PSI - Issue 5

Valeriy Lepov et al. / Procedia Structural Integrity 5 (2017) 777–784 Valeriy Lepov et al / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

The brittle fracture mechanics begins from Griffith (1920) study, when he revealed an influence of discrete invisible cracks for the materials strength. Then Ioffe (1928) shows that the small cracks arranged in a surface layer and could be managed by technology. But only past decades 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 fracture modeling noted in Fracture (1968), by Cherepanov (1977), Broek (1982), than in last years by Arkhangelskaja and Lepov (2003), Lepov (2008), Hell (2015), and other. The significantly loss of safety and economic efficiency is occurred in the operation of transport infrastructure such as railway equipment and bridge bearing in extreme climatic conditions. This leads to grows of energy and resource intensity of transportation. Special actuality of this problem is considering the construction of new rail lines and increase of cargo turnover. One of the most important units of railway equipment for example is the tire and rail system for low climatic temperatures, many tests was made by Grigoriev and Lepov (2012). For the bridges it was just shown for the weld steel bearing by Lepov and Mbelle (2017). Their durability and reliability significantly affect operating costs, and destruction is unacceptable because they pose a clear threat to traffic safety. The paper also underlines the significance of structural approach for strength and lifetime of construction members and system elements on the base of stochastic modeling of damage accumulation and fracture processes did by Broberg (1990) and Lepov et al (2007, 2016). New visualization possibilities of the Web-oriented programming for the fracture modeling due to developed algorithm of stochastic growth of the microcracks and micropores has been presented. 2.1. Structural modeling approach The importance of considering internal heterogeneity and material structure leads to the development and wide use of the so-called structural models of damage accumulation and fracture. The main advantage of using a structured approach, including evaluation of the lifetime of metal structures, is to overcome known limitations of the semi empirical models that do not include an explicit description of the physical phenomena occurring in the material. Another important aspect when modeling of the processes of damage accumulation and destruction in real materials is the taking into account the hierarchy of structural damage and non-homogeneity of material properties at different levels described in detail in Computational and Experimental Methods (2009). In many cases the statistical modeling only could resolve this problem. Those models that are based on the simple generalization of experimental data on scattering characteristics of material resistance to deformation and fracture, are essentially the same semi-empirical as based on the hypothesis of homogeneity. The models considering the statistical variability of the properties of materials at micro - and macroscopic levels are more physically reasonable and give more opportunities to predict the effect of anisotropy of properties on the regularities of deformation and fracture of materials. The important advantage of combined structural models should be mentioned also, such as the ability to find a way of experimental data obtained for one of the kinds of loading and behavior of one material to another independently of composition and structure, and combine experimental data related to various kinds of stress state and external influence. The scheme of assessment of limit state and resource of structure element on the basis of structural and stochastic modeling shown on Fig. 1a. The initial and boundary data for the models are the sizes and quantitative characteristics of the distribution of defects at different structural levels or scales obtained by the scanning probes and optical microscopy and fractography methods, characterized quantitative by fractal dimension, and it in situ evolution during the damage accumulation process. Structural level or scale in this case means the area of the extent to which the prevailing is a certain defect structures (for example, vacancy, dislocation, the accumulation of dislocations, crack, micropore, strip shift, non-metallic inclusion, etc.). Deformation surface image of plane probe of low-alloyed steel near the rupture presented on Fig. 1b made by Lepov et al (2008, 2016). The image was obtained by STM and has 5×5 µm size. Just through the middle of the image a grain boundary passed with some structural defects around (small cracks, slip bends, twins etc.). 2. Materials and methods