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Advanced Structures for Future Aerospace Engineering Haiyan Hu* and Xinwei Wang

Laboratory of Structural Mechanics and Control, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

*Phone: 86-25-84893278, *Fax: 86-25-84891512, *E-mail: hhyae@nuaa.edu.cn

Abstract: The development of aerospace engineering gives rise to an increasing demand for advanced structures made of laminated composite materials, textile composite materials and smart materials. The Laboratory of Structural Mechanics and Control (LSMC) at Nanjing University of Aeronautics and Astronautics has been focusing on maturing the technologies of the advanced structures over the two past decades. This paper describes the related research activities and achievements at LSMC, with an emphasis on the recent advances in smart structures, including those embedded with the functions of health monitoring, vibration control and flutter suppression.

Introduction

As is known, the development of high-performance aerospace vehicles gives rise to an increasing demand for advanced structures made of laminated composite materials, textile composite materials and smart materials. The Laboratory of Structural Mechanics and Control (LSMC) at Nanjing University of Aeronautics and Astronautics has been focusing on the researches of advanced composite structures and smart structures over the past two decades.

The purpose of this paper is to provide the aerospace research community with the information on the research activities related to “Aerospace System- Structures and Materials” at LSMC. The other purpose is to share the recent achievements related to the advanced structures made at LSMC with others. The paper will focus on the two major categories: composite structures and smart structures.

Composite Structure Technology

The first test using a full-size wing composite box in China was performed at LSMC several years ago to identify some critical issues in the damage tolerance and durability design of composite structures. To get more information from the expensive full-size testing box, a program integrating the static, damage tolerance and durability tests was proposed and successfully verified. From the static, damage tolerance and durability experiment, it was found that man-made de-laminations did not propagate during the entire fatigue test. In other words, the composite structure was safe if it passed the test of static strength since the design was conservative enough. Thus, new design criteria should be established. It was also identified that proper design of connections via either bolts or adhesive was critical and important, and that new NDT methods, other than C-scan and acoustic emission, to detect damages were also needed.

Analysis approaches for composite structures with multiple inclusions or cracks

For the proper design of connections in composite structures, various specimens as shown in Fig. 1 were made and tested to understand the behavior of composite connections with help of different methods. Although FEM codes enable one to make the stress analysis for mechanical connections

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shown in Fig. 1, but they are not efficient and appropriate for design purpose. Hence, Xu et al. [1,2] proposed an efficient method to determine the accurate stress distributions in a finite laminate plate containing multiple inclusions and cracks arbitrarily located. The method uses the complex potential in the plane theory of elasticity of an anisotropic body, together with the Faber series expansion and the least squares boundary collocation technique. They also developed the codes for the stress analysis of mechanical connections in composite structures with bolts modeled as hard inclusions so as to determine the stress distributions around the holes easily. They analyzed a great number of real connections and their numerical results well agreed with experimental data. As shown in Fig. 2, their method is also able to predict the residual strength of the delaminated composite with an appropriate strength criterion if the delaminated area is modeled as a soft inclusion.

Figure 1 Various jointed composite specimens Figure 2 Residual strength prediction [2]

New NDT approaches

Figure 3 Damage detection by using a Laser-Shearography System [3]

As is well known, the invisible damages, caused by an improper manufacturing process or a low energy impact such as a tool dropping in everyday maintenance, are extremely harmful to the safety of composite structures. The invisible de-lamination of composite material may greatly reduce its

a. A helicopter blade b. The shearography diagram of marked region

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compressive strength. Thus, it is may not be possible to guarantee the structure integrity even under normal operation conditions if the structure involves some invisible de-laminations. Therefore, advanced NDT technologies are essential to ensure the safety of the composite structures.

There are several NDT technologies available for detecting the damages in the laminated composites. C-scan and acoustic emission are popular techniques. However, it is difficult to use them to detect the damage occurring during the experiment. For example, C-scan technique can only detect the damages of a specimen at very low scanning speed, and is not applicable to a full-size wing composite box. Laser-Shearography technique is a relatively new technology and has several advantages over C-scan technique. It is able to detect the damages of a specimen in air at much faster scanning speed, and is even applicable to outdoor tests. Tang and Wang [3], therefore, turned to the damage detection by using a Laser-Shearography system (Q-800) made in Germany, and performed a few tests to check if the Laser-Shearography system is able to find the real invisible damages or defects. They checked the quality of a pilot-less helicopter blade as shown in Fig. 3a by means of the Laser-Shearography system and spent only 15 minutes to complete the manual check for the entire blade. The butterfly image in Fig. 3b indicates a defect or damage in the rectangular region in Fig. 3a. They did find a small inclusion of 3.5-4mm in diameter later. Preliminary testing data show that the Laser-Shearography system does have the above-mentioned advantages. However, one cannot judge, from the butterfly image alone, how big the damage or defect is. This open problem needs further investigations.

The above NDT technologies, such as C-scan, acoustic emission, and shearography, are falling into the category of passive methods. The active techniques, such as structure health monitoring, will be addressed in the section of “Smart Structure Technology”. Crashworthiness of composite structures

Crashworthiness, as a major concern in the design of helicopters, has promoted the study on the energy-absorbing capability of composite structures since advanced composites are brittle materials. Different composite waved beams has been manufactured and tested at LSMC over the past years to identify their energy-absorbing capability as they are frequently used in the sub-floor structures. Fig.4 shows three typical case studies in the crush tests of composite waved beams. The tests indicated that the crushing process was stable as shown in Fig. 4a and Fig. 4b if the waved beam was properly designed, and unstable as shown in Fig. 4c if the beam was improperly designed. In a stable crushing process, the specific energy-absorbing capability of composite was better than that of metal structures since the mass density of the composite is much lower than those of metals. The energy-absorbing mechanism of composite materials is quite different from that of metals. The former is due to crushing and the latter is due to buckling and plastic deformations. From the tests, two key factors to success were identified, namely, the proper stacking sequence of the laminations and the proper design of the triggers. As the strength and the crashworthiness of composite structures are contradicted each other, how to balance them remains a challenging task for the designers. A research on controllable or smart triggers is being undertaken at LSMC to accomplish this challenging task.

At LSMC, the energy-absorbing capability of composite structures was also investigated by using FEM codes. For example, MSC/DYTRAN was used to study the crashworthy behavior of some typical composite structures for different design parameters, such as the material properties, lay-ups, structural element formats and different trigger geometries. Wang et al. [4-6] proposed a trigger element to model the trigger mechanism and established a damage model to capture the crushing

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process of the composite materials [5, 6]. Fig. 5a shows the experimental load deflection curve for a beam shown in Fig. 4b. Fig. 5b shows the simulated load deflection curve for the same beam. It is easy to see that the simulated peak load and the average load during crushing process agree well with those of experimental data. The simulated deformation shown in Fig. 5c is similar to the test configuration shown in Fig. 4b. Therefore, the simulated information enables designers to reach a better crashworthy design for composite structures.

a. Crushing from top b. Crushing from bottom c. Broken at the middle

Figure 4 Quasi-static crushing test of weaved beams made of composite materials

a. Test data b. Numerical results c. Numerical simulations

Figure 5 Experimental data and numerical simulations of the composite waved beam [5]

Textile composite materials

To overcome the inherent de-lamination in the laminated composite materials and to raise the material’s impact resistance, the substitutions are textile composite materials, such as stitched laminates, 2D or 3D woven laminates, 2D and 3D braided composite materials and integrated composites. Fig. 6 gives examples of several kinds of composite materials investigated at LSMC. Fig.6a shows a 3D braided specimen fabricated at LSMC by RTM (Resin Transfer Molding), a low-cost composites fabrication method. Fig.6b shows six stitched laminated composite specimens. Fig.6c shows two fabricated integrated specimens with different thickness, and Fig. 6d shows various

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products fabricated at LSMC. To characterize these new kinds of composite materials, a series of standard material tests have been and are being performed. As an example, the preliminary experimental results of the integrated specimens showed that the tensile strength along the thickness direction of the integrated material increased by 75% as compared with that of the honeycomb sandwich plate with same faces and thickness. The compressive strength along the thickness direction and the double shear strength of the new material also increased by about 10% and 20%, respectively, as compared with those of the honeycomb sandwich plate with same faces and thickness. The bending stiffness of the new material was similar to that of the honeycomb sandwich plate. Relatively speaking, the material properties of the integrated composite materials are better than those of the honeycomb sandwich plate.

It is a fact, however, that the theoretical developments for textile composites are far more left behind than their practical applications. The reason is perhaps due to their inherent complex architectures. Besides, it is difficult to measure the internal non-uniform strains experimentally [7] and to observe the damage mechanism at compressive or shear loadings. Hence, little work has been successfully done on the

a. 3-D braided specimen before fabrication by RTM

b. Stitched laminated composite specimens

c. Two integrated specimens with different thickness

d. Integrated specimens with various shapes

Figure 6 Braided and integrated materials

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theoretical development for the strength prediction. For effective use and design of textile composites, designers should understand their mechanical behavior clearly. Currently the mechanical behavior is determined via various material tests. This way is obviously expensive and inefficient. Hence, Wang et al. [8-11] made great efforts to study textile material characterizations via numerical simulations.

Figure 7 Selection of RUCs [9]

The textile materials have obvious four scales, namely, the fiber scale, fiber yarn scale, meso-scale

(fiber yarns and matrix) and macro scale. Thus, multiple-scale analyses have been performed to obtain textile materials’ properties, including the stiffness and strength. Multiple-scale analyses are based on the model of Repeated Unit Cells by assuming a periodical distribution of the reinforcing phase. Thus, how to select RUCs and apply the periodic conditions properly is the key to success. Recently, Xia et al [9] proposed a simple way to correctly apply the periodical conditions in the stress analysis of finite element models. The proposed method results in not only displacement continuity along the boundary as achieved by other available methods, but also stress equilibrium along the boundary as not achieved by some methods. Hence, the unique solution could be obtained for the same material even with different selections of RUCs. Fig. 6a shows two selections of RUCs for an ideal plane weaved material, i.e., hexagon (A) and parallelogram (B). Fig. 6b shows the established finite element model of parallelogram (B). The predicted results by the finite element method, e.g., stresses and strains

a. Different RUCs b. FE model of parallelogram (B)

Figure 8 Deformation under pure shear loading [9] Figure 9 Comparisons of test data with FE predictions

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along line AB, are the same by using the proposed method to apply the periodical conditions. It is also observed from Fig.7 that the assumption on “plane keeping plane” made by some researchers is no longer valid for pure shear loading. With help of commercial FE codes and the simple way to apply the periodical boundary conditions, multiple-scale analyses have been successfully performed to predict the material properties for stitched laminates and weaved composites [9-11]. Fig. 9 gives the comparison of experimental data with FE predictions for the stitched composite shown in Fig. 6b [10]. As can be seen, the predicted inter-lamina strength agrees well with the test result. It is also shown that the inter-lamina strength of the stitched composite increases about 20% as compared with the un-stitched laminated composite.

It should be pointed out that it is not an easy task to establish the finite element model of an RUC

of textile materials. Fig. 10 shows a finite element model of the RUC for a plane weaved composite. The complicated cross-sectional shape of fiber yarns renders the modeling a difficult task. Recent experiment showed that the cross-sectional shape of fiber yarns changes at different positions for a 3D braided composite material. Fig. 11 shows the cross-sections at two different locations. To see the cross-section of the fiber yarns clearly, a few carbon fiber yarns (black color) were used besides the glass fiber yarns. It can be seen that the shape of cross-section of fiber yarns are complex, either elliptical, or rectangular or irregular. Therefore, how to model the shape changes of cross-sectional shape is a challenging task in FE modeling. The research is undertaken to establish the relations among the cross-sectional shape of the fiber yarn with the braiding craft and RTM technics.

Smart Structure Technology

Smart structure technology is a promising technology for aerospace engineering. However, the design and manufacture of smart structures are far more difficult than those of composite structures since the function failure in a smart structure alone may give rise to catastrophic failure of the entire aerospace vehicle [12]. LSMC is the first Chinese team to study smart structure technology [13, 14], and has been focusing on four topics, i.e., sensor and actuator technology and system integration, structural health monitoring and management, adaptive wing and smart rotor, active vibration control and flutter suppression. In what follows, the paper describes some related research activities and achievements on the smart structure technology at LSMC.

Figure 10 A FE model for an RUC of weaved composite [11]

Figure 11 Different cross-sectional shapes of fiber yarns in the same 3D braided material

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Sensors, actuators and system integration

Smart structures are based on various kinds of sensors and actuators, such as PZT or PVDF sensors and actuators, MR dampers, SMA actuators, and different optical fiber sensors, including distributed, Bragg, B-P, and polarization optic fiber sensors. Some of them have been commercially available, and others have been made at LSMC.

Although PVDF fibers and PVDF films posses the advantage of easy-to-integrate with host structures and are commercially available, their output force is too small for practical actuator applications. On the other hand, PZT materials have greater piezoelectric constants and Young’s modulus, but it is more difficult to fabricate PZT materials in sheet and fiber geometries. Thus, the development of piezoelectric materials and devices with geometrical shapes of easy integration has become an important subject of researches. For easy-to-integrate with host structures, PZT smart layer is a choice to make smart structures. Yuan et al. [15] developed the smart layers to integrate the PZT sensors/actuators and/or optical fiber sensors and applied them successfully to developing several structural health monitoring systems. Fig. 12 shows the PZT smart layers (Fig. 12a), and the PZT and optic fiber smart layers attached on structures (Fig. 12b and Fig. 12c) for the purpose of damage detection and load identification.

a. PZT smart layers b. PZT smart layers on a structure c. Optic fiber smart layers

Figure 12 Smart layers [15]

LSMC members have made great efforts in developing other piezoelectric ceramic actuators of high performance. For example, Qiu et al. [16] developed the fabrication technique, as shown in Fig. 13, of thin PZT (Pb(Zr53Ti47)O3) and PNN-PZT (Pb(Nb2/3Ni1/3)55-(Zr30Ti70)45O3) sheets (each about 20∼100µm in thickness). The piezoelectric property was improved by as much as 40% with the hybrid sintering process as compared to the traditional sintering method. Such kind of thin piezoelectric sheets has the advantage of easy integration when they are used as sensors and actuators in multiplayer smart composite materials.

Qiu et al. [17] also invented a new kind of piezoelectric fibers with metal core so as to enhance the strength of PZT fibers. Fig. 14 shows the configuration of the apparatus for extrusion of green fibers. The extruded green fibers were dried at 80°C and then heated to 800°C to burn out the organic

Pre-calcination

Crushing

1st Mixing

Raw Powders PbO, TiO2, ZrO2

Nb2O5 , NiO, CdO,SiO2

Ball milling:24hr

Temperature:1023, 1073, 1273K

Ceramic Mortar

Filtering

Deair

Casting

Punching

Sintering

Vacuum Stirring

Doctor BladeThickness :30µµµµm

Temperature:1173 ~ 1473K

Fabrication proceduresFabrication procedures

3rd Mixing

2nd Mixing

Ball milling:24hr

Dispersant Solvent

Plasticizer Binder

PSZ Dispersion elements

Figure 13 Fabrication procedures of sheets [16]

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additives. Fibers of 250 µm in diameter and a few centimeters long were fabricated without any cracks. Fig. 15 shows the cross-section between the core and Pb(Nb,Ni)O3-Pb(Zr,Ti)O3 (PNN-PZT) ceramic. The boundary between the metal core and PNN-PZT ceramic in the figure looks perfect.

a. Apparatus for extrusion b. The Pt core inside the core guide

Figure 14 Configuration of the apparatus for extrusion of green fibers [17]

a. Cross-section of a fiber b. Cut in the longitudinal direction

Figure 15 Cross-section between the core and PNN-PZT ceramic [17]

Recent years have witnessed an increasing interest toward Magneto-Rheological (MR) dampers in the semi-active control of vibration. However, an MR damper usually has a complicated nonlinear relation between the restoring force and the input voltage so that either a neural network or a fuzzy logical system is often used to determine the input voltage for a required restoring force in semi-active vibration control. Sun and Hu [18] invented a stepped MR damper of rotary type in Fig. 16a, which has 12 input coils inside as shown in Fig. 16b. Hence, the restoring torque of the stepped MR damper has 13 levels, including the level without input voltage. The simplified model of the MR damper in Fig.16c gives the description of the restoring torque as following

120))(sgn()12( ≤≤+−+= NFtFNNFF SNBD ω ,

ωηωτθθτ )(32))(sgn(dd),(2 3

132

21

220

2

1

RRH

RRrrrF Y

R

R BB −Θ

+Θ−== ∫ ∫Θ

,

10

)()(3

2dd),(2 31

32

00

2

1

tRRH

rrrFR

R NN ωηθθτ −Θ

== ∫ ∫Θ

,

where FS is the torque caused by the friction in seals and bearings, BF is the restoring torque of the Bingham fluid subject to the magnetic field of a powered coil, NF is the restoring torque of the Newtonian fluid corresponding to a coil with power off, N is the number of powered coils, R1 and R2 are the inner radius and outer radius of a coil,• is the radial size. The experiments well support the above description. Hence, the control system using such a stepped MR damper requires no neural network or fuzzy logical system to determine the input voltage, but select different sets of coils only.

a. Outside of the MR damper b. Inside of the MR damper c. Model of analysis

Figure 16 A stepped MR damper [18]

As is known, Ni-Ti SMA is one kind of actuating material widely used in smart structures. At LSMC, SMA wires have been used to develop the pipe-like actuators shown in Fig. 17 so as to produce torsional deformation. Fig. 18 shows the measured torsional angle with an increase of temperature. The maximal torsional angle can reach about 1.8° at 80°C.

Structural health monitoring

As mentioned above, structure health monitoring is a new technology of active category. Identifying the magnitude and position of impacting loads, detecting the de-lamination or crack are falling into the active category of structure health monitoring. In what follows are a few examples of researches and achievements made at LSMC.

Figure 17 SMA tubular actuators Figure 18 Torsional angle-temperature

10 20 30 40 50 60 70 80 90 100Temperature (C)

0.00.2

0.40.6

0.81.0

1.21.4

1.61.8

2.0

Tor

sion

ang

le (d

eg.)

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To test the applicability of newly fabricated piezoelectric fibers used as sensors, Qiu et al. [17] performed an experiment to identify the impacting loads on a composite plate. They fabricated the GFRP composite board from four layers of prepreg with two different orientation angle, namely, [0°/90°]s, and placed the PZT fibers with Pt core on the second layer whose orientation angle is 90° related to the first layer, as shown in Fig. 19. The four layers of prepreg and fibers were laminated and cured in an autoclave. They paid attention to the case studies for different diameters of Pt core and different lengths of the fiber, and embedded 4 pieces of PZT fibers in the GFRP composite board, as shown in Fig. 20. Fiber A is 170µm in diameter of Pt core and 40mm in length, fiber B is 225µm in diameter and 40mm in length, fiber C is 170µm in diameter and 60 mm in length, and fiber D is 245µm in diameter and 40 mm in length. The length, width and thickness of the GFRP composite board are 250 mm, 250 mm and 1 mm, respectively. Experimental results showed that the position of the impact could be determined with relative errors of less than 11% in the distance between the true and identified locations [17]. How to improve the identification further by using piezoelectric fiber sensors is being undertaken.

Figure 19 Schematic diagram of GFRP board [17] Figure 20 Impact position on smart board [17]

An early study on the structural health monitoring at LSMC was to integrate various sensors and actuators into a composite plate and to realize the functions, such as the load identification and the loosing bolt detection. For this purpose, LSMC established an experimental system for the composite structure with self-diagnostic and adaptive strength functions. Fig. 21 shows the sketch of this composite structure health monitoring system, where piezoelectric elements and strain wires were embedded into a laminated composite plate as sensors arranged in dot matrix, and one PZT element was embedded in the laminated composite plate as an actuator. On the basis of such a composite structure health monitoring system, Yuan et al. [19] recently developed the methods of characteristic signal extraction and recognition with wavelet and neural network for damage detection. They also studied other signal processing method, such as Lamb wave based diagnostic method [19] and Hilbert-Huang Transform approach [20], for damage detections. Experimental results showed that the experimental system had the capability of identifying the magnitude and position of a concentrated static load accurately, locating a loosing bolt each time, and determining the location of an impacting load. Fig. 22 show the identified locations of a loosen bolt and a hammer impact, respectively.

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Figure 21 Sketch of an experimental system for composite structure health monitoring

Figure 22 Two typical functions of the composite structure health monitoring system

For applying the structural health monitoring system to a real aerospace structure, different kinds of density sensor networks using different theories and having different functions are required to monitor various parameters, such as stress, stain, displacement, acoustic, pressure, and temperature [15]. However, the information sensed and the local signal processing ability are limited, while real aerospace structures are very complicated systems and have a strict restriction on their weight. It is a challenging task, therefore, to reduce the weight of the sensor networks, to coordinate and manage the density sensor networks, and to fuse the information from different kinds of sensors taking the advantage of various estimation methods in order to make a reliable estimation for the entire lightweight aerospace structure at an acceptable speed.

At LSMC, Yuan et al. [15] studied the distributed structural health monitoring technology for a real-scale wing shown in Fig. 23. They bonded the PZT and optic fiber smart layers, shown in Fig. 12, on the inside face of the skin in Fig. 23 for detecting the damages and identifying concentrated loads. Fig. 24 shows a sketch used for the health monitoring system of wing box based on the PZT sensor array. They used the active Lamb wave diagnostic method [19] for damage detections and HHT approach [20] for de-noising purpose. In this system, there are totally 24 PZT sensors to form the

a. Detecting a Loosing bolt b. Detecting the position of an impacting load

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monitoring network for a quarter of skin as shown in Fig. 25. The circular pad of PZT has 8mm in diameter and 0.8mm in thickness. Any of the 24 PZT sensors can be assigned by the computer to act as either an actuator to excite the skin or a receiver to sense the lamb wave of skin. The damage monitoring process is controlled by an integrated sensor array scanning and damage diagnostic system including hardware and diagnostic software.

Figure 23 A wing box with stiffened composite skins [15]

Figure 24 Sketch used for PZT sensor array based structural health monitoring system [15]

Fig. 25 shows the software interface of the damage diagnostic software. The right-top subfigure gives the excitation signal, and the right-bottom subfigure is the monitored signal before the damage and after the damage. The left subfigure is the on-line monitoring result. The system can monitor the emergence of multiple damages because it is a scanning system,. Fig. 26 shows the main hardware used in the structural monitoring system.

Fig. 27 shows the entire experimental set-up; where the damage can be simulated by stick a thick

a. Aluminum skin

b. Stiffened composite plate c. Skin dimensions

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tape on the surface. The experimental results showed that the system can detect the damage location reliably. Further research is currently going on to improve the diagnose speed, and to detect the real damage caused by the low energy impact.

The distributed structural health monitoring

based on a wireless smart sensor array is an interesting topic and has great potential. LSMC has started the research on smart wireless sensor technology. Preliminary results showed that, however, there are a number of important issues needed to be addressed for applying this new technology in practice. For example, the hardware and software of the smart wireless sensor platform need to be improved in order to develop complex structural health monitoring systems, and most smart wireless sensor platforms commercial available at present have some limitations, such as limited sampling rate, limited memory, limited processing speed and limited data transmission ability. Smart rotor and vibration control

To study the smart rotor, Chen et al. [21] invented a non-contact rotor testing system. Fig. 28 is a schematic diagram of the signal modulation and transmission system. Multiple testing signals and power supplies can be transmitted between the rotating blades and the fixed base via a novel non-contact electronic magnetic coupling system, as shown in Fig. 29. The testing system shown in Fig. 29b contains 18 channels for signal transmitting and 6 channels for power supply transmitting. The signal and power supply transmitting system is much lighter than the slipping rings conventionally used. They proposed the adaptive filtering feed forward control [21] and performed a

Figure 25 The software interface [15] Figure 26 The hardware [15]

Figure 27 The experimental set-up [15]

See Fig. 21

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number of vibration control tests for rotating blades on the test rig. Their experimental results showed that controlling the piezoelectric actuators could reduce the vibration of composite rotor blade, and that adaptive filtering feed forward control was a powerful real time strategy when the main vibration frequency was related to the rotating speed.

Fig. 28 Schematic diagram of signal modulation and transmission system [21]

a. Sketch of non-contact rotor testing system b. Test rig

Figure 29 Experimental study on smart rotor system [21]

0 . 0 0 1 0 . 0 0 2 0 . 0 0 3 0 . 0 0 4 0 . 0 0 5 0 . 0 0T i m e ( S ) - 4 . 0 0

- 2 . 0 0

0 . 0 0

2 . 0 0

4 . 0 0

V o l ta

g e

( V )

Figure 30 Test rig of active vibration control for a flexible structure

Figure 31 Time histories of uncontrolled (black color) and controlled (red color) beams

Adaptive

Vibration Monitoring

Conditioning

Data

and Transmission

Measurement Signal

Power Coupling

Control Signal Coupling

Sensor/Actuator Adaptive Blade Model

Reducer Motor

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Fig. 30 shows the test rig of active vibration control for flexible structures, such as the power sail of satellite, at LSMC. Fig. 31 shows the time histories of uncontrolled (black color) and controlled (red color) beams. In this study, a separation principle and optimization method was proposed to determine the locations for sensor and actuator.

Recently, Sun and Hu [22] proposed the technique of semi-active flutter suppression through the use of the stepped MR damper mentioned above, and conducted the wind tunnel tests for the wing section including a control surface as shown in Fig. 32. They established the aero-elastic model of the wing section with a stepped MR damper installed between the wing section and the control surface, and designed several control strategies, such as the on-off control, sub-optimal control, and robust control. Both numerical simulations and the wind tunnel tests showed that the semi-active control based on the stepped MR damper was able to control the flutter successfully as shown in Fig. 33, and to increase the critical flow speed by 26.2% in the wind tunnel test.

0.0 0.1 0.2 0.3 0.4 0.5

-0.04

-0.02

0.00

0.02

0.04 Controlled No control

Flap

/(rad

)

Time/(s) Figure 32 A wind tunnel test for

semi-active flutter suppression [22] Figure 33 Time histories of uncontrolled flutter

and controlled flutter [22]

Closing Remarks The Laboratory of Structural Mechanics and Control (LSMC) at Nanjing University of Aeronautics

and Astronautics has made great efforts, over the past decade, to promote the technologies of advanced composite and smart structures for future aerospace engineering. However, there are still a great number of open problems before those advanced structures become matured. At current stage, it seems more and more necessary to initiate the joint projects among universities and companies, and among those from different countries, to focus on the applications of those structures to real aerospace systems.

Acknowledgements

The authors greatly appreciate the kind help of their colleagues, especially Professors XW Xu, GM Zhou, CW Zhou, SF Yuan, JH Qiu, and RW Chen, who provided their published papers or unpublished contributions.

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