FATIGUE PROPERTIES OF HOLES WITH RESIDUAL-STRESSES

Residual stresses were induced onto pre-machined holes using the balli sing process in which a slightly over-sized tungsten carbide ball was pushed through them without rotation. Residual stresses at the vicinit y of a hole, evaluated using a fracture mechanics approach, were found to be compressive at the surface and reached maximum tensile stress s ome 3.5 mm away from the edge of the hole. The compressive stress rend ered a decrease in the stress intensity factor at the tip of a crack e manating from the hole surface. Fatigue testing of the ballised and un ballised holes showed that the compressive residual stress, together w ith the improvement in surface roughness of the ballised hole, resulte d in a significant increase in the fatigue fives of the ballised holes . With a 3.5% interference between the hole and the ball, a minimum tw o-fold increase was found. It is also shown that fatigue performance i s dependent upon whether a ballised hole is complete and unbroken or i s 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 f atigue lives of ballised holes that have been slit into halves.