Ultrasonic guided wave propagation and detection of ...

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VIIIth International Workshop NDT in Progress (NDTP2015) Oct 12-14, 2015, Prague - www.ndt.net/app.NDTP2015

Ultrasonic guided wave propagation and detection of high density core region in a honeycomb composite sandwich structure using embedded piezoelectric transducers 1

2

Shirsendu SIKDAR , Sauvik BANERJEE 1,2

Indian Institute of Technology Bombay , Powai, Mumbai-400046, India; Phone: +91 22-25767343, Fax: +91 22-25767343; Email: [email protected], [email protected]

Abstract An organized numerical and exp erimental study is carried out in order to understand ultrasonic guided wave (GW) p ropagation and interaction with a high density (HD) core region in a honey comb comp osite sandwich structure (HCSS). Also the location of HD core region in a HCSS using embedded p iezoelectric wafer transducer (PWTs) is investigated in this study . Due to comp lex structural characteristics, the study of guided wave (GW) prop agation in HCSS with HD-core region inherently carries many challenges. Therefore, a three-dimensional (3D) numerical simulation of GW p rop agation in the HCSS with and without HD core region is carried out using embedded PWT network. It is observed that the p resence of HD core significantly reduces the amp litude of the p rop agating GW modes. In order to verify the numerical results, exp eriments are conducted in the laboratory. A good agreement between the numerical and exp erimental results is observed, in all the cases studied. Finally , based on the change in amp litude of the received GW modes, the location of an unknown HD core region, within the PWT array is determined by ap p ly ing a p robability based signal difference coefficient (SDC) algorithm. Keywords: Honeycomb comp osite sandwich structure (HCSS), guided wave (GW), high density core, p iezoelectric wafer transducer (PWT)

1. Introduction HCSS is a sp ecial kind of comp osite structure in which, thin fiber reinforced comp osite skins are bonded to the two faces of relatively thick and very lightweight aluminum honey comb core using adhesive. This novel material is globally adop ted in marine and aerosp ace industries as a sp ecialized lightweight construction material [1]. The higher strength-to-weight ratio makes it suitable for construction of some of the major structural comp onents such as flight wings, blades, fuselage, etc. and the high energy-absorp tion cap ability makes it attractive for impact mitigation and p rotection related ap p lications. In some structural ap p lications, the HCSS involves the use of various cores with different densities in the same sandwich element [2]. In order to introduce electronic and electrical devices, backing p lates, fasteners, stiffeners for attachment or rigging p urpose, etc., in these structures, it is suitable to use HD core sections. A p art of the original core in sandwich structure is substituted by different core inserts like stiffeners, backing p lates and many more [3,4]. Sandwich p anels and beams with sy mmetric

faces and cores of different stiffness were studied by Bozhevolnaya et al. [5]. In case of sandwich beam under 3-point bending, the closed form estimation of stress-strain field due to local effects was calculated. The faces close to the core junctions show significant rise in bending stress. Typ ical sandwich beams with glass fiber reinforced p lastic face sheets and core junctions between p oly mer foams of different densities and rigid aluminum were tested under quasi-static and fatigue loading conditions by Johannes and Thomsen [6]. The Ultrasonic GW based techniques are p roved to be an efficient and accurate p rocedure for nondestructive evaluation (NDE) and structural health monitoring (SHM ) of comp osite structures [7, 8]. The general features of GW p rop agation that can be transmitted in isotrop ic and anisotrop ic media have been documented in many classical textbooks [9, 10]. Among the current methods, dep loy ment of surface mounted broadband transducers or embedded PWTs [11, 12] are notable. Giurgiutiu et al. [13] reported review of the GW p rop agation technique for large area NDE of the laminated comp osite structures using surface bonded p iezoelectric sensors. M aslov and Kundu [14] have shown that the tuning of GW modes in a received signal play s an imp ortant role for identification of hidden defects/discontinuities in laminated comp osites. The disp ersive p rop erties of axially sy mmetric surface waves subjected to p oint loading on an isotrop ic elastic body with dep th dep endent elastic moduli and mass density was analy zed by Balogun and Achenbach [15] in the high frequency range [16]. The numerical and exp erimental studies of GW prop agation in honey comb sandwich structures (HSS) was conducted and reported by Hosseini and Gabbert [17]. Song and Huang [18] used a surfacebonded p iezoelectric actuator/sensor system to investigate GW p rop agation mechanism in aluminum skin–hexagonal Nomex core sandwich structures, both numerically and exp erimentally . This study used a SDC to represent the differential features of debonding. Recently , Sikdar et al. [19] develop ed a baseline free damage index algorithm for detection of disbond in an HCSS using a PWT actuator/sensor network by considering time-frequency information of the received GW signals. From the brief review of p ast literature, it ap p ears that research dealing with GW p ropagation in HCSS is very few in number. Also no theoretical and/or numerical model is available to carry out elasto-dy namic analy sis of a HCSS with joint core of vary ing densities. Therefore, the p resent research is motivated by the need to study the GW field produced by PWT sources on the surface of HCSS with a soft-dense-soft core for NDE/SHM ap plications. Towards this, the characteristics of the p rop agating GW are studied numerically , and the numerical results are successfully verified with the laboratory experiment. Also a SDC based algorithm is ap p lied to identify the location and size of HD core region.

2. Experimental set-up and data acquisition Exp eriments are carried out on a HCSS samp le plate using embedded PWTs (20mm × 20mm × 0.4mm) and a NI set-up as shown in Fig. 1(a). The PWTs can serve as a transmitter as well as a sensor, and they are op erated using the NI-instrumental set-up. The NI set-up is a PXI sy stem that consists of an 8-channel oscilloscope (SCOPE), a multip lexer switch, an embedded arbitrary function/signal generator (FGEN) and a desktop monitor [19,20].

(a) (b) Fig. 1 (a) NI experimental set-up and (b) Schematic rep resentation of the HCSS with PWT locations. A Hanning window modulated 5-cy cle sine p ulse is injected into the HCSS by the FGEN soft front-p anel and a SCOPE soft front-p anel is ap plied to collect the output GW signal. The PWTs are bonded on surface of the sample p late (600 × 450 × 13.5 mm) with a known HD core zone in order to generate GW. A schematic arrangement of PWTs against the HD core zone is shown in Fig. 1(b). In order to obtain the op timum frequency range of the PWTs, a frequency modulation is carried out by p lacing two PWTs (transmitter and sensor) at 200 mm distance on the surface of the samp le p late, as shown in Fig. 2(a). The frequency vs. amp litude grap h (also known as PWT calibration curve) is p lotted in Fig. 2(b) for a range (0.05 to 0.230 M Hz) of central frequencies, in order to identify the op timum excitation frequency . It is noticed that the PWTs show maximum resp onse at 150 kHz frequency. Therefore, a 150 kHz 5-cy cle sine pulse is selected for all the exp eriments and numerical simulations. The inp ut signal and its frequency -spectrum are shown in Fig. 3.

Normalized response

1.0 0.8 0.6 0.4 0.2 0.0 0

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100 150 200 Frequency (kHz)

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(a) (b) Fig. 2 (a) Exp erimental set-up for frequency modulation and (b) corresponding calibration curve.

1.0

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0.8 Normalized amplitude

Normalized load amplitude

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(a) (b) Fig. 3 Inp ut signal of (a) 150 kHz five-cy cle sine p ulse in a Hanning window, (b) its frequency sp ectrum. In the exp eriment, the FGEN of the NI set-up is ap p lied to p roduce the 150 kHz input signal as shown in the Fig. 3(a). The outp ut signal from PWT p ath: 1-2 can be assumed as baseline signal, as the transmitter-sensor path is sufficiently away (180 mm) from the HD-core region, as shown in the Fig. 1(b). Whereas, the outp ut-signal from PWT p ath: 3-4 can be considered as affected signal due to HD core.

3. Finite Element Modeling The finite element method is used as an alternative to numerically solve this class of p roblems. The finite element software ABAQUS/6.12 is used in this study to simulate the GW prop agation in HCSS. In order to know the HD core effects on the p rop agating GW signal, a 3D numerical model (600 mm × 450 mm × 13.5 mm) of the exp erimental HCSS p late is made in ABAQUS, as shown in Fig. 5. Table 1 Elastic p rop erties of the HCSS Material CP-lamina UD-lamina Soft-core HD-core Adhesive

E1 (GPa) 110.31 60.212 0.0804 0.734 0.0486

E2 (GPa) 110.31 60.212 0.0804 0.734 0.0486

E3 (GPa) 18.247 10.252 1.6121 1.846 0.0486

G12 (GPa) 42.41 18.20 0.0321 0.0763 0.0174

G23 (GPa) 4.136 3.611 0.0964 0.4826 0.0174

G13 (GPa) 4.136 3.611 0.0964 0.4826 0.0174

ν 12

ν 13

ν 23

ρ

(kg/m3 ) 0.30 0.12 0.12 1.65 0.20 0.03 0.03 1.42 0.25 0.025 0.025 0.032 0.34 0.042 0.042 0.128 0.40 0.40 0.40 1.25

The HCSS model consists of two 0.74 mm thin Grap hite-ep oxy fiber-reinforced comp osite skins, 0.02 mm thin adhesive layer (at the interface) and a 12 mm thick soft aluminium-core (Al5056) with a HD region (100 mm × 125 mm × 12 mm). The details of the elastic material p rop erties of different layers in the HCSS are tabulated in Table 1. The PWTs (20 mm× 20 mm× 4 mm) are

t (mm) 0.17 0.08 12 12 0.01

modeled on the surface of the HCSS, using the ABAQUS imp licit code. The SP-5H PWT material p rop erties are used [21] and the numerical simulations are carried out with the 150 kHz inp ut signal. In ABAQUS, the C3D8R (8-noded linear brick element with reduced integration and hourglass control) are used for modeling the honeycomb core, adhesive, skin and electrode p art, and the C3D8E elements (8-noded linear p iezoelectric brick element) are used for p iezoelectric p art. In order to model the PWT excitations, the horizontal sy mmetric surface source excitation is ap p lied on the top surface of the HCSS p late model.

Fig. 4 Numerical model of HCSS with HD-core and PWT network in ABAQUS.

4. HD core detection algorithm In order to locate the disbond region a disbond imaging algorithm is used, which works on the basis of an SDC. The Wavelet transform of the exp erimental sensor signals are used for the characterization of disbond in HCSS. The SDC based on the transformed GW signals in the time domain is used to cap ture the differential features of the disbond. To image the disbond region, the damage p robability distribution is comp uted by using the extracted SDC as inp uts, at each p ixel. The quality of the final disbond image is imp roved by using the fused image at individual frequency . The damage localization p robability , D d, of any arbitrary position (x, y), within the sensor network is exp ressed as (Zhao et al 2007)

 β − Aij (x, y)  Dd (x, y) = ∑ iN=−11∑ Nj =i +1Dij ( x, y) = ∑ Ni=−11 ∑ Nj=i +1sdcij ( x, y)    β −1 

(1)

where, D ij(x,y) represent the damage distribution p robability , measured from actuator-sensor p air: i-j and sdcij(x,y) is the signal different coefficient, which is the difference in amplitude area with disbond and without disbond for a particular GW mode. The SDC can be exp ressed as:

∫

t2

(sd − sb )dt

t1

sdcij =

∫

t2

t1

d

[ sd ]2 dt (2)

b

where, s and s are the guided wave signal with disbond and without disbond, resp ectively , t1 is ( , )

the time arrival of signal for p articular mode and t2 = (t1 + bandwidth of signal),

is the

sp atial distribution function, which has contour in the shap e of ellip se with a non-negative value, and

Aij (x, y) =

{

Pij ( x ,y ), Pij ( x , y ) < β β, Pij ( x , y ) ≥ β

(3)

where,

Pij (x , y ) = [ ( x − xi ) 2 ( y − yi )2 + ( x − x j ) 2 ( y − y j )2 ]/ pij

(4)

where, Pij is the distance between actuator ‘i’ and receiver ‘j’, and the β is a small scaling p arameter that reduces the size of the affected zone and it is indep endent of wave velocity . The value of β is determined emp irically and in this study it is selected as 1.05 [22].

5. Results and discussions In order to study the HD core effect on the prop agating GW signal, numerical simulation is carried out for 3D HCSS models with a HD core region, as shown in Fig. 5. The p lots in Fig. 5 indicate the p resence of multip le GW modes. In order to clearly understand the multimodal behaviour, wavelet transform (WT) is p erformed with AGU-Vallen Wavelet [23] on the signals in Fig. 5(a), and are presented in Fig. 5(b). 0.00006

1

1

Without HD-core With HD-core 0.5

2

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WT Coefficient

Normalized s urface dis placement

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4 0.0

Without HD core With HD core

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(a) (b) Fig. 5 Comparison of (a) numerical outp ut signals and (b) their Wavelet transform.

250

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Four individual wave mode are observed in both the cases and the modes are designated as 1, 2, 3 and 4. From the comp arison in Fig. 5, it is observed that the presence of HD core significantly reduces the amp litude of the received signals. In order to validate the numerical simulation results, the laboratory exp eriments are conducted on the HCSS samp le plate for both the without HD core (actuator/sensor p ath: 1-2) and the with HD core (actuator/sensor path: 3-4) case (Ref. Fig. 1(b)). The outp ut signals for both the cases are p resented in Fig. 6, which confirms the p resence of all four wave modes as observed in the numerical signals. Similar behaviour with simulation results is noticed in terms of considerable reduction in amp litude of the GW modes. 1.0

0.000225

1 Without HD-core With HD-core

Without HD core With HD core

0.000175

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WT Coefficient

Normalized displacement

2

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0.000200

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0.000125 0.000100 0.000075

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(a) (b) Fig. 6 Comparison of (a) exp erimental outp ut signals and (b) their Wavelet transform. 1.0

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Numerical Experimental

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(a) (b) Fig. 7 Comp arison of numerical and experimental response in the (a) ‘without’ and (b) ’with’ HD-core case.

The comp arison of numerical and exp erimental outp ut signals of the ‘without’ and ‘with’ HD core cases, are shown in Fig. 7(a) and 7(b) resp ectively . A good agreement between the numerical and exp erimental results is found in both the cases. Therefore it can be stated that the numerical results are successfully validated by the laboratory exp eriments.

5.1 Detection of HD core in HCSS In order to identify the exact location of the HD-core zone, the transformed received signals corresp ond to mode-1 along the path 1-2, 3-4, 5-6, 7-8, 1-8 and 2-7 are ap p lied (Ref. Fig. 4) as inp ut to the SDC based algorithm in MATLAB. The SDC map is shown in Fig. 8, which rep resents the maximum SDC intensity value (negative) close to the HD-core location in the HCSS.

HD-core region

HD-core region

(a) (b) Fig. 8 SDC map in (a) contour and (b) grid p attern, showing the exact location of the HD core region. In order to clearly understand the SDC behavior, a 3D rep resentation of the SDC map is also obtained, as shown in the Fig. 9. It is exp ected that the availability of baseline data will significantly improve the HD-core detection cap ability and possibly size it with some degree of confidence. However, it is found

that the algorithm is cap able to identify HD-core size and location using the change in modal amplitude of the received GW signal with a minimum number of transmitter/sensor p aths available.

Fig. 9 3D rep resentation of the SDC map.

6. Conclusions In order to understand the mechanism of GW prop agation in a HCSS in the p resence of a HD core region, a combined numerical and exp erimental study is carried out. Significant variation in wave mechanism is noticed due to the increase in core density. A good agreement between the 2D numerical and exp erimental results is found in all the cases studied. Due to the presence of the HD core zone substantial reduction in amp litude of the GW modes in the received signal is observed. Therefore, it can be concluded that the increase in mass density of a p articular region in HCSS p late leads to decrease in the amp litude of the p rop agating GW. The SDC based HD core identification technique shows its efficiency to accurately identify the HD core location and also the ap proximate size of the HD core zone. Acknowledgement The authors wish to thank the Indian Space Research Organization (ISRO) for sup porting this work under grant 11ISROC001. References 1. S T Peters, ‘Handbook of composites’, Chapman and Hall, Boca Raton, 1988.

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