FGA formation mechanism for X10CrNiMoV12‐2‐2 and 34CrNiMo6 for constant and variable amplitude tests under the influence of applied mean loads

Abstract

AbstractThe aim of the present work is to clarify the fine granular area (FGA) formation mechanism in two steels (tempered 34CrNiMo6 and X10CrNiMoV12‐2‐2) causing grain refinement in the early state of fatigue for internal crack initiation and propagation in the very high cycle fatigue regime at pure tension‐compression loading (R = −1) and for applied mean stresses (R ≠ −1). Fatigue tests were performed with constant and variable amplitude at several R values using ultrasonic fatigue testing setups. Failed specimens were investigated using high‐resolution scanning electron microscopy and focused ion beam technique with special attention paid to the crack origin and the surrounding microstructure. To prove models for FGA formation proposed in literature, a numerical model to evaluate effective R values and contact stresses between the fracture surfaces depending on the crack length has been realised. The aim of these investigations is to estimate the influence of crack closure effects on FGA formation. FGA formation due to repeating contact of the fracture surfaces according to the model postulated by Hong et al correlates well with the findings for numerical simulations.