EFFECT OF RESIDUAL-STRESS ON THE FATIGUE PERFORMANCE OF THE SURFACE OF A BALLISED HOLE

Ballising, the process Of forcing a precision-ground tungsten carbide ball through a slightly undersized pre-machined hole, refines the surface structure of the hole and renders a plastically deformed hole surface where protrusions generated by the drilling or the boring of the hole before ballising are displaced plastically to fill up depressions. Consequently, not only are compressive residual stresses induced on the surface due to the cold-working that the hole surface has experienced, but significant improvements in surface finish, roundness of the hole and dimensional tolerance are also achievable. The present study investigates the effect of the residual stress on the fatigue performance of a ballised hole. It has been found that the fatigue performance is dependent upon two factors, namely, the completeness of the ballised hole and the interference between the bore and the ball. The fatigue life was, expectedly, observed to increase with the increase in interference, but when the ballised hole was broken, the fatigue life decreased to below that of an unballised specimen having approximately the same range of surface roughness. Residual stress studies using a fracture mechanics approach were conducted to evaluate the residual stress on the ballised hole surface. The result showed that when the hole is complete, compressive residual stress is induced at the hole surface, but when the hole is broken, the compressive stress is redistributed to give rise to a state of tensile stress at the hole surface. This finding is consistent with, and explains, the result of the fatigue tests.