Journal of Sandwich Structures and Materials Interfacial ...nilanjan/Patra_Mitra_2017.pdfIt should be noted in this regard that various other types of damage detection methodologies

Original Article

Interfacial delaminationin glass-fiber/polymer-foam-core sandwichcomposites usingsinglemode–multimode–singlemode optical fibersensors: Identificationbased on experimentalinvestigation

Nilanjan Mitra1, Alak K. Patra1, SatyaP Singh2, Shyamal Mondal2, PrasantaK Datta2 and Shailendra K Varshney2

Abstract

Identification of interfacial delamination in the glass fiber/polymer-foam-core sandwich

composites is difficult if the delamination does not propagate to the side surface of the

specimen. However, these damages may eventually lead to compromising the sandwich

composite structural component. A cost-effective novel embedded fiber optic sensor is

being proposed in this manuscript, which works on the principle of multimode inter-

ference, to perform distributed sensing of interfacial delamination within the sandwich

composites while in service. Even though this easy to use methodology has been used

to identify interfacial delamination, this methodology can also be used for different

other types of interfacial/interlaminar distributed strain sensing of samples under

mechanical as well as thermal loads.

Keywords

Optical fiber, singlemode–multimode–singlemode fiber sensor, interfacial delamination,

sandwich composite, embedded sensor

Journal of Sandwich Structures and Materials

0(00) 1–15

! The Author(s) 2017

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DOI: 10.1177/1099636217733983

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1Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India2Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India

Corresponding author:

Nilanjan Mitra, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India.

Email: [email protected]

Introduction

Sandwich composites are used in many different industrial sectors (ranging frominfrastructure to space vehicles) primarily due to their light weight and highstrength feature. Depending on application, the components of the sandwich com-posite vary with regard to the core (which can either be foam or honeycombmaterials made of polymers such as PVC, PMMA; metals and its alloys such asAl, Ti-6Al-4V; ceramics such as SiC, B4C, etc.), face sheet material (glass fiber,carbon fiber, Kevlar, different metallic and ceramic sheets, etc.) along with the gluesystem (epoxy, polyester, vinyl-ester, etc.) used and the method of fabrication(hand layup, vacuum resin infusion, autoclave, etc.). Light weight and highstrength typically equate to easy transportability and modular construction (forinfrastructural purpose), low fuel consumption and higher payloads (for vehicleswhich maybe automobiles, marine, aerospace, space vessels), and so on. In spite ofnumerous benefits, one of the major problems of these structures is the propensityfor interfacial delamination between the face sheet and the core material which maybe either due to manufacturing defects or may occur during service (such as a resultof some low velocity impact). These interfacial defects might eventually propagateas a result of normal loading conditions which the structure or component mightencounter in service (refer to literature by the author [1–4] as well as literature citedwithin those papers). Therefore, there is a need to track interfacial delaminations inreal time, while the structural component is in service.

It should be noted in this regard that various other types of damage detectionmethodologies for composites [5 and references therein] (such as ultrasound C scan,acoustic emission, X-ray, thermography, eddy currents, different types of guidedwaves, etc.) do not work for polymer foam cored sandwich composite materials asconsidered in this manuscript. The primary reason for non-suitability of thesemethodologies is the absorbance of the generated signal by the core layer whichis typically a polymeric foam material. Usually one of the most widely adoptedmethodology to determine the delamination cracks in sandwich composites is byembedding the optical fiber sensors in the interfacial region between the face sheetand the core at the time of fabrication [5–10]. Since the diameter of the opticalfibers is nearly the same as that of the glass fiber materials used as face sheets,introduction of these materials at the time of construction does not induce anysignificant stress concentrations in nearby regions. Fiber Bragg grating (FBG)sensors are widely used in sandwich composites [6–11] and stiffened laminatedcomposites [12–14] for mechanical strain and temperature sensing. But themethod of writing of Bragg grating in optical fibers is costly, and thereforewidespread usage of the sensor material for large structural components may notbe cost-effective. Hence, there is a need to develop cheap optical sensors (low costand easy fabrication) for health monitoring and damage assessment of sandwichcomposite structures.

In this manuscript, an alternative method based on multimode interference(MMI) is used to detect interfacial delamination in sandwich composites.A singlemode–multimode–singlemode (SMS) system is typically composed of

2 Journal of Sandwich Structures and Materials 0(00)

lead-in and lead-out single mode fibers (SMFs) spliced with a section of multimodefiber (MMF).

The applications of refractometer, filters, and temperature sensors [15–19] withSMS have recently been reported in literature. Li [20] measured the wavelengthshift due to strain and temperature using step index SMS fiber structure. Sensorcharacteristics (measured wavelength shift calibrated to obtain changes in mech-anical strain and temperature) have also been studied by Tripathi et al. [21] inthe SMS fiber structure with graded index MMF. An SMS fiber structure incombination with an FBG has also been used to enhance the sensitivity of strainmeasurement [22]. Simultaneous strain and temperature measurements havebeen done with the FBG plus SMS fiber structure [23]. Even though this sensingmethodology is quite well known within the photonics/optics community, therehave not been any reported studies on usage of these sensors for delaminationdetection of sandwich composites (or in a more general terminology of damagedetection as well as structural health monitoring in sandwich and laminatedcomposites) which this manuscript aims to achieve. It should also be pointed outthat the SMS fiber sensor eventually provides a distributed strain rather thanlocalised strains at a point (as observed from FBG which eventually raises issuesassociated with optimisation of number and placement of Bragg Gratings in theoptical fiber).

Theory of MMI

Interference is the outcome of superposition of two or more waves and their prod-uct to form one single wave. MMI technique is based on one main principle, calledthe self-imaging phenomenon. Here, a single transverse mode propagating throughan SMF enters in an MMF. At that time, due to larger core diameter of MMF, theenergy of the previous fundamental mode is divided into many higher order modesin the fiber. Then again at the interface of the MMF–SMF, all or a fraction of thetotal energy of different modes sum-up and propagate through the SMF. In amultimodal waveguide, due to its near parabolic graded index refractive indexnature, the entrance field is replicated in one or multiple images in periodic inter-vals along the propagation direction of the waveguide. The image replication is dueto both, constructive and destructive interference between the waves that is presentall along the multimodal waveguide. In 2D waveguides, the self-image generationcan be analysed using the modal propagation analysis, MPA [24], under a hybridmethod [25], as well as using the beam propagation method, BPM [26]. Figure 1shows the illustrative schematic diagram of SMS structure (single mode–multi-mode–singlemode), where the input fundamental mode of the SMF produces sev-eral higher order modes and those modes propagate in a repetitive manner. Due tochange in pressure and/or temperature in the MMF, the refractive index and thelength of the fiber changes affect the phase relationship of MMI process. As aresult, the output power varies with the phase-shift of multi-mode condition. Toobtain a single image, the difference in phase between all the modes of MMF has to

Mitra et al. 3

be a whole number, a multiple of 2�, thus, each mode can interfere in phase and asa result, the total field can enter into the SMF again; otherwise, loss of power isobserved. If a broadband light is sent at the input, the shift of maximum availablepower with respect to wavelength is also observed [27].

Care is to be taken during splicing of the SMFs to the MMFs such that they areaxially aligned. This ensures that only the symmetric modes of the MMF areexcited and the well-known linear polarisation approximation can be used to ana-lyse the modal propagation. It should be noted that transverse misalignment willlead to excitation of modes other than axially symmetric modes. Kumar et al. [28]theoretically investigated on the transmission characteristics of the fiber optic SMSstructure with mismatch between the spot sizes of fundamental modes in singlemode and MMFs. They demonstrated that such type of fiber structure can be usedas efficient fiber optic micro-bend sensor and novel wavelength filters.

The normalised fundamental mode field of the SMF is taken as �s rð Þ and thefield of the MMF is expressed as �M ¼

Pi ai�i, where ai and �i are the field

amplitudes at the lead-in splice and the normalised axially symmetric field of theith mode, respectively. Field amplitude ai is determined by the modal overlapbetween the fundamental mode of the SMF and the concerned ith mode of theMMF (��i ), i.e.

ai ¼ 2�

Z 10

�s��i r dr ð1Þ

Each guided modes will have different propagation constants (�i) and eventu-ally they will develop a certain phase difference as they travel along theMMF length. At the lead-out splice, these fields eventually couples back

Figure 1. Illustrative schematic diagram of SMS structure.


to the fundamental mode of the SMF and the power in the lead-out SMF can bewritten as

PSMS ¼ a20 þ a21eið�0��1ÞL þ a22e

ið�0��2ÞL þ � � �� 12 ð2Þ

It should be noted that the power is dependent on the length of the MMFsection (L). The normalised fundamental modal field of SMF is taken as

�sðrÞ ¼

ffiffiffi2

�

r1

wse� r2

w2s ð3Þ

where ws is the Gaussian spot size of the SMF and is taken as

ws

as¼ 0:65þ

1:619

V3=2s

þ2:879

V6s

� �; 0:8 � Vs � 2:5

where as and Vs represent the core radius and the V-number of the SMF,respectively.

The output power depends on the phase difference (��) developed between itssuccessive modes. The parabolic graded refractive index profile of the MMF is

nðrÞ ¼ n0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� 2�M

r2

a2M

� �s; r � aM

nðrÞ ¼ n0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� 2�M½ �

p; r4 aM

ð4Þ

where �M ¼n20�n2

cl

2n20

and aM is the core radius of the MMF. The term n0 represents

the refractive index at the axis of the core and term ncl represents the claddingrefractive index.

The Gaussian spot size of the fundamental mode of the MMF (wM) is given as

wM ¼2aM

k0n0ffiffiffiffiffiffiffiffiffiffi2�M

p

� �1=2¼ aM

ffiffiffiffiffiffiffiffi2

VM

rð5Þ

where VM represents the V-number of the MMF defined as VM ¼ k0aM

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffin20 � n2cl

q,

with k0 the free-space wavenumber. The propagation constants of the mth sym-metric mode are given as

�m ¼ k0n0 1�2 2mþ 1ð Þ�M

k0n0ð Þ2

� �1=2; m ¼ 0, 1, 2, . . . ð6Þ

Mitra et al. 5

where �M ¼k0n0aM

ffiffiffiffiffiffiffiffiffiffi2�M

p¼ VM

a2M

¼ 2w2M

.

The normalised field distribution of the MMF can be expressed as

�MðrÞ ¼

ffiffiffi2

�

r1

wMLM

2r2

w2M

� �e� r2

w2s ð7Þ

where LM is the Laguerre polynomial of degree M.Using the above equations, the power in the lead-out SMF of the SMS setup can

be calculated.

Materials and manufacturing

Semi-rigid closed cell PVC foam with a thickness of 30mm, cell size of approxi-mately 400 mm, density of 100 kg/m3 and trade name of Divinycell H100 manufac-tured by DIAB Inc. is used as the core material in this research work. The PVCfoam core is sandwiched between fiberglass face sheets on both sides. Each of theface-sheets typically comprises of two alternate layers of 995 gsm stitched combin-ation mat (Woven Roving glass fiber mats with 276 number of threads per meter aswarp and 237 threads per meter as weft at 90 angle are stitched and assembled withChopped Strand glass fiber mat) and 1.4mm thick 1161 gsm resin-treated plainWoven Roving glass fiber mat (552 threads as warp and 276 threads as weft at 90angle). Epoxy resin (Araldite CY 230-1 IN of Huntsman Advanced Materials with1300–1800 mPa s viscosity and 1.13-1.16 g/cc specific gravity at 25�C) and hardener(Aradur HY 951 IN of Huntsman Advanced Materials with 10–20 mPa s viscosityand 0.97–0.99 g/cc specific gravity at 25�C) in 10wt% of epoxy resin are mixed bystirring at 150 r/min for 15min. Each individual component is shown in Figure 2.The sample is casted through hand lay-up technique with the prepared resin systemand allowed to cure for more than 48 h at room temperature (25�C). It should benoted that there are many other methods of sample casting such as vacuum resininfusion as done by authors [1–4], but those advanced techniques have not beenused in this research since the objective of this paper was to determine the suit-ability of this sensor methodology as an effective delamination detection technique.Initial delamination in the form of pre-crack required at one end for the modifieddelaminated DCB sample is introduced by coating the upper surface of thecore (just below the lower surface of the face sheet) with 80 mm thick non-stickimpermeable Teflon sheet as shown in figure. The dimension of the final fabricatedsample is 25mm� 36mm� 170mm (where width (b)¼ 25mm, thickness(T)¼ 36mm and length (L)¼ 170mm).

Prior to using hand-layup technique to fabricate the material, the MMF (GIF 50Cof Thorlabs) is glued to the core using the resin mix. The length of the MMF is 1mand therefore significantly larger than the dimension of the sample. The core, clad-ding and coating diameter of the MMF are 50mm, 125 1mm and 245 5mm.


The MMF is spliced (using Fujikura splicing machine FSM 60s) on both sides withSMF (SMF 28 of Thorlabs) so as to develop an experimental setup as shown inFigure 3. One end of the SMS system is attached to the input part of the opticalspectrum analyser (OSA, Make and model: YOKOGAWA AQ6370) through theFC/PC end of the single mode patch cable (Part no. P1-1550A-FC-1 supplied byThorlabs). The other FC/PC end is connected with the output end of the OSA asshown in Figure 3(a). The final fabricated sample with embedded fiber optic SMSsensors is as shown in Figure 3(b). It should be noted that OSA typically records thepower vs. wavelength at different time instants of load application to the sample fromwhich the wavelength shift and eventually the strain can be evaluated in the sample.

Experimental investigation

Sandwich samples of width (b)¼ 25mm, thickness (T)¼ 36mm and length(L)¼ 170mm are cut from the initially manufactured plates. The initial

Figure 2. Component layup for sandwich composite manufacture with embedded SMS

sensor.

Figure 3. (a) Illustrative setup of the experiment; (b) fabricated sample with embedded fiber

optic SMS sensor.

Mitra et al. 7

delamination length is kept within 50 1mm. Tensile load is applied to the speci-men through the aluminum hinge tabs attached on both top and bottom faces ofsandwich specimens through pneumatic grips and 1 kN load cell of InstronElectropulse 1000 UTM as shown in Figure 4.

The constant cross-head speed is maintained at 0.5mm/min. The load and cross-head displacements at discrete points are recorded by the UTM, whereas crackinitiation and propagation for at least 20mm from the end point of initial delam-ination are recorded by camera attachments with the help of a software utilityservice. The results presented in the manuscript are mean values of at least eightsamples (with results between mean and two times standard deviation). A finaldeflected shape of the sample is shown in Figure 4(b).

The experiment is conducted under transmission mode, and the data for thespectrum are recorded by the OSA at the same points of crack initiationand propagation as recorded by the UTM. In addition, spectral data at signifi-cant points of change in spectrum are also recorded during experiment.The wavelength shift and fluctuation in power at different wavelengths at dis-crete points of time are recorded by the OSA; the load vs. displacements arerecorded by the UTM, whereas crack initiation and propagation are recordedby camera attachments. Indication of crack initiation and propagation isobserved in the OSA prior to realisation through camera attachments/visualobservation.

Figure 4. (a) Initial experimental setup. (b) Final deflected shape.


Results and discussions

Typical load deflection curve obtained from the UTM is shown in Figure 5. Anabrupt change in slope is observed in-between a cross-head displacement of 3 and3.5mm. However, a careful observation of the side of the specimen through micro-scope reveals that the small hairline crack touching the first marked position of thecrack (or the end point of delamination) is arrested at that position. As the cross-displacement moves from 3.5mm to 4.0mm, the slope further reduces (obviouslysignificantly different from the initial part of the curve); the hairline crack growswhich can be clearly observed to cross the first marked position of crack at about across-head displacement of 3.8mm as shown in Figure 6(a).

In between the cross-head displacement of 4.0mm and 4.5mm, the crack growsmore and propagates further. In between the cross-head displacements of 4.5 to4.7mm, a sharp drop in load is observed and visually a major macrocrack isobserved (refer Figure 6(b)) which suddenly moves from 52.5mm mark to thatof 57mm mark in the specimen. It should be noted that visual observance ofmicrocrack is not easy to observe without a high resolution microscope, providedthe crack is exposed to the side of the specimen. The formation of microcrack mayreveal that the crack has not initiated throughout the entire width section of thespecimen but might have occurred at certain regions along the width at that lon-gitudinal position of the specimen.

These visual observations (which in many cases are difficult to perform) mayeasily be complemented from spectrum data obtained from the optical spectrumanalyser recordings in terms of shifts in peaks or troughs of the output spectrumfrom SMS sensor fiber due to the applied strain (as shown in Figure 7).

Figure 7(a) through (c) shows the spectrum data of the SMS system as obtainedfrom the Optical spectrum analyser in between wavelengths 1440 nm and 1650 nm.

Figure 5. Experimental load displacement plot.

Mitra et al. 9

Figure 7. Power vs. wavelength and wavelength shift vs. displacement curve at specified

displacement levels.

Figure 6. (a) Experimental setup with crack crossing the first marked position of crack.

(b) Sudden movement of crack from 52.5 mm to 57.5 mm.


Apart from observance of several side peaks, it should be noted that there are twopeaks corresponding to 1551.501 nm and 1567.391 nm for cross-head displacementof 0.1mm which show the maximum values of power (as shown in Figure 7(b)).Similarly, maximum values of power are depicted by two peaks corresponding tocross-head displacement of 2.97mm at 1562.969 nm and 1549.12 nm, respectively,as shown in Figure 7(c). On the application of load/strain to the fiber-optic sensor,the entire spectrum is observed to undergo a blueshift in wavelengths as shown inFigure 7(a) through (c). Instead of measuring the wavelength shifts correspondingto the two crest peaks, we basically observe the change in wavelength associatedwith the trough in between the two crest peaks. The wavelength shift of the troughis measured for different cross-head displacements. The wavelength shifts vs. dis-placement plot is shown in Figure 7(d). From the change in wavelength, one canmeasure the strain through a direct relation as follows

�ll0¼ " 1� peð Þ ð8Þ

where l0 is the initial central wavelength for maximum power, �l is the wavelengthshift " is the axial strain along z-direction of the fibre and pe is the photo elasticcoefficient given by

pe ¼n2eff2

p12 � � p11 þ p12ð Þ½ � ð9Þ

where neff is the effective refractive index of the core of the unstressed fibre, p11 andp12 are the strain optic coefficients or Pockels constants, � is the Poisson’s ratio. TheSMF 28TM fibre is used both at the input and output ends of the SMS system. Thecore region of both the SMF and the MMF is considered to be made of dopedsilica (13.5mol% GeO2), and the cladding region is made of fused silica.Furthermore, it is assumed that the refractive index is reduced due to doping[30] in comparison to bulk silica (in which case the values generally used are:p11¼ 0.121 and p12¼ 0.27 respectively [30]). The core and clad diameters of theMMF are considered as 50 mm and 125 mm, whereas for the SMF, it is taken as8.2 mm and 125 mm. The following values of the different coefficients and constantsare used for the calculation of the strain values for SMF 28TM fibre obtained fromthe paper of Lai et al. [29]: neff¼ 1.468 (typical value for an optical fiber, i.e. theCorning SMF-28TM fibre) and p11¼ 0.113 and p12¼ 0.252 for GeO2 doped silica[29,30]. The value of Poisson’s ratio is taken as 0.17 for fused silica from the ‘FusedSilica Material Properties’ published by Accuratus Ceramic Corporation, 35Howard Street, Phillipsburg, New Jersey.

Using the above equations (8) and (9) and the changes in wavelength, one canobtain the strain values as shown in Figure 8. The wavelength shift vs. displacementplot (Figure 7(d)) is similar to strain vs. displacement plot (Figure 8) which isobvious since there is a linear relationship in between the wavelength shift and

Mitra et al. 11

the strain values. It can be observed that there is change in slope at a cross-headdisplacement of 3.5mm indicating the initiation of crack. However, it should benoted that the initiation of crack is visually observed near 3.8mm cross-head dis-placement. Thus, from Figure 8 obtained from the spectrum data of OSA, theinitiation of crack can be identified much ahead of visual observation. At across-head displacement of 4mm and beyond the slope is observed to be negativeindicating a growth of a macrocrack.

Although the main focus of this manuscript was on experimental investigation,it should be realised that numerical methods are complementary to that of experi-mental investigations. Some adaptive FEM and/or isogeometric analysis tech-niques [31,32] might be helpful for modelling delamination problems of sandwichcomposite structures.

Conclusion

An alternative, low cost, effective methodology has been presented in this manu-script to identify the interfacial delamination cracks in sandwich composites. Themethodology, typically composed of lead-in and lead-out SMF spliced with a sec-tion of MMF, relies on use of embedded optical fibers within the sandwich com-posites at the face sheet/core interface and is based upon the principle of MMI. Themethodology is able to identify interfacial cracks much ahead of visual observationsimilar to that of the FBG sensors. Shift in wavelength typically determines averagestrain developed on the entire length of MMF. In comparison to the FBG sensors,the methodology is much cheaper since it does not require fabrication of Bragggratings. The methodology provides a measure of distributed sensing over a certain

Figure 8. Strain vs. displacement plot for the specimen.


region instead of point-based sensing as used in the FBG sensors, thereby removingquestions with regard to optimal placement of FBG when sensing a larger region.

Acknowledgements

Several students such as Dr. Kajal Mandal, Suma Sindhu and the lab staff at Prof. Mitra’ssandwich composite lab are acknowledged for helping at different phases of carrying outexperimentation and sample preparations.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, author-ship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, author-

ship, and/or publication of this article: This work has been funded by VSSC cell of IndianSpace Research Organization (ISRO) under award no. IIT/KCSTC/Chair/New:Appr./13-14/70.

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Documents

Journal of Sandwich Structures and Materials Interfacial ...nilanjan/Patra_Mitra_2017.pdfIt should be noted in this regard that various other types of damage detection methodologies