The path independence result of Eq.(12)is understandable because
the integrand of the critical damage DI
crit associated with crack
opening mode I (Eq. (6)) has the form (A + B/ˇ) and, therefore, it is
also independent from the loading path history.
The above analytical procedure and the resulting Eq. (12) can
also be applied to double notched circular test specimens loaded
in plane torsion because the critical damage DII
crit associated with
crack opening mode II(Eq.(7)) has also the form (A + B/ˇ). The only
importantdifference thatneeds to be takeninto considerationis the
utilization of different values of fracture toughness R for tension
and shear loading conditions because the energy to nucleate and
propagate cracks should vary as a function of the crack opening
mode.
In connection to what was said above, it is worth noting that the
necked down region can also be formed as a result of changes in
the relative ease of plastic flow in width and thickness directions
rather than on classical mechanisms based on the development of
unstable plastic deformation (Isik et al., 2015).
Now, by taking into consideration that fracture toughness is a
material property, which is not influenced by strain loading paths
and by combination of in-plane loading and bending effects, it follows
that Eq. (12) can be utilized to determine the fracture loci.
In this way, the effective strains at fracture εf for strain loading
paths giving rise to fracture by crack opening modes I and II can be
estimated from