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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6 b , c ). The smoothness of the walls and the general view of the resulting cracks confirm the brittle mode of crack opening by cleaving of crystallites. Accordingly, at -60 ° C the zone II metal showed the lowest toughness. . a b

c

d

Fig. 4. Coalescence of micropores ( a , b – inside the dashed circumference) and their development into clefts ( b , d ) in metal of zone III at test temperature -20 °C ( a , b ) and at test temperature -60 °C ( b , d ). The arrow shows the cleft formed by the merger of several pores.

a

b

d

c

Fig. 5 . Fracture surfaces of zone I ( a , b ) and zone II ( c ) metal specimens, tested at -20 ° С .

Obviously, this may be due to a smaller number of both large and small pores. Strictly speaking, it is necessary to consider the strain-stress state and metal hardening near the pore and take into account their shape, orientation, etc. The question of the formation and evolution of pores in externally loaded metals and their influence on properties is one of the core points of strength theory. The results of this article show that the properties of spring steel in the considered operating conditions are really largely determined by the evolution of microporosity that has stipulated the satisfactory resistance to the propagation of both fatigue and brittle cracks. Thus, due to the microdeformation processes as a mechanism helping the material for adapt to loads, especially in the vicinity of the pores, the considered spring steel has a certain immunity to crack propagation. At the same time, taking into account its use in elastic elements, the fabrication technology should provide a high relaxation ability of steel, that is, sufficient resistance to small plastic strains while minimizing porosity.