6. Discussion
6.1. The stress and temperature dependence of constant
To ensure experimental damage is unity when failure occurs, the constant is defined as . The constant shows the dependence on stress and temperature as described in Figure 6. With the decrease of stress, the constant increases. Cano23 tried to use Eyring function to account for the stress dependence at a single temperature. However, Eyring function is not suitable to describe the co-dependence of constant on stress and temperature in this study. Hence, an empirical function is constructed as follows
where , , , and are fitting constants. And above constants obtained by fitting constant of specimens 1-12 are ,,,. Using this empirical function, the critical experimental damage values of specimens 13-18 are listed in Table 5. It can be observed that the critical experimental damage values are close to unity, which indicates that the selected empirical function is reasonable. After careful observation, the critical experimental damage value of specimen 15 is relatively far from unity, which can be attributed to the abrupt failure and lower rupture strain as shown in Figure 1.