Research on pipeline stress test method of civil aircraft hydraulic system

The hydraulic system is an important component of the aircraft, whose performance status directly affects the aircraft’s flight safety, and aircraft hydraulic system piping is the main component of the hydraulic system. In the development process, civil aircraft must be tested and analyzed on the hydraulic system piping vibration and stress. This paper takes the hydraulic system pipeline of civil aircraft as the research object, carries out the research of pipeline stress test method, and provides the stress test method and reliable test data for verifying whether the design of the hydraulic system pipeline is reasonable.


Introduction
A hydraulic system is an aircraft system that converts the acquired energy into hydraulic energy and transmits it to various types of hydraulic actuators on the aircraft [1], and the hydraulic system pipeline, as an important part of the hydraulic system, is the channel for hydraulic energy transmission, which is widely distributed in various parts of the aircraft, just like the blood vessels of the human body [2].The hydraulic system pipeline is shown in Figure 1.In aircraft in flight, the hydraulic system pipeline may be influenced by temperature, medium pressure, vibration, and other load coupling [3].If it cracks or fractures in flight, it will lead to hydraulic oil leakage, thereby jeopardizing flight safety [4].Therefore, it is of great significance to carry out stress tests on the pipeline of aircraft hydraulic systems during the development of civil aircraft.
Relevant research has been carried out both domestically and internationally on the stress analysis of hydraulic system pipelines.Abroad, Prof. Azar et al. [5] used the finite element method to predict the stress of marine pipelines, and Prof. Ermolenko et al. [6] used the floating string method to analyze the stress of the submarine pipeline installation process.While in China, Shi et al. [7] carried out theoretical calculations and simulation analysis on the pipeline stress of a certain type of aircraft hydraulic system and also carried out stress tests for verification.Huang et al. [8] reduced the hydraulic pipeline stresses by studying the hydraulic pipeline orientation and clamp position.
This paper takes the hydraulic system pipeline of civil aircraft as the research object and carries out the research on the stress test method of hydraulic system pipeline, including the design of hydraulic system pipeline stress test system, the selection and paste of strain gauges, the implementation of the stress test and the application of the test method, etc., to provide the stress test method and effective test data for the design and optimization of the hydraulic system pipeline.

Test principle and method
The hydraulic system pipeline stress test adopts the strain electrical measurement method [9], which is a method of converting mechanical quantities into electrical quantities.This method is by pasting strain gauges on the hydraulic system pipeline, when the force of the hydraulic pipeline is deformed, the resistance of the strain gauges will change accordingly, and the amount of change in this resistance is measured by the strain test analyzer.The strain analysis software will be converted into a strain value, and then calculated through Formula (1) to get the corresponding value of the stress: (1) where  is the hydraulic system pipeline stress, E is elastic modulus, and  is the hydraulic system pipeline strain.Obtaining stress by resistive strain measurement is one of the most widely used measurement methods in stress measurement analysis.Strain gauges have a wide measuring range, good frequency response, and the ability to achieve dynamic strain measurements at high and low temperatures as well as in vibration environments, making them ideally suited for stress measurements in the pipelines of aircraft hydraulic systems.

Test system
The stress testing system for aircraft hydraulic system pipelines consists of a strain gauge, strain testing analyzer, and computer, as shown in Figure 2. The strain gauge measures the strain that occurs in the hydraulic system pipeline under stress and converts it into a change in resistance.The strain testing analyzer collects the change in resistance output from the strain gauge and converts it into voltage.At the same time, the signal is amplified.The strain analysis software on the computer records the voltage and converts it into strain.

Strain gauge selection
When conducting stress testing on aircraft hydraulic system pipelines, strain gauges should be selected regarding the following:  We select strain gauges according to the purpose of strain measurement, the material of the tested piece, its stress state, etc.According to the demand for hydraulic system pipeline stress tests, we generally choose unidirectional strain gauges;  According to the material of the tested piece, paste area, installation conditions, etc., we choose the appropriate sensitive grid length of the strain gauge.If the tested piece of uniform material can be selected, we choose a small grid length of strain gauges, which applies to the hydraulic system pipeline strain measurement;  According to the conditions of use, power consumption, and other selection of the appropriate nominal resistance of the strain gauge, in the hydraulic system pipeline stress test, the strain gauge with a nominal resistance of 120 Ω or 350 Ω is preferred;  The resistance value of ordinary strain gauges will change with the temperature of the measured piece, so it is necessary to choose strain gauges with temperature compensation functions according to the type of material of the hydraulic system pipeline.

Strain gauge paste
The quality of strain gauge paste is one of the key factors to determine whether strain can be measured accurately, so the correct paste method is very important.The main procedures of strain gauge paste include cleaning the surface of the hydraulic system pipeline, pasting the strain gauge, checking, and protecting.

Clean the surface of the hydraulic system pipeline
The hydraulic system pipeline surface cleaning procedure is as follows:  We use sandpaper to slowly wipe the pipeline strain gauge paste area.If the pipeline surface is smooth and flat, the operation can be ignored;  We spray a degreasing agent on the pipeline strain gauge paste area to remove oil and clean it, then wipe it clean with dry gauze;  We use a drawing pencil to make a positioning mark on the pipeline strain gauge paste area, and align the center of the strain gauge with the positioning mark as much as possible when the strain gauge is pasted;  We wipe the pipeline strain gauge paste area with an alcohol cotton ball to wipe away any contaminants that may have been introduced during previous operations.

Paste strain gauge
In the strain gauge base surface and hydraulic pipeline measurement point (strain gauge paste area) surface each coated with a thin and uniform layer of strain gauge adhesive, the adhesive should be uniformly covered with the entire strain gauge base surface.Then the strain gauge center position aligned with the positioning mark and pasted on the hydraulic pipeline.We pad to take the appropriate size of polytetrafluoroethylene film, finger gently squeezed along a direction, and squeeze out the mixed air bubbles and excess adhesive.

Check
Before starting the test, the quality of the strain gauge installation should be checked for the following items:  Strain gauge resistance measurement.We measure the strain gauge resistance after installation with a digital multimeter, which should be within the set tolerance;  Visual inspection of the strain gauge is firmly affixed, whether there are residual air bubbles in the piece;  We visually check whether the position of the strain gauge is accurate.

Protect
Strain gauge-sensitive grids are susceptible to corrosion and other effects leading to changes in strain gauge resistance and zero drift, so a special protective layer needs to be applied to the surface of the strain gauge to protect against mechanical damage, corrosion, and oxidation effects.

Strain gauge preparation
We select strain gauges according to Section 3. Appearance inspection and resistance measurement should be carried out before pasting.Appearance inspection can be carried out visually or with the aid of a magnifying glass to observe whether there are any defects in the sensitive grids and whether there is any damage to the substrate and cover layer.We use resistance measurement to check whether the sensitive grid is broken or short-circuited.The strain gauges of the same measurement must have the same sensitivity coefficient.

Confirmation of measuring points and pasting of strain gauges
According to the key control parts of the aircraft hydraulic system pipeline stress, the measurement points are mainly distributed in the pump outlet, before and after the pipeline joints, pipeline bends, etc.The specific measurement points can also be combined with the actual test site environment to decide.According to Figure 3 and Section 4 to complete the strain gauge paste, each test point pastes two axial strain gauges and one circumferential strain gauge, two axial strain gauges in the pipeline cross-section angle as far as possible into 90 degrees.

Strain testing analyzer settings
The strain gauges are connected to the strain testing analyzer through the test cable, and the strain testing analyzer is connected to the computer.The strain test analyzer software sets the appropriate sampling rate for the test channel.The sampling rate is generally set to 5-10 times the frequency of the signal under test, the bridge type is set at the same time, and the 1/4 bridge is generally selected in the stress test of the hydraulic pipeline.

Testing and data analysis
We carry out the hydraulic system pipeline strain test, and obtain the strain time-domain diagram through the strain test analysis software, and finally obtain the stress according to Formula (1).

Application of the test method
Taking the stress test of a certain pipeline in an aircraft hydraulic system under certain installation conditions as an example, we set up four measurement points, the arrangement of measurement points is shown in Figure 4 and the strain gauges of the four measurement points are pasted in Figure 5.

Test procedure
The test procedure is as follows:  Hydraulic system unpressurized;  We loosen the locking nut of the frame joint on the right side of the pipeline and the pipe nut of the connecting pipeline, and loosen the clamp that fixes the pipeline;  We select strain gauges according to Section 3 and perform visual inspection and resistance measurement;  We paste strain gauges at four measuring points according to Section 4 and Figure 3;  We connect the strain testing analyzer, set the sampling rate, select the 1/4 bridge circuit, and perform equipment zero adjustment;  We start data testing and collection;  We operate with a double wrench and set the force as required.In the installation condition where the wrench used to fix the pipe joint does not actively exert force, we tighten the locking nut of the frame joint on the right side of the pipeline and the pipe nut of the connecting pipeline, and then fix the clamp of the pipeline;  We hold for one minute after completing the previous operation, stop data collection, save the data, and the test is completed;  The data were analyzed to obtain the strain time-domain diagram, and we converted it to stress according to Formula (1).

Data analysis
In the "double wrench operation and according to the requirements of the fixed force, in which the fixed pipe fitting wrench is not active force" installation conditions measured strain data are analyzed to obtain four measurement points of the strain time-domain diagrams, which are shown in Figures 6-9, and then according to Formula (1), they are converted to obtain the stress.

Conclusion
We carry out the research of aircraft hydraulic system pipeline stress test method, which provides reliable and effective test data for hydraulic pipeline design in the process of aircraft development and verifies whether the pipeline design is reasonable and meets the requirements.At the same time, in the hydraulic system pipeline hydraulic oil leakage and other faults, the test method can also be carried out through the pipeline stress test for the aircraft fault troubleshooting, and pipeline design optimization to provide effective test data support.

Figure 2 .
Figure 2. Composition of stress testing system.

Figure 5 .
Figure 5. Strain gauge paste for four measurement points.

Figure 6 .
Figure 6.Time domain diagram of strain at measurement point 1.

Figure 7 .
Figure 7. Time domain diagram of strain at measurement point 2.

Figure 8 .
Figure 8.Time domain diagram of strain at measurement point 3.

Figure 9 .
Figure 9.Time domain diagram of strain at measurement point 4.