An examination of the relevance of co-indentation studies to incomplete coverage in shot-peening using the finite-element method

Abstract

The achievement of complete coverage during the application of shot-peening to critical components represents a major concern in the aircraft and nuclear industries. It has been accepted, for many years, that complete coverage of the component is needed to attain the beneficial effects of the treatment upon the fatigue life of such components. It has also been proposed, by a number of industrialists, that incomplete coverage may apparently shorten the fatigue life of the component because of the presumed presence of tensile residual stresses in the uncovered areas. The complete and accurate solution of this problem is very complex. However, in order to simplify the situation, an elasto-plastic finite-element analysis of simultaneous indentations of a bounded solid, by two smooth, flat, rigid punches under plane-strain conditions was considered. Two aspects of the co-indentation process were accordingly examined in the initial analysis of the results presented: (i) the influence of punch separation upon the resulting residual-stress field, and (ii) the successive development of the plastic zone for different incremental-indentation pressures. The results of the work were also extended to take into account the relationship of separation between the punches to incomplete coverage in the shot-peening process. In particular, it was desired to determine the critical separation beyond which the relevant residual stresses change from compressive to tensile. The present work highlights the fact that compressive residual stresses resulting from incomplete coverage were attained even where the separation between the punches was four times the punch width.