Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 15

The creep curve is basically the outcome of the competition between the
processes of strain hardening and recovery. Materials usually strain harden
during plastic deformation due to dislocation multiplication. The strain
hardening is a kind of 'defense' mechanism in response to an applied stress.
Further plastic deformation can occur only if the applied stress exceeds the
increase in fl ow stress of the material due to strain hardening. Alternatively,
deformation can proceed at the initial applied stress if the material soft-
ens. The mechanism of recovery acts to soften a deformed specimen thus
allowing further plastic deformation. In the primary stage, the rate of strain
hardening is greater than the rate of recovery. This is due to the formation
of a more resistant creep substructure. The substructure could be the forma-
tion of dislocation networks or the arrangements leading to the formation
of subgrains. In the secondary stage of creep, the rate of strain hardening is
balanced by the rate of recovery due to dislocation annihilation and defor-
mation occurs at a constant strain rate. In the tertiary stage of creep, the
increase in applied stress due to a reduction in specimen cross-sectional
area surpasses the increase in fl ow stress due to strain hardening. The reduc-
tion in specimen cross-sectional area can be due to necking or internal void
formation . The tertiary creep is often associated with metallurgical changes
such as the coarsening of precipitate particles, recrystallization or diffusional
changes in phases present, void formation and so forth.
Figure 3.2 depicts four types of creep curves that have been generally
observed. 4 The shape of the creep curve is dependent on the initial condition

0.20

D

A

0.15

￿ ￿ ￿ ￿ ￿ ￿

B

0.10

C

0.05

0.00

0 200 400 600

Time, t

800 1000

3.2 Illustration of different types of creep curves. 4