The growth of short fatigue cracks in a medium carbon steel.

Short crack growth behaviour was studied using a method
of plastic replication on hour-glass shaped specimens of a
medium carbon steel which were subjected to push-pull fatigue
testing at ambient temperature. Crack lengths were measured
from replicas using an optical microscope from which the growth
rate could be calculated.
A theory for short crack growth is presented which may be
expressed mathematically by the equation:-
= (for a < d) da
dN
where (a) is crack length, (d) is a characteristic dimension
between adjacent microstructural obstacles to crack
propagation, C1 is a function of stress or strain range and a is
a constant. For the medium carbon steel used in this study (d)
was equated to the ferrite band length which contained the
growing crack. This theory was used to model short crack
growth in an Aluminium alloy T6-7075 Al and for the medium
carbon steel used in this project.
iii
By using a second equation to describe "long" crack
growth of the form:-
da
dN
=
where C2 is a function of the applied strain range and 0 is a
constant, it was then possible to describe the complete history
of crack growth. By obtaining short crack growth data for
different stress levels, quantitative expressions of these two
equations were calculated from which fatigue lifetime
predictions could be made by integration of the equations for
any stress level. Using this method an estimate could also be
made of the percentage of fatigue lifetime spent in the
initiation and growth of short cracks.