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Study of residual stress introduced by laser shock peening in wide chord fan blades by neutron and synchrotron diffraction

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Study of Residual Stresses Introduced by Laser Shock Peening in Wide Chord Fan Blades by Neutron and Synchrotron Diffraction A. KINGa,*, A.D. EVANSa,b, M. PREUSSa, P.J. WITHERSa and C. WOODWARDc aUniversity of Manchester & UMIST, Grosvenor St, Manchester,M1 7HS, UK; bInstitut Laue-Langevin, BP156, F-38042 Grenoble cedex 9, France; cRolls-Royce plc, Derby, UK Laser shock peening (LSP) can improve the fatigue performance of components by introducing compressive residual stresses to the surface. Residual strain has been measured in LSP’d Ti-6Al-4V fan blade root style test-pieces. Neutron diffraction (ND) has been used to measure residual strain in three orthogonal directions, which is necessary to calculate stress. This is more difficult using high energy synchrotron X-ray diffraction (SXRD) because of the characteristically low diffraction angles. Instead plane strain measurements have been made in transmission through the thickness of a sample. The out-of-plane strain was calculated using stress equilibrium arguments, and this calculation was evaluated and refined using finite element modelling. The resulting stress maps of LSP’d blade roots reveal the pattern of overlapping laser impacts. The compressive stress appears to extend more than 1 mm below the surface. Keywords: Laser shock peening; Laser; Neutron; Synchrotron; Residual stress; Fan blades INTRODUCTION Laser shock peening (LSP) has been proposed as a means of introducing compressive stresses in the near surface region of fan blade roots in order to increase their fretting fatigue resistance. LSP is capable of introducing significant compressive surface stress in flat plates to a much greater depth (typically 1–2 mm) than with conventional techniques such as shot peening (limited to ,0.25 mm) [3]. Since fatigue cracks usually initiate at the surface, the fatigue strength of a component can be greatly increased by LSP. The tolerance of the component to surface damage is also improved [1]. However, until n

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