PSI - Issue 65

Maslov S.V. et al. / Procedia Structural Integrity 65 (2024) 139–146 Author name / Structural Integrity Procedia 00 (2024) 000–000

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To assess the creep of strain gauges when processing the findings of field studies, the following data must be used:  measurements of creep during cyclic tests at a temperature maximum for the time interval under consideration (building sinusoidal wave envelopes);  calculated values obtained by formula (4) for the maximum values of temperature and deformation in the considered area;  the maximum possible creep of strain gauges for a fixed loading time, if these data are available in the data sheet for a batch of strain gauges. To minimize the creep estimates of strain gauges, the maximum number of possible combinations of estimates must be used for the accepted time intervals of the analyzed process The operating conditions of the power plants under development imply the impact of high temperatures on the structural elements in combination with significant thermomechanical loads, which can lead to both elastic and elastic-plastic deformations (Makhutov N.A., Zatsarinnyy V.V., 2019; Makhutov N.A., Zatsarinnyy V.V., 2020). Therefore, the algorithm for processing experimental information in studying full-scale structures should take into account changes in the mechanical properties of materials and changes in the dependences of the determined stresses on deformations during the periodic occurrence of plastic zones at measurement points. In tensometric studies of structures with no plastic deformations, the formulas for the relationship between the main stresses σ 1,2 and the deformations ε 1,2 measured in their directions have the following form: 5. Determination of stresses during elastic-plastic deformation of the structure where E t and µ are the temperature-dependent modulus of elasticity and the Poisson's ratio, respectively. Usually, with small temperature changes, the dependence of the modulus of elasticity on temperature can be neglected, its value is assumed to correspond to the minimum temperature for the analyzed process. In this case, the calculated stresses may be slightly higher than the true ones. When studying new power plants where plastic deformations may occur in their elements, constant SSS monitoring is necessary to determine the moment when the material goes from an elastic state to an elastic-plastic state and vice versa. In this case, the model of the relationship between stresses and deformations should be as close as possible to the real one. Linear hardening deformation models are unsuitable for many modern materials, in particular for most stainless steels. Therefore, in order to calculate stresses from measured deformations in strain gauge studies, it is necessary to use available experimental data (Bragov A.M. et al, 2015) to construct a deformation diagram that establishes a relationship between the intensities of deformations and stresses. The further procedure for calculating stresses is as follows. 1. The stresses at the measuring points are determined according to the formulas (5) applicable to the elastic deformation region. 2. The criterion for maintaining the elasticity of the material is checked, namely the stress intensities obtained are compared with the yield strength of the structural material at the current temperature. 3. After exceeding the stress intensity of the yield point, further calculation is done according to the formulas of the deformation theory of plasticity, which have the following form for a 2-axis stress state: 1,2 1,2    2,1 2 ( ) (1 ) t E     (5)

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