This work deals with the effect of heat treatment on the fatigue strength of a rear axial beam and aims to propose a methodology suitable and reliable for fatigue design. The rear axial beam is made of sheet metal (22MnB5); the initial microstructure is ferrito-pearlitic and martensitic after the treatment. A vast experimental campaign has been undertaken to investigate the behaviour and more specially the fatigue damage mechanisms observed (with material treated and no treated) under different loading conditions: tension and shear test with different load ratios. To test a sheet on shearing an original test is also used. SEM observations of fatigue failure surfaces, for both heat treated and nontreated specimens, show that the fatigue cracks initiate from inclusions for the specimens loaded in tension. The experiments show that the damage mechanism depends on the applied loading condition: for shear loadings no inclusions are observed at the crack initiation site. A probabilistic approach using the weakest link concept is used to model the fatigue. This approach leads naturally to a probabilistic Kitagawa type diagram, which in this case explains the relationship between the influence of the heat treatment and the microstructural heterogeneities.
