PSI - Issue 20

Valeriy Lepov et al. / Procedia Structural Integrity 20 (2019) 57–62 Valeriy Lepov et al. / Structural Integrity Procedia 00 (2019) 000–000

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b

a

Fig. 5. Brittle fracture and microcracks: (a) transgranular in the grain body; (b) typical for fatigue fracture.

The preliminary microstructural analysis of the breaks and studying of their character by an optical microscopy method showed that material of the studied locomotive wheel is strengthened and rather ductile at the same time. However under the cyclic variable loading in the low climatic temperature conditions the structure of material has been degraded, so that is characterized by the set of pores germination (Fig.5b). Besides the influence of such an external factor as low temperature is promote the prevalence of the brittle fracture mechanism. Obviously, in grain body the tension stress concentrate and the submicrocracks formed to initiate the splitting destruction. To estimate the influence of impact toughness drop due to ductile-brittle transition in locomotive steel, the stochastic modeling of crack growth was carried out by the algorithm proposed by Broberg (1990) and modified by Lepov et al (2016). The parameters to simulation takes from the optical and electron fractigraphy (fig. 3-5). The crack length varied from 10 to 50 mkm, with the distance about 50 mkm. The rresults of crack and pore growth visualization according to proposed and practically proved by Lepov et al. (2007, 2018) stochastic model in ductile and brittle case have shown on Fig. 6a and 6b subsequently.

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a

Fig. 6. Stochastic modeling results of ductile (a) and brittle (b) character of defects: growth in locomotive steel.

Despite of the same defects size and distance between them the results of modelling showed significant differences. The crack propagation velocity in ductile materials was about 0,22 meters per second, but in brittle close to 0,5 m/s. So the energy dissipation in brittle state (low temperature) seen to be much lesser than that in the ductile (room temperature). It is a reason for many failures of structures at low temperature eligible by many faults also. So the extreme environmental conditions such as low temperature and thermal shock could worsen the failure possibility.