A study of shot peening effects on fretting fatigue.

The high clamping forces in lap-joints in the body and wings of an aircraft result in fretting fatigue damage at the contacting surfaces. Shot peening may provide an efficient technique to delay or eliminate fretting fatigue failures. In this study, the fretting fatigue behaviour of unpeened and peened aluminium alloy 2024 T351 in flat-on-flat contact with rectangular bridge pads of aluminium alloy 2024 T351 was investigated and compared with results from previous research where steel pads were used. Contact normal pressure was maintained at a constant value during the tests while the axial load was cyclic with a stress ratio of -1 and a sinusoidal waveform of 20 Hz. The friction force acting over the contact area was measured by strain gauges mounted on the bridges. There was a considerable reduction in fatigue strength due to fretting, although improved performance for the peened condition. In all cases, fretting fatigue life was found to decrease with an increase in the normal pressure, up to a critical value of normal pressure. Above this critical value, a further increase in normal pressure tended to increase fretting fatigue life. This behaviour was observed at three different axial stresses and indicates a changed sensitivity to pressure for the fatigue damage mechanism. In order to distinguish and quantify the parameters that control such a phenomenon, both an experimental and an analytical methodology have been utilised. Fretting fatigue life was investigated in terms of the contact region and the specific roles of friction, crack growth and surface modification. The test data was used to develop and assess a fracture mechanics model based on the mechanics of the fretting fatigue process, constructed to predict fretting fatigue life. Components of friction stress estimated from surface roughness profiles, axial stress and normal stress were combined to account for the biaxiality of the loading and the benefits of shot peening were modelled by incorporating the effect of a residual stress.