Fatigue performance of holes with different finishing processes

Abstract

The fatigue performance of a hole in a component is related to its surface roughness and surface integrity as well as to the presence of residual stress, these factors being very much governed by the manufacturing processes employed in machining or finishing the holes. The present study examines the effect of five such processes on the fatigue lives of pre-machined holes in plates of Assab 760 steel, the processes investigated being electro-discharge machining, drilling, reaming, ballising and polishing. Fatigue tests on the hole surfaces were carried out using an applied tensile cyclic stress with mean and dynamic loads that were set and maintained constant for all specimens so that the fatigue lives of the test specimens could be compared. For all of the hole finishing processes applied, the results showed the expected trend of the fatigue life decreasing as the surface roughness increases. Holes that had been ballised generally showed the longest fatigue lives, whilst those manufactured with electro-discharge machining showed the shortest, polishing, reaming and drilling, in the order of decreasing fatigue life, being observed to be sandwiched between those for ballising and electro-discharge machining. Ballising appeared to be the best hole-finishing process, not only because it enhances the fatigue performance of the holes through compressive residual stress, but also because it can produce good surface finish economically and quickly. Conversely, the effect of tensile residual stress produced as a result of the re-cast layer rendered a poor fatigue performance in specimens that had been machined using electro-discharge machining. © 1994.