Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement
Author: Daniel J. Chadwick, Siavash Ghanbari, David F. Bahr, Michael D. Sangid
Source: Fatigue Fract Eng Mater Struct. 2017;1–13.
Doc ID: 2017150
Year of Publication: 2017
Abstract:
Shot peening is a dynamic coldâ€working process involving the impingement of
peening media onto a substrate surface. Shot peening is commonly used as a surface
treatment technique within the aerospace industry during manufacturing to improve
fatigue performance of structural components. The compressive residual stress
induced during shot peening results in fatigue crack growth retardation, improving
the performance of shotâ€peened components. However, shot peening is a compromise
between the benefit of inducing a compressive residual stress and causing
detrimental surface damage. Because of the relatively soft nature of AA7050â€
T7451, shot peening can result in cracking of the constituent precipitate particles,
creating an initial damage state. The aim of this paper is to understand the balance
and fundamentals of these competing phenomena through a comparative study
throughout the fatigue lifecycle of baseline versus shotâ€peened AA7050â€T7451.
Microstructure and surface topology characterization and comparison of the baseline
and shotâ€peened AA7050â€T7451 has been performed using scanning electron
microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and
optical profilometry techniques. A residual stress analysis through interrupted
fatigue of the baseline and shotâ€peened AA7050â€T7451 was completed using a
combination of Xâ€ray diffraction and nanoindentation. The fatigue life performance
of the baseline versus shotâ€peened material has been evaluated, including crack
initiation and propagation. Subsurface particles crack upon shot peening but did
not incubate into the matrix during fatigue loading, presumably due to the compressive
residual stress field. In the baseline samples, the particles were initially intact,
but upon fatigue loading, crack nucleation was observed in the particles, and these
cracks incubated into the matrix.
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