Local strain field evolution at shot-peened 7075 Al-base notches upon tensile loading using in-situ synchrotron X-ray μ-diffraction

Shot peening (SP) is commonly applied to Al-base alloys during manufacture to enhance their fatigue tolerance and notch sensitivity in aerospace engineering components with geometrical discontinuities. We have mapped with high spatial resolution the local strain field induced by surface shot peening around notches in aerospace 7050-T7451 alloy, and determined the local mechanical behaviour of the surface affected layer during room-temperature tensile testing. For this purpose, we have performed in-situ synchrotron X-ray μ-diffraction during tensile loading of notched specimens in the as-machined condition, and also for two different shot peening intensities. The behaviour of the notched specimen not affected by machining and SP was also simulated by finite element modelling to decouple the local surface mechanical response from the bulk material behaviour. The results confirmed that shot peening induces compressive residual stresses close to the notch tip, and also changes in notch geometry and surface topography. Shot-peened surfaces experience accommodation of plastic effects at the early stages of the tensile test, and they yielded at ~70-80% higher applied nominal tensile stresses than the as-machined material. When increasing the applied stress, the maximum longitudinal elastic strain shifts beneath the surface, and the magnitude of the peak strain and its position converge to the same value for the as-machined and shot-peened specimens. Beyond yielding, there is an exponential increase in plastic strain close to the notch tip, irrespective of the initial surface condition. The overall elasto-plastic response of the material is not affected by shot peening, but compressive residual strains significantly retard yielding close to the notch tip under loading.