Surface crack initiation mechanism for body centered cubic materials in the gigacycle fatigue domain

A BCC polycrystalline single phase Armco iron was tested in the Very High Cycle Fatigue (VHCF) régime using plate specimens and a load ratio of R = −1. Throughout testing until fracture, optical observations of the specimen surface (on one side) and temperature measurements and recordings (on the other specimen side) were performed. After failure, the fracture surfaces were observed via SEM. The macroscopic observations showed an early occurrence of Persistent Slips Bands (PSBs). The observations of fracture surfaces clearly revealed an initiation and propagation stage. In the initiation stage, which ranges from about 200 to 550 μm, the grain traces are visible with intergranular, transgranular and mixed fracture. The comparison with the thermal recordings leads to the conclusion that the crack initiation stage is completed near the end of the fatigue life. Interrupted tests show damage in the bulk specimen before the crack initiation.