Composite Structures 132 (2015) 833–841

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Composite Structures

journal homepage: www.elsevier .com/locate /compstruct

Sensing and healing of disbond in composite stiffened panel usinghierarchical system

http://dx.doi.org/10.1016/j.compstruct.2015.06.0740263-8223/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: minakuchi@smart.k.u-tokyo.ac.jp (S. Minakuchi).

Naoya Sakurayama, Shu Minakuchi ⇑, Nobuo TakedaDepartment of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan

a r t i c l e i n f o

Article history:Available online 3 July 2015

Keywords:Self-healingMicrovascular systemSensingOptical fiber sensorDisbond

a b s t r a c t

Our previous study proposed a hierarchical sensing–healing system combining a microvascularself-healing material and a fluid distribution system with local pressure monitoring. This study demon-strated the hierarchical system in a structural composite element. First, the overview of the system wasgiven for the sake of the completeness and the technical merits were discussed. Next, double cantileverbeam (DCB) tests were conducted to confirm the system’s feasibility. Finally, the sensing/healing perfor-mance was evaluated using a compression after impact (CAI) test of a composite stiffened panel. Eventhough further optimization of the healing resin and the microvascular channel network is necessary,the hierarchical system restored the structural stability of the damaged panel and recovered the degradedstrength, confirming the high potential of the hierarchical system under practical conditions.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Various self-healing concepts have been proposed and demon-strated during the last two decades [1–4]. In the structural com-posites field, microvascular systems to mitigate resin-dominateddamage have recently attracted considerable attention [5–12].When a crack breaches embedded vasculatures in a compositematerial, a reactive fluid (i.e., healing agent) flows from the brea-ched points into the cracked region, repairing the damage. It hasbeen demonstrated that vascular systems can almost fully recoverthe strength of aerospace-grade composite laminates and sand-wich materials with moderate impact damage [6,9]. They thushave great potential to change the current design philosophy,which could lead to more lightweight and efficient compositestructures [9,13]. Meanwhile, several researchers have recentlyaddressed the potential ability of integrated sensing–healing sys-tems [14–20]. These integrated systems are expected to increasethe efficiency and reliability of healing and to enhance the perfor-mance and adaptability of the host materials.

These recent remarkable advancements in self-healing materi-als and integrated sensing–healing systems have been achievedthrough intensive research activities focusing on localsensing/healing mechanisms (e.g., mechanical interactionsbetween sensing/healing devices and damage, chemical reactionof healing agents within damage, and strength recovery of small

coupon specimens). Currently, system-engineering approaches todesign the system architecture and manage the entire scaled-upcomplex system are urgently needed for the use of sensing–healingcomposite materials in practical smart structures. The authors’research group recently proposed an autonomous sensing–healingsystem applicable to large-scale composite structures [21]. Amicrovascular self-healing material was combined with our hierar-chical fiber-optic-based sensing system. The hierarchical system isa fluid distribution system with local pressure monitoring [22,23],and thus the combined system offered the ability to sense/healcomposite delamination and to self-diagnose the system’s condi-tion. Our previous paper validated the proposed system throughdetection and infiltration of damage [21]. Following the fundamen-tal evaluation, this study presents actual healing of structural com-posites. This paper begins by giving the overview of thehierarchical system for the sake of the completeness. Next, doublecantilever beam (DCB) tests are conducted to confirm the feasibil-ity of the hierarchical system. Finally, its sensing/healing perfor-mance is evaluated using a compression after impact (CAI) test ofa composite stiffened panel. This is, to the best of the authors’knowledge, the first attempt to demonstrate a sensing–healingsystem in a practical structural element.

2. Overview of hierarchical system for autonomous sensing–healing of composite delamination [21]

Fig. 1 depicts the schematic of the hierarchical system. It isimportant to note that the implementation procedure is highly

Fig. 1. Hierarchical system for autonomous sensing/healing of delamination inlarge-scale composite structure [21].

834 N. Sakurayama et al. / Composite Structures 132 (2015) 833–841

compatible with current composite manufacturing and the signif-icant process change is not necessary, as will be demonstrated inthe following demonstration. Only machining of the structural sur-face and the device installation are the additional work, and thematerial cost is sufficiently low. The system employs the vaporiza-tion of sacrificial components (VaSC) technique [7,8,11,24–26].First, sacrificial fibers (catalyst-impregnated polylactide (PLA)fibers) are woven into a dry fabric. The interval of the sacrificialfibers is determined by unacceptable damage size. After resin infu-sion and curing, the sacrificial fibers are removed by heating thepanel to vaporize the PLA, yielding empty channels in the panel.Cross-sections of these empty channels are then exposed to the

panel surface. The panel has a surface sacrificial layer [22], whichis normally glass fiber-reinforced plastic (GFRP) and does not bearload, and the channels partially go through the sacrificial layer. Byslightly machining the panel surface, one can expose thecross-sections of necessary channels without damaging the struc-tural layer (e.g., carbon fiber reinforced plastic (CFRP) layer).Furthermore, one can divide a long channel into short segmentsby closing the exposed cross-sections with sealant. Several channelsegments are united through a surface groove to become one vas-cular module, and are then connected to the main supply tubethrough a check valve. Meanwhile, shallow holes are created onthe other side of the vascular modules, and fiber-optic-based pres-sure sensors are installed on them. The size of the module determi-nes the spatial resolution of the sensing system.

First, pressurized air is injected into the channels from the sup-ply tube or an additional air supply tube, and a pressurized healingagent is then circulated in the supply tube. The pressure of thehealing agent is lower than the pressure in the channels; therefore,the healing agent does not flow into the channels, and the checkvalves keep the pressurized air in the channels from flowing back-ward into the supply tube. When delamination is induced, chan-nels located in the delaminated area are breached, and thepressure in the damaged vascular module drops as the pressurizedair leaks into the delamination. The healing agent then flows intothe breached channels and damage, initiating healing. At the sametime, the pressure sensors detect pressure reduction in the brea-ched channels. Strain of the optical fiber changes in the damagedmodule; thus, one can easily identify the damaged module bymonitoring the strain distribution along the optical fiber using adistributed sensing system [27]. The identified area can be theninspected in detail using non-destructive evaluation techniquesto check the damage severity and healing state. If necessary, addi-tional repair (e.g., patch or scarf) is performed on the healed region,along with appropriate measures to regenerate the sensing/healingfunction of the damaged module. Furthermore, it is possible toreduce operational loading on the structure, based on the monitor-ing result, until the structural strength fully recovers by healingand repairing.

The hierarchical system is analogous to the blood circulatoryand nervous system in vertebrates. It hierarchically combines sev-eral specialized devices to form the network. The supply vesselwith a large diameter, which corresponds to an artery, globally dis-tributes the healing agent over the whole structural area from thepump and the resin reservoir, which correspond to a heart. Thethin channels branching from the main supply tube, which corre-spond to capillary blood vessels, locally delivers the healing agentto the damaged area. This hierarchization maximizes the flow effi-ciency, minimizes degradation of the structural performance dueto vascularization, and increases the robustness against damage[21]. At the same time, the embedded channels double as sensorynerve cells and detect damage. The fiber-optic network, which cor-responds to a spinal cord, gathers damage signals and transmitsthe information to a measuring instrument (i.e., brain). As a resultof this hierarchization, the sensing system has better repairability,higher robustness, and a wider monitorable area than conventionalsystems [23].

In our previous study [21], the autonomous sensing/healingsystem was validated by focusing on phenomena before curing ofthe healing agent. Specifically, water-based viscous liquid was uti-lized instead of a resin to demonstrate detection and infiltration ofdelamination. It was successfully confirmed that the hierarchicalsystem can detect delamination from the strain change in the opti-cal fiber, and can fill the damage with the viscous liquid. In the fol-lowing sections, the actual healing is demonstrated to confirm thesystem’s feasibility and to evaluate its performance in a practicalcondition.

Fig. 3. Overview of hierarchical system deployed on DCB specimen.

N. Sakurayama et al. / Composite Structures 132 (2015) 833–841 835

3. Feasibility study using DCB test

3.1. Materials and methods

The DCB test was conducted by following ASTM D 5528 [28].Fig. 2 depicts the schematic of the specimen fabricated usingvacuum-assisted resin transfer molding (VaRTM). The materialsused were a plain-woven glass fabric (M200 � 104H3, Unitika,Ltd.), a PLA monofilament fiber (Unitika, Ltd., 200 lm diameter)and an epoxy resin (XNR6813K/XNH6813K, Nagase ChemteX Co.,210 �C glass-transition temperature). First, the PLA fibers weretreated with tin(II) oxalate (SnOx) to decrease their depolymeriza-tion temperature, with reference to the method reported in Ref.[24]. The fibers were immersed in a stirred solution of 50 ml triflu-oroethanol (Wako Pure Chemical Industries, Ltd), 25 ml water and6 g SnOx (Wako Pure Chemical Industries, Ltd). After 24 h of thechemical treatment, the fibers were dried in an oven. The treatedPLA fibers were then manually stitched into a stacked dry glassfabric ([(0,90)16]). Two additional plies ([(0,90)2]) were laid onboth surfaces of the stacked specimen and the final laminate stack-ing sequence was [(0,90)20] (5 mm thickness). The epoxy resin wasinfused into the stacked fabrics using VaRTM. After impregnation,the specimen was pre-cured in an oven at 80 �C for 4 h and120 �C for 2 h. The specimen was then post-cured at 180 �C for12 h and microvascular channels were created by vaporizing theembedded PLA fiber. The vaporization temperature of PLA wasbelow the glass-transition temperature of the epoxy matrix(210 �C), and thus the property degradation was negligible. Twoshallow holes were formed on the specimen surface using a millingmachine to expose the cross-section of the embedded hollow chan-nel (Fig. 2). Hole A at the crack side was 2 mm in diameter and0.8 mm in depth, and its effect on the mechanical behavior of thespecimen was minimal due to its small size. Hole B (8 mm diame-ter, 0.9 mm depth) was relatively large, but was positioned at thestress-free region of the specimen. Hole B was covered with asquare acrylic substrate (20 mm side length, 1 mm thickness)and a fiber Bragg grating (FBG) sensor (Technica S.A., 150 lmdiameter, 1 mm gauge length) was boned on the plastic cover.Fig. 3 depicts the overview of the hierarchical system deployedon the specimen. The FBG sensor was connected to an optical sens-ing interrogator (sm130, Micron Optics Inc, 10 Hz sampling speed)and the measurement result was obtained from the analysis soft-ware (Enlight ver1.5, Micron Optics Inc). When the internal pres-sure of the sensor increases, the plastic cover deforms upward,inducing tensile strain in the bonded FBG sensor. In contrast, when

24.6

5.0

147100

59

Polyimide film 12.5μm thick

12.3

44

PLA ϕ 200μm FBG sensor

Unit:mm

Supply tube

a

Fig. 2. Schematic of DCB specimen. Flow channel was created by vaporizingembedded PLA fiber. Pre-crack was formed from polyimide film.

delamination breaches the embedded channel and the internalpressure of the sensor drops, the strain of the bonded optical fiberdecreases. Thus, one can easily detect the delamination by moni-toring the strain of the FBG sensor. Hole A was connected to theresin supply tube (3 mm internal diameter) via the check valve.A relatively large check valve was used to visually determine themoment of channel breaching. Room-temperature curing,ultra-low-viscosity epoxy resin (E205, Konishi Co., Ltd, 100 mPa sat 20 �C) was selected for the healing agent, after consideringdiameter of the embedded flow channel (200 lm), the mechanicalproperty, and the practical healing condition (i.e., room tempera-ture and atmospheric pressure).

Tensile loading (loading speed 1 mm/min) was applied to thespecimen using a material testing system (AG-50kN, ShimadzuCo.), and the crack length a (Fig. 2) was monitored. The DCB testbegan after forming a precrack (a = 50 mm) from a release film(polyimide film, 12.5 lm thickness) implanted at the midplane ofthe specimen during fabrication. The embedded flow channelwas filled with compressed air (0.1 MPa gauge pressure) using acompressor and the resin in the supply tube was pressurized(0.09 MPa gauge pressure) as illustrated in Fig. 3. After the firstDCB test, the infused healing agent was cured at 25 �C for 24 hunder atmospheric pressure. The DCB test was then performedagain using the healed specimen to evaluate the strength recovery.The healing efficiency was evaluated using fracture toughness GIC

calculated using the modified beam theory [28].

3.2. Results

Fig. 4 shows the crack detection result. The strain in the bondedFBG sensor increased to 9000 le after pressurizing the flow chan-nel and kept the constant value during the early stage of the crackpropagation, confirming the air tightness of the system. When acrack breached the channel at a = 85 mm, however, the strainimmediately dropped, successfully demonstrating crack detection.The strain value did not reach zero probably because the resin fill-ing the crack blocked air leakage from the breached channel, andthus a small amount of compressed air remained in the pressuresensor. In our previous research [21], the sensor response wasrather slow since a pressurized liquid was injected into the chan-nels to visually determine the moment of the channel breach.The viscous liquid flowed slowly in the thin channel after thebreach, and pressure change in the sensor was gradual. The resultabove indicates the clear advantage of filling the flow channel withcompressed air to improve the sensor response. Fig. 5 presents theresin flow triggered by the channel breach. The spring coil in thecheck valve moved due to the pressure difference induced and

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Fig. 4. Result of detection. Strain dropped immediately after crack breached embedded flow channel at a = 85 mm.

836 N. Sakurayama et al. / Composite Structures 132 (2015) 833–841

the healing agent flowed into the specimen. The healing systemstarted without time lag after the channel breach and the healingagent reached the damage in a moment. Sufficient amount of theresin flowed into the crack within five minutes. High responsespeed is one of the key features of this system, which is beneficialfor rapid healing of large damage in practical conditions.

Fig. 6 compares load–displacement curves and GIC values of thevirgin and healed specimens. The healed specimen almost fullyrecovered the initial stiffness and the fracture toughness at theearly stage of the test. However, the crack unstably propagatedat d = 14 mm and the load suddenly dropped. As a result, the frac-ture toughness value at a = 70–85 mm could not be obtained in thehealed specimen. This unstable crack propagation may be attribu-ted to non-uniform thickness of the healing resin layer betweenthe crack surfaces, incompatibility between the host resin andthe healing resin (i.e., weak interface), and/or brittleness of thehealing resin. An additional DCB test was thus conducted to con-firm the full potential of the proposed system. The specimen con-figuration was the same as the test above, but the epoxy used tofabricate the specimen (XNR6813K/XNH6813K, Nagase ChemteXCo.) was utilized for the healing agent. Therefore, compatibility

RfCheck

valve

Fig. 5. Resin flow triggered by channel breac

between the host resin and the healing agent was high. After theDCB test and the resin impregnation, the specimen was heated inan oven at 80 �C for 4 h, 120 �C for 2 h and 180 �C for 2 h to fullycure the healing agent. Fig. 7 compares load–displacement curvesof the virgin and healed specimens. The healed specimen perfectlyrecovered the strength, confirming that full strength recovery ispossible using the hierarchical system with an optimized healingagent.

To summarize the DCB tests, the feasibility of the hierarchicalsystem was successfully validated. The system could detect thedamage using the pressure change induced by the breach of theflow channel. The self-healing was then automatically triggeredand the response speed was very high. The healing completed at25 �C for 24 h under atmospheric pressure. Even though unstablecrack propagation occurred in the healed specimen, the fracturetoughness almost recovered (a = 55–65 mm and a = 90 mm) andthe high potential of the hierarchical system was confirmed. Inthe next section, the hierarchical system is applied to a compositestiffened panel to sense and heal the critical damage; disbondbetween the skin and the stiffener. The system performance isevaluated in a practical condition.

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Fig. 6. Result of healing. (a) Load–displacement curves of DCB tests and (b) fracture toughness GIC.

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Fig. 7. Load–displacement curves obtained in additional DCB tests. The same resinwas used for both specimen fabrication and healing.

Fig. 8. Schematic of CFRP stiffened panel.

N. Sakurayama et al. / Composite Structures 132 (2015) 833–841 837

4. System demonstration using CAI test of composite stiffenedpanel

4.1. Materials

Fig. 8 depicts the schematic of the CFRP stiffened panel. Thepanel was manufactured using VaRTM and composed of aT-shaped stiffener and a skin. The single stiffener configurationsimilar to the one in Ref. [29] was adopted after considering theload capability of our testing system (AG-50 kN, Shimadzu Co.).The panel was 400 mm in length and 100 mm in width.Aluminum jigs were attached to both edges of the specimen toapply compressive loading. The materials used were non-crimpfabrics (SAERTEX GmbH & Co.) of aerospace-grade carbonfibers (Toray Industries, Inc., T800S) and an epoxy resin(XNR6813K/XNH6813K, Nagase ChemteX Co.). Stacking sequenceof the skin and the stiffener was quasi-isotropic([�45/+45/0/90]S), and additional plies of plain-woven dry glassfabrics (M200 � 104H3, Unitika, Ltd., [(�45,45)3]) were laid onthe surface of each stiffener flange as the sacrificial layer (Fig. 9).Two chemically-treated PLA fibers (Unitika, Ltd., 200 lm diameter,80 mm length) were woven into the stiffener flange in the longitu-dinal direction. After the cure and heat treatment to vaporize thePLA, flow channels were formed in the interface between thestiffener and the skin (Fig. 10). The length of the channels wasdetermined from the damage survey in preliminary tests. Bothedges of the flow channels were positioned within the sacrificiallayer and four shallow holes were formed on the surface using amilling machine to expose the cross-section of the embeddedchannels (Figs. 8 and 10). In the following test, only the channel

close to the center of the stiffener was used. Again, the smaller hole(2 mm diameter, 0.8 mm depth) was connected with the supplytube for the healing agent. And the larger hole (8 mm diameter,0.9 mm depth) was covered with a square acrylic substrate(20 mm side length, 1 mm thickness) and an FBG sensor(Technica S.A., 150 lm diameter, 1 mm gauge length) was bonedon the plastic cover. The FBG sensor was then connected to an opti-cal sensing interrogator (sm130, Micron Optics Inc, 10 Hz samplingspeed) and the measurement result was obtained from the analysissoftware (Enlight ver1.5, Micron Optics Inc).

4.2. Impact test and damage healing

First, disbond between the skin and the stiffener was intro-duced using a drop-weight impact tester (Yonekura MGF Co.,Ltd). It is important to note that in this demonstration the disbondwas not healed immediately after its occurrence, which is in con-trast to the DCB tests above. The damaged panel was first scannedby a 3D ultrasonic inspection system (Matrixeye, Toshiba Corp.) to

Fig. 9. Cross-section of stiffened panel.

Fig. 10. Flow channel formed in interface between skin and stiffener flange.

838 N. Sakurayama et al. / Composite Structures 132 (2015) 833–841

evaluate the damage state, which can be compared with the stateafter healing. After the flow channel was filled with compressed airof 0.1 MPa gauge pressure, impact energy of 25 J was applied to thecenter of the flange width (Fig. 8). Fig. 11 presents the impact loadcurve and the FBG response measured during the test. Again thestrain dropped immediately after the impact, successfully detect-ing the damage. After the impact test, both surfaces of the speci-men were visually checked. At the impact point (Fig. 12(1a)),there was a small surface dent with depth of 0.3 mm and thusthe damage introduced was barely visible impact damage (BVID).Furthermore, a long interface crack (80 mm length) was seenbetween the stiffener flange and the skin (Fig. 12(2a)), implyingthat large disbond was introduced in the interface between theskin and the stiffener. Fig. 12(3a) presents a photograph of the

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internal damage observed by the Matrixeye. A semicircular dis-bond reaching the channel position was observed, indicating thatthe flow channel was successfully breached by the disbond.

The damaged panel was then healed by injecting a healingagent (E205, Konishi Co, 100 mPa s at 20 �C) that was used in theDCB test. The injection was stopped when the gap (i.e., crack)between the flange and the skin was filled with the healing agent(visually checked). The healing agent was cured at room tempera-ture for 24 h under atmospheric pressure. Fig. 12(1b) and (2b) pre-sent photographs of the specimen after healing. The gap betweenthe flange and the skin was successfully closed with the resin(Fig. 12(2b)). Furthermore, a cured resin was observed in theresidual dent on the impacted surface (Fig. 12(1b)), indicatingthat the resin injected from the back surface flowed in thethrough-thickness direction and finally reached the opposite sur-face. This is attributed to internal cracks induced by the impact.Delamination and matrix cracks in the skin formed the flow pathsand the resin flowed from the skin/stiffener interface to theimpacted surface (Fig. 10). Fig. 12(3b) presents the result of theultrasonic inspection. The disbond disappeared, and the delami-nated area in the skin decreased. Even though perfect infiltrationof the damage including the skin delamination was not achieved,the hierarchical system demonstrated its high performance toinfuse large damage in practical structures.

4.3. Compression test

Finally a compression test was performed using a material test-ing system (AG-50kN, Shimadzu Co., Fig. 13). Loading speed wasset to 1 mm/min. A random dot pattern was applied to the speci-men surface to monitor out-of-plane displacement of the skinusing digital image correlation (ARAMIS, GOM mbH). In addition,three strain gauges (Kyowa Electronic Instruments Co., Ltd.,KFG-10-120-C1-11L3M2R, gauge length 10 mm) were boned on

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Fig. 12. Comparison of damage states after impact and healing.

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the skin to measure the strain development under the compressiveloading.

Fig. 14 presents the strain change measured by the three straingauges. The bonded positions are illustrated in Fig. 8. The panelbegan to globally buckle at 19 kN and the buckling mode changedto a one half-wave mode at 23 kN. The load then increased whilekeeping the same buckling mode. However, the load suddenlydropped at 32.7 kN, and the panel failed at 34.41 kN. Fig. 15 pre-sents photographs of the healed flange under compressive loading.At 32.7 kN, an interface crack occurred between the skin and theflange from the edge of the healed disbond (Fig. 12(2b)), resultingin localization of the buckling deformation (Fig. 16). The healedregion lost the structural support from the stiffener and the final

Aluminum fixture

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Fig. 13. Compression test setup. Random dot pattern was applied to the specimensurface to measure out-of-plane displacement of the skin using digital imagecorrelation.

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Fig. 14. Strain change measured by three strain gauges bonded on specimensurface (Fig. 8).

failure occurred. Preliminary tests confirmed that equivalent stiff-ened panels with and without damage had the strength of 26 kNand 43 kN, so the strength partially recovered by healing but thefull recovery could not be achieved. This may be attributed totwo reasons. One is that the healing agent utilized was weakand/or incompatible with the host resin. As indicated in the DCBtests, a more appropriate resin would be needed to fully recoverthe strength. Second reason is the resin impregnation direction.The healing agent flowed into the disbond from the breach pointof the channel nearest to the supply tube (Fig. 12(2b)), thus theimpregnation speed and pressure of the resin was significantlylow at the pressure sensor side, where the interfacialcrack occurred during the compressive loading. Consequently,small non-impregnated area (i.e., origin of crack) might be gener-ated even though ultrasonic inspection could not detect it(Fig. 12(3b)). This implies the necessity to design more complex

34.4kN(Destruction)

5sec5sec 17sec

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Impact point

32kN 32.7kN 32.6kN

Fig. 15. Photographs of healed flange under compressive loading. At 32.7 kN, interface crack occurred between the skin and the flange from the edge of the healed disbond(Fig. 12(2b)).

Fig. 16. Out-of-plane deformation just before final failure (34.4 kN). The bucklingdeformation at the left side localized around the healed region, leading to finalfailure.

(a) Unhealed (b) Healed

Fig. 17. Comparison of out-of-plane deformation at 25 kN.

840 N. Sakurayama et al. / Composite Structures 132 (2015) 833–841

channel network to fully infuse complex damage in practicalstructures.

Nevertheless, the hierarchical system showed a high potentialto sense and heal disbond in the practical structural element. The

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impact damage was detected immediately after its occurrence andthe degraded compressive strength was recovered. Fig. 17(a)depicts the out-of-plane displacement of an equivalent specimenwith damage, which was measured in a preliminary test. Theunhealed panel locally deformed around the impact damage, andthe damage gradually expanded with cracking sound until the finalfailure at 26 kN. In contrast, the healed panel suppressed the localdeformation around the damage (Fig. 17(b)) and made no soundbefore the occurrence of the interface crack, indicating that thehealing restored the structural stability of the damaged panel. Inour future research, the healing performance will be furtherenhanced by optimizing the healing agent and the design of theflow channel network.

5. Conclusions

This study presented the first attempt to demonstrate a sens-ing–healing system in a structural composite element. First, theoverview of the hierarchical system was given for the sake of thecompleteness and its technical merits were discussed. Next, doublecantilever beam (DCB) tests were conducted. The hierarchical sys-tem detected the damage using the pressure change induced bythe channel breach. The self-healing was then automatically trig-gered and the response speed was very high. Even though unstablecrack propagation occurred in the healed specimen, the fracturetoughness almost recovered and the high potential of the hierar-chical system was confirmed. Finally, the sensing/healing perfor-mance was evaluated using a compression after impact (CAI) testof a composite stiffened panel. Even though further optimizationof the healing resin and the channel network design is necessary,the hierarchical system restored the structural stability of the dam-aged panel and recovered the degraded strength, confirming thehigh potential of the hierarchical system under the practicalcondition.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number25709082. The authors acknowledge the support of MicronOptics, Inc. on fiber-optic measurement and the support of Dr.Yosuke Nagao on fabrication of stiffened panels.

References

[1] Zwaag SVD, editor. Self healing materials: an alternative approach to 20centuries of materials science. Dordrecht: Springer; 2007.

[2] Trask RS, Williams HR, Bond IP. Self-healing polymer composites: mimickingnature to enhance performance. Bioinspiration Biomimetics 2007;2(1):1–9.

[3] Blaiszik BJ, Kramer SLB, Olugebefola SC, Moore JS, Sottos NR, White SR. Self-healing polymers and composites. Annu Rev Mater Res 2010;40:179–211.

[4] Meng H, Li G. A review of stimuli-responsive shape memory polymercomposites. Polymer 2013;54(9):2199–221.

[5] Olugebefola SC, Aragón AM, Hansen CJ, Hamilton AR, Kozola BD, Wu W, et al.Polymer microvascular network composites. J Compos Mater 2010;44(22):2587–603.

[6] Williams HR, Trask RS, Bond IP. Self-healing sandwich panels: restoration ofcompressive strength after impact. Compos Sci Technol 2008;68(15):3171–7.

[7] Esser-Kahn AP, Thakre PR, Dong H, Patrick JF, Vlasko-Vlasov VK, Sottos NR,et al. Three-dimensional microvascular fiber-reinforced composites. Adv Mater2011;23(32):3654–8.

[8] Hamilton AR, Sottos NR, White SR. Pressurized vascular systems for self-healing materials. J R Soc Interface 2012;9(70):1020–8.

[9] Norris CJ, Bond IP, Trask RS. Healing of low-velocity impact damage invascularised composites. Compos Part A: Appl Sci Manuf 2013;44:78–85.

[10] Chen C, Peters K, Li Y. Self-healing sandwich structures incorporating aninterfacial layer with vascular network. Smart Mater Struct2013;22(2):025031.

[11] Patrick JF, Hart KR, Krull BP, Diesendruck CE, Moore JS, White SR, et al.Continuous self-healing life cycle in vascularized structural composites. AdvMater 2014.

[12] Coope TS, Wass DF, Trask RS, Bond IP. Repeated self-healing of microvascularcarbon fibre reinforced polymer composites. Smart Mater Struct2014;23(11):115002.

[13] Williams HR, Trask RS, Bond IP. A probabilistic approach for design andcertification of self-healing advanced composite structures. Proc Inst Mech EngPart O J Risk Reliab 2011;225(4):435–49.

[14] Garcia ME, Lin Y, Sodano HA. Autonomous materials with controlledtoughening and healing. J Appl Phys 2010;108(9):093512.

[15] Hurley DA, Huston DR. Coordinated sensing and active repair for self-healing.Smart Mater Struct 2011;20(2):025010.

[16] Kirkby E, de Oliveira R, Michaud V, Månson JA. Impact localisation with FBG fora self-healing carbon fibre composite structure. Compos Struct 2011;94(1):8–14.

[17] Brandon EJ, Vozoff M, Kolawa EA, Studor GF, Lyons F, Keller MW, et al.Structural health management technologies for inflatable/deployablestructures: integrating sensing and self-healing. Acta Astron 2011;68(7):883–903.

[18] Wu AS, Coppola AM, Sinnott MJ, Chou TW, Thostenson ET, Byun JH, et al.Sensing of damage and healing in three-dimensional braided composites withvascular channels. Compos Sci Tech 2012;72(13):1618–26.

[19] Norris CJ, White JAP, McCombe G, Chatterjee P, Bond IP, Trask RS. Autonomousstimulus triggered self-healing in smart structural composites. Smart MaterStruct 2012;21(9):094027.

[20] Trask RS, Norris CJ, Bond IP. Stimuli-triggered self-healing functionality inadvanced fibre-reinforced composites. J Intell Mater Syst Struct 2014;25(1):87–97.

[21] Minakuchi S, Sun D, Takeda N. Hierarchical system for autonomous sensing–healing of delamination in large-scale composite structures. Smart MaterStruct 2014;23(11):115014.

[22] Minakuchi S, Banshoya H, Ii S, Takeda N. Hierarchical fiber-optic delaminationdetection system for carbon fiber reinforced plastic structures. Smart MaterStruct 2012;21(10):105008.

[23] Minakuchi S, Tsukamoto H, Banshoya H, Takeda N. Hierarchical fiber-optic-based sensing system: impact damage monitoring of large-scale CFRPstructures. Smart Mater Struct 2011;20(8):085029.

[24] Dong H, Esser-Kahn AP, Thakre PR, Patrick JF, Sottos NR, White SR, et al.Chemical treatment of poly (lactic acid) fibers to enhance the rate of thermaldepolymerization. ACS Appl Mater Interfaces 2011;4(2):503–9.

[25] White SR, Moore JS, Sottos NR, Krull BP, Santa Cruz WA, Gergely RCR.Restoration of large damage volumes in polymers. Science 2014;344(6184):620–3.

[26] Gergely RC, Pety SJ, Krull BP, Patrick JF, Doan TQ, Coppola AM, Thakre PR,Sottos NR, Moore JS, White SR. Multidimensional vascularized polymers usingdegradable sacrificial templates. Adv Funct Mater 2015;25(7):1043–52.

[27] Glisic B, Inaudi D. Fibre optic methods for structural healthmonitoring. Chichester: Wiley; 2008.

[28] Handbook of the American Society for Testing and Materials: Standard D-5528, West Conshohocken, PA, USA: ASTM International; 2007.

[29] Orifici AC, de Zarate Alberdi IO, Thomson RS, Bayandor J. Compression andpost-buckling damage growth and collapse analysis of flat composite stiffenedpanels. Compos Sci Technol 2008;68(15):3150–60.