It is important to have relatively uniform residual stresses across the treated area on a part, and LSP provides this. The surface stress distribution along the radius of a laser shock peened spot is shown in Figure 1 for an aluminum alloy. This is typical for all other alloys where surface distributions were determined. The stress distribution is essentially uniform except for a slight dip in the center of the spot. There has been no evidence that this dip has affected fatigue behavior. In the next section, it will be seen that the dip extends only a few thousandths of an inch below the surface.
In Figure 1, the residual stresses in both the tangential and radial directions are shown. These stresses are nominally equal in the interior of the laser treated spot, but at the spot’s outside edge, they tend to differ. Immediately outside the treated spot a surface tensile residual stress forms tangentially to compensate for the compressive stresses inside the spot, whereas in the radial direction this effect is often absent. To avoid having this peripheral tensile residual stress affect the post-processed properties, the treated area must be large enough to move this stress outside the fatigue-critical area.
Larger areas are laser peened with overlapping spots. Extensive measurements have shown that there is no tensile residual stress around the edges of the spots either on the surface, or in the subsurface compressive stress layer within the overlapped area. The residual stresses within the laser peened areas created by overlapping spots is uniform across the group of spots.
In addition to laying out a treatment pattern consisting of multiple spots, specially tailored spot shapes can be used to provide different surface and in-depth distributions of the residual stress. Figure 3a shows an example of an annular-shaped spot around a hole, with the inside diameter of the laser shocked region slightly larger than the diameter of the hole. Figure 3b shows the resulting residual stress distributions along a radius through the annular spot at the surface and at depths of 0.0035 and 0.0125 inches (0.09 and 0.31 mm) below the surface. At 0.012 inches and more below the surface, the stress is reduced more rapidly around the edges of the annulus than in the middle. This is caused by the peak pressure being decreased by release shock waves traveling in from the edges of the shock wave.
It is noteworthy that the compressive residual stress is increased between the inside diameter of the annular shocked region and the hole, even though this area was not shocked (the unshocked surface residual stress is about -8 ksi). The hole was drilled before LSP. Increasing the intensity of laser shock peening increases the magnitude of the surface compressive residual stresses.