Study of distributions of local elastic and plastic deformations at fracture of specimen with damage

It is the purpose of this paper to present an analysis of experimental data obtained during the fracture of 17G1S (GOST steel grade) steel specimen with pre-applied damage (deep scratch across the width of the specimen). The loading process was monitored using a MicroScan 7600 PRO thermal imager with thermograms recorded in the areas of temperature increase, i.e., in the fracture area (the place of pre-applied damage). The increase in specimen temperature is due to thermoplastic effect in the areas of plastic deformation. The peculiarities of deformation localization were investigated using statistical characteristics, Minkowski connectivity assessment, thermal field clustering, and determination of average temperature values along the length of the specimen. Studies of thermograms obtained in the process of loading the specimen showed that during elastic deformation, local levels of elastic deformation are redistributed at the initial stage without increasing the coefficient of variation. At the stage before the occurrence of plastic deformations, the coefficient of variation decreases somewhat, i.e., the unevenness in the distribution of local elastic deformations decreases, and subsequently, with an increase in the level of plastic deformations, their localization increases in the area of pre-applied damage. Cluster analysis showed that as the load increases, the temperature field changes its character. The number of levels in which clusters are defined changes. At the same time, the number of clusters changes at each level. The statistical characteristics of the local distribution of deformations also change, as a rule, the coefficient of variation increases, the standard deviation increases too. As the load increases, the type of distribution density of local deformations changes. If in the elastic area of the specimen loading the distribution is unimodal, then in the plastic distribution area it has a more complex type — the modality of the distribution of temperatures (deformations) increases when a crack is formed — in this case the distribution is three-modal. There are distribution areas corresponding to the elastic deformation zone and two plastic deformation zones. The average distribution corresponds to the crack appearance zone, where in the fracture area the specimen has a lower temperature (a crack is formed).

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