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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2

models of SSS changes at dangerous points for calculating accumulated damage and determining the residual life of the structure after a specified period of operation. Currently, further improvement of experimental control tools that are operable in new conditions is required due to developing new power plants operated under higher operating parameters (temperatures, mechanical and thermal loads) compared to the previous-generation plants.

2. Problem setting

An example of new power plants with high operating parameters is liquid-metal-cooled fast nuclear reactors operated at temperatures up to 550 °C and in non-stationary thermal conditions. The strength and service life of such plants are usually validated during their design by computational analysis methods using, if necessary, physical modeling methods in conditions that are considered similar to those implemented in a full-scale design. At the same time, during the actual operation of the structure, its loading conditions do not always change in accordance with the accepted design options, therefore, the service life assigned during the design requires clarification based on experimental information obtained during SSS field studies under real operational loading (Gadenin M.M. et al., 2021). Structural elements where dangerous stresses may occur include: pressure vessel elements, sealing welds, and interfaces of heat exchanger elements having different heat capacities. Under certain loading conditions, due to high temperatures (up to 520–540 °C) and rapid changes in the coolant parameters, these elements may undergo high local deformations, which in some cases are inelastic (Razumovsky I.A. et al, 2014; Maslov, S.V., 2023). The analysis showed that strain-gauge studies in such conditions required advanced measuring instruments and techniques, which have not been previously necessary. The objective of this paper is to determine the factors affecting the accuracy and reliability of field study findings under these conditions, as well as to develop algorithms that allow estimating and minimizing measurement errors. The conditions for full-scale strain gauge studies within this paper are as follows:  temperature range: 400–550 °C;  deformations in the areas where strain gauges are installed: up to 0.3%;  period of strain gauge measurements: up to 0,75 hours (2700 s);  the type of strain gauges used: wire welded strain gauges with a sensing element made of nickel-molybdenum alloy NM23XU with an organosilicate carrier. One of the main factors making it difficult to unambiguously interpret changes in the output signal of strain gauges exposed to high temperatures and deformations is the high creep of strain gauges. The creep (Karl Hoffmann, 1989) is an uninformative component of the output signal caused by a change in the rheological characteristics of the measuring grid and the carrier when exposed to deformation. The creep is usually defined as a change in its output signal over time when exposed to constant deformation and fixed influencing quantities (temperature, magnetic field intensity, radiation exposure, and other factors) and is expressed by the formula 3. Creep measurement methods of strain gauges

  ,       

     0

   

0

, Cr



(1)

where   , Cr   is the creep of the strain gauge during deformation  over time  ;   0   is the output signal during deformation  at the initial moment of time;   ,    is the output signal after exposure to constant deformation  over time  . Such a definition is not suitable for determining the amount of distortion of the strain gauge output signal caused by a change in the mechanical characteristics of the carrier/measuring grid system with varying measured deformation, although the physical causes of this phenomenon remain the same. Therefore, in this paper, creep is understood as a value defined by the following expression: