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ScienceDirectProcedia Engineering 00 (2014) 000–000

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“APISAT2014”, 2014 Asia-Pacific International Symposium on Aerospace Technology, APISAT2014

Fatigue Testing and Analysis of Aircraft Structure Subjected to Aerodynamic and Buffet Loads

Li Yixuan*, Zhang Zhijun, Shao Chuang Laboratory of Aeronautical Acoustics and Dynamics, Aircraft Strength Research Institute, Xi’an 710065, China

Abstract

The aircrafts experience the combined effect of severe buffet and aerodynamic loads in flight, which must be considered in the fatigue life analysis and testing of the aircraft structure. So studying the coupling loading method of buffet and the aerodynamic loads, and its feasibility is necessary. The article detailed the design and implementation of the test rig developed to verify the feasibility of the coupling test. A new air bag loading system other than the more conventional hydraulic loading system was used to apply the aerodynamic loads, and the buffet loads were applied by high powered, high displacement electromagnetic shakers. In the coupling testing, the conclusions can be drawn that: 1)Compared with the natural frequency, the first 3 frequency of the article loaded with the aerodynamic loads by the airbags changed within 4%, that means the loading airbags would not change the structure inherent dynamic characteristics largely and the loading environment simulated here is close to the reality; 2)The number and the configuration of the loading airbags influenced the dynamic response of the structure, the pressure reservoir should be added and the reasonable load position should be selected to decrease the influence of the airbags to the structural dynamic characteristics.© 2014 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

Keywords: aerodynamic loads; buffet loads; coupling loading; dynamic response testing

1. Introduction

The aircrafts experience the combined effect of severe buffet and aerodynamic loads in flight, which must be considered in the fatigue life analysis and testing of the aircraft structural parts. So studying the coupling loading method of buffet and the aerodynamic loads, and its feasibility is very necessary. The article detailed the design and implementation of the test rig developed to verify the feasibility of the coupling test. A new air bag loading system

* * Corresponding author. Tel.: +86-18502902430; .E-mail address: liyixuan0430@126.com

1877-7058 © 2014 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

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other than the more conventional hydraulic loading system was used to apply the distributed aerodynamic loads, and the buffet loads were applied by high powered, high displacement electromagnetic shakers.

2. Test Rig Design

2.1. The article design

In consideration of the test aim and the spatial configuration, the cantilever beam utilizing a rectangular tube section was chosen as the article, a steel plate was attached to the free end of the cantilever to support the airbags. Its details are shown in Fig. 1.

Fig. 1 the three-dimensional diagram of the cantilever beam

2.2. The buffet loads simulating

The loading system used was an electromagnetic shaker with 4000kg capability, and in the trial test a steel sperical stucture was used to deliver buffet loads to the article from the shaker, but the mode was soon proved at question. The energy could not be delivered completely because of the gaps between the ball and the article. And the phenomenon of ball’s deflection and loose also happened during the vibration. The second choice was the rigid connection of the shaker and the article avoiding the ball’s question, but during the vibration, the phenomenon appeared again because the crash. The details of the two buffet excitation modes are shown in Fig. 2.

Summarizing the problems in the trial test, the Hydrostatic Spherical Coupling was used to connect the shaker and the article. The airsprings loading would lead to the bending deformation of the cantilever beam, and the Hydrostatic Spherical Coupling was used to transmit dynamic loads while allowing angular misalignment to ensure the test proceed normally and protect the shaker from damage due to misalignment or angular motion. The energy also could be delivered completely by using the Hydrostatic Spherical Coupling.

(a） spherical coupling conection excitation (b） rigid connection excitation

Fig. 2. the two buffet excitation modes in the trial test

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2.3. The static loads/the conventional fatigue loads simulating

The usual method of applying aerodynamic loads by waffle trees attached to the test article could not be used. Apart from the difficulty in isolating the waffle tree from excessive vibration, the natural response modes of the article would be altered by the added stiffness of the attachments. A new approach based on the use of soft airspring actuators would be used and verified in the test.

2.4. The test rig configuration design

The cantilever beam was clamped to the load bearing wall, three pairs of airsprings were used to load static loads or conventional fatigue loads to the article, and their maximum output were 2000kg ， 1000kg and 200kg. An electromagnetic shaker was used to load the aerodynamic loads, and the load position was in the middle of two pairs of airsprings. The layout diagram of the test rig was referred in Fig .3, and the scene of the validation test was referred in Fig. 4.

Fig. 3. the validation test rig layout diagram

Fig. 4. the scene of the validation test

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3. Dynamic Response Testing

3.1. dynamic response testing with no airsprings loading

The first three frequencies and two points’ acceleration magnification data of the cantilever beam were tested while there were no airsprings pairs loading loads, the data in detail were referred in Table 1.The positions of the two test points were referred in Fig. 3.

Table 1 the frequecies and accelaration magnification data of the cantilever beam while no airsprings pairs loading loads

the 1st frequency the 2nd frequency the 3rd frequency

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

Point 1 28.45 7.13 157.34 7.27 321.55 3.90

Point 2 33.67 2.23 14.77

3.2. dynamic response testing with airsprings loading static loads

The first three frequencies and two points’ acceleration magnification data of the cantilever beam were tested while there were airsprings pairs loading static loads of 4 different conditions. The data in detail were referred in Table 2.The positions of the two points were referred in Fig. 3. The 4 different loading conditions were listed below: one airsprings pair: the middle pair loaded 100kg and 500kg static loads; double airsprings pairs: the middle pair loaded 100kg and 750kg static loads, the free end pair loaded 10kg and

100kg static loads; three airsprings pairs(the 1st condition): the middle pair loaded 100kg and 750kg static loads, the free end pair

loaded 10kg and 100kg static loads, the root pair loaded 10kg and 1000kg static loads; three airsprings pairs(the 2nd condition): the middle pair loaded 100kg and800kg static loads, the free end pair

loaded 10kg and 100kg static loads, the root pair loaded 10kg and 2000kg static loads.

Table 2 the frequecies and accelaration magnification data of the cantilever beam while airsprings pairs loading static loads

The 1st frequency the 2nd frequency the 3rd frequency

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

a Point 1 28.45 7.57 158.24 5.03 316.06 4.77

Point 2 39.10 1.4 12.8

b Point 1 28.50 7.77 157.34 3.57 310.60 4.40

Point 2 39.80 1.27 11.60

c Point 1 28.45 7.90 158.24 3.83 311.27 4.83

Point 2 40.13 1.43 14.60

d Point 1 28.72 7.13 158.24 7.8 311.24 4.93

Point 2 39.80 1.77 15.4

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3.3. dynamic response testing with airsprings loading conventional fatigue loads

The first three frequencies and two points’ acceleration magnification data of the cantilever beam were tested while there were airsprings pairs loading conventional fatigue loads. The data in detail were referred in Table 3.The positions of the two points were referred in Fig. 3.The conventional fatigue loads ,the load amplitude were 80kg and the load frequency were 0.3Hz, were loaded by the middle airsprings pair.

Table 3 the frequecies and accelaration magnification data of the cantilever beam while airsprings pairs loading conventional fatigue loads

The 1st frequency the 2nd frequency the 3rd frequency

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

Frequency

(Hz)

Acceleration

Magnification

Point 1 28.04 7.83 158. 4 5.6 324.6 4.83

Point 2 39.80 1.52 12.93

4. Test Result Analysis

4.1. the frequency change rate

The first three frequencies of the cantilever beam with airspings pairs loading static loads or conventional fatigue loads were compared with its inherent frequencies, and the details was referred in Fig.5.It can be seen that the frequency change rates during 5 different loading conditions changed within 4%, especially the first two frequencies changed even within 1%.The analysis was as below:

Fig.5. the frequency change rate of the cantilever beam while airsprings pairs loading static loads or conventional fatigue loads

The new approach based on the use of soft spring pneumatic actuator indeed added less stiffness to the cantilever beam than the usual method of applying aerodynamic loads by waffle trees, the stiffness of the airbag can be giver by the formula (1) [1,2]:

(1)

Where P is the airbag pressure; A is the cross-section area of the bag; V is the bag/reservoir volume.

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Considering the restraint of the Hydrostatic Spherical Coupling to the cantilever beam, the real modes of the cantilever beam were analyzed in the finite element software, the first three modes were one bending mode, two bending mode and three bending mode separately. The influence on cantilever beam’s frequency while loading static loads or conventional fatigue loads can be expressed by the formula(2) [3], the inherent frequency ignoring the shearing elasticity and the inertia moment can be expressed by the formula(3) [3]:

(2)

(3)

Where: k is the section parameter; is the modal order; E is the elasticity modulus; G is the shearing elasticity

modulus; A is the cross sectional area; ρ is the material density; is the length;J is the inertia moment.

It can be seen from the formula (2) ，the modal frequencies decreased while considering the shearing elasticity modulus and the inertia moment,whose reason is that the effective mass increased and the effective stiffness decreased. And the high-order frequencies decreased more. Analyzing the testing result which is referred in Fig.5, the change rates of the first two frequencies during 5 different loading conditions varied few, and the change rates of the 3rd frequency during the single airspring loading , including the static loads loading and the conventional fatigue loads loading,varied also few.However ,under the situation of the multiple airsprings loading ,the change rates of the 3rd frequency changed more obviously, almost three times of the 3rd inherent frequency. Consequently, the high order frequencies were influenced more when loading multiple airsprings.

4.2. the acceleration magnification change rate

(a) the acceleration magnification changes of the response point 1 while airsprings pairs loading different loads

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(b) the acceleration magnification changes of the response point 2 while airsprings pairs loading different loads

Fig. 6. the acceleration magnification changes of the two response points

The acceleration magnification change rates of two test points(Fig. 3) during 5 different loading conditions varied smooth and steady, except that of the test point 1 under the 2nd frequency. The more details were referred in Fig.6 and the analysis was as below. The acceleration magnification of two test points during the static loads loading and conventional loads loading

remained stable; Referring to the site specific conditions, the reason of the large fluctuating data of the test point 1 under the 2nd

frequency was that no pressure reservoirs were added, and the airsprings without the pressure reservoirs might damp the vibration.

Under different conditions, some acceleration magnification increased and some other acceleration magnification decreased. So it can be concluded that the number and the configuration of the loading airsprings influenced the acceleration magnification, the reasonable load position should be selected to decrease the influence.

5. Conclusion

Aiming at the dynamic response testing of a cantilever beam while the static loads or the conventional fatigue loads were loaded on it, the design and implementation of the test rig was developed. It can be concluded through the testing results: The first three frequencies of the cantilever beam with airbag pairs loading static loads or conventional fatigue

loads were compared with its inherent frequencies. It can be seen that the frequency change rates during 5 different loading conditions changed within 4%, that means the loading airbags would not change the structure inherent dynamic characteristics largely and the loading environment simulated here is close to the reality;

Compared with the first two frequencies, the third frequency changed more when loading the same static loads or conventional fatigue loads, almost 3 times of them.

The number and the configuration of the loading airbags influenced the dynamic response (the natural frequency and the acceleration magnification) of the structure, the pressure reservoir should be added and the reasonable load position should be selected to decrease the influence of the airbags to the structural dynamic characteristics.

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References

[1] D.Graham, D.Symons, D.Sherman, ARL F/A-18 Full Scale Fatigue Test [C], 5th Australian Aeronautical Conference, 13-15 September 1993, Melbourne, Australia.

[2] D.P.Consor, A.D.Graham, C.J.Smith, C.L.Yule, The Application of Dynamic Loads to A Full Scale F/A-18 Fatigue test Article [R], AMRL Report

[3] Jia Qifen, Liu Xijun, Mechanical and Structral Vibration [M], Tianjin, the Tianjin University Press, 2007