PSI - Issue 30

S.P. Yakovleva et al. / Procedia Structural Integrity 30 (2020) 193–200 Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2020) 000–000

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Table 1. Microdamage accumulation coefficient, average microhardness and porosity characteristics of spring steel in zones I, II and III. Parameter I II III k 1.95 1.89 2.07 Н 100 , MPa 3590 3720 3796 V total = V fine + V crs /quantity 1.9 / 1348 1.8 /1081 2.2 /1021 V fine / quantity 1.2 / 1258 0.9 / 980 0.8 / 857 V crs / quantity 0.7 / 90 0.9 / 101 1.4 / 164 The results of impact toughness tests of samples with different levels of operational micro- and mesoscale damage are shown in Fig. 3. Despite structural damages, the spring metal shows a fairly high impact toughness at 20 °C. At low temperatures the impact toughness of the samples of groups II and III rather monotonously decreases; for metal zone I (zone of pre-fracture state) values of impact toughness at negative temperatures change slightly, and at -60 °C become higher ones of the zone II, which has lower rates micro- and mesoscale damage. Taking into account the ambiguous dependence of the impact toughness on micro scale damage and the fact that the most significant differences in the structure of the three groups metal consist in the number of fine and coarse pores, we can assume the determining influence of mesoscale damage on its improvement. To explain the mechanism of the positive influence of fine and coarse pores on the resistance of metal to nucleation and the development of brittle fracture, fractographic studies of Charpy specimens fracture surfaces were carried out. 3.2. Impact toughness of fatigue-damaged spring steel

Fig. 3. Change in impact toughness of metal of the spring different zones when the test temperature decreases.

3.3. Influence of porosity on fracture micromechanisms and brittle failure resistance of spring steel

Microfractographic analysis revealed the presence of numerous secondary cracks in the fracture surfaces of all impact samples (see Figs. 4, 5, 6). As shown on Fig. 4, they were formed by coalescence the pores. The microstructure of secondary cracks reflects the energy intensity of the fracture development process of each of the samples. For zone III metal, this process was accompanied by a noticeable plastic deformation – elongation of the pores until they merge and the deforming of the formed cavities walls. The negative influence of microscale damage the highest degree of which was registered in zone III was neutralized by the possibility of strain processes in the vicinity of many large pores found in that zone. Deformation of the material under the elongation and coalescence of pores was also observed in the fractures of the specimens of zones I and II tested at -20 ° С , but the secondary cracks that appeared in the pre-fracture zone I much shorter (see Figs. 5 a , b ). At -60 ° С their path becomes zigzag (see Figs. 6 a , b ), which is indicative of the material’s declining deformability. The union of pores and secondary cracks occurs by cracking of the jumpers between them (see Figs.