Fatigue properties of cold worked holes

Abstract

The introduction of holes in a body would inevitably raise the stress locally around the holes. This stress concentration would in turn affect the performance of the body under static and fatigue loading conditions. The aim of the present paper is to investigate how three cold working processes, namely, shot peening, wet blasting and ballising would affect the fatigue lives of holes. Fatigue tests on 3mm thick tensile specimens of Assab 760 steel showed that ballising with interference between 1 and 3.3% enhanced the fatigue lives of the holes by about 60 to 220%. For shot peening with pressure from 138 to 414 kPa, the enhancement was about 30 to 100%, while for wet blasting at pressure between 69 and 690 kPa, 10 to 50%. The residual stresses around the holes as a result of the cold working processes were evaluated using a fracture mechanics approach. It was found that all the three processes studied induced compressive residual stress field at the surfaces of the holes. The compressive stress decreased and became tensile as the distance from the surface of the hole increased. The compressive stress was observed to increase with pressure in the shot peened and wet blasted holes and with interference in the ballised holes. Within the range of pressure and interference investigated, residual stress measurements revealed that ballising induced the greatest amount of compressive residual stress while wet blasting, the lowest. This is well manifested in the fatigue lives of the holes.