PSI - Issue 59

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

256

4

Directly before the beginning of the tests, mechanical polishing of the working surface of the specimen was carried out based on 20 mm along its length. Surface cleanliness for hardness measurement met the requirements of the COMPUTEST operating instructions and was 1 μm. The load was carried out at a constant speed up to a certain amount of longitudinal deformation, after which the specimen was unloaded at the same speed and the hardness of the metal was measured on the surface of the working part of the specimen within the limits of the base for measuring the deformations along the creation line, as well as along the circle of the diameter (Fig. 2). The specified procedure was repeated at all subsequent levels of loading until the deformation neck of the specimen was formed at the longitudinal deformation  ε zp = 13.5 – 18.5%.

Fig. 2. Directions for measuring the hardness of the specimen’s metal: 1 – along the creation line; 2 – along the circle.

3.2. Experimental Results and Discussion We assume that at the initial stage of loading the specimen, due to the inconsistency of the geometric shapes of the structural elements of the metal, separate stress peaks occur (Giginyak et al. (2010), Giginyak and Bulakh (2012)). These stresses lead to local plastic deformations with subsequent loss of plastic resistance of individual structural elements of the metal. further loading is accompanied by an increase in the number of such defects, but the growth rate of their number in the process of deformation decreases due to the mutual adaptation of structural elements. This period corresponds to stable elastoplastic deformation at the stage of material strengthening, and accordingly, we have a constant volume of metal cross-section of the specimen V = const . In a certain interval of macro-deformations, the structure is saturated with defects to a level characteristic of this material. As a result of the merging of individual pores and cracks of structural elements, the increase in the number and size of discontinuities, micro-looseness is replaced by looseness. The loosening rate begins to increase with the growth of macro-deformation and, ultimately, leads to the fracture of the specimen. The analysis of the results presented in Table 1 shows that for each tested steel at all values of the longitudinal deformation reached at each stage of the load, the values of the relative coefficients of homogeneity will be different along the axis of the specimen and the ring in the neck of the specimen. At the same time, a stable trend is maintained: at all levels of the achieved deformation, the absolute value of m rel.  is always greater than m rel.z . Table 1. The nature of the change in the homogeneity coefficient of the investigated steels in the process of uniaxial tension.

No specimen

Material

m 0

m z

m θ

m rel.z

m rel. θ

m z /m θ

ε zp , %

N1

143.44

26.591 87.217 29.286 49.436

52.766

0.185 0.484 0.163 0.259 0.185

0.367

0.503 0.698 0.538 0.584 0.503

18.05

6.06 18.0 14.0 16.8 18.7 6.73

124.782 54.435

0.693 0.303 0.443 0.369

N3

179.97

15Kh2MFA

N4 N6 N1 N2

190.69

84.567

195.3 99.27

36.29 13.94 59.44

72.22 23.22

0.14

0.24

0.6

10GN2MFA

105.59

78.048

0.562

0.739

0.761