Impact Damage Detection in Composite Chiral Sandwich Panels

Using Nonlinear Vibro-Acoustic Modulations

Andrzej Klepka1,a, Wieslaw J. Staszewski1,b, Dario di Maio2,c, Fabrizio

Scarpa2,d

1Department of Robotics and Mechatronics, AGH University of Science and Technology,

Al. Mickiewicza 30, 30-059 Krakow, Poland

2Department of Aerospace Engineering, Bristol University, Queens Building, Bristol BS8

1TR, UK

Abstract

This paper reports an application of nonlinear acoustics for impact damage detection in a

composite chiral sandwich panel. The panel is built from a chiral honeycomb and two

composite skins. High-frequency ultrasonic excitation and low-frequency modal

excitation were used to observe nonlinear modulations in ultrasonic waves due to

structural damage. Low-profile, surface-bonded piezoceramic transducers were used for

ultrasonic excitation. Non-contact laser vibrometry was applied for ultrasonic sensing.

The work presented focuses on the analysis of modulation intensities and damage-related

nonlinearities. The paper demonstrates that the method can be used for impact damage

detection in composite chiral sandwich panels.

Keywords: composites, sandwich panels, chiral structures, impact damage, damage

detection, nonlinear acoustics, vibro-acoustic modulations

 

 

aklepka@agh.edu.pl, bw.j.staszewski@agh.edu.pl, cDario.DiMaio@bristol.ac.uk, df.scarpa@bristol.ac.uk,

1. Introduction

Composite materials are very attractive for structural design due to their interesting

properties such as low weight, fatigue strength, large damping, corrosion resistance or

flexibility to form complex shapes for various applications. Composites are widely used in

civil engineering (e.g. bridge reinforcements), process engineering (e.g. tanks, pipes) and

transportation (e.g. boats, racing cars, aircraft). The most demanding application is related to

space exploration where for example reusable launch vehicles are designed and fabricated

using many different types of composite materials. Composites are often reinforced using

sandwich-structured configurations. Thick and light cores are sandwiched between two thin

and stiff composite plates to form panels that exhibit low weight, high bending stiffness and

good energy absorption. Various open- and closed-cell structures - such as foams and

honeycombs - are used as cores. The main common feature of honeycomb cores is an array of

hollow cells. Different geometries, design topologies and materials are used for honeycomb

cores.

Recent studies in the area of honeycomb composites include topologies featuring unusual deformation mechanisms. A novel class of sandwich composites with chiral behaviour is an

example [1-6]. The chirality, which in this case is an atomic or structural rotative but not reflective symmetry, is an important characteristic of these honeycomb cores. The chirality is utilised to obtain auxetic cores that exhibit a negative Poisson's ratio. Such cores expand in all

directions when pulled in only one direction. This is achieved either through a micro-structural material composition or macro-structural hierarchy. The latter includes various

types of cell geometries, as illustrated in

Figure 1. Composite chiral sandwich panels offer improved: shear stiffness, fracture

toughness, shock and vibration absorption and resistance to impact damage. These panels are

very attractive for applications that involve highly-curved surfaces (e.g. large space antennas

or automotive panels).

It is well known that most composite structures absorb mechanical energy by elastic

deformation or damage mechanisms. In metals plastic deformation is possible and the energy

absorption mechanism is different. Various forms of structural failures can be observed in

composite honeycomb structures. This include through-thickness buckling, core crushing, de-

bonding between core and skins or damage of composite skins such as indentation,

delamination or fibre/matrix cracking. All these damage modes can significantly modify

structural properties leading to residual strength reduction . Structural damage due to impacts

is one of the major concerns in aerospace. Impact damage is caused by bird strikes, tool drops

during servicing or loose runway stones during take offs, as discussed in [7]. Skin-core

interface damage - such as debonding of the core from composite skins due to impacts - is in

fact one of the major failure modes in composite sandwich structures. This type of damage

can also result from manufacturing due to poor or incomplete adhesive distribution. It is

important to note that core-skin debonding is difficult to detect visually.

Health monitoring of composite structures is an important area of research and

application, as discussed in [7]. Various passive and active methods can be used to monitor

composite structures for impact damage. Passive approaches include methods that are based -

on acoustic emission [7-8] and strain measurements. Strains can be used to obtain information

about structural loads and structural usage [7]. Operational Load Monitoring (OLM) - used in

military aircraft structures - is a good example. Strain waves can be utilised for impact

damage detection [7,9] that leads to location of impact damage and estimation of impact

energy. Active methods involve vibration/modal analysis and various Non-Destructive

Testing (NDT) approaches. The latter includes classical approaches such as X-rays, C-

scanning [7,10], vibrothermography [11-12], ultrasonic testing [13] and new methods based

on guided ultrasonic waves and laser vibrometry [9,14-16]. Recent damage detection

applications are based on Structural Health Monitoring (SHM) philosophy. SHM methods

often use a network of structure-integrated, low-profile piezoceramic transducers [7,17-18] or

optical fibre sensors [19] and utilse guided ultrasonic waves. Lamb waves are the most widely

used guided ultrasonic waves for damage detection in composite structures.

More recently nonlinear acoustics has been applied for impact damage detection in

composite structures. Nonlinearities are manifested in damaged structures through different

effects, including higher and sub-harmonic generation, frequency shifting, signal modulations,

modulation transfer or hysteretic behavior. Various physical phenomena are involved, as

discussed in [20] and reviewed in [21]. The method based on nonlinear-vibro-acoustic

modulations [22-31] is particularly attractive and has been used for impact damage detection

in composites plates [32] and composite sandwich panels [33]. Relative implementation

simplicity and good sensitivity - if compared with classical linear methods - is a major

attraction of this approach.

This paper reports an application of the nonlinear vibro-acoustic modulation technique for

impact damage detection of composite chiral sandwich panels. Low-profile piezoceramic

transducers are used for ultrasonic wave propagation. Monitoring of structural debonding

between the composite skin and the chiral core is the major focus. An attempt is made to

explain the major cause of the nonlinear behaviour.

The work presented in this paper consists of four major parts. Firstly, the composite chiral

sandwich panel used is in the current investigations is presented and described in Section 2.

Then the nonlinear vibro-acoustic technique is briefly explained in Section 3. The

experimental arrangements and procedure are described in Section 4. Finally, experimental

results are presented and discussed in Section 5 and the paper is concluded in Section 6.

2. Composite Chiral Sandwich Panel

The chiral composite sandwich panel (Fig. 2a) tested was manufactured using a truss-core with an anti-tetrachiral configuration (

Figure 1c and Fig. 2b). Two quasi-isotropic composite plates were used to produce the

skins. The plates were manufactured using 8552/IM7 prepreg with a 0/-45/+45/90/90/+45/-

45/0 stacking layer sequence. The truss-core core was manufactured using a Rapid

Prototyping (RP) SLA technique and polyamide sintered powders (Schneider Prototyping,

Poole, UK). The aspect ratio α of the cells was equal to 6, with a relative thickness β equal to

0.4 [6,35]. The length of the major side of the unit cell L was 25.4 mm, with a gauge

thickness of 25 mm. The diagram explaining the parameters of the anti-tetrachiral topology

is presented in Fig. 2c. The face skins and the core were attached with an epoxy-resin based

adhesive and cured in autoclave at 120 °C. The entire honeycomb panel was made of 5 x 3

unit cells. The Young’s modulus of the core material measured on a BSI standard sample

under tensile loading was 1.6 GPa. The PI Ceramics PIC155 transducers (diameter 10 mm;

thickness 1 mm) was surface-bonded to the top skin of the panel, as shown in Fig. 2a.

3. Nonlinear acoustics

Various approaches have been developed for damage detection in nonlinear acoustics.

Generally these approaches can be divided into two major groups of methods. The first group

is based on classical nonlinear phenomena manifested for example by higher harmonic

generation or frequency mixing. The classical theory of elasticity explains well the process of

higher harmonic generation due to a nonlinear form of the Hook’s law describing the

relationship between stress and strain [36-37]. These nonlinear phenomena are known for

many years and used for material testing applications. The second group of methods includes

non-classical phenomena based on various contact-type nonlinearities related to crack-wave

interactions such as contact acoustic nonlinearity, vibro-acoustic modulations, cross-

modulation transfer, energy dissipation via hysteresis or the so-called L-G effect [20-22,38-

41]. These methods are less understood and various theoretical models - that explain non-

classical nonlinearities have been proposed, as reviewed in [21,42]. Often analysis of

harmonics and sidebands can reveal important information on different types of

nonlinearities, as demonstrated in Table 1.

The method based on nonlinear vibro-acoustic wave interactions is used in this paper for

damage detection. This approach is based on nonlinear modulation interactions of high-

frequency ultrasonic waves and low-frequency vibration/modal excitation. Both excitations

are introduced to a monitored structures simultaneously, as illustrated graphically in Fig. 3.

Usually one of the modal frequencies is used as the low-frequency excitation to "move"

damage. However, it is important to note that the level of excitation is relatively small if

compared with classical approaches. It is anticipated that in metallic structures crack surfaces

are only perturbed, i.e. no opening-closing action of crack is observed. Similarly in

composites structures it is anticipated that delaminated layers are also only perturbed, i.e. no

permanent gap between delaminated layers is maintained. The high-frequency ultrasonic sine

wave is used for damage detection. When the monitored structure is intact or undamaged, the

high-frequency wave is unchanged – only low and high frequency components are visible in

signal spectra. However, when the structure is damaged the high-frequency ultrasonic wave is

modulated by the low-frequency vibration flexural wave. The damaged structure displays

additionally frequency sidebands around this main ultrasonic component. The intensity of

modulation strongly relates to damage severity, i.e. the number of sidebands and their

amplitudes increases or changes with the advancement of damage. Frequencies of the

subsequent (n =1, 2, 3, …, ) sidebands are equal to

𝑓!! = 𝑓! ± 𝑛𝑓! (1)

where fH is the frequency of the ultrasonic wave and fL is the frequency of modal excitation.

Often the intensity of modulation is described by the parameter that is defined as

𝑅 = !!!!!!!

(2)

where A0 is the spectral amplitude of the carrier ultrasonic frequency, A1 and A2 are the

spectral amplitudes of the first pair of sidebands. Alternatively, the intensity of modulation

can be analysed in the time domain using the instantaneous amplitude and frequency, as

described in [43].

4. Impact damage detection using nonlinear acoustics

The nonlinear vibro-acoutsic wave modulation technique - briefly described in Section 3 -

was used to detect impact damage in the composite chiral sandwich panel presented in

Section 2. The work was focused on debonding between the composite skin and the chiral

core. This section describes experimental arrangements and presents damage detection results.

4.1. Modal analysis

Experimental modal analysis was performed initially to obtain information about structural

vibration modes of the composite chiral sandwich panel and select frequencies for low-

frequency excitation in nonlinear acoustic tests.

The panel was freely suspended using elastic cords to avoid nonlinearities from

boundaries. The TIRA S503 electromagnetic shaker was used to excite the structure. A white

noise signal was used to excite the panel. The TIRA BAA120 amplifier were used to generate

the excitation signal. A PCB Piezotronics 208A15 force sensor was used to measure dynamic

forces. A Polytec PSV-400 laser vibrometer was used for non-contact measurements of

vibration responses. The velocity range for the vibrometer was 20 mm/s, corresponding to an

output voltage of 5V. Fig. 4 shows the entire experimental arrangements.

The Frequency Response Functions (FRF) was estimated from the acquired excitation and

response vibration data using Polytec PSV 8.8 software. The FRF was averaged using fifty

sets of data to improve the signal-to-noise ratio. The FRF magnitude - presented in Figure 5a

exhibits a number of vibration modes. The frequencies of three vibration modes (Figure 5b),

i.e. 1512, 2363 and 3801 Hz were selected for nonlinear acoustic tests. The first selected

mode was related to global torsion, the second mode shapes was of flexural nature whereas

the third mode shape was a combination of both movements.

4.2 Impact damage

A series of simple drop-weight impact tests was performed in order to introduce impact

damage to the composite chiral sandwich panel. A steel cylinder of Young’s modulus equal to

21 MPa was used as an impactor. Three consecutive impacts of 2, 9 and 30 J were introduced

to the panel in the middle of the top composite skin. The major interest in these investigations

was the accumulated damage resulting from repeated impacts at one position.

Since no visible effect of impact was observed on the surface of the composite chiral

sandwich panel, vibrothermography was used to monitor for possible presence of damage

after each impact. A prototype ultrasonic excitation system (in-house design) and a high

performance thermographic camera (Cedip Silver 420M) were used for damage imaging The

ultrasonic generator with operating output power up to 2000W was used to introduce 28-

kHz sinusoidal signal into the structure. The system was equipped with a pneumatic press

system which allowed to control clamping force between the sonotrode and the structure. The

infrared camera was applied to acquire temperature evolution over the plate surface. The top

and bottom skins were scanned using a sonotrode (Fig. 6a). A clear debonding could be

observed between the skin and the core after 30 J impact. Fig. 6b displays a contour plot of

the surface temperature distribution in the area of damage; a clear impact damage can be

observed in the upper part of this figure. The relevant temperature values - measured

horizontally and vertically across indicated A-B and C-D profiles - given in Fig. 6c also

confirm the presence of damage.

4.3 Nonlinear acoustic tests

The nonlinear vibro-acoustic modulation technique was performed for the composite chiral

sandwich panel before and after each consecutive impact. The tests were performed using the

experimental set-up shown in Fig. 7.

The high-frequency (HF) ultrasonic wave and the low-frequency (LF) vibration were used

simultaneously for excitation. Both excitation signals were harmonic sine waves. The HF

excitation was generated using the Agilent 33522A signal generator and amplified using the

EC Electronics PAHV 2000 high-voltage amplifier to the voltage level equal to 60 V. The PI

Ceramics PIC155 transducer was used for the HF excitation. Once the ultrasonic wave was

propagating in the top composite skin, the entire panel was modally excited (LF excitation)

using a TIRA electromagnetic shaker. The LF excitation position was selected to avoid

excitation at nodal points. Single harmonic excitation was applied using one of the three

selected modal frequencies. The Total Harmonic Distortion (THD) test was performed for the

LF and HF excitation to avoid nonlinearities associated with the instrument chain used. The

THD coefficient for all test configurations (different frequencies and amplitudes) was much

less than 1%. A Polytec PSV-400 laser vibrometer was used for non-contact measurements of

ultrasonic responses in a pre-defined position near the area of damage.

5. Nonlinear acoustics damage detection results

This section presents the results from nonlinear acoustic tests used for damage detection.

Firstly, power spectra are analysed to reveal nonlinearities, i.e. higher harmonics and

modulations. Then intensity of modulation is analysed for various vibration mode excitations

and severities of damage. Finally, amplitudes of higher harmonics and modulations sidebands

are analysed in more details to reveal possible types of nonlinearities.

5.1 Analysis of power spectra

Ultrasonic responses from nonlinear acoustic tests were used to compute power spectra. Fig.

8 shows examples of power spectra clearly displaying higher harmonics of LF excitation. The

results are presented for the 2nd vibration mode excitation (2364 Hz). Higher harmonics can

be observed after each impact. This is particularly visible for the damaged panel after 30 J

impact in Fig. 8d. Fig. 9 shows power spectra zoomed around the main 60 kHz ultrasonic HF

component to reveal possible modulation sidebands. The example results are presented for the

2nd mode of vibration excitation (2364 Hz). The fundamental 60 kHz harmonic of the

ultrasonic wave dominates all zoomed power spectra. A clear pattern of sidebands can be

observed around the 60 kHz carrier component for the impacted plate results in Fig. 9b-d. The

frequency spacing of these sidebands corresponds to the modal frequency of LF excitation,

i.e. 2364 Hz. When the relevant power spectrum is analysed in Figure 9a for the undamaged

specimen, similar sidebands can be observed. This is probably due to intrinsic (e.g. material)

nonlinearities. However, the amplitude of the first pair of sidebands is much lower if

compared with the results in Fig. 9b-d, where the first two sidebands are predominant. Some

other sideband components can be observed in Fig. 9a for the undamaged specimen.

However, these components do not correspond to any harmonics of the LF modal excitation.

5.2 Analysis of modulation intensity

A series of nonlinear acoustic tests was performed to analyse modulation intensity for various

levels of LF excitation amplitude. The HF excitation amplitude was kept constant in these

tests. Modulation intensity was assessed using the R parameter, defined in Section 3.

The results are presented in Fig. 10 for different vibration modes and damage severities

investigated. When the plate is undamaged modulation intensity does not increase with the

excitation level. Similar results are obtained for all impact energies investigated when the

panel is excited with the 3rd vibration mode in Fig. 10c. Modulation intensity increases

significantly for the damaged panel after the 30 J impact but only for the 1st vibration mode

excitation (Fig. 10a). However, when the 2nd mode vibration excitation is used modulation

intensity exhibits significant monotonical increase only for the 9 J impact, after 17 V of

amplitude excitation. For this vibration mode modulation intensity remains relatively stable -

after the initial jump - for the 30 J impact and increases relatively slowly after the 2 J impact.

In summary, the results show that when the composite skin is debonded from the chiral

core after the 30 J impact, modulation intensity increases significantly with the amplitude of

the 1st vibration mode excitation. The 2nd vibration mode excitation exhibit significant

increase of modulation intensity for the 9 J impact. The 3rd vibration mode does not produce

any significant modulations and therefore the results for this mode are not analysed any

further.

5.3 Analysis of higher harmonics and modulation sidebands

The experimental results for the 1st and 2nd selected vibration modes excitation have been

analysed in more details to investigate whether classical or non-classical nonlinearities are

involved in damage-wave interactions. Amplitudes of higher LF harmonics and the first two

pairs of modulation sidebands have been analysed for various levels of the fundamental LF

amplitude. The results - presented in Fig. 11-14 - can be compared with the theoretical

amplitude dependence characteristics given in Table 1.

For the 1st vibration mode excitation, when the panel is undamaged, the amplitude of the

2nd harmonic displays quadratic behaviour in Fig. 11a whereas the amplitude of the 3rd

harmonics remains relatively unchanged. The amplitude of the first two pairs of sidebands in

Fig. 12a is relatively scattered if compared with the remaining results in Fig. 12b-d. This

suggests that nonlinearity involved probably relates to intrinsic effects, e.g. material

nonlinearity. When the panel is damaged - after the 30 J impact - the 3rd harmonic exhibits

cubic dependence in Fig. 11d; the slopes for the amplitude of the 1st and 2nd pair of

sidebands are equal approximately to 1 and 2 respectively, when the linear parts of all curves

are analysed for larger excitation levels in Fig. 12d. After the 9 J impact the amplitude

behaviour of the 2nd and 3rd harmonics (Fig. 11c) and the second pair of sidebands (Fig. 12c)

is similar to the damaged panel. These results suggest that - for the case investigated - the

classical nonlinear perturbation is involved when the panel is damaged. Although, damage

was not revealed after the 9 J impact when vibrothermography was used in Section 4.2, its

severity could be very small (e.g. partial or weaker bonding only involved between the skin

and the chiral core). This needs further investigations. The amplitude of the higher harmonics

and sidebands after the 2 J impact do not reveal anything interesting in Fig. 11b and Fig. 12b.

For the 2nd vibration mode excitation, when the panel is damaged (after the 30 J impact)

the 2nd harmonic displays quadratic dependence in Fig. 13d; the slope for the first and second

pair of sidebands is equal approximately to 1 and 2, respectively in Fig. 14d. This suggests

that the classical perturbation nolinearity is involved, similarly to the results obtained for the

1st vibration mode. After the 9 J impact the 2nd harmonic exhibits a quadratic behaviour in

Fig. 13c and the slope for the second pair of sidebands is equal approximately to 1 in Fig. 14c.

This is similar to a non-classical hysteretic behaviour (i.e. nonlinear dissipation via

hysteresis). It is important to note that, modulation intensity after the 9 J impact was

dominant for this vibration mode and reached the maximum recorded level of 0.16 in Fig.

10b. Interestingly, the nonclassical hysteretic dissipation was also the major cause of

nonlinear modulation when the crack-wave nonlinear interaction was investigated in [20].

6. Conclusions

Nonlinear acoustics was used for impact damage detection in the composite chiral sandwich

panel. The method involved the analysis of higher harmonics and nonlinear vibro-acoustic

wave modulations.

The study shows that invisible debonding between the composite skin and the chiral core

can be detected by the method. However, proper selection of modal excitation is important for

the success of the method investigated. Damage detection result was confirmed by the

classical vibrothermographic analysis.

A significant pattern of sidebands around the carrier ultrasonic frequency was observed in

the power spectrum of the vibro-acoustic response when the panel was impacted. The

intensity of modulation increased with the modal excitation amplitude. Amplitude analysis of

higher harmonics and modulations sidebands showed that the classical nonlinear perturbation

could be associated with the damage-wave interaction mechanism. However, some evidence

of nonlinear hysteresis was also observed. This effect - associated with the largest modulation

intensity observed - could indicate possible damage. Unfortunately the presence of damage

could not be revealed with the virbrothermographic test used, probably due to poor damage

sensitivity.

The results are similar and confirm some previous findings related to the nonlinear crack-

wave interaction described in [20]. Further work is required to confirm the conclusions.

Acknowledgements The authors would also like to acknowledge the financial support from the Foundation for

Polish Science under the WELCOME research project no. 2010-3/2 (Innovative Economy,

National Cohesion Programme, EU).

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Figure 1. Examples of typical chiral core topology: a) hexachiral, b) tetrachiral, c) anti-tetrachiral.

 

 

Fig. 2. Chiral sandwich composite panel: a) the view displaying piezoceramic transducer used for high frequency ultrasonic excitation, b) anti-tetrachiral core bonded to a composite skin,

c)geometry of anti-tetrachiral topology.

 

Table 1. Amplitude dependence for various nonlinearities in nonlinear acoustics.

 

 

 

 

 

 

 

 

 

 

 

Fig. 3. Example illustrating the nonlinear vibro-acoustic wave modulation technique used for impact damage detection.

 

 

 

 

Fig. 4. Experimental arrangements used for modal analysis.

 

 

 

Fig. 5. Experimental modal analysis results for the composite chiral sandwich panel: (a) Frequency Response Function magnitude; (b) selected vibration modes.

 

 

 

 

Fig. 6. Damage detection using vibrothermography (a) experimental arrangements; (b) contour plot of surface temperature distribution display damage area; (c) temperature

distribution.

 

 

 

 

 

 

Fig. 7. Experimental set-up used for damage detection based on nonlinear vibro-acoustic modulation tests.

 

Fig. 8. Power spectra for the 2nd LF excitation for various damage severities: (a) undamaged panel; (b) after 2 J impact; (c) after 9 J impact; (d) after 30 J impact.

 

 

 

 

 

 

Fig. 9. Zoomed power spectra for the 2nd mode LF excitation for various damage severities: (a) undamaged panel; (b) after 2 J impact; (c) after 9 J impact; (d) after 30 J impact.

 

Fig. 10. Intensity of modulation vs. low-frequency excitation: (a) 1st vibration mode; (b) 2nd vibration mode; (c) 3rd vibration mode.

Fig. 11. Analysis of low-frequency harmonics amplitude for 1st vibration mode: (a) undamaged panel; (b) after 2 J impact; (c) after 9 J impact; (d) after 30 J impact.

Fig. 12. Analysis of low-frequency harmonics amplitude for 2nd vibration mode: (a)

undamaged panel; (b) after 2J impact; (c) after 9J impact; (d) after 30J impact.

 

Fig. 13. Analysis of first and second sidebands amplitude for the 1st vibration mode: (a) undamaged panel; (b) after 2J impact; (c) after 9J impact; (d) after 30J impact.

Fig. 14. Analysis of first and second sidebands amplitude for the 2nd vibration mode

excitation: (a) undamaged panel; (b) after 2J impact; (c) after 9J impact; (d) after 30J impact.