Deformation and fracture behaviour under combined creep and fatigue

An examination of deformation and fracture has been made for the nickel base superalloy IN597 at 850°C for relatively high stress short life cyclic torsional loading programmes. The cycles combine low cycle fatigue with creep and relaxation dwell from the repeated application of a closed hysteresis loop under strain control. From a consideration of the changing shape of the hysteresis loop throughout cyclic life the effects of dwell stress, inelastic strain range and cycle number on creep deformation are examined together with the variation of peak stress softening with cycle number and relaxation time. Their effects on fracture in a high-temperature oxidising environment are appraised in relation to various predictive techniques; the Coffin-Manson equation, the universal slopes method, the linear damage rule, strain range partitioning and various incremental life prediction laws. Observations made on transgranular and intergranular crack paths are shown to provide the necessary metallographic evidence for the relative contributions to fracture from the repeated action of creep, fatigue and relaxation processes.