PSI - Issue 36

V. Kharchenko et al. / Procedia Structural Integrity 36 (2022) 277–283 A.V. Sereda et al. / Structural Integrity Procedia 00 (2021) 000 – 000

278

2

1. Introduction and Problem Statement The main quality criteria for structural materials are mechanical characteristics. Different methods and types of mechanical tests are used to determine them. One of the basic types of material testing is uniaxial tension. Based on its results, the characteristics of strength and plasticity are determined, which are basic ones in the strength calculations for machine components and structural elements. The static uniaxial tension test method is regulated by standard ISO 6892-1: 2019 in Ukraine. These tests are labor-intensive, time-consuming, and costly since they involve the production of standard specimens. Two types of specimens are used to determine the strength characteristics of sheet products made of high-strength steel: specimens with a cylindrical gauge length and specimens with a flat gauge length. Manufacturing of such specimens is connected with difficulties of machining due to high metal strength characteristics. Therefore, it is necessary to use more high-tech and expensive methods of machining, in particular laser, plasma, or EDM, not to expose the specimen to undesirable thermal and mechanical effects that can change the structure of the metal and its mechanical characteristics both in the surface layers and in the entire volume. In addition, uniaxial tensile testing of high-strength steel specimens requires powerful special experimental equipment. One of the alternative types of mechanical tests on metal products is hardness tests. The most common metal hardness measurement, which is based on the process of indenter pressing by a static load, is the Brinell method according to GOST EN ISO 6506-1: 2019. As a result, only the surface layer of metal within the indenter contact zone undergoes deformation, while an imprint remains on the surface of the sheet metal, which, if necessary, can be easily aligned. The hardness value is given in certificates for metal products. For many metals and alloys, the correlation dependencies are experimentally established between the Brinell hardness and tensile strength characteristics, which have mainly a linear nature. Based on them, the tables for transforming the hardness values into strength and yield limits have been made and used in engineering practice to assess the mechanical characteristics of metal structures and equipment (1.2.1.02.019.1121-2016, SOU-N NAEK 133:2016). Noteworthy is that the correlation dependencies, which were obtained at different times, are constantly analyzed and refined. A large number of existing dependencies, as well as a significant discrepancy between them in different manuscripts, complicates the calculation of strength characteristics using the hardness measurements and their practical application. In addition, it is impossible to establish a single correlation dependence for a wide class of materials, and in some cases, such dependences may differ depending on the structure and stress state (for a particular steel grade), which is influenced by the delivery condition and heat treatment mode (Matuynin and Marchenkov, (2016), Katok et al. (2014)). The test methods demonstrate considerable errors, especially the measurement of the obtained hardness and strength values. There is a need for the development of new methods that ensure more accurate determination of the mechanical characteristics with lower expenses for testing. The simplicity of the method and technical means of the hardness measurement of structural materials makes them very convenient for quality control of metal products and elements of complex structures for special purposes. At the same time, the needs of production constantly put new challenges for scientists to expand the field of application, improve the accuracy of hardness control, and development of methods for determining the mechanical characteristics via non-destructive testing. In turn, these circumstances create requirements for the development of new express methods for the evaluation of mechanical characteristics, which increase the reliability and efficiency of metal control in the production of military products. The authors of the paper believe that the instrumented indentation is more accurate and less expensive as one of the non-destructive control methods. It is based on the discontinuous registration of the material local deformation via indenter in the form of the diagram « load on indenter F – indenter penetration depth h » (SОU -56-28-2018). The advantage of this method lies in the possibility to directly register the entire process of discontinuous local material deformation via indenter. It also demonstrates the simplicity and high speed of testing due to its automatization. Moreover, the correlation dependencies for the determination of the strength characteristics by the indentation diagrams are valid for a wide class of materials. The goal of this paper is to perform complex investigations on the determination of the strength and yield limits of high-strength steels from the Brinell hardness measurement results and illustrations of the instrumented