Analysis of Cross-Spectrum of Phase-Frequency Spectra of Strain-Gauge Signals of Deformations of Small-Diameter Liquid-Propellant Engine Pipelines in Order to Assess the Residual Life of Fatigue Strength

Язык труда и переводы:
УДК:
621.454.2
Дата публикации:
16 января 2022, 20:03
Категория:
Секция 03. Основоположники аэрокосмического двигателестроения и проблемы теории и конструкций двигателей летательных аппаратов
Авторы
Suleymanov Ildar Mnipovich
JSC "NPO Energomash"
Аннотация:
It is proposed to use the methods of spectral analysis (analysis of the cross-spectrum of phase-frequency signals of strain strain gauging), along with the use of traditional measuring instruments, to determine the residual service life of liquid-propellant engine cooling pipelines during ground fire tests. The dependences of the spectral densities of strain strain diagrams of thin pipelines located in symmetrically arranged places of the engine structure and having a geometric and physical similarity are analyzed.
Ключевые слова:
liquid-propellant engine, vibration, strain gauging, spectral analysis, Cross-Spectrum
Основной текст труда

The finite element method (FEM) simulates the stress-strain state (SSS) of pipelines of small diameter under the influence of vibration [1]. The experimental data obtained as a result of ground firing tests of the liquid-propellant rocket engine are investigated by the methods of spectral analysis [2]. Some dependences of the spectral densities of strain diagrams of thin pipelines located in symmetrically arranged places of the engine structure and having a geometric and physical similarity are analyzed. The presented graphs of the cross-spectrum demonstrate the similarity of the deformations of such pipelines. A computational and experimental method for predicting the state of a pipeline with repeated use of the engine in ground fire tests or as part of the returned elements of a rocket and space transport system is proposed.

In this case, strain gauges are a means of diagnosing the SSS of liquid-propellant engine elements. During the fire test, the relative displacements (compression-tension) of the critical points of the pipes are measured: dynamic strain gauge. In addition to this, static strain gauging is preliminarily carried out at the stage of installation of these lines to fix the installation residual stresses on an inoperative engine. In conjunction with the data of dynamic strain gauging, data from the signals of the accelerometers installed on the units mating with the pipelines are also used. The dependences of the cross-spectrum of signals from accelerometers and strain gauges informatively enrich the unique «vibroacoustic portrait» of the engine during its operation [3].

Data processing was carried out in the WinPos program in terms of phase-frequency and amplitude-frequency characteristics [4, 5]. It seems interesting to compare the cross-spectrum of geometrically completely identical pipelines. The form of the phase-frequency components of the cross-spectrum very slightly depends on the operation profile, duration and features of the motor control settings. The most pronounced form of the spectrum is determined by the spatial geometry of the pipe, the material, the places and the way of installing the sensors (mirror or diametrically symmetrical). The factor of residual installation stresses and its influence on the natural shapes and frequencies of pipelines were considered in particular detail. Violation of the integrity of a pipe or sensor immediately manifests itself in a modification of the cross-spectrum for a particular moment in time. Monitoring its appearance during the operation of the liquid-propellant engine can be used to monitor the integrity of the pipe (and sensor) and the good condition of the pipeline as a whole for the emergency engine shutdown system during fire tests.

The phase spectrum shows how much lagging or leading in phase such a component in one of the series is, the corresponding component in the other series for a given frequency. In the WinPos software tool used for signal analysis and processing, it is possible to obtain mutual phase and mutual amplitude spectra from two series of discrete sequences of signal values. The generally accepted terminology for this is the cross-spectrum, respectively, of the phase and amplitude components. It is of interest to compare the mutual phase spectra of geometrically identical «twin» pipelines.

Reciprocal amplitude spectra are notable for one or two peaks in the 600...700 Hz band and vary in modulus from 2 to 250 mm/mm. We observe some similarity in the mutual spectral density of signals from a pair of strain gauges of the same cross-section in comparison with its mirror pair. Places of installation of strain gauges 1 and 2 are located diametrically 180° opposite 9 and 10, on the “twin” pipelines, respectively, of the 1st and 2nd gauges .

Let's note the following: the similarity in the most interesting range of 500-800Hz is quite close. The difference in individual fragments (frequency bands) of the mutual spectral density can be explained by the errors in the geometry of the spatial arrangement of the nozzles and the difference in the installation stresses of the two pipelines. Even more remarkable is the comparison of the mutual spectral density of the same pair of strain gauges during a series of several sequential fire tests without reassembling this line between tests, and, accordingly, at constant initial residual mounting stresses.

All of the above for the cooling pipelines of the jackets of the gas supply unit is also true for the cooling pipelines of the swinging combustion chambers.

Литература
  1. Pastukhov V.I. Ustalostnaya prochnost' truboprovodov, ispol'zuemykh v ZhRD [Fatigue strength of pipelines used in liquid rocket engines]. Trudy GDL-OKB [Proceedings of GDL-OKB], 2001, no. 19, pp. 239–250. (In Russ.).
  2. Tkach V.V. Otsenka povrezhdaemosti i resursa elementov konstruktsii ZhRD posle ognevykh ispytanii [Assessment of damageability and service life of LPRE structural elements after fire tests]. Trudy GDL-OKB [Proceedings of GDL-OKB], 2016, no. 33, pp. 166–174. (In Russ.).
  3. Vorob'ev E.E., Martirosov D.S. Funktsional'naya diagnostika ZhRD na osnove ekvivalentnoi razreshayushchei sposobnosti izmeryaemykh parametrov [Functional diagnostics of liquid-propellant engine based on the equivalent resolution of the measured parameters]. Trudy GDL-OKB [Proceedings of GDL-OKB], 2006, no. 23, pp. 95–113. (In Russ.).
  4. Dzhenkins G., Vatts D. Spektral'nyi analiz i ego prilozheniya [Spectral analysis and its applications], iss. 2. Moscow, Mir Publ., 1971, pp. 98–173. (In Russ.).
  5. Bendat Dzh., Pirsol A. Prikladnoi analiz sluchainykh dannykh [Applied analysis of random data]. Moscow, Mir Publ., 1989, 540 p. (In Russ.).
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