Abstract
A new life prediction method was proposed for high strength materials under fatigue-creep interaction (FCI) based on their tensile total strain energy behavior.
The net tensile hysteresis energy is now being used to evaluate FCI life in the frequency modified damage function rule developed by W.J. Ostergren. For high strength materials, however, this damage rule seems to require some modification to give good life assesment, because the elastic strain range is much larger than the inelastic strain range in the FCI behavior of these materials.
In this paper, strain controlled tensile or compressive strain hold tests with F-F, F-S and S-F wave forms were performed for Cast René 80. The total strain energy including a large elastic strain energy was defined, and FCI life was obtained against this strain energy. Multiple regression analysis was carried out to assess the relationship between strain energy and cycles to failure and was expressed numerically to improve life estimation accuracy.
Finally, a frequency modified total strain energy parameter was defined to evaluate the failure lives (Nf) of high strength materials such as René 80 under the FCI condition. The total tensile strain energy (ΔWTT) and the time (τTT) under tensile stress loading were used to define this tensile strain energy parameter. Thus,
ΔWTT·τnTT·Nmf=C
where, n, m and C are constants. A similar relation was also obtained between total strain energy (ΔWTT) and crack initiation life (Nc).
