13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- for creep crack growth properties of the materials in the same way as creep strain rate and rupture for uniaxial creep tests. These data can be employed directly in crack initiation and growth models described in the different codes [11-15] to estimate residual lives in components. For components such as pipes and plates, on the other hand, C* must be determined from finite element analysis or reference stress methods. For this the reference stress procedure is adopted in line with that used in the defect assessment codes [11-15]. With this approach C* is expressed approximately as [16]: 2 ref ref ref * ref K C (4) where ε ˙ref is the creep strain rate at the appropriate σref for the component and K is the stress intensity factor corresponding to the applied loading. When the creep strain rate ε ˙ at an applied stress σ can be described in terms of the Norton creep law [16]: n A (5) where A and n are material constants at constant temperature. Thus, Eqn. (4) can be rewritten as: 1 2 C A K n ref * ref (6) The typical value for n is between 5 and 12 for most metals. In addition, the concept of the average creep rate, ε ˙Ave, obtained directly from rupture data, has been used to account for all three stages of creep as an approximate method for estimating the average creep rate ε ˙Ave as shown and defined by Ave Ave n Ave n o R f Ave A t 0 (7) where σ is the applied stress, εf is the uniaxial failure strain, tR is the time-to-rupture and AAve, nAve, σo and ε ˙0 are material constants. 2.3. Creep Crack Initiation (CCI) analysis When a structure containing a defect is first loaded the stress distribution is given by the elastic K-field or the elastic-plastic J-field. Therefore, time is required for the stresses to redistribute to the steady-state creep stress distribution controlled by C*. During this period, transient conditions exist which are not uniquely defined by C*. In addition, a period of time is needed for creep damage to develop around the crack tip [16]. Furthermore due to the practical limitations of crack detection equipment, the initiation of crack growth is difficult to determine precisely. Typically, this ranges between an extension Δa of between about 0.1 and 0.5 mm depending on component and crack
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