Residual stress relaxation in shot peened high strength low alloy steel

The relaxation of residual stress in a shot peened high strength low alloy steel in response to applied loads has been investigated for both single shot and cyclic loading. The near surface residual stress was measured predominantly by X-ray diffraction (first 10 mu m) but supplemented for some conditions by measurements using high resolution, fine increment hole drilling to yield depth profiling. The compressive residual stress showed a peak at similar to 50 mu m below the surface. Under single shot compressive loading, relaxation became significant when the combined surface and applied compressive stresses exceeded the proportional limit of the unpeened surface. The steel showed good resistance to residual stress relaxation under cyclic loading conditions, with only a 13% decrease despite cyclic load amplitudes at 50% of the 0.2% proof stress in compression. Significant relaxation did occur at applied stress amplitudes of similar to 80% of the 0.2% proof stress in compression. While large single shot or cyclic stresses would represent rare occurrences, they could lead to a marked loss in beneficial compressive stress and should be accounted for in structural integrity assessment.