Ultra-High Cycle Fatigue in High Strength Steels

MIRCO DANIEL CHAPETTI

Santiago, Chile

Congreso; CONAMET-SAM 2006; 2006

CONAMET-SAM

Fatigue fracture origins in ultra-long cycle fatigue in high strength steels are mostly at non-metallic inclusions due to the influence of the hydrogen trapped by them. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was often observed inside a “fish-eye” mark for specimens with a long fatigue life, which represents the particular morphology associated with the mechanism of failure involved. Murakami and coworkers have named this area as “Optically Dark Area” (ODA), and the related mechanism of failure as “hydrogen embrittlement assisted by fatigue”.  Any estimation of total fatigue life associated with the failure produced by cracks initiated at internal inclusions will succeed only if the crack initiation period can be properly estimated. The crack initiation period can be defined by the number of cycles necessary to create a crack length given by the optical dark area (ODA). The number of cycles to form a crack length given by ODA size, is defined by the type and size of inclusion, the tensile residual stresses around it, the amount of hydrogen, the applied nominal stress range and amplitude, some particular features of the microstructure related to the hydrogen trapping places and the threshold for pure fatigue crack propagation.   In this work a model to estimate the threshold stress to obtain fatigue fracture from cracks initiated at internal inclusions (internal fatigue limit), is presented. The defined internal fatigue limit is obtained from: (a) the threshold for pure fatigue crack propagation, (b) the maximum inclusion size in the material, and (c) a defined critical size of the ODA.