## 5 Conclusions  
In this work, the results of experimental work and numerical simulations relevant to the determination of fracture initiation during hot forging process are presented. This experiment serves as an improved compression test to estimate the limits of the plastic deformation without failure and the formability of materials. Investigations were carried out for two different aluminum alloys and the results of the strain distribution obtained during the experiments by the DIC system were directly compared to the numerical results in an effort to explain the reasons for crack initiation and to successfully detect the position of fracture based on the strain values. From this study several conclusions can be drawn:* The developed numerical simulation model exhibited a high degree of accuracy compared to the experimental results of strain distribution for both aluminum alloys. Comparison of the experimental results from two different angles showed sufficient similarity with the numerical ones regarding the position of the highest strain concentration. * The prediction of forging load for the two workpiece materials and the prediction of strain variation on the position where the fracture occurs, showed sufficient levels of accuracy indicating that the developed model could represent the experimental findings in a reliable way. Thus, it can be used for fracture prediction and subsequently for the optimization of the hot forging process. * The comparison between two different fracture criteria for the simulation model indicated that although there are some differences in their distribution, they can both reliably determine the areas with a high probability of fracture, as was determined experimentally from the DIC results. * After the validity of the proposed approach was evaluated, it was applied toa more complex industrial case, that of backward extrusion which combines different, simpler steps of plastic deformation processes, involving a high probability of material fracture. It was proven that the simulation model successfully determined the regions with fracture risk and provided insight into the reason for the fracture as the high increase in strain in the internal surface of the workpiece in comparison to the strain in the external surface. * The main objective of the research, from the industrial point of view, is to determine the value of the crack criterion accompanying the initiation of cracks and to determine areas with an increased risk of cracks. On that basis, it is possible to determine the correctness of the applied technology. During detailed analysis of the process, it is possible to optimize the process parameters on this basis and, in critical cases, to change the technology, for example, by introducing an additional operation into the analyzed production technology. * In a future study, the authors plan to focus on the detailed microstructure analysis of the deformed specimens, including more inhomogeneous structures. Additionally, the residual stresses will be analyzed in the light of the industrial process. The results of these additional investigations will be later employed in order to improve the studied process by selecting the optimal parameters which allow the prevention of cracking. Conceptualization, L.L. and P.L.-G.; methodology, L.L. and P.L.-G.; software, L.L. and P.L.-G.; validation, L.L., P.L.-G. and N.E.K. ; formal analysis, L.L. and P.L.-G.; investigation, L.L. and P.L.-G.; resources, N.E.K. ; data curation, L.L. and P.L.-G.; writing--original draft preparation, L.L., P.L.-G. and N.E.K. ; writing--review and editing, L.L.,P.L.-G. and N.E.K. ; visualization, L.L. and P.L.-G.; supervision, L.L. and N.E.K. ; project administration, L.L. and N.E.K. ; funding acquisition, L.L. and P.L.-G. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.