Browsing by Author "Ferreira, J.A.M."
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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.
- Effect of adherends and environment on static and transverse impact response of adhesive lap jointsPublication . Reis, P.N.B.; Soares, J.R.L.; Pereira, António; Ferreira, J.A.M.Impact response of adhesive joints has received limited attention compared to quasi-static loading. On the other hand, there are very few studies combining moisture and its effect on the impact strength. Therefore, the present paper aims to study the effect of moisture on the tensile and impact strength of single lap joints with different adherends (high limit elastic steel and a commercial composite). It was possible to conclude that adhesive joints with steel adherends are very sensitive to the environment and exposure time. For adhesive joints with composite adherends, the water showed a marginal effect. A marked hygrothermal effect was observed for all joints. For impact loads the environment effect is similar, but much more severe than that observed in tensile tests. For both tests, adhesive failures occurred for adhesive joints with steel adherends and delaminations for joints involving composite.
- Fatigue behaviour of selective laser melting steel componentsPublication . Santos, L.M.S.; Ferreira, J.A.M.; Jesus, J.S.; Costa, J.M.; C. CapelaSelective laser melting (SLM) is a laser based rapid manufacturing technology that builds metal parts layer-by-layer using metal powders and a computer controlled laser. Various important aspects strongly affect the mechanical properties of sintered metal components, such as: porosity, surface roughness, scan speed, layer thickness, and residual stresses. Therefore, properties of SLM manufactured parts must be carefully analysed, particularly under fatigue conditions. The purpose of this work was to study the effect of scan speed, porosity and microstructure on the mechanical properties and fatigue strength of sintered laser samples. Sintered laser parts were manufactured in maraging steel AISI 18Ni300. Fatigue behaviour is related to process parameters, such as: surface residual stresses, microstructure and porosity. The results showed that a very high scan speed (400 or 600 mm/s) causes the appearance of high percentages of porosity and a consequent drastic reduction of tensile strength and stiffness. Fatigue behaviour was assessed in terms of the traditional S-N curves and of the da/dN–DK crack propagation curves. Fatigue life predictions based on Hartman and Schijve’s equation underestimated significantly fatigue lives, particularly for low stress levels. The results of the tests performed at variable amplitude loading were well fitted by Miner’s law.
- 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.
- Improvement in fatigue life of Al 7475-T7351 alloy specimens by applying ultrasonic and microshot peeningPublication . Ramos, R.; Ferreira, N.; Ferreira, J.A.M.; Capela, Carlos; Batista, A.C.Shot peening and ultrasonic peening are widely used mechanical surface treatment in the automotive and aerospace industries to improve the fatigue life of metallic components. This work aims to further improve the mechanical properties of the AA7475-T7351 alloy by applying three different peening processes: Ultrasonic Peening (USP) and Microshot Peening with two different beads size (MSP). A systematic study was carried out on the roughness, surface hardening, residual stress profiles and fatigue life. Uniaxial fatigue tests were conducted. Residual stresses were evaluated by X-ray diffraction, and the fracture surface was observed and analysed with a Scanning Electron Microscope. The microstructures of the specimens were observed by optic microscope. Microhardness and roughness were also measured. All of the surface treatments enhanced the specimens fatigue lives, despite having created different superficial conditions. In addition, the experimental results and analytic predictions based on the Molski-Glinka approach were compared, and in general, agree well.