An interpretation of the C-t parameter for increasing load conditions

The prior assessment equations for C-t, which is a well-known fracture para meter for characterizing creep and creep-fatigue crack growth rates, have a pplicability to constant loading conditions only. However, crack growth due to creep can also occur under varying load conditions during a fatigue cyc le when the loading (or unloading) rate is slow enough such that creep defo rmation can occur near the crack tip. Hence, the applicability of the C-t p arameter should be extended to varying load conditions. In this study, a me thod of extending the use of the C-t parameter to increasing load condition s is proposed. Based on the concept of Irwin's effective crack size, new eq uations for estimating C-t under increasing load conditions are derived and denoted as (C-t)(r). Finite element analyses were also performed under var ious increasing load conditions. From the analysis, the variation of (C-t)( r) values during the load rise period is obtained and the difference betwee n the (C-t)(r) value at the end of the load rise period and the C-t value a t the beginning of the succeeding load hold period is discussed. A generali zed creep-fatigue crack growth model which employs (C-t)(r) as a parameter characterizing crack growth rate during the rise time is also discussed.