Life prediction techniques for combined creep and fatigue

This paper contains a review of the techniques that are currently available for predicting the life of metallic materials when they fail at high temperatures under the combined action of creep and fatigue. The work concentrates on those theories that are relevant to engineering design without specifically employing fracture mechanics concepts. As a consequence this generally limits the prerequisite data to that of low cycle fatigue under high rate strain cycling conditions and monotonic creep at a given temperature. Some techniques require additional or alternative data on tensile ductility, high cycle fatigue, cyclic plasticity and creep.Each technique is appraised from the results of laboratory experiments. It is shown that life may be predicted quite reliably in one instance but not in another. Some attempt is made to provide explanations and recommendations where these anomalies occur. The range of application is also clarified. Many approaches are limited to strain controlled cycling under uniaxial stress conditions. Only a limited number may further be applied to predict life for cycles in which ratcheting occurs. At present it appears that very few techniques are available to predict the creep-fatigue life of materials under high temperature multiaxial stress states. under high temperature multiaxial stress states.