PSI - Issue 50

R.S. Akhmetkhanov et al. / Procedia Structural Integrity 50 (2023) 6–10 R.S.Akhmetkhanov / Structural Integrity Procedia 00 (2019) 000 – 000

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(plastic strain) of the structural material in the vicinity of the crack into thermal energy and consequently increases the average temperature in the area of these strains. Thus, at failure there is a breach of thermodynamic equilibrium of the object with the environment on its surface there is an excessive temperature field, which allows to obtain information about the level of plastic deformations and, therefore, to conduct non-destructive testing (NDT) and diagnostics of structures by the thermal method. The main thermal processes in a solid body occur in the volume of plastic deformed material located along the slip bands, where there are increased plastic deformations. In these areas (sliding strips), further deformation of the specimen results in the formation of free surfaces and the development of fractures in the part. Fig. 1 shows graphs of temperature measurement in the area of deformation (elastic and plastic) of the material when loading the sample under tensile load. The graph also shows the change in the coefficient of relative contraction of the specimen cross section ψ when the simple stress-strain state is realized. In the course of this loading, the specimen passes through three stages before fracture: I - cooling of the specimenunder elastic deformation; II - process of increasing plastic deformation; III - transition to the fracture stage of the specimen (Nesteruk D.A., at etc., 2007., Moiseichik A.E., at etc., 2014). The process of heat generation in material volumes as a result of plastic deformation is a "natural manifestation" of physical-chemical and mechanical changes occurring within these materials. These material properties are caused by thermos-elastic and thermoplastic effects (Kovalenko A.D., 1970). Under elastic uniaxial tension of a steel specimen (stage I) there will be a decrease in the temperature of the deformed volume and an increase under elastic compression. During elastic-plastic and plastic deformation of the body (stages II, III), the body is heated. Stage III is characterized by a large gradient increase in the temperature of the sample. Let's consider a model example - tension (simple loading) of a plate with holes. To determine the stresses in the plate, a calculation was performed using the ANSIS program. The calculation determined the distribution of stresses at different values of the applied tensile load (fig. 2). The greatest stresses and strains are in the area of holes (stress concentrators) and defects of material structure. Figure 2 shows zones with the highest values of stresses at various levels of loading (two views 1 and 2); view 1 is stresses caused by stress concentrators, view 2 is lines (bands) of sliding at plastic deformations.

Fig.1. Temperature change T of the sample with increasing strain (uniaxial loading): σ u - tensile strength; σ y - elastic limit; ψ - relative contraction of the sample cross section 2. Material Consider the thermograms when loading the plate with holes. The plate is steel - material St3 sp. Figure 3 shows thermos-grams of the sample under load at different levels of plastic deformation (stage 2). At lower load there are zones with increased temperature, which belong to the area of stress concentration. With further increase in tensile load, there is an increase in temperature - areas with maximum temperature appear.