Prediction of threshold value for FGA formation

► The mechanism and threshold of FGA formation in high-strength steels is studied. ► The value of Δ K FGA of different materials keeps constant and is close to Δ K th,0. ► Crack initiation progress in FGA corresponds to the crack propagation below Δ K th,0. ► A theoretical model depending only on Burgers vector and shear modulus is proposed. ► The predictions by the model are in good agreement with many metallic materials. This paper studies the formation mechanism of fine granular area (FGA) in high-strength steels and predicts the threshold value of its formation. Fatigue experiments are carried out by using rotating bending and ultrasonic fatigue testing machines on a high carbon chromium steel (GCr15) with three different heat treatments. The results show that the range of stress intensity factor at the periphery of FGA (Δ K FGA) keeps constant with an average value of 5.2 MPa m 1/2, which is close to the traditional crack growth threshold (Δ K th,0), 5.0 MPa m 1/2. A theoretical model based on the plastic zone at crack tip is proposed to predict the value of Δ K FGA and the predictions are in good agreement with experimental data.