6.2. Nonlinear creep damage accumulation effect
It can be seen from Table 4 that constant also exhibits the co-dependence on stress and temperature, which can be described in Figure 7 accordingly. Constant increases with the decrease of stress at a given temperature. Haque attributed the increase of to the accommodation of longer creep life22. However, according to Eq., constant controls the curvature of damage evolution curve, which means that the greater the constant , the greater the curvature of the damage evolution curve. In essence, the nonlinear creep damage accumulation effect is revealed implicitly. The damage evolution with normalized time of specimens 1-12 under different stress at 600℃ and 650℃ are shown in Figure 8. Obviously, the nonlinear damage accumulation effect can be observed. A schematic diagram of nonlinear damage accumulation is shown in the Figure 9. For a two-step loading condition, if the specimen is firstly loaded at high stress level for normalized time , the damage will evolve from zero to point A. Subsequently, when low stress level condition is applied for normalized time up to failure, the damage will evolve from point A to point B horizontally, and finally reach to unity. In this case, the total life is reduced. This nonlinear damage accumulation effect has been evaluated quantitatively by using isodamage line theory for superalloy GH4169 in recent research59, i.e. the total life will be reduced when high stress condition was applied first. It should be pointed out that the opposing situation was also found for some other materials60. This means that the variation trend of the constant with stress can be essentially related to the different responses of the material to the load sequence effect.