CREEP-FATIGUE CRACK-GROWTH BEHAVIOR IN 1CR-1MO-0.25V STEELS .1. ESTIMATION OF CRACK-TIP PARAMETERS

Non-linear finite element (FEM) analyses, involving various creep defo rmation laws, as well as experiments with hold times of 100 s and 15 m in were performed on compact type specimens with stationary cracks. Th e work was aimed at developing accurate expressions for estimating the small-scale creep parameters, (C-t)(avg), for 1Cr-1Mo-0.25V steel at 538 degrees C (1000 degrees F). Here (C-t)(avg) is a representative va lue for the small-scale creep parameter C-t, averaged over a hold peri od under the conditions of creep fatigue loading. These expressions we re then applied to crack growth data obtained from specimens tested un der the various hold times. When an elastic-plastic-primary creep-seco ndary creep constitutive model was used in the FEM analyses, the calcu lated values of C-t compared well with the measured values. The FEM re sults also showed that the accumulated creep deformation during the ho ld time was not significantly reversed during the unloading portion of the creep fatigue cycle for this material. Therefore, a new method of estimating (C-t)(avg) is Proposed on the basis of these numerical and experimental results.