4. Results and discussion
4.1. Comparison of theoretical models for predicting fracture loci
Fig. 6 provides a schematic comparison of the theoretical estimate
of the fracture loci (FFL and SFFL) obtained from ductile
damage and void coalescence/growth models associated to crack
opening modes I and II (Sections 2.1 and 2.2) with the theoretical
estimates provided by fracture toughness based models with
values determined from double-notched test specimens loaded in
tension and plane torsion (Section 2.3).
As seen in Fig. 6, the fracture loci obtained from fracture toughness
tests in crack opening modes I and II (Eqs. (13) and (14)) are
represented in the principal strain space as two different ellipses
(ε = f(RI) and ε = f(RII)) with major axis along pure shear ε1 = −ε2
and minor axis along equal biaxial stretching ε1 = ε2. This allows
concluding thatthe slope ‘+1’ ofthe in-plane shear fracture forming
line (SFFL) associated to crack opening mode II and to the ductile
damage criterion based on the ratio of the shear stress to the effective
stress (Eq. (7)) exactly matches the slope of the derivative to