On the Fatigue Strength of Ball Burnished Mechanical Elements

Abstract

This work presents the quantification of the improvement in the high-cycle fatigue strength of machine parts manufactured with medium-carbon steels by means of low-plasticity burnishing. A complete experimental study has been performed on specimens: surface integrity, in-depth residual stresses, residual stresses relaxation, alternating bending fatigue tests and fractographic analysis. The tests show that the fatigue strength of ball-burnished components is significantly improved in the range 104 to 106 cycles, and the fatigue limit is increased by 20%. The residual stresses in the surface layer are non-isotropic biaxial compressive stresses varying in time because of stress relaxation. The quantitative data here provided are the modifying coefficient for the Marin equation and a uniaxial effective mean stress for each stress level. For fatigue calculations, this effective constant mean stress is equivalent to the effects produced by ball burnishing, and is an alternative to the calculations based on modifying coefficients.