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.