PSI - Issue 18

Tretyakov M.P. et al. / Procedia Structural Integrity 18 (2019) 816–822 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Durability; safety; relaxation; postcritical deformation; tension; high temperatures.

1. Introduction Currently, considerable attention of researchers is focused on the study of the processes of deformation and destruction of materials from the point of view of safety and reducing the catastrophic failure of structures in emergencies. Increasing requirements for reliability, resource and long-term durability of critical products determine the relevance of experimental and theoretical studies of the patterns and characteristics of rheological behavior of materials in conditions of postcritical deformation. The deformation of materials at the postcritical stage is accompanied by actively occurring processes of damage accumulation and plasticity is characterized by a decrease in load with increasing elongation, and precedes the moment of sample failure. As part of the study, the task of assessing and accounting for the rheological behavior of materials under conditions of postcritical deformation is posed for the first time, which is a necessary element for the further development of models of softening media for analyzing and predicting the behavior of critical structures during failure processes. To date, the foundations of the mathematical theory of the processes of stable postcritical deformation of softening media have been developed by Vildeman et al. (1997, 2008 and 2013). Experimental data on the postcritical behaviour of structural steels and alloys at different temperatures and types of stress-strain state, and loading conditions have been obtained in works of Tretyakov et al. (2016, 2018), Wildemann et al. (2014) and Wildemann and Tretyakov (2019). A comparison between the criteria for the transition of the deformation process to the postcritical stage for various types of stress-strain state is made. Analytic and numerical solutions of boundary value problems, illustrating the realization of the reserves of bearing capacity and increasing the survivability of structures and bodies with cracks, taking into account the postcritical deformation of materials are obtained. When analyzing deformation and fracture of structures that are operated under high temperatures and loadings applied for a long period of time, special attention is paid to studying the aspects of material behaviour connected to rheological effects, which are characterized by a variation of deformations within time under constant stresses or a decline of stresses within time under fixed deformation. Whereas materials behaviour is considered both under uniaxial tension and under complex stress-strain state, as well as the applicability of different variants of creep theories. A wide range of researches in this area has been carried out by Lokoshchenko (2016), Agahi et al. (2009), Radchenko et al. (2012). Great attention of researchers is paid to studying creep processes not only during testing of smooth samples but also of samples with concentrators of stresses. Most often, the concentrators are considered in the form of an undercut in a section of a cylinder sample during tension and torsion tests. The influence of concentrators on time until fracture is studied for such a sample under creep, as well as the applicability of different methods aimed at a quantitative estimation of this effect by Namestnikova (1985), Shesterikov and Lokoschenko (1996). Both a negative and positive influence of concentrators on a long-term durability of materials is observed in different types of tests. The aim of the work is studying the features of the rheological behavior of steels under conditions of postcritical deformation at high temperatures, which is connected with the solution of methodological issues of testing and obtaining new experimental data at the stage of deformation softening. 2. Methods and materials Experimental studies were conducted on the above samples of heat-resistant high-alloyed corrosion-resistant steel martensitic class X15CrNi12-2. The chemical composition of the steel is given in Table 1. The samples are made by a lathe method from a rod with a diameter of 16 mm in the delivery condition without subsequent heat treatment. In the manufacture of samples, the material was minimally heated in the working section. Before testing, the samples are marked and measured the actual dimensions of the working part with an accuracy of 0.01 mm. In the working part of the tensile test specimens, mechanical punching was performed to determine the residual elongation after rupture.