13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Very high cycle fatigue (VHCF) behavior of structured Al 2024 thin sheets Sebastian Stille*, Tilmann Beck, Lorenz Singheiser Institute of Energy and Climate Research (IEK-2), FZ Juelich, D-52425 Juelich, Germany * Corresponding author: s.stille@fz-juelich.de Abstract In the present study the VHCF behavior of Al 2024, a standard material in aerospace application, is investigated. The focus of the presented work is the influence of riblet-like surface structures on fatigue life and damage mechanisms. The cold-rolling process used for riblet forming results in slight work hardening and near-surface residual stresses. Furthermore the influence of pure Al-claddings on the VHCF behavior of Al 2024 sheets is examined. Fatigue tests are performed using an ultrasonic fatigue testing system at a frequency of around 20 kHz. The present work contributes to a deeper understanding of riblet structures and surface claddings on the VHCF behavior of Al 2024. Keywords VHCF, Al 2024, surface structure 1. Introduction An innovative approach for aerodynamic drag reduction, e.g. of airfoils, are surface riblet structures [1] combined with transversal surface waves with amplitudes of few 1/10 mm at frequencies of some 100 Hz [2]. The resulting very high cycle fatigue (VHCF) loading at cycle numbers beyond the value of 2x107, taken as ultimate cycle number in most studies on fatigue of Al sheet alloys for aircraft application, has to be considered as a limiting factor for this approach. Al 2024 is a standard material in aerospace applications. Its VHCF behavior has been evaluated in detail in different studies [3-7]. These contributions show that material failure takes place even beyond 2x107 load cycles and that the S/N curve does not approach a horizontal fatigue limit. Fatigue experiments on cladded sheets have been performed in references [8-10]. Merati et al. [8] observed multiple crack initiation sites at the cladding layer in the regime up to 6x105 cycles. Edwards et al. [9] report that the first crack in the cladding layer can be detected as early as 1 % of the total lifetime of the samples and that clad specimen are less sensitive to notching effects than bare Al 2024 sheets. One intention of the present work is the investigation of the VHCF behavior of clad Al 2024 material up to cycle numbers distinctly beyond the existing data. Furthermore, the influence of riblet structures on the VHCF life of Al 2024 sheets needs to be determined for future application of active drag reduction. The experiments conducted in the frame of the present study cover nano-indentation, X-ray stress analysis, ultrasonic fatigue experiments and electron microscopy based fractography. 2. Material The experiments were performed on the aluminium alloy Al 2024. The sheet material has been supplied by ALCOA Mill Products in the T-351 state (solutions heat treated at 493 °C and quenched in water, subsequently stress relieved by stretching to a controlled deformation and finally naturally aged for 8 days [11]). The sheets are supplied in two different thicknesses, 1.6 and 2 mm. Furthermore, sheets with a cladding layer of 64 µm of pure aluminium on one side were tested.
RkJQdWJsaXNoZXIy MjM0NDE=