Creep rupture behavior of modified 9Cr-1Mo steel under multiaxial stress and its modeling

Structural materials experience various multiaxial stress states and their integrity under such conditions needs to be evaluated in design and life management. Especially creep rupture behavior is known to be quite sensitive to the stress multiaxiality. To systematically evaluate the multiaxial effect on creep rupture behavior of modified 9Cr-1Mo steel, a number of creep tests were conducted on round-bar specimens with circumferential notches. Strong effects of temperature and inelastic strain rate on rupture strain were observed that their synergetic effect was modeled by a simple expression. Then crack growth in compact tension specimens was simulated by finite element analysis incorporating primary, secondary and tertiary creep stains to derive ductility under higher stress triaxiality. Finally, true rupture strain was expressed as a function of temperature, inelastic strain rate and triaxiality factor and its validity was demonstrated through finite element analyses on notched bar and compact tension specimens employing it as a load ductility criterion. (author)