An interpretation of the C_t parameter for increasing load conditions

The prior assessment equations for C_t, 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 C_t parameter should be extended to varying load conditions. In this study, a method of extending the use of the C_t parameter to increasing load conditions is proposed. Based on the concept of Irwin's effective crack size, new equations for estimating C_t under increasing load conditions are derived and denoted as (C_t)_r. Finite element analyses were also performed under various increasing load conditions. From the analysis, the variation of (C_t)_r values during the load rise period is obtained and the difference between the (C_t)_r value at the end of the load rise period and the C_t value at the beginning of the succeeding load hold period is discussed. A generalized creep-fatigue crack growth model which employs (C_t)_r as a parameter characterizing crack growth rate during the rise time is also discussed.