13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- steady state conditions have been reached, the creep fracture mechanics term C* is used [8,16-20]. During the intervening stage damage formation and stress redistribution is occurring at the crack tip. The parameters are validated by the right usage of parameters to describe creep brittle and creep ductile crack growth [8]. 2.1. Steady state CCG analysis Creep crack growth rate under steady state for a creep ductile material is usually analysed using the fracture mechanics parameter C* [16-20]. The derivation for C* which is analogous to J is well documented [16,17] and will not be detailed in this paper. Once a steady-state distribution of stress and creep damage has been developed ahead of a crack tip, it is usually found that creep crack growth rate a ˙ can be described by an expression of form [16-20]: a D C* (1) where D and are material constants. Where creep dominates most often the constants in Eqn. (1) are obtained from tests that are carried out on compact tension (C(T)) specimens based on the recommendations of ASTM E1457 [8] standard and hence, C* is estimated experimentally from measurements of creep load-line displacement according to the experimentally determined value of C* given by n 1 n B (W a) P C n c * (2) where c is the load line displacement rate due to creep alone, Bn, W and a are the specimen net thickness (accounting for side-grooves), width and crack length, respectively, n is the creep stress exponent. The geometry function from [16,21-22] is given as d aW dm m 1 . (3) Where m is a function of collapse load of the cracked body [22]. Solutions for the functions, in different geometries, based on analytical solutions (limit load analyses) and finite element calculations are available [22]. From Eqn. (2) therefore providing that the displacement rates can be measured it is possible to simply derive C* experimentally [23,24] for subsequent use in Eqn. (1). 2.2. Reference stress method of estimating C* The data obtained from C(T) specimens using Eqn. (1) is considered as ‘benchmark’ material data
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