PSI - Issue 13

Valeriy Lepov et al. / Procedia Structural Integrity 13 (2018) 1201–1208 Valeriy Lepov et al/ Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

The brittle fracture mechanics begins from experimental study (Griffith, 1920), that revealed an influence of invisible cracks for the materials strength. It has been shown then that the small cracks arranged in the surface layer (Ioffe, 1928) and could be eliminated by fine polishing. But only the last decades research shows the whole spectrum of sizes and forms of defects and significance of damage accumulation processes in structural materials due to both the experimental and numerical mechanics development, including the electron microscopy achievements and multiscale modeling approach, noted at the end of XX cenury (Fracture, 1968; Cherepanov, 1977; Broek, 1982), than in last years (Arkhangelskaja, Lepov, 2003; Lepov, 2008; Hell, 2015, Fracture at all Scales, 2017, and other). To apply the fracture mechanics and damage mechanics approaches for structure members especially taking into account the multiscale model of behavior of materials it is required to know the internal structure, mechanical properties and deformation states at different scales. Most of the mechanical properties and external loading aspects could be described by quantitative microstructural analysis [Ghomashchi, M.R., 1998; Lepov et al, 2016; 2017]. Nevertheless the problem of observation and description of microstructure at different scales is stay one of the significant to understanding how the multiscale model could be build and operate. The optical and electronic microscopy for surface of fracture has been used usually, like fractography, and it is very significant for fracture mechanism and failure reasons analysis. But for the model building not only qualitative analysis but also the quantitative characteristics and dependences are necessary. The importance of quantitative dependences for physical description in brittle and dynamic fracture processes was underlines by many authors (Cherepanov, G.P., 1974; Bell, J. F., 1973; and other). Last one gives evidence that even in homogeneous materials at normal temperature the plastic deformation is heterogeneous and should include the second order phase transition not to mention the anisotropic defective steel and low temperature conditions. The paper also underlines the significance of stochastic approach for crack propagation modeling by mechanism of small defects coalescence (Broberg, 1990; Lepov et al, 2007). New visualization possibilities of the Web-oriented programming for the fracture modeling due to developed algorithm of stochastic growth of microcracks and micropores has been presented also (Lepov et al, 2016). Known approaches for static loading of heterogeneous viscous media cannot take into account the low-cycling and dynamic loading that is quite important in severe environment conditions (Lepov and Loginov, 2012). Other significant factors should be taking into account for multiscale model building are the structural heterogeneity due to the weld joints and branching of the crack path (Lepov et al, 2017). The railroad located on territory of the northern Russian territory - Republic of Sakha (Yakutia), is distinguished by low climatic temperatures and acutely continental climate, where the period of subzero longs about 210 days and the minimum of temperature reaches 60 ˚С below zero, and difference of the average temperatures achieve more than 70 ˚С per season. So most structural steel experience the ductile-brittle transition that made worse the crack resistance. Other problem is an extreme uncertainty of locomotive tire temperature and loading during the operation. It is required to use the Bayesian probability approach (Grogoriev, Lepov, 2017).

Nomenclature ASM atomic-force microsope KCV impact toughness measured by dynamic bend test on V-notched probe [J/m 2 ] k Boltzmann’s constant, 1.381 × 10-23 [J/  K]. SEM scanning electron microscope T temperature [  C]  lifetime, [days, months or years] U internal energy, J  surface energy, J  elongation [%]  B breaking stress [MPa]  Y yielding stress [MPa] ε deformation  damage