The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel

The underlying motivation for this work was to test
the conventional wisdom that 100% coverage by shot
peening is required to achieve full benefit in terms of
compressive residual stress magnitude and depth as
well as fatigue strength. Fatigue performance of
many shot peened alloys is widely reported to
increase with coverage up to 100%, by many
investigators and even in shot peening manuals.(1)
The fatigue strength of some alloys is reported to be
reduced by excessive coverage(2) Aerospace(3,4),
automotive(5), and military(6) shot peening
specifications require at least 100% coverage.
Internal shot peening procedures of aerospace
manufacturers may require 125% to 200% coverage.
Most of the published fatigue data supporting the
100% minimum coverage recommendation was
developed in fully reversed axial loading(2,7) or
bending(8,9 ) with a stress ratio, R= Smin / Smax, of -1.
The residual stress field arising from an individual
shot impact is much greater in extent than the
physical size of the impact crater and the resulting
surrounding ridge of raised material.(10) Hence, at
least some degree of undimpled surface area, less
than 100% coverage, should be tolerable in terms of
residual stress and fatigue strength achieved by
peening. Accordingly, residual stress-depth
distributions were determined for specimens peened
to various coverage levels. Fatigue performance was
tested at R>0, so that the shot peened surface was
loaded only in tension. Additionally, cold work-depth
distributions and the effects of thermal relaxation on
both residual stresses and cold work were
determined.