Peen forming can be carried out by using either large or small indent ratios. The magnitude of the curvature induced will be proportional to the total amount of work done by peening. A minor proportion of the curvature is temporary, since that relates to the residual stress distribution in the curved component, and can be removed by stress-relief annealing. The major proportion of the curvature is permanent, as it relates directly to the non-uniform plastic deformation. The net plastic deformation involved with large indent ratios is much greater than it is with small ratios. Greater curvatures can therefore be induced. Large indent ratios require, however, that the indentation diameters are relatively large. The corresponding unevenness of the surface may be a disadvantage. Peening of Almen strips is a type of peen forming. It follows that very high Almen intensities cannot be accurately detected if it involves large indent ratios. That is why there has to be a range of Almen strip thicknesses - currently N, A and C. Even the thickest C strips would be inappropriate for large indent ratios. For a given shot size, type and velocity the indent ratio is reduced as the strip thickness increases. Hence, an even thicker gage of strip than C would be needed for very high Almen intensities. Indent ratio also affects the 'ridges' that normally surround peening indentations. Above the critical aspect ratio, ridges disappear - which confirms the slip line field prediction given in fig. 8. It should be noted that 'Slip line theory' is a convoluted area of indentation theory. It is virtually impossible to produce accurate field diagrams because of their dependence on imprecise variables such as work hardening, friction between sphere and surface, velocity effect and proportion of impact energy converted into heat. Nevertheless, the theory does give useful explanations of observed indentation phenomena.
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