## 3 Results and Discussion  
### Comparison of Experimental and Numerical Results  
The initiation of a controlled fracture in a defined place was achieved thanks to the precisely defined position of the sample on the lower tool. The DIC system was used for analyzing the strain distribution on the sample just before the fracture. Then, the determined strain at the moment of fracture was correlated with the time recorded on the press. Based on this, the displacement and the force needed to induce the fracture were determined. These data were used to accurately reproduce the process in the numerical simulation and to determine the fracture criterion values for the analyzed samples. More specifically, Figure 7 shows the results of the study on the rheological characteristics of the tested material at elevated temperature. Two positions of the sample relative to the camera of the DIC system were analyzed. The obtained results were then compared with the results of the computer simulations carried out with the use of the commercial QForm software package. In both analyzed cases, the formed cracks were observed near the top corners of the specimens, as expected. In the case of the sample with its edge facing the camera (Figure 7a-d), the strain concentration was visible already at the initial stage of compression (Figure 7a,b). In the final phase of the tests, the strain accumulated near the corner of the specimens (Figure 7c,d), where finally cracking occurred. The results of DIC correspond to the results obtained from the numerical simulation as the strain distribution, values and areas of concentration of high strains were similar. In the next case of the analysis of the sample, with the orientation in relation to the camera position (with the side wall towards the camera--Figure 7e-h), similar conclusions can be drawn. In the final phase of the test, a clear localized intensification of strains is visible (Figure 7), which initiates the material cracking process in the corner of the sample. Comparing the two positions of the sample in relation to the DIC camera, it can be stated that in the case where the sample is directed with its edge towards the camera, it is possible to simultaneously observe two side walls. This minimizes the risk of not capturing the crack initiation point when recording the test. Apart from comparing the results of DIC analysis and numerical simulations, the focus was also set on controlling the parameters of the compression test itself. It should be emphasized that the models adopted in the computer simulation describing the tested materials and process, friction conditions, device kinematics, etc., were largely reflected in the laboratory test conditions. This is confirmed by the comparison of the forging load curve during the test and the experiment, which showed a high similarity, capturing not only the force variations during different test stages but also the magnitude of load with a high accuracy (Figure 8a). Figure 8: Comparison of the deformation process parameters under the experimental conditions with those calculated in the computer simulation: (**a**) change in load with time during compression for Grade A and B, (**b**) principal strain change at the point of fracture—Grade A and B.  
Figure 7: The results of the compression test of samples with: (**a**–**d**) the side edge facing the camera—Grade A, (**e**–**h**) the side wall facing towards the camera—Grade B. It can be seen that the increase in the load value during the process is close to linear. This is due to the shape and size of the working surface of the upper tool. In the case of the conventional upsetting test, this graph takes the parabolic form. The high agreement of the results between the experiment and the simulation is important, which confirms the correctness of the assumptions made.