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.