Browsing by Author "Ferreira, J. A. M."
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- 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 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 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.
- 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.