PSI - Issue 50

S.V. Maslov et al. / Procedia Structural Integrity 50 (2023) 178–183 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

179

2

and operation. Despite the current progress in the development of numerical methods for analyzing the stress-strain state (SSS) of complex structures, the experimental control of operating units has not lost relevance (Makhutov N.A. et al., 2020; Razumovsky I.A. et al., 2014), since it allows obtaining data in real operating conditions of power plants, which differ from the calculated ones. This is especially true for structures with complex geometry subject to unpredictable changes in temperature state that cannot be predicted theoretically. Unfortunately, previously available competencies of research and design centers in the creation and application of experimental control means applicable in extreme operating conditions - at temperatures above 450 °C and the presence of aggressive coolant (Daychik M.L. et al., 1989; N. A. Makhutov, ed., 1992; Maslov S.V., 2019) are currently lost in many cases and need to be restored and further developed. The present work is related to the development of means and algorithms of SSS calculated-experimental research applicable to operating conditions of fast-neutron BN, BREST reactors, and similar types in which liquid metal (sodium, lead, etc.) is used as a coolant. 2. Development and bench testing of measuring instruments It is necessary to use strain gauges with stable metrological characteristics at temperatures of 520 – 540 °C to measure strain under the above conditions. The only Russian type of strain gauges with characteristics close to those required are NMT-450. Since the maximum operating temperature of these strain gauges is lower than required, it was necessary to develop a special method of their preheating, which comprises a long exposure at 540 ° C and selection of "active-compensation" for three parameters: the initial resistance, the temperature response, and drift parameters. As a result, it was possible to create strain gauges with an extended length of the metal substrate suitable for manufacturing hermetic strain gauges (GTR) with an integrated thermocouple to monitor their temperature state. Their distinctive feature is low thermal inertia due to the autonomous protection of the coolant foil made of steel X18H10T with a thickness of 0.1 mm. The strain gauge is connected to the protective tube through an adapter unit attached to the upper part of the foil. Several options were considered for the production technology of the hermetic sensor to ensure the tightness of the connection area of the adapter with the foil: soldering with silver solder spot contact welding with overlapping welding points and soldering with nickel solder according to GOST R 53542 2009. To verify the performance of the developed measuring instruments manufactured by different sealing options, they were tested on a test bench with a real coolant under thermal cyclic loads (heating to 540 ° C with subsequent rapid cooling down to 250 ° C at a rate of 5 – 10 o C/s). The results shown in Table 1 are bench test results lasting 500 hours. Table 1. Violation ( – ) or preservation (% of good samples) of strain gauge sealing made by different technologies of connecting the adapter to the foil. Sealing option

Exposure time 50 hours, temperature 250 ° C

Tightness after 50 cycles

Tightness after 100 cycles

Tightness after 500 cycles

Soldering with silver Soldering with nickel

0%

-

-

-

100% 100%

100% 100%

100%

92% 42%

Spot welding

84%

As the table shows, the use of silver solder technology does not lead to acceptable results due to the washout of the solder from the soldering area. Using contact welding with continuous weld spots is possible when the duration of exposure to the heat transfer medium is short and the number of thermal cycles is small. An analysis of the state of the strain gauges after the tests showed that the breach of tightness occurred in the contact welding area and was associated with the cyclic fatigue of the metal in the area of stress concentration. The best results are obtained by using nickel soldering technology in a protective gas or vacuum. Fig. 1 shows photos of the surface of the upper foil of strain gauges before and after the tests, obtained with an optical "StereoScan" type microscope (magnification 2000). The photos show the gradual emergence and growth of cracks on the surface of the foil in the area of initial microdefects. Further analysis allowed selecting the foil steel grades for which this effect is minimal. Strain gauges