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published online 23 May 2014Journal of Intelligent Material Systems and StructuresNirupam Aich, Eunho Kim, Mohamed ElBatanouny, Jaime Plazas-Tuttle, Jinkyu Yang, Paul Ziehl and Navid B Saleh

through triboluminescent-based imagingreinforced polymer specimens−Detection of crack formation and stress distribution for carbon fiber

  

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Original Article

Journal of Intelligent Material Systemsand Structures1–8� The Author(s) 2014Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/1045389X14535017jim.sagepub.com

Detection of crack formation andstress distribution for carbonfiber–reinforced polymer specimensthrough triboluminescent-basedimaging

Nirupam Aich1, Eunho Kim2, Mohamed ElBatanouny3,Jaime Plazas-Tuttle1, Jinkyu Yang2, Paul Ziehl3 and Navid B Saleh1

AbstractThis article demonstrates the ability of surface-coated triboluminescent materials to detect damage in carbonfiber–reinforced polymer specimens. An experimental protocol was developed to test the efficiency of thetriboluminescent-based diagnostic method using carbon fiber–reinforced polymer coupons under combined bending–compression conditions. Luminescence, emitted from the triboluminescent coatings under quasi-static loading, wasdetected by capturing digital images. We employed image processing software to quantify change in luminescence as a func-tion of triboluminescent concentration. We observed that 10%, 20%, and 30% triboluminescent coating resulted in 25.3,27.9, and 40.4 (arbitrary units) total luminescence, respectively, which shows a positive correlation of triboluminescentconcentration with luminescence. Finite element simulation was also performed to understand the stress and strain distri-bution and to aid in understanding and correlating light emission regions on the carbon fiber–reinforced polymer couponsunder bending deformation. This work represents a step toward the development of a robust technology that employs tri-boluminescent materials for early damage detection, consistent with theoretical predictions of damage occurrence.

KeywordsPolymers, sensor, structural health monitoring

Introduction

Fiber-reinforced polymeric (FRP) materials are widelyused in many engineering applications, including aero-space, automotive, marine, and civil engineering. TheseFRP materials are typically composed of high-strengthfibers (e.g. glass or carbon) embedded in a relativelysoft polymer matrix, such as epoxy, polyester, and vinylester. The high strength-to-weight ratio of FRP compo-sites along with their long life expectancy and corrosionresistance makes them a suitable substitute for metalsin many applications. However, they also pose unprece-dented challenges for inspection and maintenance dueto their unique failure modes; such as fiber breakage,matrix cracking, and delaminations (Davies et al.,1998; Hamdi et al., 2013). Any undetected damage canjeopardize the safety and reliability of structural mem-bers that employ FRP composites, potentially leadingto catastrophic failure. Therefore, reliable and real-timestructural health monitoring (SHM) methods are

strongly desired to detect the unique damage modes inFRP composites.

The last two decades have seen numerous studiesfocused on the development of SHM methods for FRPmaterials (Austin et al., 2013; Fowler et al., 1989; Ziehland Fowler, 2003). The most widely investigated non-destructive evaluation (NDE) methods are based onultrasonic waves that propagate and interact with struc-tural damage in composites, hence enabling condition

1Department of Civil, Architectural, and Environmental Engineering,

University of Texas, Austin, TX, USA2William E. Boeing Department of Aeronautics & Astronautics,

University of Washington, Seattle, WA, USA3Department of Civil and Environmental Engineering, University of South

Carolina, Columbia, SC, USA

Corresponding author:

Paul Ziehl, Department of Civil and Environmental Engineering,

University of South Carolina, 300 Main Street, Columbia, SC 29208, USA.

Email: ziehl@cec.sc.edu

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assessment of the integrity of the tested component(Hoon and Sang Jun, 2010; Keilers and Chang, 1995).Other techniques, for example, thermography, eddycurrent method, and X-ray-based imaging, have alsobeen pursued (Adams and Cawley, 1988; Giurgiutiu,2007; Qin and Bao, 1995). While each of these technol-ogies provides diagnostic information with uniqueadvantages and shortcomings, most methods necessi-tate complicated software for postprocessing and/orbulky and expensive hardware that often limits theirapplications. Thus, novel SHM methods capable ofreal-time damage sensing based on simple detectiontechniques and inexpensive devices are needed to over-come the limitations of large dataset analysis and theuse of complex damage detection equipment.

Triboluminescent (TL) materials offer an alternativesolution, where real-time crack detection can be accom-plished at low cost and with rather simple evaluationmethods. TL materials release energy via luminescence,when the crystal planes are subject to strain or fracture.A wide range of TL materials, commercially known asphosphors (Phosphor Technology, Hoddesdone, Herts,UK), are being employed for different applications; forexample, manganese-doped ZnS (ZnS:Mn) (Aich et al.,2013; Chandra et al., 2010; Olawale et al., 2012), euro-pium tetrakis (Hurt et al., 1966), and terbium com-plexes (Sage et al., 2001). Different materials have beenused to extract luminescence under mechanical stress.However, ZnS:Mn with two to three orders of magni-tude higher luminescence when compared to the otherphosphors has been the most widely used for SHMapplications (Olawale et al., 2011). ZnS:Mn has thusbeen selected as the material of choice for this study.Previous studies have demonstrated that these TLmaterials, including ZnS:Mn, can effectively serve foroptical or visible damage detection and sensing (Sageand Bourhill, 2001).

The advantageous properties of TL materials haveintrigued researchers to study them for detection ofmechanical or structural damage in TL films under lowvelocity impact (Dickens et al., 2011), in aluminumplates coated with TL material under low velocity(Sage et al., 1999) and hypervelocity impact (Bergeronet al., 2006; Sage et al., 1999), in cementitious materialsunder low velocity impact (Olawale et al., 2012) andcompressive loading (Aich et al., 2013; Olawale et al.,2012), and in FRP composites (Sage et al., 2001). Thesewere also used for visualization of stress distributionon the surface of structures under external loading (Xuet al., 2000). However, research on TL applications inFRPs is currently limited to preparation of TL-mixedFRPs and their preliminary evaluation for damagedetection (Dickens and Okoli, 2011; Sage et al., 2001).The current gap in the technology development is lackof systematic evaluation and correlation between TLintensity for the case of lower rates of loading, such asthose that would normally be applicable as part of a

routine maintenance or evaluation procedure. In addi-tion, correlation of luminescence associated with spe-cific damage and failure mechanisms is not currentlyavailable. This article demonstrates the ability of sur-face coated TL materials to detect severe deformationand damage in carbon fiber reinforced polymer(CFRP) specimens. We developed an experimental pro-tocol to test the efficiency of the TL-based diagnosticmethod using CFRP coupons under combined bend-ing-compression conditions. Luminescence, emittedfrom the TL coatings under quasi-static loading, wasdetected by capturing digital images. Finite elementsimulation was also performed to understand the stressand strain distribution and to aid in understanding andcorrelating light emission regions on the CFRP cou-pons under bending deformation. The current workrepresents a step toward the development of a robusttechnology that employs TL material for early damagedetection, consistent with theoretical predictions ofdamage occurrence.

Materials and methods

Carbon fiber–reinforced polymer and TL materials

Carbon/polyphenylene sulfide (PPS) plastic unidirec-tional blank coupons (Fiberforge, Glenwood Springs,CO, USA) with dimension of 100 mm 3 10 mm 3

2.1 mm were used as test specimens. These specimenswere purchased as panels and cut to the desired dimen-sions using a diamond saw. The stacking sequence ofthe specimen was [0/90/0/45/245/90/0/90/0/90/245/45/0/90/0]. ZnS:Mn crystals with a median size of 8.5 mmobtained from Phosphor Technology were used as TLcoating materials. Characterization protocols andresults have been discussed in detail in a previouslypublished article (Aich et al., 2013). IVEX-C410ArmorStar� polystyrene composite resin from CCPComposites (Kansas City, MO, USA) and Hi-Point 90resin additive from Pergan Marshall LLC (Marshall,TX, USA) were obtained for use in ZnS:Mn dispersionand composite preparation.

Specimen preparation and combined compression–bending testing

ZnS:Mn material was first suspended in 40 mL IVEX-C410 (ArmorStarVE) by magnetic stirring for 5 minand then 0.6 mL Hi-Point 90 resin catalyst was addedto the mix followed by further stirring for 2 min. Thesuspension was bath sonicated for 5 min to ensurehomogenized dispersion of TL particles. Three differentsets of suspensions were prepared with 4.6, 10.4, and17.8 g ZnS:Mn to obtain 10%, 20%, and 30% (w/w)TL concentrations, respectively. The CFRP specimenswere immersed in each suspension for 1 h and thendried in room temperature in an arrangement, as shown

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in Figure 1(a). During the drying process, each side ofthe coupons was retouched with the TL suspensionusing a fine brush to achieve strong binding with theCFRP and uniformity in coating thickness. To ensureevenness of the coatings on both sides of the specimens,the upper-facing sides were altered every 15 min.Uniform coating of ZnS:Mn suspension with 0.25 mmthickness on each side of the specimens was achieved.Coated specimens with 10% TL materials are shown inFigure 1(b).

The TL-coated specimens were then loaded in acombined compression–bending test using a TiniusOlsen TI-5000 electromechanical load frame (TiniusOlsen Inc., Horsham, PA, USA). The overall test setupis presented in Figure 2(a) to (c). Both ends of the spe-cimen were fixed in the grips, which were free to rotate(Figure 3(a)). Also, a 0.2-mm offset from the center ofthe end cross section was given to both ends to inducea bending deformation under compression loading (thered line in Figure 3(a)). Details of the testing setup aredescribed in Guo et al. (2013). Quasi-static compression

loading was applied to the grips with a 2.54-mm/minloading rate until the specimen failed through bendingdeformation. The applied force and compression dis-placements were recorded during the test through aNational Instruments data acquisition system con-trolled with LabVIEW software (National Instruments,Austin, TX).

Imaging and analysis

Light emission from the TL coating layers on theCFRP surfaces under combined compression–bendingstress was captured in a sufficiently darkened roomusing two Nikon D-7000 digital single-lens reflex(DSLR) cameras (Nikon Inc., Melville, NY, USA).The cameras were placed on opposite sides (compres-sion and tension sides of the specimen) of the loadframe at equal distances from the specimens with simi-lar ISO values, focal length, and shutter speed to main-tain controlled image collection (Figure 2(c)). Theimaging for each specimen was started at the beginning

Figure 1. (a) Arrangements of painting and drying of the TL-epoxy coating on the CFRP coupon surfaces and (b) prepared CFRPcoupons uniformly coated with 10% TL materials.TL: triboluminescent; CFRP: carbon fiber–reinforced polymer.

Figure 2. Specimen condition (a) before loading and (b) after loading under combined compression–bending stress and fractureformation causing buckling. (c) DSLR camera positioned at equal distance on the opposite sides of the TL-coated CFRP specimen forimaging.DSLR: digital single-lens reflex; TL: triboluminescent; CFRP: carbon fiber–reinforced polymer.

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of loading and continued for 3 min taking sequentialimages every 20 s (i.e. the exposure time of each shot was20 s). Image analysis was performed with ImageJ soft-ware (NIH) to analyze captured luminescence. The three-dimensional (3D) profiles for luminescence intensity foreach of the samples were generated to measure the effectof increasing TL concentrations, and a MATLAB pro-gram was used to quantify luminescence intensity.

Finite element simulation

A finite element (FE) program (ABAQUS/Standard)was used for the compression–bending simulation ofthe composite specimen. The material properties ofCFRP materials were as follows: E11 = 134GPa,E22 = 9:0GPa, G12 = 3:8GPa, v12 = 0:27, andG23 = 3:2GPa, where E, G, and n represent Young’smodulus, shear modulus, and Poisson’s ratio, and thesubscripts 1, 2, and 3 denote fiber direction, transversefiber direction, and through-thickness direction, respec-tively. The strengths of CFRP material were as follows:ST11 = 1:78GPa, SC11 = 1:6GPa, ST22 = 77MPa, andSC22 = 30MPa, where subscript T and C denote tensionand compression, respectively. The mechanical proper-ties were given from the supplier of CFRP (Fiberforge)except the shear stiffness values which were from Kimet al. (2013). The TL coating was also accounted for inthe FE model. The volume ratio of TL particles in thecoated layer is small. Therefore, we used Young’s

modulus E = 4:0GPa and Poisson’s ratio v= 0:3 ofcured IVEX-C410 resin (ArmorStarVE) for the TLlayer. Each composite layer was considered as homoge-neous and was modeled with solid elements, typeC3D8R, supported by ABAQUS/standard. In thethrough-thickness direction, one solid element was usedfor each ply.

To simulate the compression–bending behavior,boundary conditions (BCs) were applied to a line thathad a 0.2 mm offset from the center of the cross section,as shown in Figure 3(b). For the BC in the lower endsection, x, y, and z degrees of freedom are fixed, whileonly x and z degrees of freedom are fixed at the BC inthe upper end section. The Y direction displacement ofthe upper BC line was controlled to invoke compressionin the specimen. The two end sections of the specimenare free to rotate under the compression loading due toapplied BCs (see Figure 3(b)). Geometric nonlinearitywas considered in the FE simulation for large bendingbehavior. FE results were used to obtain stress distribu-tion on the CFRP coupons, and failure mechanismswere excluded in the numerical studies.

Results and discussions

Progressive failure detection

The imaging technique using DSLR provides the capa-bility to detect the effect of loading and damagethrough TL photo-emission from the compression

Figure 3. BCs for CFRP beam (a) in test setup and (b) FE model. x and z degrees of freedom are fixed in upper boundary conditionand x, y, and z degrees of freedom are fixed in lower boundary condition.CFRP: carbon fiber–reinforced polymer; FE: finite element; DOF: degrees of freedom; BC: boundary condition.

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surface of the CFRP specimens. In previous work withconcrete specimens, one single image per specimen wastaken for the entire duration of the test, that is, fromthe initiation of loading until the occurrence of speci-men failure (Aich et al., 2013). This single exposure didnot provide information of damage progression overtime. Rather, it described damage over the course ofthe entire test. In the current investigation, sequentialimaging each 20 s during the loading period showsincremental TL emission as a function of compressiveloading. Figure 4(a) to (e) shows the captured imagesof the incremental TL emission from the compressionsurface of a CFRP specimen coated with 30% TLmaterial under gradually increasing compressive load.The corresponding load–displacement curve is pre-sented in Figure 4(f), and associated time scales, displa-cements, and applied forces are listed in Table 1.Figure 4(a) shows an image taken at 80 s after loadingwas applied. This time scale corresponds to a displace-ment of 2.53 mm and a force of 458 N. From that pointforward, the TL emissions were continuously observeddue to the CFRP surface stresses as depicted in Figure4(a) to (e). These time scales are marked with arrows toshow the corresponding displacements of 2.53, 3.44,4.38, 5.01, and 5.96 mm, respectively, for Figure 4(a) to(e); while corresponding time scales are 80, 100, 120,140, and 160 s, respectively (Table 1). Quantified valuesof luminescence are also presented in Table 1.

The detection of light depends on the sensitivity ofthe camera and intensity of the emitted light from TLmaterials. It has been reported that crystals of ZnS:Mnrequire approximately 1 MPa for elastic-tribolumines-cence; the limit of elasticity is approximately 30 MPa(Chandra et al., 2010; Osip’yan et al., 1986). Also,approximately 100 MPa is required for fracto-triboluminescence (Olawale et al., 2012). It has beenreported that about 6% of triboluminescence occursduring elastic deformation, 14% at fracture, while 80%occurs during plastic deformation (Alzetta et al., 1970;Olawale et al., 2012). Figure 5(b) represents the stressdistributions obtained from FE simulation in the TLlayer, located on the compression side of the specimenunder five different displacements; that is, 0.3, 0.93,1.88, 2.53, and 5.01 mm, corresponding to the timeintervals of 20, 40, 60, 80, and 140 s, respectively. Whenthe compression of the specimen reaches around 1.88mm, the maximum stress in the TL layer exceeds 40MPa in the center (red-colored region). This impliesthat light emission captured in Figure 4(a) to (e) ismainly due to the plastic deformation of TL, while thelow-intensity light emitted during elastic deformationof TL is difficult to be sensed with the camera. It shouldbe noted that according to the FE simulation, the firstcrack in CFRP beam appears around 0.48 mm com-pression at the first 90� layer from the compressionside. This is in agreement with the experimental obser-vation that the first failure appears around 0.5 mmcompression. The stress distributions obtained from FEsimulation without considering damage (Figure 5(b))may differ from those in the experiment. Nonetheless,we found that the FE simulation well captured themoment of strong light emission in the TL layer, imply-ing that minor cracks appearing in the weakest layersdo not affect the stress distribution in the relatively softTL layer prior to the sudden fracture of the specimen.

Figure 4. (a-e) Captured digital images and (f) Increasing TLemission from the compression surface due to increasing loaduntil failure indicating possible early detection of overload andcrack detection during the compression–bending testperformed on a CFRP specimen coated with 30% TL material.TL: triboluminescent; CFRP: carbon fiber–reinforced polymer; DSLR:

digital single-lens reflex.

Table 1. Load–displacement results for the compression–bending test performed on a CFRP specimen coated with 30%TL material.

Time (s) Compression(mm)

Force (N) Intensityvalues (a.u.)

20 0.3069 410 040 0.9355 449 060 1.8766 456 0

80 2.5319 458 18.17100 3.4425 458 20.42120 4.3823 458 18.60140 5.0122 457 18.67160 5.9622 81 40.40

180 6.9033 84 0200 7.5281 84 0220 8.4679 82 0240 9.4052 82 0260 10.0084 83 0

CFRP: carbon fiber–reinforced polymer; TL: triboluminescent.

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We found nearly no light emission from the 180-stime point (Figure 4(f) and Table 1), implying that thefailure of the coupon occurred in the time windowbetween 140 and 160 s. It should be noted that the firstdetection of light occurred between 40 and 60 s, whichis much earlier than the failure of the coupon. Thisimplies that TL materials may be used to signal the pro-gressive failure of composites in the host structure priorto fracture. This concept may be helpful for detectingfirst ply failure within composites, given optimal con-figurations of TL doping on the surface of compositematerials. Recently, high durability of TL systems wasreported in the literature (Moon Jeong et al., 2013),where light was consistently emitted under cyclic load-ing up to ~30,000 cycles. This is promising for the struc-tural prognosis related to fatigue loading, throughcorrelation of the number of cycles and the correspond-ing light emission.

Detection of fracture

As shown in Figure 4(f), the applied force does notchange significantly after it reaches about 450 N due tobending deformation and suddenly decreases to 81 Nwith the fracture of the specimen. At the moment offracture, TL materials emit exceptionally high-intensitylight (flashing) as depicted in Figure 4(e) with the light-intensity value of 40.40 arbitrary units (a.u.) (Table 1).We found that the TL coating layers were also frac-tured and debonded from the CFRP coupons as theCFRP specimen underwent this rupture. The resin-mixed TL coating exhibited sufficient adhesive strengthwith the CFRP surface and failed only when fractureappeared in the CFRP structure.

Figure 5(a) shows the 3D profile of light emissionobtained from the 30% TL-coated CFRP surface at thetime of failure (i.e. at the displacement of 5.96 mm,

corresponding to 160 s). The left-side image in Figure5(a) indicates the selected areas for analysis of the TLemission from the CFRP surface. On the right side ofthe figure, the 3D profile is shown where the height inthe Z direction indicates the intensity of the lumines-cence originating from the surface and Z = 0 indicatesthe TL-coated compression surface of the CFRP speci-men. The Y direction represents the vertical directionof the CFRP specimen during loading. A and D are thetwo ends of the CFRP specimen attached to the jig,and B and C are the areas near the center. It is notedthat the TL emission intensity is highest near the centerof the CFRP compressive surface, that is, near B andC. This luminescence map indicates the stress distribu-tion in the CFRP during failure.

The stress distribution of the specimen in terms ofluminescence is compared with the FE simulation, asshown in Figure 5(b). Combined compression–bendingwith free rotational end conditions induces bendingdeformation without stress concentration at the ends.Therefore, the maximum stress and strain appear at thecenter of the specimen in the X–Y domain. We observequalitative agreement between the experimental (Figure5(a)) and numerically predicted results from the FEsimulation (Figure 5(b)). Comparison of the lumines-cence profile in Figure 5(a) with the stress distributionprofile in Figure 5(b) indicates the capability of TLmaterials to effectively detect stress distribution. Thisalso demonstrates the compatibility of the TL-embedded epoxy coating with the CFRP specimens.Such compatibility is needed for the determination oflocal stress distribution.

This sequential imaging technique and the TL coat-ing provide unique advantages not only to detect thecritical deformation but may also be effectively utilizedto track the time of failure for CFRP specimens under

Figure 5. Comparison of the (a) image analysis results for TL emission distribution obtained by image analysis and (b) FE simulationfor stress distribution in TL layer. The stress distributions at five different compressions, 0.30, 0.93, 1.88, 2.53, and 5.01 mm arepresented in (b). The TL emission image in (a) was obtained from the compression surface of a CFRP specimen coated with 30% TLmaterial at the time of failure during the compression–bending test performed on a CFRP specimen coated with 30% TL material.TL: triboluminescent; FE: finite element; CFRP: carbon fiber–reinforced polymer.

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loading. If a calibration curve is generated using displa-cement and illumination intensity, it may be possible toimage CFRP specimens under stress and to project theremaining time or load to failure. In this way, a predic-tive tool may be developed for monitoring of structuralsystems in real time, or alternately for routine inspec-tion/evaluation of composite components. This willrequire advancement in the area of high-speed, high-exposure cameras for the capturing of fractions of sec-onds with high-quality output.

Luminescence intensity–TL concentration relation

Figure 6(a) to (c) presents DSLR images of the TLemission from stressed CFRP coated with 10%, 20%,and 30% TL and Figure 6(d) to (f) presents the corre-sponding 3D profiles. It is evident that with the increas-ing TL concentration, stress-induced luminescence isincreased. When analyzed to quantify total lumines-cence using MATLAB, total intensities for these speci-mens with 10%, 20%, and 30% TL concentrationswere found to be 25.3, 27.9, and 40.4 a.u., respectively.Similar observations of TL concentration-dependentluminescence variations were made during a previousstudy related to concrete cubes (Aich et al., 2013).Optimization of TL concentration for actual TL-basedsensing systems will require further investigation usingdifferent concentrations of TL materials and rates ofloading.

Conclusion

This study presents a novel imaging technique using aTL material, which can be used to visually detectsequential stress accumulation during loading and fail-ure of a CFRP element. The technique also enablesvisual mapping of stresses along the CFRP surface,which has been correlated to an FE model. Moreover,the increase in TL concentration in the amount of 10,20, and 30 wt% resulted in more intense luminescencequantified at 25.3, 27.9, and 40.4 a.u., respectively.Such correlation of luminescence with TL concentra-tion indicates opportunities of optimization for TLmaterial usage in large-scale applications. Thisapproach has potential application for SHM in aero-space, civil, and other structural systems. It is alsopotentially well suited as a low-cost, rapid evaluationtool as part of a routine maintenance procedure or dur-ing fabrication. Optimization of the process regardingapplicable TL concentration and controlling TL coat-ing thickness will be the premise of future TL-basedmonitoring/evaluation research.

Acknowledgements

The authors thank Dr Michael Sutton, Mr Siming Guo, andMr David Westbury in the Department of MechanicalEngineering at the University of South Carolina for their gui-dance and help with testing CFRP coupons in the Tinius

Figure 6. (a–c) DSLR images of various concentration TL-coated CFRPs during failure and (d–f) corresponding 3D luminescenceprofile obtained by analysis of the presented DSLR images showing TL emission from the compression surface of a CFRP specimenat the time of failure obtained through image analysis.DSLR: digital single-lens reflex; TL: triboluminescent; CFRP: carbon fiber–reinforced polymer; 3D: three-dimensional.

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Olsen TI-5000 electromechanical load frame. The authors alsothank Mr Ifat Jahangir, graduate student in the Departmentof Electrical Engineering, for the help with MATLAB compu-tations. Authors N.A. and E.K. contributed equally to thiswork.

Declaration of conflicting interests

The authors declare that there is no conflicts of interest.

Funding

Portions of this work were supported through the Universityof South Carolina, Vice President for Research, ASPIRE-Iprogram.

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