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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were m iz – the homogeneity coefficient, calculated from measurements of the hardness of the specimen along its axis based on the measurement of the established limit value of longitudinal deformation ε zp ; m i θ – the homogeneity coefficient, calculated from measurements of the hardness of the tested specimen at its smallest diameter in the deformation neck after reaching the set amount of longitudinal deformation ε zp . We use the obtained values of the relative homogeneity coefficients in (2) m rel.z and m rel. θ as the initial values for the calculation of the participation of the different in direction and absolute values of the deformation in the process of reaching the limit state of the metal at the structural meso-level. m rel.z / m rel. θ = m μ , (3) Based on the above considerations, experimental studies of the kinetics of metal damage in the area of the specimen neck formed during uniaxial stretching were performed in the tangential (annular - ε θ ) and longitudinal (along the axis of the specimen – ε z ) directions. The validity of such an approach to the experimental evaluation of metal damage is confirmed by Lebedev and Chausov (2004). They also proposed an experimentally substantiated model of the accumulation of damage in metal, according to which the main parameter of the current state of the metal, that is, the degree of its loosening, is matched by the coefficient of transverse deformation at the appropriate stage. The results obtained by Lebedev and Chausov (2004) show that in a certain interval of macro-deformations, the body is saturated with micro defects to some characteristic level for this material. The number of micro-defects is replaced defects by due to the growth of their sizes, including by the merging of individual micro-pores and micro cracks. The rate of structure loosening with the growth of macro-deformation begins to increase and eventually leads to the formation of a macrocrack. The above considerations lead to the conclusion that it is possible to establish the effect of shear deformation on the damage parameter adopted in our studies - the relative homogeneity coefficient m rel. We assume that under the conditions of uniaxial tension before the fracture of the specimen, the interaction of transverse and longitudinal deformations is most fully realized in the smallest diameter of the deformation neck. At the same time, we take into account that these deformations have different signs, and the strain intensity of the specimen ε i for both directions of deformation remains the same. 3. Experimental Technique 3.1. Specimens, Material and Experimental Equipment The tests were performed using the SNT-8U complex in automatic mode up to the specified value of longitudinal deformation ε z , which allowed loading and unloading with a constant rate of deformation έ z =0.025% s -1 . The tests were performed on solid cylindrical specimens made of 10HN2MFA steels. 15X2MFA and 15X2NMFA (Fig. 1) under stepwise uniaxial tension at room temperature. At each step, after unloading, the required number of hardness measurements were performed using a COMPUTEST (Switzerland) hardness tester. Specimens for experiments were made from blanks of each steel of one metal melt.

Fig. 1. Specimen for testing.