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complex, non-steady stress-temperature conditions encountered in service
conditions.
In addition to these methods, creep life predictions are also guided by
damage mechanics. The irreversible material damage caused by mechan-
ical loading and environmental features during creep eventually leads to
very high strain rates of deformation and failure. Damage could be due
to cavity formation, microcracks and gross deformation such as strain-
or ageing-induced. A materials scientist viewpoint on micromechanical
causes of damage is given by Le May.
94
In addition to creep damage, other
mechanisms of damage such as fatigue, surface oxidation and internal cor-
rosion are also important. Although some of these phenomena are not
temperature dependent, their interactions with creep, such as creep-fatigue
interaction, can have signifi cant effects on high temperature damage accu-
mulation. The different damage processes constitute ductile creep rupture,
intergranular cavitation during creep, continuum creep rupture, continuum
fatigue damage, environmental damage and age- and strain-induced hard-
ening and softening. In contrast to creep life predictions based on mecha-
nistic models, continuum damage mechanics (CDM) attempts to provide
a holistic view of the damage process and accordingly models the useful
creep life of a material. By accepting the fact that damage is a result of
the complex interactions between different mechanisms, CDM provides
greater accuracy in creep life estimation in comparison to models based
on a single mechanism of creep, namely grain boundary sliding or dislo-
cation creep. While there have been many continuum damage mechanics
models advocated over the years, a unique model is the one proposed by
Kachanov,
95
later elaborated by Rabotnov
96
and commonly referred to as
the Kachanov-Rabotnov model. A brief review of the Kachanov-Rabotnov
model is presented below.
3.6.1 The Kachanov-Rabotnov CDM model
Kachanov represented continuum damage as an effective loss in mate-
rial cross-section due to the formation and growth of internal voids.
Consequently the internal stress corresponding to a nominal externally
applied load increases with increasing damage. Kachanov assumed that
damage could be represented by a quantity which he called the 'continuity.'
The continuity is essentially the ratio of the remaining effective area
A
to
the original area
A
0
. With accumulation of damage, the resulting internal
stress (
σ
i
) increases from initial value
σ
0
to a value given by
A
A
0
[3.54 ]
σσ
0
.
i
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