Environmental Engineering Reference
In-Depth Information
Figure 3.38
Constant strain testing for stress corrosion cracking: (a) U bend; (b) C
ring; (c) bent beam; (d) tensile.
Different metallurgical and electrochemical variables affect
t
f
, but it is difficult
to ascertain these effects separately on
t
i
and
t
p
. In smooth specimens usually
t
i
t
p
, whereas in practical situations a pit or a surface roughness feature acts
as an already initiated crack and the question of its propagation under different
variables assumes more importance. The use of precracked specimens, notched
or fatigue-precracked, and the application of linear elastic fracture mechanics
(LEFM) technique in stress corrosion crack propagation have evolved as a conse-
quence.
Tests on statically loaded precracked samples
are usually conducted with a
constant applied load and the velocity of crack propagation as a function of stress
intensity factor,
k
, is measured. The value of
k
is calculated from
k
σ
C
1/2
,
where
is the applied stress and
C
is the crack length. Figure 3.39 gives the
schematic representation of a typical
da
/
dt
versus
k
plot. Three regions in the
plot are identified as stage I, II, and III. No crack propagation is observed below
some threshold stress-intensity level
k
ISCC
. In stage 1, the crack propagation rate
increases rapidly with the stress-intensity factor. In stage 2, the crack propagation
rate approaches some constant velocity, referred to as the plateau velocity, which
is characteristic of the alloy-environment combination and is a result of rate-
limiting diffusion of the reactant species to the crack tip. The effect of variations
in composition, changes in heat treatment, electrochemical variables, and changes
to the environment gets reflected in the plateau velocity and such data become
helpful in alloy selection. Alloys that have a low-plateau velocity can be chosen
σ
