Steels_ Metallurgy and Applications, Third Edition

112 Steels: Metallurgy and Applications

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All dimensions in mm

Figure 1.111 Compact tension specimen used for crack growth test 167

geometry and the difference between the minimum and maximum loads applied.
During the test the crack length is monitored using a travelling microscope and
a plot of crack length versus number of cycles enables the crack growth rate
per cycle to be obtained. A fatigue threshold is defined as the maximum stress
intensity factor below which an existing crack does not propagate. It represents
a fatigue limit related to crack growth.

The cyclic stress-strain curve

The method of obtaining the cyclic curve mentioned above involves the use
of several specimens. Alternative methods have been proposed for obtaining
the cyclic curve from a single specimen. Some of these involve the waveforms
illustrated in Figure 1.112.
It was indicated above that the cyclic stresses developed as a result of the
application of a constant cyclic strain may often vary during the test. The impli-
cation is that the cyclic stress-strain curve is often different from the monotonic
tensile curve obtained from a conventional tensile test. The monotonic curve
would correspond with the first part of the first cycle in a fatigue test. Examples
of cyclic softening, cyclic hardening and cyclic stability, or a mixture of all three,
have been observed for various types of steel when plastic strain is involved in
the test but the strength is stable when the deformation is substantially elastic. It
is found that the effects tend to be associated with the strengthening mechanisms
used in the steel. 168 Strengthening by solid solution effects, grain refinement and
precipitation, for example, tend to be stable in the presence of plastic defor-
mation whereas strengthening by temper-rolling or bake-hardening effects tend
to be unstable. It must be remembered, however, that components made from
bake-hardening steels are not subject to plastic deformation over their main area
during use. The increase in dent resistance arising from the bake hardening will
not, therefore, be affected by fatigue. Studies of the cyclic stability of steel cut