Percorrer por autor "Costa, J.D."
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- 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 Crack Propagation in Shot Peened al 7475-t7351 Alloy SpecimensPublication . Ferreira, N.; Ferreira, J.A.M.; Antunes, P.V.; Costa, J.D.; C. CapelaThe approach to engineering design based on the flaws propagation assumption applying the principles of fracture mechanics is commonly used in aluminum structures for aerospace engineering, in which surface shot peening is an attractive method of improving fatigue performance, because it promotes the retardation of the crack initiation and earlier crack growth. The main purpose of present work was to analyze the effect of the surface shot peening on the fatigue crack propagation of the 7475 aluminum alloy with a T7351 heat treatment. Two types of fatigue tests were performed: constant amplitude and variable amplitude loading in which periodic overload blocks of 300 cycles are applied with intervals of Nint cycles. Surface micro shot peened promoted an increasing in micro-hardness only in order or 6% and created negative surface residual stresses in order of -174 MPa, which compare with the positive residual stresses of +291 MPa on the machined specimens. For tests at constant amplitude loading the effect of surface peening on da/dN-ΔK curves is quite limited, particularly for R = 0.4. However, this beneficial effect increases significantly near the threshold. Repeated overload block reduces significantly the fatigue crack propagation rate, being this effect particularly dependent of the intervals between the blocks. The maximum reduction of crack propagation rate and retardation effects were obtained for Nint = 7500 cycles.
- Fatigue Performance of Hybrid Steel Samples with Laser Sintered ImplantsPublication . Santos, L.M.S.; Ferreira, J.A.M.; Costa, J.D.; C. CapelaLaser sintering metal has recently been used in the manufacture of metallic structural hybrid components comprising two different materials obtained by two distinct technological processes. This process allows to obtain productivity gains reducing sintering time and hence the cost. In current study it was used a machined substrate in which it is built by sintering the remaining part. The purpose of present work was to study the effect of the substrate material and interface microstructure on the fatigue performance under constant and variable block loadings. 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. Fatigue strength will be quantified in terms of S - N curves. The results show that tensile properties of sintered specimens and of the hybrid parts was similar. Fatigue strength for short lives, of the sintered specimens and hybrid parts was quite similar. However, the fatigue strength of hybrid parts tends to decrease, for long lives, when compared with single sintered specimens. The fatigue tests under block loadings leads to indicate that the application of Miner’s law is adequate to predate fatigue life in hybrid components with sintered implants, despite having been observed a tendency to be conservative for long life.
- Fatigue strength of tubular carbon fibre composites under bending/ torsion loadingPublication . Capela, C.; Ferreira, J.A.M.; Febra, T.; Costa, J.D.Carbon fibre reinforced polymer composites have been increasingly used on structures frequently subjected to biaxial fatigue loadings. This paper studies the fatigue behaviour of tubular carbon fibre composites under in phase biaxial bending/torsion dynamic loadings. Particularly, it was analysed both the torsion stress and mean stress effects on the fatigue strength and failure mechanisms. Fatigue strength decreases significantly with increased torsional/bending stresses ratio, while the damage becomes faster. For the cases in which a torsion loading was applied the effect of the mean stress on the fatigue strength seems to be well fitted by using a quadratic equation.