Mechanisms of laser shock peening residual stress and the influence on life enhancement

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    In recent decades, the manufacturing process of Laser Shock Peening (LSP) has been used to extend the service life of very particular aircraft components. This process introduces compressive residual stresses in the metallic components, effectively increasing its fatigue life and the resistance to stress corrosion cracking. Advancements in laser technology have now provided the ability to systematically study the influence of numerous LSP parameters on the formation of residual stresses and to develop novel uses of the induced stresses to increase the safety of aircraft structures.

    This research aimed to determine the influence of laser temporal profiles on the LSP induced residual stress and explore the use of different LSP strategies to produce a redirection in the trajectory and extension in crack growth performance of aircraft fuselage structures. The experimental results of these studies were used to develop predictive methods to determine the outcome of the LSP processing and the behaviour of fatigue cracks in the presence of residual stresses.

    The study of the mechanisms of LSP used 6 mm thick 2524-T351 clad aluminium samples with four different variations of the laser pulse temporal profiles (flat top, standard Gaussian, two variations of a double Gaussian). The maximum pulse power density, pulse duration, pulse size and spatial distribution were fixed so that the influence of the temporal profiles were isolated. Incremental hole drilling was used to measure the induced residual stress, whilst the surface deformation caused by the laser shot was characterised by interferometry techniques.

    The results showed, no single tested temporal profile resulted in a significant increase in comparative performance of the LSP processing, as all deviations in the residual stress distributions were within the experimental scatter of the measurements. A physics-based finite element model was developed that accounted for the laser temporal profile and variations in the spatial distribution. The simulated and measured results indicated good agreement, with the proposed finite element model increasing the predictive accuracy of the post-LSP deformed surface.

    The crack growth life extension and modification in the trajectory of the fatigue crack study focused on the application of LSP on 2 mm thick 2024-T3 clad aluminium. LSP patches were orientated at various angles and distances from the central crack notch, to evaluate the effectiveness of LSP residual stresses to force a modification of the crack trajectory.

    Residual stress at multiple points on the sample was measured using incremental hole drilling and neutron diffraction techniques. Constant amplitude fatigue testing was conducted at two different stress ranges, to assess the influence of loading conditions on the crack growth rate and crack trajectory.

    Results showed that the LSP increased the crack growth life by as much as 9.5 times compared to the unpeened samples. It was found that the LSP residual stress tended to deviate the crack towards the centre of the LSP patches as the crack propagated under mode I cyclic loading. The highest crack growth life extension was achieved by placing the LSP patch as close to the initial notch. The largest crack deviation was achieved by placing the LSP patches further along the crack trajectory. The largest crack deviation tended to occur as the crack approached the edge of the LSP patch, this was due to the adjacent balancing stresses which occurred due to the LSP compressive residual stress. Residual stress models were coupled with linear elastic fracture mechanics finite element simulations to characterise the complex loading condition which occurred due to the superposition of the applied load and residual stresses. The crack growth life and crack deviation potential were predicted.
    Date of Award2021
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SponsorsAirbus Operations GmbH
    SupervisorMichael Fitzpatrick (Supervisor), Niall Smyth (Supervisor) & Xiang Zhang (Supervisor)

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