STRESS ANALYSIS OF BALLISED HOLES

Abstract

Ballising, a reliable manufacturing process, involves pushing a slightly over-sized ball made of hard material, though a hole without rotation. Through the ballising process, the surface roughness, hardness and topography of the surface of a hole can be improved. In addition, compressive residual stresses are introduced onto the hole surface as a results of the interference between the hole and the ball. These factors give rise to significant enhancement in the fatigue performance of the ballised hole in comparison to that of unballised hole. This present study aims to investigate the residual stress distribution around a ballised hole. A theoretical approach has been employed to estimate the stress distribution. This is later extended to the case involving two ballised holes. Elastic-plastic theory was used to evaluate the stress fields associated with the ballising process. For the purpose of theoretical simplification, a ballised hole was looked upon as a cold-expanded hole in an axially symmetrical circular thin plate. The residual stress distribution surrounding a ballised hole was derived by the theory of plasticity. In the estimation, only the diameter of the hole before and after ballising needs to be measured to determine the residual stress fields. With the basic theory, a series of specimens was ballised and investigated. The effects of different interference levels and specimen size on residual stress were studied. The overall residual stresses of plate with two ballised holes were obtained by the numerical superposition of the residual stresses induced by single ballised hole. The distance between the centres of the two holes was varied in the computation for different interference levels. Two computing programmes in FORTRAN language were complied to calculate the residual stress fields for plate with one and two ballised holes respectively. This study confirms that ballising introduces large residual compressive stresses on the edge of the hole. As the approach presented is a non-destructive method, it offers a convenient way to arrive at a theoretical estimation of the residual stresses.