Fatigue properties of holes with residual stresses

Residual stresses were induced onto pre-machined holes using the ballising process in which a slightly over-sized tungsten carbide ball was pushed through them without rotation. Residual stresses at the vicinity of a hole, evaluated using a fracture mechanics approach, were found to be compressive at the surface and reached maximum tensile stress some 3.5 mm away from the edge of the hole. The compressive stress rendered a decrease in the stress intensity factor at the tip of a crack emanating from the hole surface. Fatigue testing of the ballised and unballised holes showed that the compressive residual stress, together with the improvement in surface roughness of the ballised hole, resulted in a significant increase in the fatigue lives of the ballised holes. With a 3.5% interference between the hole and the ball, a minimum two-fold increase was found. It is also shown that fatigue performance is dependent upon whether a ballised hole is complete and unbroken or is slit. When the hole is slit, the compressive stress is redistributed, giving rise to an entirely tensile stress state at the hole surface. This stress state unambiguously manifests itself in the decrease in fatigue lives of ballised holes that have been slit into halves.