PSI - Issue 80

6

Vaclav Sklenicka et al. / Procedia Structural Integrity 80 (2026) 493–500 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

498 ( ̇ m )

a t (3) the exponent a is usually found to range from about 0.8 to 0.95 (Cadek (1988)), and C MG is Monkman- Grant constant depending on temperature and the material. Fig. 4(a) shows in a double logarithmic plot the dependence of the time to fracture, t f , on the minimum creep rate, ̇ m , applied to the creep fracture data of the alloy under investigation. From Fig. 4 (a), it can be seen that nearly all the experimental data can be fitted with very good coincidence by a single line over the whole loading conditions. Later on, Dobeš and Milička (1976) suggested modification of Eq. (3) by introducing the creep strain to fracture, ε f (ε f )/t f = C MMG ( ̇ m ) b , (4) where the exponent b is close to unity, and C MMG is a temperature-independent constant. Nevertheless, the modification suggested by Dobeš and Milička does not improve the prediction capability of the M -G relationship since the modification also requires a further parameter- the strain to fracture, ε f. The relation between the mean creep rate, ε f /t f , and the minimum creep rate, ̇ m , is shown in Fig. 4 (b). It can be seen from Fig. 4 that the exponents a and c are the same and very close to unity. Therefore, Fig. 4 demonstrates that the Monkman - Grant relationship and his modified version are applicable with the slope values a = 0.98 and b = 0.98. The fact that the experimental results obey the Monkman-Grant relation supports a close relationship between creep deformation and fracture processes. f = C MG,

Fig. 4. Relation between creep deformation and fracture: (a) Monkman-Grant relationship, and (b) relation between the mean creep rate , ε f /t f , and the minimum creep rate, ̇ m . For b = 1, Eq. (4) predicts the temperature and stress dependence of time to fracture to be inverse to the minimum creep rate unless the strain to fracture, ε f , is temperature and stress-dependent. 3.4. Microstructural and fractographic observations The grain size was measured on the longitudinal metallographic section of the specimens using the linear intercept methods using SEM. An average grain size of 4- 5 μm was slightly changed during creep due to polyg onization processes inside the grain. The fraction of high-angle grain boundaries decreases, and the (sub)boundary distribution significantly changes during creep. The microstructural characteristics of the precipitates were investigated by