Browsing by Author "Capela, C."
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- 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.
- 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.
- 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.
- Fracture toughness of the heat affected zone on Nd-YAG laser welded jointsPublication . Costa, J.M.; Ferreira, J.M.; Capela, C.; Capela, CarlosLaser deposit welding based on modern Nd-YAG lasers is a new mould repair process with advantages relatively to the traditional methods (Micro-plasma and TIG methods). For moulds steels there are no research studies about the laser deposit welding process effects in respect to fracture toughness, hardness and residual stresses variations in the laser-deposited layer and in the heat affected zone. These variations will have an important influence in the thermal-mechanical fatigue strength of the moulds parts. This paper is concerned on the study of the fracture toughness of transient microstructure regions in Nd-YAG laser welded joints performed in mould steels. Fracture toughness tests were performed in two hot-working tool steels: AISI H13 and AISI P20. Small welded specimens were prepared with U notches and filled with laser welding deposits. The hardness profiles were obtained at the middle cross-section of specimens against distance to the surface in order to identify the microstructures present at the crack tip region. Fracture toughness was evaluated and plotted against the Vickers hardness measured at the fatigue crack tip. Depending of the crack tip depth two very different toughness levels were observed in the H13 steel, while a lower variation of toughness was observed for the P20 steel. The values of fracture toughness observed in each specimen are consistent with the correspondent crack tip microstructure hardness.
- Mechanical behavior of high dosage short carbon fiber reinforced epoxy compositesPublication . Capela, C.; Oliveira, S. E.; Ferreira, J. A. M.Short fibers are effective reinforcements in strengthening and toughening polymer materials, even small amounts of fibers. However, for high fiber dosage the achieved dispersion and interface adhesion is quite poor reaching to lower stiffness and strength efficiency. In high dosage composites, the effects of the fiber length and volume fraction on mechanical properties is not entirely consensual; nonetheless this paper intends to contibute for a better understanding of this occurrence. Composite plates were manufactured by compression moulding, using short carbon fibers reinforcements (2, 4 and 6 mm in length) with 45-75 % wt fiber fraction. Tensile and dynamic mechanical analysis (DMA) specimens were machined from molded plates for the desired dimensions. DMA was performed with bi-supported specimens in temperature ranged from 20 to 180 oC. High dosage composites exhibit very low efficiency parameters both in stiffness and particularly in tensile strength. However, the stiffness increases in order of 25 % when fiber length increases from 2 mm to 4 mm, but afterwards tends to decrease for 6 mm fiber length composites. The same tendency was observed for the tensile strength. For very high fiber dosage composites, tensile strength decreases and Young’s modulus tends to remain at constant values. However, the results of DMA indicate that bending modulus storage increases when fiber length increases from 2 mm to 6 mm.