Finite element analysis of effect of triaxial state of stress on creep cavitation and rupture behaviour of 2.25Cr–1Mo steel

Effect of triaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel has been investigated by performing creep tests on plain and circumferential U-notch specimens of different notch acuity ratios ranging from 1 to 20. The creep rupture life of the material was found to increase in the presence of notch and the strengthening effect tends to saturate at higher notch acuity ratio. Fractographic observations revealed the plasticity induced intragranular ductile failure with dimple appearance for relatively shallow notches (low notch acuity ratio) and creep cavitation induced intergranular brittle failure for relatively sharp notches (high notch acuity ratio). The creep damage initiated at the centre of notch plane for shallow notches and at the notch root for sharper notches. In order to understand the effect of notch sharpness on stress distribution and damage across the notch plane, finite element (FE) analysis was carried out. The peak value of maximum principal and hydrostatic stresses was found to increase and shift from the centre of the notch plane to close to notch root with the increase in notch acuity ratio. On the other hand, the von-Mises stress decreased with increase in notch acuity ratio but remained nearly uniform across the notch plane for shallow notches and high at the notch root for sharp notches. The difference in creep fracture mode of the notched specimen with notch acuity ratio has been explained based on the FE analysis. The skeletal point stresses obtained from FE analysis were used for predicting the relative contribution of each stresses on governing the rupture life of notched specimens.