13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 4. Creep crack growth rate characteristics C* [11-14], dδ/dt [15-17] and Q* [18-19] have been proposed as the fracture mechanics parameter to describe the creep crack growth rate. The C* parameter for a circular notched specimen is given by Eq. (1). dt d n n C net δ σ 2 1 2 1 * − + = , (1) where n is creep exponent, σnet is the net section stress (MPa). The creep crack growth data for both base metals and weldmens (indication data are for the specimen with notch located in the middle part of HAZ) estimated by C* and dδ/dt are shown in Figs. 5 and 6, respectively. The characteristics of creep crack growth rate estimated by C* and dδ/dt show typical dual behavior which takes major portion of total crack growth life, approximately 60-70% of total life. An example of dual behavior on C* is shown in Fig. 7. By the way, Q* concept enables crack growth in the steady state regions, which correspond to the circular region in Fig. 7, to be characterized uniquely [18-19]. The creep crack growth rate for creep-brittle materials is represented by the Q* parameter as follow [19], ( ) ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ Δ − = = RT H A Q A K dt da g n in exp exp * * , (2) where A and A* are constant, Kin is the initial stress intensity factor (MPam1/2), n is the exponent of initial stress intensity factor, ΔHg is an activation energy (kJ/mol), R is the gas constant (=8.3145J/Kmol), T is the absolute temperature (K). By taking logarithms of both sides of Eq. (2), Eq. (3) is obtained. ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ Δ − + = RT H A n K dt da g in log log log * . (3) The bracket portion of Eq. (3) corresponds to the Q* parameter which has been proposed as an estimation parameter of creep crack growth rate [18]. On the basis of Q* concept, creep crack growth rate in this region is well predicted for base metals and weldments including the data for specimen with the notch located in the fine-grained HAZ, as shown in Fig. 8, regardless of the life of creep crack growth are different between them. 5. Characteristic of creep crack growth life In the section 3, the possible presence of an incubation time of creep crack growth, regardless of variation in notch location, was shown and we suggested that in order to predict the fracture life for weldment, it is important to clarify the incubation process in order correctly determine the incubation time of creep crack growth. The comparison of creep crack growth behavior for weldment with that for base metal under the same condition (T=625°C, Kin=12MPam1/2) is shown in Fig. 9. In Fig. 9, in spite of the fact that the creep crack growth rate in the constant region of creep crack growth both for weldment and base metal are the same, the life of creep crack growth for WM was shorter than that for BM. This will be due to the occurrence of the rapidly acceleration of creep crack growth for WM. In Fig. 9, the region of restrained creep crack growth rate due to the HAZ will correspond to the crack length of 100 μm. Therefore, this region is considered to be within an incubation region of creep crack growth which can be termed creep crack initiation for circular notched round bar specimen.
RkJQdWJsaXNoZXIy MjM0NDE=