## 4 Results and Discussion
Figure 18: _Cont._  
Figure 19: T6 Al4V fracture surface calibrated through [PERSON]’s tests, CC model: (**a**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**b**) 2D view of fracture surfaces for Driemeier surface; (**c**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**d**) 2D view of fracture surfaces for Mohr surface. Figure 18: T6 Al4V fracture surface calibrated through [PERSON]’s tests, MMC model: (**a**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**b**) 2D view of fracture surfaces for Driemeier surface; (**c**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**d**) 2D view of fracture surfaces for Mohr surface. Side-by-side comparisons of the surfaces coming from the different calibrations for the three damage models (MMC, CC, and HC) and for both materials (Ti6 Al4V and 17-4 PH) are provided in Figure 21. Overall, the five fracture surfaces identified by the different combinations of experimental points are very close to each other. This suggests that no significant differences exist in terms of ductility prediction when a damage model is calibrated through the [PERSON]'s multiaxial specimens in place of the conventional PS and Tors specimens. The conventional surfaces are slightly higher than the other ones, which is a consequence of the plastic strain values of the PS and Tors tests being higher than those obtained with the multiaxial specimens, though belonging to the same region of the \(T\),\(X\) domain. Although the surface obtained with conventional tests was chosen as the reference surface for the comparisons, it is worth a reminder that the surface obtained using all tests should be the most accurate, being based on the largest amount of data. Compared to this last fracture surface, the difference with the [PERSON]'s calibrated surfaces is even smaller. This further confirms that the selected damage models can be calibrated adopting [PERSON]'s multiaxial specimens in place of conventional specimens such as PS, Tors, or other multiaxial specimens, with the advantage of using a standard uniaxial testing machine for the experimental phase. Furthermore,  
Figure 20: Ti6 Al4V fracture surface calibrated through [PERSON]’s tests, HC model: (**a**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**b**) 2D view of fracture surfaces for [PERSON] surface; (**c**) 3D fracture surface calibrated using [PERSON] specimens plus RB and RNB; (**d**) 2D view of fracture surfaces for Mohr surface. regardless of whether the [PERSON] or [PERSON]'s specimens are used, the resulting surfaces are similar in terms of predicted ductility. Figure 21: Comparison of Ti6 Al4V and 17-4 PH fracture surfaces coming from calibrations using different combinations of tests (1–5, see the list at the end or materials and methods section) for the three damage models. Fracture surfaces of: (**a**) Ti6 Al4V for MMC model; (**b**) 17-4 PH for MMC model; (**c**) Ti6 Al4V for CC model; (**d**) 17-4 PH for CC model; (**e**) Ti6 Al4V for HC model; (**f**) 17-4 PH for HC model. It should be noted that for all the proposed combinations, the damage surfaces appear to steepen for low values of triaxiality and values of the deviatoric parameter tending to one. The predicted values of plastic strain at failure by damage surfaces in these zones may provide unreasonable results. The reason lies in the lack of experimental evidence in those areas, namely the unavailability of experimental tests characterized by \(T\) and \(X\) around 0 and 1 leads to a poor calibration in these areas. Currently, no specimens have been studied in the literature that provide the capability of investigating stress states with these particular \(T\) and \(X\) values. Surely this problem could represent an interesting opportunity for future studies.