failure by fracture in sheet metal forming. In other words, they
assumed the FFL and the SFFL to meet in a point.
However, a transition loci between the FFL and the SFFL is not
only likely to existinside the grey quadrilateral of Fig. 8 as its occurrence
makes sense due to the existence of infinite ellipses located
in-between the upper and lower ellipses corresponding to crack
opening modes I and II.
The above conclusion may justify future interest in extending
the overall procedure based on the utilization of double notched
test specimens loaded in tension and plane torsion to include the
double notched staggered test specimens loaded in tension that
were originally proposed by Cotterell et al. (1982) to produce fracture
in mixed modes I and II along the inclined direction, which
forms at an angle to the tips of the notches.