Browsing by Author "Ferreira, N."
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- 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 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.
- 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.