RELATION BETWEEN FATIGUE THRESHOLD AND FATIGUE LIMIT FOR SURFACE-ROLLED SPECIMENS

After cylinder notch fatigue specimens of 40 CrNiMo steel were rolled, their fatigue limit increased by 41%. The rolled specimens did not fracture, even though they had been loaded for 10(7) cycles under fatigue limit stress, but a non-propagating fatigue crack was generated. Thus the value of the fatigue limit depends on the fatigue threshold value DELTAK(th) of the metal of the rolled layer. Plastic deformation increased DELTAK(th) in these experiments. It can be inferred that DELTAK(th) of the rolled layer increases from the occurrence of plastic deformation and microvoids on the layer. Calculation of the effect of residual stress in the crack wake on the stress intensity factor DELTAK indicates that residual compression stress decreases DELTAK by 21.5 MPa square-root M. It was calculated that rolling induced both the length of the non-propagating crack and the increase of fatigue limit. The calculated values are in accord with experiment. Analysis and calculations indicate that the non-propagating crack is generated on the rolled layer. Thus the fatigue limit is improved because rolling produces residual compression stress in the layer (which decreases the stress intensity factor), and increases DELTAK(th) of the layer.