Environmental Engineering Reference
In-Depth Information
engineering materials such as metals and alloys are characterized by large
scale plastic deformation leading to failure. The extent of deformation is
controlled by intrinsic factors such as bond strength, presence of secondary
phases and defect concentration. At the same time extrinsic factors such as
applied loads, temperature, deformation rates and geometry of the structure
also determine the amount of plastic deformation. It has been well estab-
lished that high applied loads and temperatures generally accelerate the
rate of plastic deformation. This is because high temperatures and stresses
provide the necessary activation energy required for defects to overcome
barriers to plastic deformation. While plastic deformation at room tempera-
ture or low homologous temperatures ( T / T m ) occurs when the applied stress
exceeds the yield stress
y , deformation at high temperatures can occur at
stresses signifi cantly smaller in comparison to the yield stress. The branch of
metallurgy which attempts at understanding material deformability at high
homologous temperatures and small applied stress has come to be known
as creep. The kinetics of deformation processes become important with
increasing temperatures and hence creep is defi ned as the time dependent
plastic deformation of a material under constant load or stress.
The earliest studies on time dependence of plastic strain were carried
out by Andrade. 1 The time dependence of elongation under tensile loads
was investigated at constant temperature. Andrade observed that the total
deformation could be divided into three periods: (a) immediate extension
upon loading (mainly elastic with relatively small instantaneous plastic), (b)
an initial fl ow which gradually disappears and (c) a constant fl ow which
takes place throughout the elongation. Subsequent studies by Hanson and
Wheeler 2 showed the presence of a period where the extension increases
continuously until fracture. This period was found to occur following
the period of constant fl ow and was understood to be due to decreased
cross-sectional area accompanying the elongation. At constant loads, the
cross-sectional area decrease leads to the increase in effective stress and a
corresponding increase in strain rate.
σ
￿ ￿ ￿ ￿ ￿ ￿
3.1.1 Creep curve
The time dependence of plastic strain is described by plots of strain against
time, also known as creep curves. A typical creep curve is shown in Fig. 3.1, 3
and consists of three different regions: the primary, secondary and tertiary
creep regions. Usually the primary creep region commences only after the
material has experienced an instantaneous strain,
0 which is a result of
sudden loading of the material and corresponds to period 'a' observed by
Andrade. 1 The instantaneous strain is composed of elastic (recoverable on
release of load), anelastic (recovers with time) and plastic (non-recover-
able) components. Though the applied stresses for creep are smaller than
ε
Search WWH ::




Custom Search