Machines 2022, 10, 916. https://doi.org/10.3390/machines10100916 www.mdpi.com/journal/machines Article Upgrade of Biaxial Mechatronic Testing Machine for Cruciform Specimens and Verification by FEM Analysis Ľubica Miková 1 , Erik Prada 1, *, Michal Kelemen 1 , Václav Krys 2 , Roman Mykhailyshyn 3,4 , Peter Ján Sinčák 1 , Tomáš Merva 1 and Lukáš Leštach 1 1 Department of Industrial Automation and Mechatronics, Faculty of Mechanical Engineering, Technical University of Košice, Park Komenského 8, 04200 Košice, Slovakia 2 Faculty of Mechanical Engineering, VSB-Technical University of Ostrava, 70800 Ostrava, Czech Republic 3 Department of Automation of Technological Professions and Manufacturing, Ternopil Ivan Puluj National Technical University, 46001 Ternopil, Ukraine 4 Texas Robotics, Cockrell School of Engineering, College of Natural Sciences, The University of Texas at Austin, Austin, TX 78712, USA * Correspondence: erik.prada@tuke.sk; Tel.: +421-908-310-209 Abstract: This article deals with the modernization of an existing loading system for the analysis of elastic–plastic properties of sheet metals in plane stress. The identification of the beginning of plastic deformation of sheet metal in plane strain is important in the cold pressing of sheet metal and in the assessment of the load capacity of thin-walled structures in the automotive and aerospace industry. The design of the control structure of the hydraulic part of the loading system for cross testing was carried out to automatize the whole process of experimental evaluation. For this purpose, propor- tional pressure-reducing valves together with control electronics were designed. Thus, the loading system is a control system for which a control algorithm has been designed and implemented on a PC. A computer simulation was performed to verify the functionality of the load system. An FEM simulation was performed to verify the correctness of the proposed numerical models and to con- firm the experimental results. A numerical nonlinear model of the selected material was applied for the specification of plastic deformations. From the results, it is possible to state the appropriateness of the used models as well as the appropriateness of using modernized equipment for subsequent analysis of the plastic deformation of cruciform specimens. Keywords: sheet metal; biaxial tensile test; transfer function; low-pass filter; plastic deformation; FEM analysis 1. Introduction Identification of the initial plastic deformation of sheet metals in plane stress is im- portant after cold rolling and in the assessment of the load carrying capacity of thin- walled structures. The development of numerical computational methods and mathemat- ical modelling methods requires knowledge of the behavior of sheet metal during the transition from the elastic to the plastic region at different stress states. Knowledge of the above phenomena is important due to the expanding use of sheets with higher strength properties in the manufacturing of load-bearing elements, especially in the aeronautics, automotive, construction, and mechanical engineering industries. These trends seek to reduce the weight of structures while maintaining (or even increasing) their strength and stiffness parameters [1,2]. The conditions under which a material enters a plastic state (plastic deformation be- gins) are called plasticity conditions [3]. Since the cold rolling of sheet metal changes the statistically random arrangement of crystals into an arrangement with a preferred orien- tation, textures are formed that cause textural anisotropy of the sheet metal during plastic Citation: Miková, Ľ.; Prada, E.; Kelemen, M.; Krys, V.; Mykhailyshyn, R.; Sinčák, P.J.; Merva, T.; Leštach, L. Upgrade of Biaxial Mechatronic Testing Machine for Cruciform Specimens and Verification by FEM Analysis. Machines 2022, 10, 916. https://doi.org/10.3390/ machines10100916 Academic Editor: Dan Zhang Received: 9 September 2022 Accepted: 4 October 2022 Published: 9 October 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).