Investigation of crack initiation mechanisms responsible for the fish eye formation in the Very High Cycle Fatigue regime

The present paper aims to explain the subsurface crack formation occurring in the Very High Cycle Fatigue domain for bcc steels. For this purpose, two high strength steels (a low alloyed steel AISI 5120 and a martensitic stainless steel X40CrMo13) were studied under ultrasonic push-pull fatigue tests. For all tests, a fish eye developed before fracture and, in most cases, the subsurface crack initiation originated from inclusions. Also, during fatigue tests for both materials, the thermal field on specimen surface was recorded by infrared camera and fracture surface observations were performed by Scanning Electron Microscopy (SEM). The SEM pictures reveal the presence of a Fine Granular Area (FGA) surrounding the inclusion in the crack initiation stage. Moreover, earlier in the crack process, slips markings were found in the matrix inside the inclusion dimple indicating that some similarities exist between surface and subsurface crack initiation mechanisms.