Percorrer por autor "Ferreira, J. A. M."
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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.
- Cyclic creep response of adhesively bonded steel lap jointsPublication . Reis, P. N. B.; Pereira, António; Ferreira, J. A. M.; Costa, J. D. M.The viscoelastic nature of polymeric adhesives means that the effect of fatigue frequency has to be treated cautiously. However, this subject has received limited attention and very few studies can be found. Therefore, this work aims at investigating the cyclic creep response of adhesively bonded steel lap joints. Load-controlled fatigue tests were performed with shear stresses of 9.1, 7.4, and 6.3 MPa, which are typically low cycle fatigue stresses. Only during the last 20%of fatigue life canwe observe an increase in the cycle hysteresis area due to the decrease of the shear stiffness caused by the failure mechanisms. Under fatigue load, the maximum/ minimum strain curves exhibit a shape being similar to that of the steady creep curves, inwhich occurs a second stage with nearly onstant strain rate, independently of the number of cycles and increasing with the load range. A linear relationship between the log cyclic creep rate and the log of the number of cycles to failure was observed, indicating that fatigue behaviour is strictly related to cyclic creep.
- 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 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 fiber length on the mechanical properties of high dosage carbon reinforcedPublication . Capela, Carlos; Oliveira, S. E.; Pestana, J.; Ferreira, J. A. M.Short fibers are effective reinforcements in strengthening and toughening polymer materials. It is reported that even small amounts of fibers drastically increased composite strength. However, for high fiber dosage the dispersion and interface adhesion is quite poor reaching to lower stiffness and strength efficiency. The effects of fiber length on mechanical properties of low content of short fiber reinforced composites is usually associated with a gain with the increasing of fiber length, but for high dosage this statement is not entirely consensual. This paper intends to contribute for the better understanding of the effect of the fiber length on the mechanical performance of high dosage fiber reinforced composites. Composite plates were manufactured by compression moulding, using short carbon fibers reinforcements (2, 4 and 6 mm in length) with 60% wt fiber fraction and the Biresin®CR120 resin as matrix. The specimens were machined from the plates for desired dimensions to the tensile and DMA tests. High dosage composites exhibits very low efficiency parameters both in stiffness and particularly in tensile strength. Stiffness increases in order of 25% when fiber length increases from 2mm to 4mm, but afterwards decreases for 6mm fiber length composites. The same tendency was observed for the tensile strength meaning that poor fiber dispersion and disorder was achieved for 6mm fiber length. The results of DMA indicate, however, that modulus storage increases when fiber length increases from 2mm to 6mm.
- 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.
- Effect of the mean stress on the fatigue behaviour of single lap jointsPublication . Pereira, António; Reis, P. N. B.; Ferreira, J. A. M.Steel is the most important construction material for the mass production of engineered structures, especially in the transport industry. On the other hand, adhesive joints are typically used to join load-bearing components. Therefore, this work intends to investigate the stress ratio effects on the fatigue behaviour of adhesively bonded steel lap joints. S–N diagrams of fatigue tests, under constant amplitude loading, were obtained for stress ratios ranging between 0.05 and 0.7. It was observed that the fatigue life of the adhesive joints has very little dependence on the stress amplitude, indicating that only the maximum stress is important. The combination of a linear equation with a quadratic equation seems to be the best formulation to fit the experimental results. Finally, the Palmgren–Miner’s Law is accurate enough to predict the fatigue design for sequential block loadings.
- 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.