Geology Reference
In-Depth Information
Fig. 4.2
Stages of a creep
test
Strain
ε
ε 0
Primary
Secondary
Tertiary
Time T
current cross-sectional area, and e to be natural strain, Sect. 4.2 ). In a creep test, it
is convenient to distinguish four stages (Fig. 4.2 ), characterized as follows:
1. The instantaneous strain e 0 : This is the strain that occurs during the application
of the stress and in the interval before practical strain measurements as a
function of time are begun. It corresponds more or less to the strain determined
in a short-term stress-strain test but of course it depends on the rate at which the
stress is applied and it includes some strain that might be counted as creep
strain in the initial stages of an ideal creep test in which the stress were applied
strictly instantaneously and strain recording begun immediately.
2. Primary creep. The first stage of the recorded creep curve in which the strain
rate normally decreases continuously but at a diminishing rate, as shown in
Fig. 4.2 . However, in some cases, there may be an incubation period of little or
no creep followed by an acceleration in strain rate before the stage of dimin-
ishing strain rate is entered, giving an overall sigmoidal shape to the creep
curve in the primary creep stage.
3. Secondary creep. A stage in which the strain rate reaches its minimum value
and during which the rate of change of strain rate is relatively small or sensibly
constant.
4. Tertiary creep. A final stage of accelerating creep, representing the onset of
some sort of instability leading to failure; less commonly met in compression
tests than in extension tests.
Leaving aside the consideration of tertiary creep and of any initial incubation
stage, it is commonly assumed that the creep curve can be represented as the sum
of three terms, comprising the instantaneous strain e 0 and two time-dependent
forms, a transient creep term B ðÞ; and a steady-state term e s t (e s a constant), as
follows:
e ¼ e 0 þ B ðÞþ e s t
ð 4 : 5 Þ
Search WWH ::




Custom Search