Composites Science and Technology 70 (2010) 1072–1079

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Composites Science and Technology

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Smart tetrachiral and hexachiral honeycomb: Sensing and impact detection

H. Abramovitch a, M. Burgard b, Lucy Edery-Azulay a, K.E. Evans c, M. Hoffmeister b, W. Miller c, F. Scarpa d,*,C.W. Smith c, K.F. Tee d

a Department of Aerospace Engineering, Technion Institute of Technology, Haifa, Israelb Fraunhofer IPA, Nobelstrasse 12, D-70569 Stuttgart, Germanyc School of Engineering, Computing and Mathematics, University of Exeter, EX4 4QF, UKd Department of Aerospace Engineering, University of Bristol, BS8 1TR, UK

a r t i c l e i n f o

Article history:Received 19 November 2008Received in revised form 27 July 2009Accepted 30 July 2009Available online 3 August 2009

Keywords:A. Smart materialsB. VibrationC. Sandwich structuresD. Non-destructive testingE. Injection moulding

0266-3538/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.compscitech.2009.07.017

* Corresponding author. Address: Department of Aesity of Bristol, Queens Building, University Walk, BS8 19289861; fax: +44 0117 9272771.

E-mail addresses: f.scarpa@bris.ac.uk, scarpa.fabriz

a b s t r a c t

In this work we present concepts and prototypes of a novel class of chiral honeycomb core with embed-ded sensing characteristics for potential structural health monitoring (SHM) and other multifunctionalapplications. The cellular structure concepts, all having negative Poisson’s ratio behaviour, have piezo-electric sensors and their hardware support embedded on the surface, or within the unit cell plates. Boththe sensors and the infrastructure provide not only the capability of detecting signals proportional to theexternal mechanical loading, but act also as load-bearing units. The honeycombs have been producedusing vacuum-casting techniques and resin transfer moulding methods, with micro fibre composites(MFCs) embedded in their cell walls. The sensing and mechanical performance of the prototypes are eval-uated using finite element simulations, static tests, broadband vibration excitation and impact at lowkinetic energy levels using an airgun.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Chiral structures are a subset of negative Poisson’s ratio (auxet-ic) solids, displaying rotative, but not reflective symmetry. Thenegative Poisson’s ratio (NPR) behaviour is a characteristic featureof materials that expand in all directions when pulled in only one,therefore exhibiting a counterintuitive deformation mechanismwhen compared to ‘‘classical”, i.e., positive Poisson’s ratio materi-als. NPR does not contradict classical elasticity theory, e.g. thebounds for isotropic solids are +0.5 6 m 6 �1.0, but it is only anindication of unusual volume deformation under loading condi-tions [1]. A classical example of auxetic honeycombs is the re-en-trant or ‘‘butterfly” centresymmetric configuration, which isderived by classical hexagonal unit cell shapes [2]. Centresymmet-ric honeycombs, both with positive or negative Poisson’s ratio, arespecial orthotropic materials [3]. Conversely, chiral structures areisotropic in the baseline plane, with a Poisson’s ratio close to �1– the limit of classical elasticity theory for unconstrained solids.Structural chiral honeycombs can have 6, 4 and 3 connectivities(hexachiral, tetrachiral and trichiral configurations), all of thesestructures consisting of cylinders connected with tangential liga-ments [4]. Tetrachiral and trichiral configurations can have also a

ll rights reserved.

rospace Engineering, Univer-TR Bristol, UK. Tel.: +44 0117

io@gmail.com (F. Scarpa).

further layout, the anti-chiral structure, which displays both reflec-tive and rotational symmetry [4]. As a structural concept, the hex-achiral structure was proposed by Roderic Lakes and co-workers[5,6], although the chiral topology and its NPR behaviour has beenidentified for the first time by Wojciechowski and Branka [7], andalso investigated in other works [8–10]. The flatwise compressiveproperties of hexachiral honeycombs have recently been investi-gated by Spadoni et al. [11], and Scarpa et al. [12], while Spadoniet al. have also analysed the vibroacoustic and wave propagationbehaviour [13–15]. The hexachiral cellular structure has also beenevaluated to design and manufacture innovative airfoil and wingbox concepts for morphing wing applications [13,16,17]. Apartfrom the synclastic curvature feature common to auxetic materials[1], the chiral honeycomb structures also provide the possibility topartially decouple failure loads in out of plane shear and compres-sion. This is because the cylinders provide enhanced compressivestrength, while the ligaments resist shear, enabling honeycombsandwich cores with properties tailored to a specific application[12,18] The chiral structures are also suited for use with embeddedactuators or possible morphing characteristics due to their connec-tivity. When one ligament is put into flexure, it causes the attachedcylinders to rotate, therefore causing other adjacent cylinders torotate and ligaments to flex. This deformation mechanisms act asa mechanical multiplier device, which has been recently used forshape memory alloy-based deployable truss structures [19,20].

Due to the importance of sandwich structures in aerospace [21]and marine applications, a consistent body of research work has

H. Abramovitch et al. / Composites Science and Technology 70 (2010) 1072–1079 1073

been undertaken to assess the structural integrity of these struc-tures, both with online and off-line techniques [21]. For sandwichstructures, in particular, global modal approaches have been tradi-tionally considered to detect damage or crack propagation [22].The sandwich structures are generally equipped with piezoelectricor piezoresistive sensors attached (or embedded) to the face skins,using also novel sensors and locations techniques [23]. However,the use of the face skins as platforms for the sensing capabilitytend to limit the acquisition of signals from inside the core, wheredifferent types of failures can occur, like dimpling and shear crimp-ing [27]. The positioning of a sensor platform inside the sandwichpanel (i.e., between the face skins) would also protect the hard-ware infrastructure associated with the sensing process from anyexternal loading, or harsh environmental conditions. Embeddedactuators have been incorporated inside an adaptive antennareflector, but as an active layer to provide the deployability ofthe system [24]. More recently, fibre Bragg grating (FBG) sensorshave been applied between a CFRP facesheet and the adhesive join-ing the honeycomb sandwich to track the onset of debonding be-tween the face skin and the cellular core [25]. However, to thebest of our knowledge, no attempt has been made so far to embeda sensor within the plates of a unit cell honeycomb or truss core, orbonded to the surface of the single unit cells. The presence of thesensor system inside the cell microstructure would allow to usealso the sensor, and its supporting infrastructure, as load bearingelements of the sandwich panel, without reducing the contact areabetween the cellular core and the face skins, which is one of themajor causes of debonding in sandwich elements [27]. In this work,we demonstrate how the chiral multifunctional honeycomb con-cept [4,28] could be used to produce such a cellular structure withembedded sensors for structural health monitoring (SHM), or ac-tive microwave absorption characteristics. The rationale behindthe use of the chiral structure is the relative mechanical decouplingof the ligaments and cylinders of the unit cell under out-of-planeloading, where the ligaments carry out the majority of transverseshear, and the cylinders the flatwise compression [12,18]. The chi-ral honeycombs have been manufactured using rapid prototypingtechniques, and MFC/PZT sensors embedded directly inside thecore (ligaments) [26]. Sensing and actuation capabilities have beenevaluated simulated and evaluated using finite element (FE) mod-els and experimental techniques, using Scanning Laser Vibrome-ters and acoustic waves. The frequency response functionsobtained from the embedded MFCs under random vibration excita-tion have been measured to assess the feasibility of the smart hon-eycomb for SHM techniques based on modal analysis, while thetime histories recorded under low kinetic energy impact can indi-cate the likelihood of damage tolerance monitoring [21]. The re-sults show the potential of using this type of honeycomb for

Fig. 1. (a) Layout of micro fibre composite and (

complex SHM applications, allowing the health monitoring processto be applied to the honeycomb itself, rather than relying on patchesapplied to the face skins of sandwich panels. To the knowledge ofthe authors, these are the first attempts to build a honeycomb mate-rial with sensing infrastructure embedded in the core unit cell,where the same hardware infrastructure has load bearing functions.

2. Manufacturing, modelling and testing

2.1. Sensors and tetrachiral cell prototype

The class of sensing/actuators used for the chiral honeycombs isthe Macro Fiber Composite (MFC), a planar piezoelectric deviceconsisting of rectangular cross-section PZT fibers embedded in anepoxy matrix and sandwiched between Kapton�-clad copper inter-digitated electrodes, produced by Smart Material Corp, Sarasota, FL(Fig. 1a). Its lightweight, robust construction and increased actua-tion strain energy density have made it a popular choice for struc-tural control or as sensor device in various applications in theaerospace, automotive, and consumer products industries [26].The MFCs used in this work consists of rectangular uniaxiallyaligned piezoelectric fibres sandwiched between layers of adhesiveand electrode polyimide films. Using non-metallised PZT the elec-tric field couples between neighboured finger electrodes of differ-ent polarity in the fibre direction (d33 effect), generating strainwhen embedded within a structure. The MFCs in d33 mode [26]had an operation voltage range between �500 V and +1500 V,a capacity of 0.42 nF/cm2, a piezoelectric coefficient d33 of460 pC/N and dynamic range between 0 and 4 kHz. The actuationauthority was between 0.7 and 0.9 lstrain/V, with a charging gen-erator capacity of 1670 pC/ppm above 100 V. A multi-step lamina-tion process was applied to the top MFC/electrode structure, withthe application of wires to the contact pads for the embedding intothe chiral structures (Figs. 1b and 2a).

The single cell tetrachiral prototype (Fig. 1b) was manufacturedusing a vacuum casting technique where both the mixing and cast-ing process occur under vacuum. This process prevents the airbeing mixed into the resins and removes gasses produced duringcure. The material used was 8040 2-part resin (MCP Equipment),which was chosen as it is relatively low viscosity and well suitedto casting structures with high aspect ratio features, such as theligaments in the chiral structures. MFC patches (Smart MaterialsCorp., Sarasota, FL 34236) were placed in the mould prior to castingand held in place with spacers of the bulk resin material. Resinswere mixed and cured according to the specifications providedby the manufacture, but it was necessary to provide extra ventingfor the mould to allow the gasses produced during cure to escape

b) tetrachiral unit cell with embedded MFC.

Fig. 2. FE model of the tetrachiral unit cell.

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before the resin hardened. The manufacture unit cell has a riblength of 47.5 mm, wall thickness of 2.4 mm, 19.8 mm of outerdiameter of the cylinder, and 45 mm of depth through thethickness.

A 3-D model (Fig. 2), based on the finite element method usingthe ANSYS code [30], was applied and used to prototype the staticresponse, modal analysis, and sensing and actuation authority. Thehost structures were assumed to be made of the cast resin (isotro-pic material: E = 490 MPa, t = 0.32, q = 1130), and the piezoelectricpatches were considered being made of a piezoceramic material,Macro Fiber Composite (MFC) type, with the following mechanicalproperties [29]:Ex = 30 GPa, Ey = Ez = 15.5 GPa, Gxy = 5.7 GPa,Gxz = Gyz = 10.7 GPa, q = 470 kgm�3, tyz = txz = 0.4, tyz = 0.35, e33 =15.1 C/m2, e32 = e31 = �5.2 C/m2. The host structure was modeledusing SOLID45 brick linear 8-nodes elements, and three DOFs pernode (UX, UY, UZ). The piezoelectric patch was modeled using SO-LID226 elements. These elements have twenty nodes with up tofive DOF per node (UX, UY, UZ, TEMP and VOLT). KEYOPT (1001)of this element activates the relevant DOF for piezoelectric ele-ment: displacements and voltage. A perfect bond between the hoststructure and the piezoelectric strips is defined via the GLUE op-tions. The electrodes surfaces are modeled by coupling the volt de-grees of freedom (DOF) on the major surfaces of the electroderegion. The electrodes are simulated by using CP command inANSYS. This set all the potentials on a face to the same prescribedvalue. For modal analysis: Coupled-mode natural frequencies werecalculated for short circuit (resonance) case. The closed-circuitedcase is specified by setting the potentials on both electrodes ofthe piezoelectric patch to zero, called grounded electrodes. This

Fig. 3. (a) Detail of a MFC applied to the internal cell wall of the RTM hexachiral honebroadband base excitation. The silver spots are elements of reflective paper for the lase

situation yields the resonant frequencies. For the actuator simula-tion, one electrode was grounded (voltages are set equal to zero)while a given voltage, V0, was applied to the other electrode. Forthe sensor simulation, only one electrode is grounded.

The vacuum cast sample with the embedded MFC patch wasloaded under flatwise compression test using a servo-hydraulicmaterials testing machine (Instron 8872, Instron Corp.). The testwas performed to measure the transverse stiffness of the embed-ded chiral cell/MFC unit. The load was applied using displacementcontrol at a speed of 1 mm/min using a steel rod of 6 mm diameterwith a hemispherical tip.

2.2. Hexachiral honeycomb panel and sandwich plate

A large scale honeycomb panel prototype (21 cm � 27 cm �2.5 cm) representing a hexachiral configuration with polyester/fi-bre glass core [31] was used to evaluate the sensing characteristicsof the chiral smart honeycomb configuration. Six MFC patches (ac-tive area of 14 mm � 14 mm) were glued on the surface of the sixligaments using 2-part epoxies (Bondmaster). Fig. 3a shows thearrangement of the six MFC patches in the honeycomb teststructure.

Another set of chiral cellular core was used to produced a sand-wich panel, with 1.2 mm symmetric laminate skins made by IM7/8552 carbon fibre/epoxy (0�/90�/90�/0�)sym. Before the embeddingto the cellular structure, the capacitance of the six MFC sensors wasmeasured, ranging from 0.76 to 0.84 nF. The final sandwich panel isshown in Fig. 3b, where it is possible to notice the bundle of wiresto be connected with data acquisition systems.

The sensing and actuation characteristics on the tetrachiral pro-totype (Fig. 1b) and chiral honeycomb panel (Fig. 3b) were testedusing a base-excitation system, and a broadband voltage appliedto the electrodes. The tetrachiral unit cell was attached to a forcetransducer (PCB 208C03) and a shaker using two bolts connectedto one of the cylinder of the unit cell, and a stringer s connected be-tween them. The shaker is mounted on a supporting rigid tablewith tight nuts. The electromagnetic shaker (model Ling DynamicSystems V406) is used to generate a random force to dynamicallyexcite the unit cell. The stroke of electromagnetic shaker is drivenby a signal generator (MATLAB-dSPACE Interface) and a poweramplifier (model LDS PA100E). In each vibration test, a randomforce is imparted along the axis of the cell cylinder. The resolution,force range, sensitivity and frequency range of force sensor are0.005 lb (0.022 N) r.m.s, ±500 lb (2224 N), 2.37 mV/N and 36 kHz,respectively. The signals from the force sensor pass through a sig-nal conditioner (KISTLER Type 5134), before being captured by thedata acquisition system. The signal conditioner also providespower for the force sensor to condition the output signal. Theoutput voltages from the piezosensor during vibration test are

ycomb and (b) distribution of the MFCs in the hexachiral honeycomb panel underr vibrometer.

Fig. 4. (a) Map of the MFCs locations for the sandwich panel (b) subjected to impact loading.

H. Abramovitch et al. / Composites Science and Technology 70 (2010) 1072–1079 1075

recorded and analysed in the MATLAB-dSPACE Interface. For actu-ation, voltage loading from the piezosensor is used to excite theunit cell. The piezosensor is driven by a signal generator (MAT-LAB-dSPACE Interface) and an amplifier which operates at high fre-quency from 10 kHz to 250 MHz. A scanning laser vibrometer(Polytec PSV-300) is used to sense dynamic responses on the smarttetrachiral unit cell. The velocity range which corresponded to avibrometer output voltage of 1 V is chosen at 1 mm/s. Fast FourierTransform (FFT) acquisition is performed for the signals with a se-lected bandwidth recorded from 10 to 50 kHz.

The hexachiral core panel with embedded MFCs (Fig. 3a) wasconnected to the shaker using two bolts inserted in two adjacentcylinders of the hexachiral panel and linked together. The two con-nected bolts were fixed to the shaker via a stinger and force trans-ducer. The panel was excited using a random force, with the signalsfrom the MFC sensors passed through a signal conditioner (KISTLERType 5134), before being captured by the data acquisition system.The output voltages from the six MFCs under broadband whitenoise excitation during vibration test were recorded and analysedin the MATLAB-dSPACE Interface.

0

2000

4000

6000

8000

10000

12000

14000

16000

0 0.002 0.004 0.006 0.008

Displace

Load

(N)

Fig. 5. Load–displacement curve of the static co

An air gun test rig was used to detect the behaviour of the smartchiral panel during impact. The impact gun was constituted bythree main parts: a pressure vessel capable of providing high pres-sures in the range of 200 atmospheres, a releasing mechanism anda barrel having either the diameter of 13 mm, or another barrelwith a 9 mm diameter. The velocity of the impact is measuredusing a velocitymeter mounted at the end of the barrel. The lengthof same barrel was 1.8 m, with an outside diameter of 62 mm; aninner diameter of 13 mm (or alternatively 9 mm). The diameterof the pressure vessel was 25.4 cm. For all the tests, the sandwichplate (Fig. 4b) was impacted at the centre, with a maximum kineticenergy of 0.6 J to avoid further damages to the prototype.

3. Results and discussion

The static response of the tetrachiral smart cell is shown inFig. 5. The load displacement curve features a bilinear behaviour,with a initial slope of 2.24 kN/mm up to 0.006 mm, and a secondslope of 7.45 kN/mm thereafter. The analogous FE linear elastic sta-tic simulation provides a satisfactory agreement in trend with the

0.01 0.012 0.014 0.016 0.018

ment (mm)

mpression test on the tetrachiral unit cell.

Fig. 6. Magnitude (a) and phase (b) of the transfer function (voltage/force) of the MFC embedded in the tetrachiral unit cell for different r.m.s. force values.

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fist slope of the curve, providing for 0.023 mm of imposed defor-mation a load of 28 N, instead of the experimental 35 N. Theembedded MFC provides an effective stiffening effect for smalldeformations, indicating therefore that the presence of the sensorwithin the ligament plate is beneficial in the linear elastic regime– as expected simply by considering the contribution of the sensorwith a basic rule of mixture. The first four open circuit FE frequen-cies simulating the experimental conditions for the modal analysiswere respectively 157 Hz, 264 Hz, 370 Hz and 858 Hz. The firstmode was flexural, while the upper modes were all coupled tor-sional/flexural types, with the second in particular having a domi-nant torsional motion. The experimental modes derived from themulti-point Scanning Laser Vibrometer measurements were154 Hz, 236 Hz, 470 Hz and 589 Hz respectively. The SLV was alsopositioned along a direction normal to the thickness of the tetr-achiral cell, to detect lateral motions during vibration. The discrep-ancies between FE and experimental results on the upper modesare influenced by the representation of the experimental boundaryconditions, with possible torsional rigidities in the bolt-cell assem-bly that are not taken sufficiently into account. The first two free–free FE modes were 264 Hz and 309 Hz respectively, with the firstmode providing a 2.7% error. The response of the piezosensor un-der broadband white noise excitation was measured for seven dif-

Fig. 7. Comparison between magnitude (a) and phase (b) of the transfer functio

ferent r.m.s values of input force (0.8 N, 1.3 N, 2.1 N, 2.6 N, 3.5 N,4.0 N and 4.7 N). Fig. 6a and b shows the magnitudes and phasesof transfer functions of the piezosensor output voltages for differ-ent r.m.s values of excitations. There is a very good agreement ofthe response up to 1 kHz, while for higher frequencies the trans-missibility differ for input r.m.s. forces from 3.5 N onwards. It isworth noticing (Fig. 6b), that the phase recorded by the MFC is onlylimited affected by the input force on the system.

The FE model provides a response at 5 Hz of �38 dB V for 1 N ofinput force, sufficiently close to the experimental values measuredfor 1.3 N or r.m.s. input (�35 dB V). Fig. 7a and b illustrate the com-parative response given by the Scanning Laser Vibrometer and theMFC for an input r.m.s. force of 2.1 N with broadband randomwhite noise excitation. To transform the MFC output into theequivalent of the velocity signal recorded by the SLV, the voltagesignal from the MFC has been transformed into a velocity signalby the relation:

vs ¼ ixd33Vt

h ð1Þ

where vs is the equivalent structural velocity for the frequency x,d33 is the direct piezoelectric coefficient, V the voltage recorded atthe frequency x, t the thickness of the piezosensor along the three

n (velocity/force) provided by the MFC and SLV in the tetrachiral unit cell.

0 100 200 300 400 500 600 700 800 900 10000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frequency (Hz)

Mag

nitu

de

Fig. 8. Coherence of the transfer function (velocity/force) provided by the MFCembedded in the tetrachiral cell.

H. Abramovitch et al. / Composites Science and Technology 70 (2010) 1072–1079 1077

direction, and h the width of the MFC. Eq. (1) can be derived consid-ering the relation between strain and voltage in piezoelectric, andassuming harmonic motion of the displacements ðv s ¼ ixxseixtÞ.

Fig. 9. Comparison between SLV and embedded MFC measurements for the hexachiral ho(d) for MFC # 6.

In terms of magnitude (Fig. 7a), the MFC provides transformedsignals with a trend following the one of the SLV, although withan average attenuation of 20 dB in the region of the coupled out-of-plane flexural–torsional modes (400–600 Hz), and even higheraverage discrepancy in the other bandwidths. The phase recorded(Fig. 7b) shows also almost a 180� behaviour up to 400 Hz, and fur-ther attenuation onwards. A possible explanation for this is the po-sition of the embedded MFC into the tetrachiral unit cell. Along thecell thickness direction, the MFC is 15 mm above the top surface ofthe cell, while only the electroplated electrodes with their wiringare flush to the surface itself. On the opposite, the laser beam spotof the SLV is on the outer surface of the prototype, placed in themiddle of the wire ends. The polyvinide resin provides some atten-uation to the signal from the MFC, and viscoelastic effects of thecore material are particularly evident in the phase response ofthe embedded sensor [32]. The magnitude of the MFC frequencyresponse function, however, is an indication that the embeddedsensor can provide information proportional to the global modalbehaviour of the structure.

The coherence of the transfer function between input force andMFC voltage (Fig. 8) is generally good for the overall response up to1 kHz, while the drops in coherence are localized on the antireso-nances of the specific the transfer function.

As an example of the dynamic response under random broad-band excitation of the hexachiral panel samples (no face skins),Fig. 9 shows the magnitude of the response of the MFC Nos. 5

neycomb panel. Magnitude (a) and phase (b) for MFC # 5; Magnitude (c) and phase

Fig. 10. Comparison of the time domain signals provided by accelerometers placed at the corresponding MFC locations in the sandwich panel.

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and 6 (see Fig. 3b) between 1 Hz and 4 kHz. The hexachiral panelwith no face skin attached (Fig. 3b), has natural frequencies at512 Hz, 567 Hz, 1072 Hz, 1454 Hz and 1878 Hz. The first frequencycorresponds to a torsional mode, while the second correspond to apure flexural in free–free conditions. The other frequencies havecombined torsional–flexural modes with different half-wave terms[31]. When the face skins are attached, the stiffening provided by

Fig. 11. Repeatibility of signals acquired by the piezosensors under impact l

the sandwich structures increases the natural frequencies to1066 Hz, 1478 Hz, 2266 Hz and 2388 Hz. The associated modesare respectively first torsional, flexural, second flexural and secondtorsional. Up to 4 kHz, other torsional and flexural modes can beidentified at 3313 Hz and 3834 Hz. There is a general good conver-gence between the response of the MFCs and the one provided bythe laser vibrometer in the corresponding points on the outer

oading at 0.6 J. (a) MFC # 1; (b) MFC # 2; (c) MFC # 6 and (d) MFC # 7.

H. Abramovitch et al. / Composites Science and Technology 70 (2010) 1072–1079 1079

surface of the hexachiral panel, in particular for frequency rangesabove 1.6 kHz, where the modal density of the sample starts toincrease.

It is noticeable, especially for the MFC No. 5, a very good com-parison between SLV and MFC response for the mode around3240 Hz, corresponding to a flexural–torsional modeshape forwhich the piezosensor location represents a point of maximumdisplacement. A similar trend for the magnitude is also recordedfor MFC No. 6 (Fig. 9c), although the convergence with the SLV sig-nal is slightly lower, especially below the 1.6 kHz threshold. Interms of phase response, the magnitude of the response providedby the MFCs compared to the one of the SLV is different, in a sim-ilar manner to the tetrachiral unit cell prototype. However, oppo-site to the last case, the overall trend of the MFCs responsefollows the one of the laser vibrometer, with no apparent phaseopposition. As it can be noticed in Fig. 3a, the MFCs are glued onthe internal wall of the hexachiral cells, but not embedded. Afterbonding, a general decrease in capacitance was recorded. The sen-sor C3 (Fig. 3a) with an unbonded capacitance of 0.84 nF, provideda new value once bonded to the chiral structure equal to 0.80 nF.The other MFCs recorded a general capacitance decrease of 7%,due to the epoxy bonding layer applied. Although capacitanceattenuation can be expected gluing the piezosensor to the struc-ture, there is no further constraining and viscoelastic effect givenby the embedding of the sensors.

Fig. 10 shows the reference signals by six accelerometers placedin the outer skin of the sandwich panel, corresponding to the loca-tions of the MFCs. The signals are recorded in the time domainafter the plate is impacted with the airgun. Some examples ofthe response of the MFCs are showed in Fig. 11. For each sensor,six different tests have been performed to assess the repeatabilityof the measured voltages. Being sensors C1 (Fig. 11a) close to thevicinity of the impact location (see Fig. 4a), the output voltage ishigher in terms of magnitude of sensors C7 and C8 (Fig. 11c andd, respectively), sensor C1 showing a maximum peak magnitudeof 10 V, while sensor C8 provides a maximum magnitude of 6 V.It is worth noticing that, although the kinetic energy of the impactis relatively low, the sensing voltages provided by the sensors weresufficiently large and consistent in the various different tests.

4. Conclusions

In this work a novel concept of smart honeycomb with negativePoisson’s ratio behaviour and embedded micro fibre compositesensors in the cell core has been developed using the auxetic chiraltopology. The embedded sensors, when casted within the cellularunit, show also load bearing capabilities. The smart honeycombshave shown potential for structural health monitoring applica-tions. Compared to the signal received from an external measure-ment device (the laser vibrometer), the response under broadbanddynamic excitation of the embedded sensors is influenced by thegluing of the MFCs and their embedding into the core material,providing a decrease in terms of capacitance. However, the embed-ded piezosensors can detect complex motions and strain fields,which an external sensor cannot acquire. The time response ofthe MFCs internal to the core of a sandwich panel subjected tolow kinetic energy impact provides consistent repeatability ofthe results, as well as signal amplitudes proportional to the vicinityof the sensor to the impact location.

Acknowledgement

This work has been funded by the FP6 NMP-CT-2005-013641CHISMACOMB project.

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