Discussion on the static-thermal test technology of aircraft

During the high-speed flight of the aircraft, the cabin body will bear aerodynamic loads, engine thrust loads, structural and equipment component mass inertial loads. At the same time, aerodynamic thermal loads will be generated due to high speed flight. With the development of hypersonic aircraft technology, the demand for multi-field coupling test is more and more common. Therefore, it is necessary to carry out the technical verification of the mechanics and thermal composite test on the ground to obtain the structural characteristics. In this paper, a certain type of aircraft cabin as the research object, carried out the research on the combined mechanical and thermal test technology. The results of this paper show that the displacement response of the cabin under the combined action of force and thermal load is consistent with the change trend of the force loading curve, and the force load has obvious influence on the displacement response of the cabin. The strain response is consistent with the trend of temperature curve, and thermal load has obvious influence on cabin deformation. Through this study, we can obtain the static-thermal test technology method, and provide the technical basis for the development of similar test technology.


Introduction
Static test plays an important role in the design of spacecraft and aircraft [1].It is used to determine the stress distribution, bearing capacity and stability of the main structure of the aircraft under certain loads [2].Through static loading, the stress, strain, displacement and other changes of the spacecraft structural components can be detected in real time, to achieve the structural parameters of the product verification and optimization design purposes [3].
According to "GJB 1027A-2005 Test requirements for launch, upper-stage, and space vehicles" [4], static test of structural sub-system is used to verify the static strength and stiffness characteristics of the structure, and to measure the load, stress distribution and deflection of structural elements under specified load conditions.The static test data are of special significance because the nonlinearity of the structure (including nonlinear bearings, elastic distortion of plates, clearance of preloaded interfaces and sliding of frictional joints, etc.) is often not adequately represented in modal tests.
Displacement of missile radome with different joint structure in thermal-structural load was simulated and analyzed by numerical simulation method [5].Li Jianhua used finite element simulation to analyze the thermal stress of missile hood [6].Through finite element analysis, Tang Qiqin established and optimized the model of typical projectile and arrow structure under the action of thermal-mechanical [7].Rotating body structure of missile radome was taken ad the research object, and the finite element analysis method was adopted, deformation and stress distribution of the temperature field, the static field and the thermal-mechanical coupling field of the rotating body structure were respectively obtained [8].In these references, the response of products under thermal-mechanical loading was obtained mainly through numerical simulation, but how to conduct thermalmechanical tests was rarely found in the literatures.
In the process of high-speed flight, the cabin body will bear aerodynamic loads, engine thrust loads, structural and equipment component mass inertial loads, and aerodynamic thermal loads will also be generated due to high-speed flight.Therefore, it is necessary to carry out the technical verification of the combined mechanical and thermal tests of the aircraft on the ground to obtain the structural characteristics of the aircraft under combined mechanical and thermal loading [9].Combined static and thermal test (static-thermal test) is a ground test method to simulate the distribution of static and thermal loads and their changes with time according to a certain actual working process or a given process in advance.

The composition of the static-thermal test system
The static-thermal test system is composed of a static loading system, a thermal loading system, a measurement and acquisition system and auxiliary devices, shown in Figure 1.The static loading system includes: oil source, servo valve, actuator, force/displacement sensor, force bearing rod, hydraulic loading controller and loading support device.The thermal loading system includes: thermal loading controller, power regulator, radiant heater, heat flow meter.The measurement and acquisition system includes: measurement and acquisition instrument, strain gauge, thermocouple, displacement meter; auxiliary devices include: heat insulation device, cooling circulation device, smoke exhaust device, etc.

Static loading technique
The application of force loads shall follow flight procedures or predetermined procedures.The mechanical boundary, load distribution and application mode should meet the test requirements.Static load system adopts hydraulic servo actuation system, the main factors to consider in system selection include: dynamic characteristics, instantaneous output power, control capability of the load and displacement [10].
Hydraulic loading controller realizes automatic control of static load and real-time recording of test data, at least it should have good load coordination and linkage control ability, good response time and precision performance, and with open and closed loop control function and automatic unloading ability.
The force loading fixture should meet the loading requirements and have sufficient strength and stiffness.Under the loading force, the deformation of the fixture is smaller than that of the specimen.The load bearing capacity of the support device and the reaction beam are greater than the load force of the system.

Thermal loading technique
The thermal load conditions shall specify the range of each temperature zone, and take the temperature or heat flow of representative locations within each temperature zone as the control data.Usually in the form of radiant heating, the radiant heater is divided to meet the requirements of the heating area [11].
When the thermal loading system works, the heat flow meter collects the heat flow / temperature value of the specimen, and compares it with the target value received from the computer.The PID control algorithm is used to control the output power of the power regulator, and the collected heat flow/temperature value is fed back to the computer for data preservation and processing.

Loading requirements for the static-thermal test
The loading of the static-thermal test should at least meet the following requirements.
a) Coordinated design of static loading device and heating device to avoid mutual interference.
b) The static loading device and the heating device should generally be independent of each other and have sufficient safe distance.
c) If the static loading device and heating device have follow-up requirements, they shall be designed according to the specific requirements.
d) The static loading device shall be able to withstand the test temperature assessment, and its strength safety factor shall be no less than 4, and thermal protection treatment shall be carried out if necessary.
e) The static loading device shall not change the thermal boundary of the structure and the overall thermal load distribution.

Measurement technique of the static-thermal test
The measurement of displacement, strain and temperature is usually carried out in the static-thermal tests.The measurement of displacement and strain at high temperature is the key to realize the response measurement of static-thermal tests.

Displacement measurement technique at high temperature
The commonly used sensors for displacement measurement in static test are micrometer, thimble displacement meter and laser displacement sensor.In the high temperature environment, the micrometer and the thimble displacement sensor can not directly contact with the surface of the object, so it is difficult to obtain the test results directly.Laser displacement sensor is a measurement sensor using laser technology.It is composed of laser, laser detector and measurement circuit.It can accurately measure the position and displacement of the measured object without contact [12].However, the infrared light wave of the laser displacement sensor coincides with the wavelength of the infrared heater, so it is difficult to measure the displacement when the product is heated by the infrared radiation.Therefore, a high temperature displacement measuring device is designed in this paper, as shown in Figure 2, through the ceramic rod contact the surface of the tested product, the ceramic rod is not easy to deformation at high temperature, measure the displacement of the ceramic rod, obtain the displacement of the product surface, the device can adjust the test height and angle according to the need.

Strain measurement technique at high temperature
The main difficulties of strain measurement technique at high temperature can be summarized as follows: The adaptability of the sensor installation process to high temperature and the strain transfer characteristics at high temperature need to be analyzed, the temperature compensation of high temperature strain test and the method of static heat decoupling need to be studied.High temperature strain measurement methods include: Laser interference sensor measurement, Fiber Bragg grating sensor measurement, CCD camera photography, Extensometer extraction method measurement, High temperature strain gauge measurement.The temperature applicable to high temperature strain gauges generally does not exceed 900°C.The strain measurement at high temperature of 1000°C and above can be realized by using optical fiber testing technology [13].The Table 1.lists the advantages and disadvantages of each test method.the loading force hat.The static load is realized by imposing axial force and shear force on the loading force hat, and the loading requirements of the force-load are shown in Table 2.The direction of the bending moment compresses Quadrant III.The thermal load is realized by radiant heating of quartz lamps uniformly distributed in the outer part of the cabin body.The heating area is 360° in the circumfluence direction.The heat flow loading requirements are shown in Figure 4.  5.The lamp array bracket is made of 40×40×3mm rectangular tube welded, with an envelope size of 847mm and a height of 555mm.The infrared lamp array is composed of 40 lamps uniformly distributed and controlled according to 3 heating zones.Each lamp has a length of 345mm and a power of 5kW.The spacing between each infrared lamp is 19mm, 40mm away from the product.The function of water cooling plate and water cooling sleeve is to cool the bracket through the circulation of water.

Design of static loading.
Axial force T is achieved by the vertical upward loading force F1, F2 of two 5-ton actuator cylinders.The shear Q is implemented by lateral manual loading.The bending moment M is realized by F1, F2 and Q. Where, F1 and F2 are positive values indicating the pulling load of actuator barrel, while shear Q is always pulling load.The positions of specimen and actuator barrel are shown in Figure 6, then the calculation formulas of F1, F2 and Q are as follows, and the loading forces at all levels are shown in Table 3.

F1+F2=T
(1) Where: L is the distance between the loading point and the center, 850mm; h is the distance between Q and the cross-section: 185mm.

Static-thermal loading of aircraft cabin
Step by step loading was carried out according to the test conditions in Table 3, and displacement, strain and temperature were measured.The static-thermal test of the aircraft cabin is shown in Figure 7.
The specific loading process was as follows: (1) Before the force was loaded, the measuring system was initialized, and then the measuring system was started.
(2) Load from level 0 to level 1, the loading process used 50s, the load of the force was maintained.
(3) The time history curve of force and thermal loading was initialized, and the heat flow started to load, and the force load was continued to be loaded.
(4) The force load was loaded from level 1 to level 8, and each stage was loaded for 10s; (5) The level 8 force load was maintained continuously and the temperature was monitored until the target temperature was reached; (6) In 10s, the internal force load was loaded from level 8 to level 9, and the static load and thermal load were maintained continuously; (7) After the temperature reached the target temperature, turned off the heater, turned on the cooling device, and unloaded the force load.

Analysis of the Static-thermal test result of the aircraft cabin
Figure 8 to Figure 11 respectively show the force loading curve, displacement response curve, temperature curve and strain response curve of this test.It can be seen from the Figure 8 and Figure 9, that the displacement response of the cabin under the combined action of force and thermal load is consistent with the change trend of the force loading curve, and the force load has obvious influence on the displacement response of the cabin.The strain response is consistent with the trend of temperature curve, as shown in the Figure 10 and Figure 11, and thermal load has obvious influence on cabin deformation.
The experimental data are reasonable and effective within the expected range.This test has achieved the purpose of test examination and can be used as the basis for the structural design of the aircraft cabin.

Conclusions and prospects
In this paper, an aircraft cabin is taken as the research object to carry out the research on the combined static and thermal test technology.Correctness and feasibility of the combined static and thermal test method are demonstrated.The successful application of this method can provide a new verification method for the design of aircraft and provide a technical basis for the development of similar test technology.
For special products, combustion, smoke and even toxic gas will be generated during the staticthermal, so it is necessary to do a good job of protection, and design smoke exhaust settings to ensure personal and product safety.
This paper only introduced the static-thermal test technology.With the development of hypersonic aircraft technology, the demand for multi-field coupling test is more and more common.The research on thermal-mode, thermal-noise, thermal-vibration and other coupling test technology needs to be further developed.

Figure 1 .
Figure 1.Composition diagram of static-thermal test system.

Figure 2 .
Figure 2. Displacement measurement technique at high temperature.

Figure 3 .
Figure 3. Installation requirements for test.Figure 4. Heat flux condition of aircraft cabin.

Figure 4 .
Figure 3. Installation requirements for test.Figure 4. Heat flux condition of aircraft cabin.

Figure 5 .
Figure 5. Design of heat flux loading.Figure 6. Design of static loading.

Figure 6 .
Figure 5. Design of heat flux loading.Figure 6. Design of static loading.

Figure 8 .
Figure 8. Curve of static loading.Figure 9. Curve of displacement response for different positions in the cabin.

Figure 9 .Figure 10 .
Figure 8. Curve of static loading.Figure 9. Curve of displacement response for different positions in the cabin.(°C)

Figure 11 .
Figure 11.Curve of stain response for different positions in the cabin.

Table 1 .
Advantages and disadvantages for different types of strain gauges.

Table 2 .
Requirement for loading forces at all levels.Design of heat flow loading.The heat flow simulation device is composed of lamp array bracket, U-shaped infrared lamp, water cooling plate and water cooling sleeve, shown in Figure

Table 3 .
Design for loading forces at all levels.