Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 56

·

ε R = λε

·

ε = ε s t R

t MGD t R

Time, t

3.23 Representation of creep strain growth following the
Kachanov-Rabotnov model.

.
Phaniraj et al . 98 have established a correlation between the ratio of time to
Monkman-Grant ductility ( t MGD ) and time to rupture ( t R ) and the damage
tolerance parameter as given by

the creep curve would assume different shapes for different values of λ

λ .

⎡
⎢

⎤
⎥
⎤
⎦

λ 1 t

MG D

−

1

[3.60 ]

⎡
⎣

=−

λ t R t

Figure 3.24 is based on this Equation [3.60] and shows that t MGD / t R is essen-
tially constant for

> 4. The t MGD was suggested as time for onset of true
tertiary creep damage and was considered to be an important parameter in
identifying the useful creep life of a material. It also describes the time for
which minimum creep ductility is ensured. Hence Phaniraj et al. contend
that the stress to cause t MGD in 10 5 h can be used as a useful design criterion
for creep of elevated temperature components.
Before concluding we present a few examples where the concepts dis-
cussed in the previous sections may not be directly applied. Rather subtle
modifi cations to the models are necessary in order to simulate the actual
behavior of the material.

λ

￿ ￿ ￿ ￿ ￿ ￿

3.7 Case studies illustrating the role of other factors

In the following section, the effects of impurities, second phases and multi-
axial loadings on creep of materials are discussed with examples taken from
various classes of materials including ionic solids.