Environmental Engineering Reference
In-Depth Information
Fracture
Primary
Secondary
Teritary
I
II
III
ε
0
Time,
t
3.1
A typical creep curve.
3
the yield strength of the material, the instantaneous strain is composed of a
plastic strain component.
The primary creep (also known as transient creep) region, as the name
suggests, describes the fi rst or initial stage of creep deformation and corre-
sponds to period 'b' in Andrade's work.
1
Such a region is characterized by a
strain rate decreasing with time. The decrease in strain rate continues until
the secondary stage (also known as steady-state creep) is attained. In the sec-
ondary creep region (period 'c' in Andrade's study) the strain rate of defor-
mation remains constant. This is evident in Fig. 3.1 with the secondary creep
region described by a straight line indicating a constant slope. The secondary
stage strain rate is the minimum strain rate of the creep curve. The useful
creep life of most engineering materials is generally estimated from second-
ary stage creep strain rate values. However such a methodology might not be
applicable for materials which have a large primary creep region or where
the tertiary creep region completely dominates the primary and secondary
creep stages. The tertiary creep region is the last stage of creep deformation
and concludes with the failure of the material. In the tertiary creep regime
(as identifi ed by Hanson and Wheeler
2
) the material undergoes deformation
at very high strain rates. The tertiary stage of the creep curve usually occurs
over signifi cantly smaller time periods in comparison to the primary and the
secondary stage, and is often regarded as 'fracture' mode.
Nature of the creep curve
The previous section described the different regions of a creep curve and
their corresponding characteristics. Mechanistically, the creep curve is
a result of the changes occurring in a material at a microstructural level.
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