Contact fatigue of automotive gears: evolution and effects of residual stresses introduced by surface treatments

Helical gears from an automotive gearbox, previously subjected to the surfa ce treatments of carbo-nitriding and shot-peening, were submitted to contac t fatigue tests. The X-ray diffraction technique was used to characterize t he evolution of different mechanical and metallurgical parameters as a func tion of gear damage. Particular attention was paid to residual stress relie f. A numerical model was developed to predict residual stress relaxation an d estimate the most likely localization of contact fatigue crack initiation . The stress-strain laws of the surface-treated layers were determined by m eans of two separate experimental methods, based on locally measured parame ters. The Dang Van multiaxial fatigue criterion was used to analyse the fai lure of the gears, taking into account the effects of friction and roughnes s.