UPRATING AND LIFE ASSESSMENT UNDER FATIGUE-CREEP CONDITIONS

Uprating and life assessment exercises require an understanding of the likely failure mechanisms and confidence that appropriate life predic tive expressions are being employed. Principal trends in fatigue-creep response of a wide range of engineering alloys have been identified, although it is emphasised that a single alloy may exhibit all types of response if the appropriate conditions prevail. The quality of the av ailable data has been assessed in terms of its variability, arising fr om material and testing factors, and its representation of service con ditions that allow greater scope for material ageing and deterioration . Similarly, the assumptions of popular life prediction methods based upon linear cumulative damage have been examined. Where possible, expe rimental evidence is cited to estimate the significance of such effect s. The general case of uprating, as achieved by an increase in tempera ture, strain range and dwell duration, is explored in terms of the fat igue and creep damage fractions.