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2013 24: 441 originally published online 23 October 2012Journal of Intelligent Material Systems and StructuresParviz Soroushian, Roz-Ud-Din Nassar and Anagi M BalachandraPiezo-driven self-healing by electrochemical phenomena

  

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Journal of Intelligent Material Systemsand Structures24(4) 441–453� The Author(s) 2012Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/1045389X12461081jim.sagepub.com

Piezo-driven self-healing byelectrochemical phenomena

Parviz Soroushian1, Roz-Ud-Din Nassar1 and Anagi M Balachandra2

AbstractSelf-healing structures mimic the ability of biological structures (e.g. bone) to redistribute their structural mass inresponse to dynamic service loads and damaging effects. The self-healing features yield enhanced levels of structural effi-ciency and safety in dynamic service environments. In this study, the piezoelectric effect was used to convert the dynamicmechanical energy applied to the structure into electrical energy that, in turn, was used to drive electrochemical self-healing phenomena within a solid electrolyte. A theoretical framework was developed for self-healing materials, andexperiments were conducted to verify the fundamental principles of the approach. The theoretical models confirmedthat: (1) the piezoelectric effect can, within the geometric and mechanical constraints of actual structural systems, gener-ate sufficient electric potential and charge (through harvesting the available mechanical energy) to enable electrochemicalmass transport within a solid electrolyte; and (2) the redistribution of structural mass in dynamic service environmentscan occur within viable time frames. The fundamental principles of the new self-healing materials were validated throughthe demonstration of piezo-induced electrolytic phenomena in solid electrolytes and by verifying the gains in mechanicalperformance associated with such phenomena.

KeywordsSelf-healing, piezoelectric effect, mechanical performance, solid electrolyte, electrolysis

Introduction

Polymers and structural composites have found grow-ing use in broad fields of application, including trans-port vehicles (cars, ships, and aircraft), civil engineeringstructures, and sporting goods. These materials, how-ever, are susceptible to damage induced by mechanical,chemical, and thermal effects or combinations thereof(Osswald and Menges, 2003). Many damage phenom-ena introduce microcracks in the body of the structure,which are difficult to detect and even more difficult torepair manually. Consequently, such defects underminethe performance and reliability of polymer and compo-site materials (Blassiau et al., 2007; Kessler et al., 2003).

With increasing use of polymers and composites invarious structural applications, several techniques havebeen developed and adopted for repairing them.Conventional repair methods such as welding andpatching, however, are not effective for healing micro-cracks within the structure during its service life. Theidea of self-healing polymers and composites wasinspired by biological systems that continuously adaptand remodel in response to load and environmentaleffects. The concept of self-healing materials is basedon the in-built capability to substantially recover their

load-transferring ability after damage. Such recoverycan either be autonomous or stimulus activated (Wuet al., 2008). Originally proposed by Jud et al. (1981),self-healing is viewed as a means of healing invisiblecracks in polymeric components. The more recent workof White et al. (2001), who reported the concept ofmaterial’s ability to autonomically heal cracks, and thatof other researchers (Kessler et al., 2003; Pang andBond, 2005) has increased the interest in this technique.Kersey et al. (2007) have reported two general cate-gories of the self-healing techniques available for recov-ery of mechanical properties. Self-healing is noted tooccur either through the creation of new chemicalbonds within the material, for example, by in situ poly-merization of monomers in cracks or by re-creation ofthe ruptured chemical bonds as demonstrated by Chen

1Department of Civil and Environmental Engineering, Michigan State

University, East Lansing, MI, USA2Technova Corporation, Lansing, MI, USA

Corresponding author:

Anagi M Balachandra, Technova Corporation, Turner Street, Lansing, MI

48906-4053, USA.

Email: anagi@technovacorp.com

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and Chu (2005) in their work on thermally remendablepolymers based on cycloaddition (Diels–Alder).Contrary to what has been reported by Kersey et al.(2007), Hayes et al. (2007b) identified four types of self-healing systems. The first system uses glass capillariesto contain liquid resin system that bleed into damagesites upon fracture. Bleay et al. (2001) and Pang andBond (2005) used hollow glass fibers. Healing wouldoccur in this case upon fiber rupture under load. Trasket al. (2007) have used hollow glass fibers in carbonfiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP) to impart a self-healing func-tionality to the laminate. This mechanism proved to beproblematic in some cases. This was mainly due to thefact that the curing rate of the resin employed for self-healing was faster than its diffusion rate, which led toblockage of fiber ends (Wu et al., 2008). The secondsystem comprised a liquid resin microencapsulated in apolymeric shell. A hardener or curing catalyst was alsodispersed throughout the matrix. The resin would bereleased into the matrix upon fracture of the shell; itwould then follow the path of a propagating crack. Asimilar approach is followed by White et al. (2001). Athird approach proposed by Chen and Chu (2005) usesa reverse Diels–Alder polymer, which is based onpolymerization–depolymerization equilibrium thatenables re-formation of broken bonds upon heating.Finally, Hayes and Hou (2002) and Hayes et al.(2007a) used self-healing resins toward the developmentof smart composites with inherent health monitoringand self-healing capabilities.

The self-healing kinetics have been described to playcrucial roles in determining the degree of repairachieved. In addition to avoiding unwanted side reac-tions, the chemistry of the self-healing process must bekinetically accessible on the time scale of crack propa-gation, and it must be thermodynamically viable(Kersey et al., 2007; Kessler et al., 2003). In some self-healing systems, the rate of healing is a function of theviscosity of the healing agent and the capillary drivingforces (Trask et al., 2007). An effective self-healingmaterial should be capable of sensing and respondingto damage over the lifetime of the structural system,and incorporation of the self-healing features shouldnot compromise the initial material properties. Self-healing constitutes recovery of material properties thatare critical to an application, including tensile strength,fracture toughness, and barrier attributes. A healingefficiency coefficient can be used to define the extent ofrecovery of a particular property (Wu et al., 2008). Inthe case of composites subjected to fatigue crackgrowth under high stress intensity, improvements inchemical kinetics of healing are required to prevent thecrack growth (Jones et al., 2007). The healing of poly-meric materials refers to the recovery of properties suchas tensile strength, fracture toughness, surface smooth-ness, and barrier properties. Wool and O’Connor

(1981) have described the healing extent for variousproperties as the ratio of the healed to the original levelof the property.

The self-healing and adaptive qualities of biologicalmaterials make key contributions to their survivability,damage tolerance, and structural efficiency. Bone, forexample, employs a simple but powerful controlmechanism to reconfigure its structure in order toremove the stress gradients for optimum structural per-formance. A new mechanism, inspired by the adaptivefeatures of bone, is introduced here. This mechanismprovides for redistribution of structural mass inresponse to stress gradients in a dynamic loading envi-ronment. The self-healing/adaptive mechanism involvesthe conversion of mechanical energy into electricalenergy using the piezoelectric effect. Redistribution ofmass occurs electrochemically in the context of a solidelectrolyte; it is guided and driven by piezo-inducedelectric potential and charge, respectively. The self-healing features yield enhanced levels of structural effi-ciency and safety in dynamic service environments.

Under load and environmental effects, damage initi-ates and accumulates in composites in the form ofmatrix, interface, interlaminar, and fiber cracks.Through the introduction of piezoelectric and solidelectrolyte constituents, composites assume the inherentcapability to transport structural substance toward anddeposit it at critically stressed areas in order to mitigatecatastrophic damage propagation. This self-healing/adaptive mechanism makes constructive use of the oth-erwise destructive energy concentrated at such criticalzones to effectively mobilize the material resourcesavailable elsewhere against catastrophic failure.

Self-healing/adaptive composite materials promisesignificant benefits in terms of the reliability, surviva-bility, life-cycle economy, and weight of future struc-tural systems (Noor, 2000). The self-healing/adaptivemechanism effectively mobilizes material resources tooptimize structural performance in light of altered ser-vice environments or damaging effects; mitigates cata-strophic damage growth; and benefits the reliability,efficiency, longevity, and maintenance requirements ofstructural materials.

Fracture of composites is a complex phenomenon.Wu (2000) derived expressions for stress distributionaround the crack tip in an anisotropic material. Thebackground experiments in this investigation on thecritical load and crack length at incipient rapid crackextension indicated that cracks propagate collinear withthe original crack. The cohesive zone model (CZM)originally pioneered by Hillerborg (1991) for concreteunder the name of fictitious crack model has been widelyapplied to composite materials (Blackman et al., 2003;Li and Siegmund, 2004; Tijssens et al., 2000). The CZMis a general model applicable to the process zone (Eliceset al., 2001); however, it has been criticized because itneeds the assumption of a crack path (Kennedy and

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Nahan, 1996). The models for brittle-matrix compositesare based on the fracture mechanics notion that sharpcracks propagate in the matrix with fibers bridging thecrack behind the tip (Marshall et al., 1985). Numerousother models have been proposed to analyze the dam-age zone and crack propagation in composite materials.Homma and Mitsubayashi (1989) considered stablecrack growth to evaluate the fracture toughness offiber-reinforced polymer composites. They stated thatthere is always a damage zone in the vicinity of cracktip where debonding of fibers from the matrix occurs.In their work, they analyzed the magnitude of the stressapplied to fibers in the damage zone. Kennedy andNahan (1996) proposed a nonlocal damage model toassess the progressive damage growth around the notchand to predict the failure load of composite laminate.Again, Kennedy and Nahan (1997) extended their ear-lier model to make it applicable to laminated shellstructures containing a crack. A model to characterizethe failure of unidirectional fiber-reinforced compositeswas postulated by Oguni and Ravichandran (2000).The distinct feature of their model was the analysis ofcrack branches initiating from the tip of the initialcrack. They concluded that the critical stress state waslargely dependent on the size of the dominant crackunder given loading conditions. Ju and Ko (2008) pre-sented their micromechanical elastoplastic model to ela-borate the process of partial interfacial debonding offibers from the matrix. Tzadka and Schulgasser (2009)put forward their model to examine the propagationprocess based on the energy approach. In their work,they replaced the crack system by an inclusion to facili-tate the computational process. They also consideredthe damage zone to be of rectangular shape, arguingthat this shape better illustrates the nature of damageevolution. It is generally agreed that in spite of themuch work done to investigate the failure mechanics ofcomposites, a robust model to capture the complicatedphysics associated with damage and failure of compo-sites needs to be presented (Pineda et al., 2009). In mostdamage models, emphasis is placed on the developmentof a damage zone ahead of the crack tip, which has aconsiderable influence on crack propagation. In thecase of fiber-reinforced brittle-matrix composites,microcracks initiate by strain localization and developahead of the crack tip in the region referred to as thefracture processing zone (Mehta and Parsania, 2006). Insummary, characterization of the damage/processingzone is of fundamental importance in the fracturemechanics of composite materials.

Li and Siegmund (2004) highlighted that an effectivebrittle-matrix composite reinforced with strong brittlefibers relies on a weak bond between fibers and matrix.In the event of crack propagation through matrix,some intact fibers are left behind the crack tip. Thesefibers exert closing tractions on the crack surface andwork against the frictional forces at the fiber–matrix

interface during the pull-out process. The energy dissi-pation associated with this phenomenon enhances thetoughness of composites. The work of Oguni andRavichandran (2001) identifies the presence of crackbranches that initiate from the initial crack tip. Theypredicted that the failure envelopes for a range of stressstates are dependent, beside dominant initial flaw, onthe growth and orientation of the crack branches.

Theoretical modeling and analyticalverification of self-healing composites

Matrix cracking, fiber rupture, delamination, anddebonding at fiber–matrix interfaces are the key failuremodes in composite materials. Although thermal,chemical, and other environmental factors can causedamage in composites, failures caused by impact andcyclic fatigue have received the most attention forstructural applications of composites (Baker et al.,1985). Crack propagation is the basic failure modeinvolved in both mechanisms. Consequently, crack pro-pagation in composites has been researched extensively.Cracking and fiber rupture (Figure 1) are some keymanifestations of damage in composites. They bringabout local stress concentration and thus cause furthergrowth of the damage zone.

In this study, the mechanical energy applied to thestructure is used to carry out the self-healing process.The stress gradient (concentration) within the compo-site structure (Figure 2(a)) drives the self-healing phe-nomenon, which strives to restore normalized stressdistribution through transport of structural substanceto highly stressed areas. A theoretical treatment of theself-healing phenomena is presented below.

Stress (s) as a function of the distance from crack tip(r) is given by the expression (Irwin, 1957)

s =Kc(2pr)�1=2 ð1Þ

where, Kc is the critical stress intensity factor (fracturetoughness). Equation (1) provides the basis to identifya damage zone in the vicinity of the crack tip, definedas the zone where concentrated stress exceeds a thresh-old level (sD) corresponding to damage initiation (e.g.debonding). The size of damage zone (rD) (see Figure2(b)) can be expressed as (Matthews and Rawlings,1994)

rD =K2c (2ps2

D)�1=2 ð2Þ

Considering the case of carbon fiber-reinforcedepoxy laminate [0/90]4s, a typical critical stress intensityfactor (Kc) is equal to 13 MPa m1/2 and a typical tensilestrength is 6.38 MPa. For a damage threshold stressthat is 75% of the ultimate tensile strength, equation(2) yields a damage zone size (rD) of 0.12 mm. Thedamage zone (Figure 2(b)) responsible for damage

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growth leading to catastrophic failure is thus relativelysmall. In other words, the failure of a damaged compo-site is decided by a very small fraction of its volume.The self-healing phenomenon renders strengtheningeffects to the damage zone, which mitigates the damagegrowth. The composites embodying self-healing princi-ples may incorporate carbon fibers that have received ahybrid coating comprising piezoelectric and solid elec-trolyte layers (Figure 3). The solid electrolyte layerwould incorporate close to 15% by weight of metalcations (associated with a dissolved salt); electrostrip-ping of this metal from outside the damage zone andits transport into and electrodeposition within the dam-age zone constitute the self-healing effect, which miti-gates damage propagation and catastrophic failure ofdamaged composite. For the purpose of this theoreticalevaluation, we assume that the metal to be transportedto the damage zone is nickel (which is amenable to elec-trolysis). Nickel offers yield strength and elastic modu-lus of about 350 MPa and 207 GPa, respectively. Thetensile strength and elastic modulus of carbon fibers

are about 2700 MPa and 380 GPa respectively; theyrupture at a strain of about 0.00711. These values fallin the range reported in the literature (Tagawa andMiyata, 1997) for high-modulus carbon fibers. Theself-healing phenomenon electrodeposits a layer ofnickel onto the carbon fiber coating.

The electrodeposited nickel would yield as fiberapproaches its rupture strain; thus, it develops a stressof 350 MPa. In order to add 20% to the rupturestrength of a carbon fiber, 1.08 3 1027 g of nickelshould be deposited within the 0.12-mm fiber lengthsegment within the damage zone (Figure 4). With 15%weight fraction of nickel in the solid electrolyte layer of1 mm thickness, one would need nickel from the solidelectrolyte coating within a 17-mm length segment ofthe fiber to effectively strengthen the fiber length occur-ring within the damage zone (Figure 4). The chargegenerated with each application of stress to the piezo-electric layer (over the 17-mm length segment of fiber)is 1026 C. This charge can transport 3 3 10211 g ofnickel for electrodeposition within the damage zone.

Figure 1. Basic modes of crack development in composites: (a) matrix, (b) fiber–matrix interface, (c) interlaminar, and (d) fiber.

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Hence, in order to transport a total of 1.08 3 1027 g ofnickel to damage zone (to render the targeted fiberstrengthening effect), one needs 1080 applications ofstress. Assuming that stress is applied at a frequency of10 Hz, the targeted strengthening (self-healing) effectwould be completed over a period of 108 s (1.8 min).The kinetics of the self-healing mechanism thus pro-vides for efficient fiber strengthening effects on the timescale of damage through crack propagation. Thismechanism will equally apply to the healing of matrixin case cracks develop parallel or transverse to the fiberdirection, as observed in other investigations of self-healing materials (Hayes et al., 2007; Wu et al., 2008).The piezoelectric-based self-healing has also beenreported as a means of repairing delaminated beamsthat would otherwise fail in a sliding fracture mode(Wang and Quek, 2004).

The above calculations provide a key analytical sup-port for the hypothesis that piezo-driven electrolysiswithin solid electrolyte can provide a viable basis fordevelopment of self-healing composite materials. Theintroduction of a hybrid coating onto carbon fibersyields, with a relatively small weight penalty, the desired

self-healing effect that can be completed within a rela-tively short time period.

Experimental verification of theoccurrence and the implications ofelectrolysis phenomena within solidelectrolytes

Tests were conducted in order to verify: (a) the occur-rence of electrostripping, ionic transport, and electrode-position phenomena in solid electrolytes and (b) thegains in mechanical performance associated with elec-trodeposition in solid electrolytes.

Electrochemical phenomena have been traditionallyrealized in the context of liquid electrolytes. The grow-ing demands for solid-state batteries and fuel cells, how-ever, have prompted major development efforts in thefield of solid electrolytes (Zhang et al., 2002). Solid elec-trolytes cover a host of polymer and ceramic materials(Agrawal et al., 2002; Zhang et al., 2002). This researchfocused on solid polymer electrolytes (Ji et al., 2003).

Selection and processing of solid electrolytes

The ability to dissolve salts is not limited to liquid sol-vents. The existence of polar groups in polymers is acommon feature; it may thus be expected that somepolymers will have the potential to dissolve salts andform stable ion–polymer complexes. A salt dissolves ina solid polymer only if the associated energy andentropy changes produce an overall reduction in thefree energy of the system. This arises when the interac-tion between ionic species and the coordinating groupson polymer chain compensates for the loss of salt latticeenergy. The transfer of electric current through solidelectrolytes incorporating dissolved salts can then occurthrough ionic transport. The ions are transported in apolymer electrolyte by a combination of ionic motioncoupled with the local motion of polymer segments(Figure 5) (Gray, 1991). The ion-conducting polymers

Figure 2. Stress rise and the corresponding damage zone near crack tip in composites: (a) stress rise near crack tip and (b) damagezone.

Figure 3. Cross section of carbon fiber with hybridpiezoelectric/solid electrolyte coating.

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exhibit three key characteristics: (a) atoms or groups ofatoms with sufficient electron donor power to formcoordinate bonds with cations, (b) low barriers to bondrotation so that segmental motion of polymer chainsoccurs readily, and (c) a suitable distance between coor-dinating centers facilitating the formation of multipleintrapolymer ion bonds (Gray, 1991).

Solid-state batteries and fuel cells employ solid elec-trolytes simply as fast ion conductors. The self-healingphenomenon, which is the subject of this research, how-ever, requires simultaneous occurrence of electrostrip-ping, ionic transport, and electrodeposition within solidelectrolytes. Furthermore, the mechanical performanceof solid electrolytes is of utmost importance in theirincorporation into high-performance composites. Theselection of a mechanically viable class of solid electro-lytes and validation of complete electrolysis phenomena(electrostripping, ionic transport, and electrodeposi-tion) within this solid electrolyte thus constitute criticalsteps in the development of self-healing composites.

Selection of a solid polymer electrolyte: poly(vinylidine fluoride-co-hexafluropropylene). The approach adopted was toselect solid electrolytes with high inherent mechanicalcharacteristics, which also offer high levels ofconductivity. Based on a comprehensive literaturereview, poly(vinylidine fluoride-co-hexafluropropylene)(PVDF-HFP) was selected to provide a desirable bal-ance of ionic conductivity and mechanical attributes.Because of the high density of fluorine groups, PVDF-HFP has a high coordinating ability with many metalions; this leads to relatively high levels of ionic conduc-tivity. Fluorine can further form H-bonds, which pro-vide the system with pseudo cross-linking and thusenhanced mechanical attributes.

The materials used for the preparation ofPVDF-HFP solid electrolyte included PVDF-HFP(pellets, crystalline thermoplastic copolymer, 15% HFPaverage molecular weight (Mw) ;400,000), ethylenecarbonate (EC, 98%), propylene carbonate (PC, 99%),copper(II) trifluoromethanesulfonate (CuTf, 98%), and

Figure 4. Length segments of carbon fiber subjected to electrostripping and electrodeposition.

Figure 5. Schematic presentation of the cation transport process in a PEO-based polymer electrolyte: (a) segmental motion ofpolymer chain by bond rotation and (b) transfer of cation between polymer chains.PEO: Polyethyelene Oxide.

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tetrahydrofuran (THF, 99.9+% high-performanceliquid chromatography (HPLC) grade, inhibitor free).All materials were used without any further purification.The electrodes were made of stainless steel shims (0.05mm). The three different solid electrolytes were preparedby varying the composition of copper salt, EC, and PCwhile keeping the PVDF percentage constant.

Preparation of the PVDF-HFP solid electrolyte with 3% copperion concentration. PVDF-HFP was dissolved in THF(30% by weight, 3 g) at moderately elevated tempera-ture (60�C). Subsequently, CuTf (1.8084 g), EC (3.5224g), and PC (1.7865 g) were added to the mix (70% byweight at CuTf:EC:PC ratio of 1.0:8.0:3.5) and dis-solved until a uniform solution was obtained. The solu-tion was cast on a Petri dish and left at roomtemperature until all of the THF was evaporated. Ablue/green color sheet was obtained, which was cut intopieces for use in electrochemical experiments.

Experimental verification of electrolysis phenomenawithin solid electrolytes

Experimental procedures. The PVDF-HFP-CuTf/EC/PCspecimens (25 mm 3 25 mm 3 4 mm) were sandwichedbetween stainless steel electrodes as shown in Figure 6.The electrodes/electrolyte assembly was clamped in aTeflon cell with plastic backing plates (Figure 6(b)) toensure adequate contact between the electrodes and thesolid polymer electrolyte sheet. The weights of electro-des and the polymer sheet were measured prior to elec-trolysis. A constant potential of 4 V was applied toelectrodes for a period of 15 h.

Visual assessment of the electrolysis phenomena within solidelectrolyte. The primary goal of the experimental effort

reported herein was to prove the feasibility of electroly-sis (electrostripping, ionic transport, and electrodeposi-tion) occurring in the context of a mechanically viablesolid electrolyte. Prior to application of voltage, copperions are complexed to the fluorine atoms in PVDF-HFP. When voltage is applied, due to polarity differ-ences, two ends of the positively charged copper ionswill be attracted to the negatively charged cathode.Copper ions migrate from one site to the other and getreduced at the cathode where they deposit as copper.Since the most common coordination number of cop-per is 4, each copper ion will bind with four fluorineatoms as shown in Figure 7. The maximum molar ratiobetween copper ion and polymer should thus be 2:1.This ratio guides our efforts to increase the concentra-tion of copper ions in PVDF-HFP.

After the application of a constant voltage of 4 V,the stainless steel electrodes were detached from thesolid electrolyte and observed. Figure 8(a) presents thesurface of solid electrolyte prior to electrolysis, andFigure 8(b) and (c) shows the cathodic and anodic sur-faces of the solid electrolyte after electrolysis. Copperdeposition was quite obvious at cathode, and indica-tions of the dissolution of metal were present at anode.We also observed that the cathode was adhered to thesolid electrolyte; no such adherence was observed atanode. The extent of electrodeposition was relativelylarge, yielding an obvious metallic appearance at cath-ode. A weight gain of 6%–7% was recorded at anodeas a result of electrodeposition.

Assessment of the mechanical consequences ofelectrolysis within solid electrolyte

Mechanical (hardness) tests were performed followingthe ASTM D 2240 procedures on PVDF-HFP solidelectrolytes before and after electrolysis. The polymer

Figure 6. Electrolysis setup with solid electrolyte: (a) test setup and (b) solid electrolyte electrolysis cell (the solid electrolytespecimen thickness is 4 mm, and its side dimension is 25 mm).DC: direct current.

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sample was detached from electrodes after electrolysis,and hardness of the cathode and anode interfaces weremeasured separately. The hardness tests were per-formed at three different locations on each surface. Thehardness test data are summarized in Figure 9.Hardness, as a measure of mechanical attributes, isobserved to increase markedly at cathode after electro-lysis. This finding provides further evidence for the via-bility of our approach, which relies on electrolysisphenomena within solid electrolytes to transport masstoward (and deposit it at) sites of stress rise (cathode)to render self-healing effects. The gains in mechanicalproperties associated with electrolytic mass transport(through the solid electrolyte) to cathode confirm thatthe process can actually render self-healing effects.Mechanical testing was not on the test program as thiswas a proof-of-concept study. Hardness measurementswere made to assess local mechanical properties beforeand after electrolysis. Furthermore, the gain in hard-ness at the cathode was significant (87%), indicating anincrease in the mechanical properties. Since tension test

is an important measurement, it has been added to thefuture work section.

Evaluation of ionic versus electronic transportphenomena in PVDF-HFP solid electrolyte

Solid electrolytes generally transfer electric current by acombination of ionic and electronic transport phenom-ena. While electronic transport is sustainable, ionictransport could be limited by the finite quantity ofmetals available to be ionized (and transported).Without metallic electrodes, solid electrolyte would relyonly on the dissolved metal salt as the source of metalfor ionic transport. Long-term polarization experi-ments allowed us to distinguish between ionic and elec-tronic transport phenomena.

Polarization tests were conducted by connecting theammeter between the power supply and the solid elec-trolyte cell. A bias voltage of 15 V was applied betweenthe electrodes of the cell, with current measured versustime. The time history of electric current was measuredunder a constant bias voltage over a period of 6 h.Figure 10 presents the current time history for thePVDF-HFP system with 3% copper ion concentration.A substantial drop in the current flow is evident overtime. This suggests that the solid electrolyte with dis-solved copper salt relies largely on ionic (in lieu of elec-tronic) transport of electric current, which favors theuse of PVDF-HFP in self-healing composites. The pre-dominance of ionic (vs electronic) conductivity ensuresthat the electrical energy (resulting from transforma-tion of mechanical energy by piezoelectric effect) willbe used efficiently to transport mass (in lieu of elec-trons) to render self-healing effects. It should also benoted that the relatively small fraction of electronictransport has a role to play in the system. Eventualreduction of metal cations for deposition of metal

Figure 8. Visual appearances of PVDF-HFP solid electrolyte before and after electrolysis: (a) before electrolysis, (b) cathodeinterface, and (c) anode interface (the specimens are 25 mm 3 25 mm).

Figure 7. Binding of copper via fluorine atoms.

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requires electrons; the electronic fraction of currenttransfer ensures greater availability of electrons notonly at but also near the cathode interface, therebybroadening the volume within damage zone where elec-trodeposition (and thus self-healing) occurs.

Experimental validation of piezo-drivenelectrolysis within solid electrolyte

The experimental program presented in the previoussection verified the electrochemical mass transport and

deposition phenomena within the solid electrolyte usingdirect input of electricity and evaluated the mechanicalimplications of these phenomena. The experimentalwork presented in this section is concerned with verify-ing the electrolysis phenomena in the solid electrolytedriven by the piezoelectric effect.

Figure 11(a) presents examples of piezo-inducedelectric charge generation under uniform stress. In thecontext of self-healing composites (Figure 12), a piezo-electric coating could be applied on carbon fibers; thestress gradient along fibers would then generate thepiezo-induced potential and charge, which guide anddrive the self-healing process. This unconventionalmanifestation of piezoelectric effect is schematicallypresented in Figure 11(b).

The piezo-induced potential guides the self-healingeffect in the sense that it turns zones of stress concentra-tion into cathode, toward which electrolytic mass trans-port would occur. The piezo-induced charge drives theself-healing effect in the sense that it governs the rate ofmass transport toward cathode. The piezoelectric coef-ficients governing generation of electric potential andcharge under stress are briefly introduced below.

Piezoelectric materials possess anisotropic proper-ties. Consequently, the electrical response of the mate-rial is dependent on the direction of externalmechanical loads. In order to define the electromecha-nical properties relative to prescribed body-fixed axisframes, conventions have been developed for piezoelec-tric plate-like geometries (see Figure 13(a)).Piezoelectric coefficients are defined with two sub-scripts: the first one identifies the axis of electric field,while the second one corresponds to mechanical stress.Expressions for piezo-induced electric charge (D) andpotential (E), and for a stress system that is relevant toour application are depicted in Figure 13(b), where d31and g31 are the relevant piezoelectric coefficients. Someceramics (e.g. lead zirconate titanate (PZT) andBaTiO3) and polymers (e.g. PVDF) exhibit pronouncedpiezoelectricity. The d31 piezoelectric coefficients ofPZT, BaTiO3, and PVDF are 110, 78, and 23 pC/N,respectively, and the corresponding g31 coefficients are10, 5, and 216 mV m/N, respectively.

For piezo-driven electrolysis to actually take place insolid electrolyte, it is essential that (1) the piezo-induced

Figure 11. Different manifestations of the piezoelectric effect: (a) conventional piezoelectric effect under uniform stress and (b)piezoelectric effect under stress gradient.

Figure 10. Current time history for PVDF-HFP with 3%copper ion concentration.

Figure 9. Hardness test results (ASTM D 2240).

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Figure 12. Schematic presentation of the self-healing mechanism: (a) catastrophic failure of conventional composite, (b) self-healingmechanism mitigating catastrophic failure, (c) stress rise upon fiber rupture, (d) hybrid fiber coating, and (e) piezo-driven electrolysis.

Figure 13. Some fundamental piezoelectric principles: (a) body axes defining piezoelectric coefficients and (b) piezo-induced charge(D) and potential (E).

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potential (E) is sufficient to overcome the activationand polarization overpotentials at anode and cathodeand also the i–R drop within the solid electrolyte (wherei is electric current and R is resistance); and (2) piezo-induced charge is sufficient for electrochemical masstransport at viable rates.

In order to validate piezo-induced electrolysis withinsolid electrolyte, PVDF-HFP solid electrolyte specimenswith dissolved copper salt were prepared as discussedearlier. The solid electrolyte specimens were sandwichedbetween two stainless steel electrodes. Piezoelectric(PZT fiber-reinforced composite) sheets were then sub-jected to repeated stress application, and the piezo-induced voltage was applied to the electrodes. Currentwas measured at picoampere precision (between thepiezoelectric setup and electrodes). The basic elementsof the test setup are schematically depicted in Figure 14.The current flowing through the solid electrolyte wasfound to be 20 mA; a load frequency of 3 Hz was usedin this experiment that lasted for 18 h.

The solid electrolyte surfaces adjacent to cathodeand anode electrodes were visually evaluated after theperformance of the test. Clear evidence of metal deposi-tion was observed at the cathode interface in solid elec-trolyte under piezo-driven electrolysis.

As shown in Figure 15, the cathode interface ofPVDF-HFP solid electrolyte showed obvious signs ofmetal deposition. It is important to reiterate that theelectrolytic mass transport toward and its deposition atcathode interface in Figure 15 occurred without anydirect application of electric potential but by the appli-cation of stress, with piezoelectric effect transformingthe mechanical energy into electrical energy for guidingand driving electrolytic mass transport in solid electro-lyte. After piezo-driven electrolysis, the solid electrolyteadhered to the electrode at cathode.

Material transport occurs within the solid electrolytematrix, with copper ions migrating from low-stress tohighly stressed regions. Many highly stressed regionsincorporate fractured fibers. Migration and depositionof copper in highly stressed regions thus involve somestrengthening of stressed fiber–matrix interfacial areas.It should be noted that the dissolutions at anode isinsignificant when compared with the depositionsoccurring at cathode. This is confirmed by the insignifi-cant loss of mechanical performance (hardness) experi-enced at anode, which suggests that fiber debondingdue to dissolutions is not a major concern.

Conclusion

The analytical and experimental studies were con-ducted to validate the fundamental principles of newself-healing composites where the piezoelectric effectconverts the mechanical energy applied to structureinto electrical energy which, in turn, drives electroche-mical phenomena within a solid electrolyte to renderlocal mechanical benefits. The key conclusions are asfollows:

1. Theoretical investigation of self-healing compo-site materials validated that piezo-driven electro-lytic mass transport in solid electrolyte providesa viable basis for the development of self-healing

Figure 15. Anode and cathode interfaces of solid electrolyte after piezo-driven electrolysis: (a) anode interface and (b) cathodeinterface (the specimen dimensions are 25 mm 3 25 mm).

Figure 14. Schematics of the experimental setup for validationof piezo-induced electrolysis within solid electrolyte.

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composites. These composites are inherentlycapable of strengthening damaged zones withina short time period to mitigate catastrophic fail-ure. The functional constituents rendering self-healing effects can be introduced as fiber coat-ings with minimal weight penalties; the gains inreliability associated with self-healing effects canbe used to enhance structural safety and toreduce weight.

2. Experimental investigation of selected solidpolymer electrolytes verified that electrolyticmass transport (i.e. electrostripping, ionic trans-port, and electrodeposition) can take placewithin mechanically viable solid electrolytes andcan yield marked gains in mechanical perfor-mance at electrodeposition sites.

3. Experimental studies confirmed that the piezo-electric effect can generate sufficient electricpotential and charge to guide and drive electro-lytic mass transport in solid electrolyte for self-healing through electrodeposition.

4. An experimental approach was adopted in thisarticle to provide proof for validation of thisconcept. This initial work was successful, anddetailed design is still needed for optimizing thethickness of piezoelectric coating.

Future work

The experimental work presented in this article pro-vides proof of concept for conversion of mechanicalenergy into electrical energy using the piezoelectriceffect for strengthening the highly stressed areas ofcomposites through ionic movements and depositionsin the context of a solid electrolyte. Detailed design ofintegrated systems embodying all constituents in viableproportions and processing and experimental verifica-tion of the integrated system would be a critical nextstep to demonstrate the new self-healing principle inthe application to composite structures.

Funding

The research reported herein was supported by the US Armyunder contract number W911W6-05-C-0010.

Acknowledgement

The authors are thankful to Mr Nate Bordick of the USArmy for his guidance and support throughout the project.

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