This paper examines the evaluation and interpretation of the Ct parameter that has been proposed for correlating crack growth under non-steady-state creep conditions. The study has been carried out by finite element analysis of the commonly used compact-type fracture specimen with stationary cracks.
It is shown that the loss of constraint of the creep zone growing at the crack tip region due to intense creep at the loading pins can have a significant effect on the results of the load line displacement rate which are used to calculate Ct. Since Ct is identical to C* under extensive creep conditions, these effects also seriously impact determination of other commonly used creep crack growth parameters. This study has given some insight into the proper care that should be taken in obtaining experimentally measured and numerically calculated load line displacement rates in fracture specimens so as to eliminate the undesired effects of intense creep at the loading pins and/or between the crack tip and loading pins. It is shown that compact-type specimens with a crack size to width ratio greater than 0.5 and loaded with a dead weight are best suited for obtaining creep crack growth rate data. A relationship between Ct and the crack tip opening displacement rate has also been derived that provides a more complete framework for the analytical and experimental evaluation of the Ct parameter.
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