Flat on Flat contact best replicates conditions of fretting fatigue in aircraft single or double lap joints. Fretting fatigue is characterised by a number of complex mechanisms including stick to slip transition and mixed mode crack propagation which diverts after a specific depth, defined by the contact pressure or bolt pressure, to a mode I crack propagation. Extensive experimental characterisation utilising, apart from traditional S-N curves, fractography and laser 3-D profilometry, found that the typical U shape fatigue life (at constant axial varying normal stress) primarily depends on the stick to slip transition. The above, amplified the criticality of the surface roughness and near surface residual stresses, which when compressive can increase the surface wear resistance. To address the issue, surface engineering treatment in the form of controlled shot peening (CSP) was utilised. After achieving the required level of residual stress, the surface was subjected to super-finishing conditions to deliver uniformity of the surface roughness. The experimental result indicate that CSP has the potential of increasing the fatigue life of lap joints as much as 300% and thus reducing the time-dependent statistical distribution of multiple site damage.
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