An interpretation of the Ct parameter for increasing load conditions

Abstract

The prior assessment equations for Ct , which is a well‐known fracture parameter for characterizing creep and creep–fatigue crack growth rates, have applicability to constant loading conditions only. However, crack growth due to creep can also occur under varying load conditions during a fatigue cycle when the loading (or unloading) rate is slow enough such that creep deformation can occur near the crack tip. Hence, the applicability of the Ct parameter should be extended to varying load conditions.In this study, a method of extending the use of the Ct parameter to increasing load conditions is proposed. Based on the concept of Irwin’s effective crack size, new equations for estimating Ct under increasing load conditions are derived and denoted as (Ct )r . Finite element analyses were also performed under various increasing load conditions. From the analysis, the variation of (Ct )r values during the load rise period is obtained and the difference between the (Ct )r value at the end of the load rise period and the Ct value at the beginning of the succeeding load hold period is discussed. A generalized creep–fatigue crack growth model which employs (Ct )r as a parameter characterizing crack growth rate during the rise time is also discussed.