened Al 7475-T7351 specimensPublication . Ferreira, N.; Jesus, J. S.; Ferreira, J. A. M.; Capela, C.; Costa, J. M.; Batista, A. C.The present work aims to analyse the effect of shot peening processing parameters, material and size of beads on the fatigue behaviour of aluminium alloy AA7475-T7351. A systematic study was carried out on the roughness, surface hardening, residual stress profiles and fatigue life. Fatigue tests were carried out under both three points bending (3 PB) and tensile loadings. For 3 PB tests it was concluded that shot peening does not introduce significant improvement on fatigue life and that the use of low size glass beads is potentially beneficial, with roughness being as or more important than residual stresses. All tensile treated specimens presented an improvement of fatigue life in comparison to the untreated specimens, particularly when the crack initiated internally. Internal crack propagation generates a conical fracture surface until transition to mode I propagation.
- 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.
- Environmental effect on the fatigue crack propagation of AM TiAl6V4 alloy specimensPublication . Borrego, L.P.; Jesus, J. de; Ferreira, J.A.M.; Costa, J.D.; Capela, C.Additive manufactured (AM) parts made in TiAl6V4 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. Fatigue performance of additive manufactured alloys is significantly influenced by the porosities, residual stresses, which can be reduced by optimizing the process parameters, thermal treatments or hot isostatic pressing (HIP). Those parameters can also influence significantly to the propagation of cracks under corrosion-fatigue, but the understanding of this subject still needs significant research work. This paper presents the results of a fatigue crack propagation study in titanium TiAl6V4 specimens produced by selective laser melting (SLM), under corrosive ambient. The environment solutions studied were: artificial saliva and 3.5%wt NaCl solution. Tests were performed using standard 6 mm thick compact specimens (CT) tested at R=0.05 and with frequency 10 Hz. The main objective was to study the effect of the environment solution on da/dN-∆K curves and on the fatigue failure mechanisms. Current work shows a very important accelerating effect on the crack nucleation and fatigue crack propagation for tests under corrosion ambient, particularly for 3.5%wt NaCl solution. Fatigue path shows an irregular path. Secondary cracking was observed in air, but not detected in corrosive ambient.
- 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.
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