Civil Engineering Reference
In-Depth Information
Stress
s
j
f
b
h
d
a
F
IGURE
8.17
Strain hardening of a
material
0
c
g
Strain
ε
8.4 S
TRAIN
H
ARDENING
The stress-strain curve for a material is influenced by the
strain history,
or the loading
and unloading of the material, within the plastic range. For example, in Fig. 8.17 a
test piece is initially stressed in tension beyond the yield stress at, 'a', to a value at
'b'. The material is then unloaded to 'c' and reloaded to 'f' producing an increase in
yield stress from the value at 'a' to the value at 'd'. Subsequent unloading to 'g' and
loading to 'j' increases the yield stress still further to the value at 'h'. This increase in
strength resulting from the loading and unloading is known as
strain hardening.
It can
be seen from Fig. 8.17 that the stress-strain curve during the unloading and loading
cycles forms loops (the shaded areas in Fig. 8.17). These indicate that strain energy
is lost during the cycle, the energy being dissipated in the form of heat produced by
internal friction. This energy loss is known as
mechanical hysteresis
and the loops as
hysteresis loops.
Although the ultimate stress is increased by strain hardening it is not
influenced to the same extent as yield stress. The increase in strength produced by
strain hardening is accompanied by decreases in toughness and ductility.
8.5 C
REEP AND
R
ELAXATION
We have seen in Chapter 7 that a given load produces a calculable value of stress in a
structural member and hence a corresponding value of strain once the full value of the
load is transferred to the member. However, after this initial or 'instantaneous' stress
and its corresponding value of strain have been attained, a great number of structural
materials continue to deform slowly and progressively under load over a period of
time. This behaviour is known as
creep.
A typical creep curve is shown in Fig. 8.18.
Some materials, such as plastics and rubber, exhibit creep at room temperatures but
most structural materials require high temperatures or long-duration loading at mod-
erate temperatures. In some 'soft' metals, such as zinc and lead, creep occurs over
a relatively short period of time, whereas materials such as concrete may be subject
to creep over a period of years. Creep occurs in steel to a slight extent at normal
temperatures but becomes very important at temperatures above 316
◦
C.
