PSI - Issue 59

Pavlo Bulakh et al. / Procedia Structural Integrity 59 (2024) 253–258 Pavlo Bulakh / Structural Integrity Procedia 00 (2019) 000 – 000

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Keywords: LM-hardness method, plastic deformation, damage accumulation, necking.

1. Introduction It is known, that the localization of plastic deformation develops in stages at different scale levels of the material Lebedev et al. (1982). It occurs before reaching the proof stress, and then manifests itself at the macro level and ends when the ultimate strength is reached. At the same time, the rate of damage accumulation during material deformation depends on the type of stress state, which is quite convincingly confirmed by the experimental data for heat-resistant steels presented in Giginyak et al. (2003), Troshchenko et al. (1993), Pisarenko et al. (1969). We assume that plastic deformation causes changes in the metal structure of different magnitudes and directions when deformations are in different directions – in the longitudinal and circumferential (diameter) cross-sections of the structural element. Therefore, metal structures with different levels of homogeneity can be formed in deformed structural elements. Such features of the kinetics of the homogeneity of the deformed metal influence the development of the process of reaching the limit state of the material at the moment of its fracture. As stated in Pysarenko and Lebedev (1976), reaching the limit state of a metal is due to its ability to resist both normal and tangential stresses. The specified stresses cause corresponding deformations of shear and separation in the material of the specimen, different in direction and magnitude, at the moment of reaching the limit state in the most deformed part of it, in the region of the developed deformation neck. At the same time, the deformation, which decreases in absolute value, develops along the diameter of the deformation neck ( ε θ ), but the increasing longitudinal deformation ( ε z ) in the neck develops in the direction of action of the normal tensile stress. Such a deformation process can affect the change in the homogeneity of the metal at the moment of reaching the limit state. Based on the influence on the process of the above-mentioned difference in levels of homogeneity in different directions for the deformation neck, it is possible to express some considerations regarding the possibility of assessing the contribution of tangential and normal stresses to the process of reaching the limit state of the metal. 2. Theoretical foundations In our research, as a damage parameter, we take the relative value of the metal homogeneity coefficient, which is determined by the LM-hardness method, for each specimen DSTU 7793-2015. We assume that the condition of the loosening of the material and the generation of discontinuities in it creates differences in the homogeneity of the material, which arise in the process of plastic deformation, in particular under mechanical influences. That is the relative value of the homogeneity coefficients m rel. of the studied metal in different directions of deformation (longitudinal and transverse). m rel. = m i /m 0 , (1) were m i – homogeneity coefficient of the material of the test specimen, calculated from hardness measurements, m 0 – the homogeneity coefficient, calculated from the measurements of the hardness of the tested specimen before the beginning of the tests (metal in its initial state). We accept as the main parameter of damage (looseness) of the material at the moment preceding the macro fracture to use the relative values of the coefficients of homogeneity of the metal in the longitudinal and diametrical directions of the specimen. They are determined experimentally in the minimum diameter section of the formed deformation neck of the specimen. In the direction along this diameter m rel. θ and corresponds to it the relative value of the homogeneity coefficient m rel.z along the axis of the specimen based on the measurement of the deformation of the specimen under conditions of uniaxial tension: m rel.z = m iz / m 0 ; m rel. θ = m i θ / m 0 , (2)