Helical gears from an automotive gearbox, previously subjected to the surface treatments of carbo‐nitriding and shot‐peening, were submitted to contact fatigue tests. The X‐ray diffraction technique was used to characterize the evolution of different mechanical and metallurgical parameters as a function of gear damage. Particular attention was paid to residual stress relief. A numerical model was developed to predict residual stress relaxation and estimate the most likely localization of contact fatigue crack initiation. The stress–strain laws of the surface‐treated layers were determined by means of two separate experimental methods, based on locally measured parameters. The Dang Van multiaxial fatigue criterion was used to analyse the failure of the gears, taking into account the effects of friction and roughness.

An overview of the X‐ray fractography technique, as performed on fatigue crack surfaces of several steels and Al‐alloys under different loading conditions, is presented. The plastic zone sizes of fatigue cracks, for plane strain conditions, are measured from the in‐depth distribution of residual stresses and X‐ray diffraction peak broadening. In addition to the usual monotonic plastic zone size determination methodology, a model for the estimation of the reverse plastic zone size was established in the case of fatigue softening materials. Monotonic and cyclic plastic zone sizes are related to the stress intensity by, respectively, rpm = α  (Kmax /σys )2 and rpc = α  (ΔK/2σ′ys )2. The α‐value, in the monotonic plastic zone size equation, increases as the yield strength of the material increases, following the relationship α = 0.196  [σys /(129 + 0.928σys )]2. The α‐value versus σys evolution has been understood through the influence of the hardening rate of materials on the plastic zone size. X‐ray fractography has been applied to actual failure analyses to predict some aspects of the actual loadings.