## 4 Results and Discussion  
The main findings are presented in the following order: the results of the experimental campaign, the numerical data extracted from FEA, the outcome of the 2D-DIC analysis, and finally the calibrated fracture surfaces of the different damage models along with a detailed comparison between them. Figure 7 shows the global load-displacement curves measured experimentally and calculated numerically for the two alloys, with the red dots representing the onset of ductile fracture. Of the three repetitions of each test, for the sake of brevity, only the average experimental values are reported in the figures. As for the global response, the curves obtained experimentally match those obtained via FEA. For brevity, only the results obtained by DIC-2D relative to instant of fracture of the first repetition for the [PERSON]'s specimen are shown, for the two materials (Figures 8-15). It is worth noting that in all cases, the overall distribution of the equivalent plastic strain obtained via DIC-2D is consistent with that obtained via FEA, with the local values corresponding fairly well. For both specimen types, a central zone of high plastic strain of significant extent is observed, with the deformation decreasing as one moves away from the center. The most critical point of this region, where fracture is expected to occur, is indicated by a red arrow in Figures 8-15. Regarding the results of the Mohr15 specimen obtained via FEM (Figures 11 and 15), a concentration of plastic strain in the external side regions can be observed, with values similar to those present in the central test area; this should not be an issue, however, since the size of this concentration areas is very limited compared to that of the central zone and is furthermore not confirmed by the results obtained via DIC-2D. Therefore, the central zone of the specimen is considered at all effects as the most critical one. The FEM-DIC comparison confirms the accuracy of the FE simulations also from a local standpoint. Using the FE models, the three principal stresses and the accumulated equivalent plastic strain at the instant of fracture were evaluated at the most critical point of each specimen. From these quantities, the triaxiality parameter \(T\) (Equation (2)) and the devia  
Figure 7: Experimental and numerical load–displacement curves for 17-4 PH and Ti6 Al4V alloys. The red dots indicate the onset of plastic failure. toric parameter \(X\) (Equation (3)) were derived; Table 1 summarizes the local quantities that have been obtained. These data represent the coordinates of specific points in the space (\(T\), \(X\), \(\varepsilon_{f}\)) that are employed as input data in the minimization algorithm of the calibration procedure. The average values \(T_{avg}\) and \(X_{avg}\) calculated via Equations (6) and (7) were assumed, presenting \(T\) and \(X\) limited variations with plastic strain, as confirmed by the low standard deviation values reported in Table 2. This ensures the proportional or quasi proportional hypothesis guaranteeing the validity of Equation (5). The trend of \(T\) and \(X\) with respect to the accumulated equivalent plastic strain is also reported in Figures 16 and 17. The graphs of the evolution of \(T\) and \(X\) with plastic strain were plotted on a \(y\) axis scaling from 0 to 1, which is the typical range exhibited by these quantities, in order to highlight the quasi-constant evolution. Table 3 provides the experimental displacement and load values at failure (\(d_{exp}\), \(F_{exp}\)), the load at failure obtained via FEM (\(F_{FEM}\)) and the average value of the plastic deformation in the critical zone of the specimen calculated via DIC (\(\varepsilon_{f-DIC}\)). Figure 8: FEM-DIC comparison using Ansys Mechanical APDL for Driemeier Pos10, Ti6 Al4V. The area of fracture is indicated by a red arrow, while in the legend the red box indicates the critical value of plastic strain to failure. The same applies to the following figures.