A new statistical framework for the determination of safe creep life using the theta projection technique

In this article a new estimation framework is put forward for the well-known theta projection technique which enables, for the first time, levels of confidence to be associated with the creep property predictions made using this technique. The predictions made from the resulting model are in the form of distributions, which is a substantial advance on existing life assessment methodologies used in high-temperature applications such as the disks and blades used in aero engines. This additional information should prove invaluable for questions related to the issues of possible life extension. When applied to data on Ti.6.2.4.6, accurate interpolations and extrapolations could be made of the actual distribution of the minimum creep rate, which in combination with the Monkman-Grant relation, also enabled accurate predictions to be made for the time to failure. For example, when extrapolating to a stress of 480 MPa, the time to failure was predicted to follow a non-normal distribution with a median time of 3450 h and a 95% confidence interval of 3250-3800 h. The single experimental data point available at this stress was consistent with such an extrapolation.