Cmpsite Materials with Self-Cntained Wireless Sensing Netwrks Kristin Schaaf, Rbert Kim, and Sia Nemat-Nasser Department f Mechanical and Aerspace Engineering, Center f Excellence fr Advanced Materials, University f Califrnia, San Dieg, 9500 Gilman Drive, La Jlla, CA 92093-0416, USA ABSTRACT The increasing demand fr in-service structural health mnitring, particularly in the aircraft industry, has stimulated effrts t integrate self sensing capabilities int materials and structures. This wrk presents effrts t develp structural cmpsite materials which include netwrks f sensrs with decisin-making capabilities that extend the functinality f the cmpsite materials t be infrmatin-aware. Cmpsite panels are utfitted with netwrks f self-cntained wireless sensr mdules which can detect damage in cmpsite materials via active nndestructive testing techniques. The wireless sensr mdules will cmmunicate with ne anther and with a central prcessing unit t cnvey the sensr data while als maintaining rbustness and the ability t self-recnfigure in the event that a mdule fails. Ultimately, this research seeks t create an idealized netwrk that is cmpact in size, cst efficient, and ptimized fr lw pwer cnsumptin while prviding a sufficient data transfer rate t a lcal hst. Keywrds: Structural health mnitring, self sensing, cmpsite materials, nndestructive testing, wireless sensr mdules 1. INTRODUCTION A cmpsite is an artificially cnstructed material system cnsisting f multiple cmpnents designed t attain specific prperties superir t thse f the individual cnstituents. Cmpsites have unique advantages ver mnlithic materials including high strength, high stiffness, lng fatigue life, lw density, and adaptability t the intended functin f the structure 1. Because f these prperties, cmpsite materials lend themselves t a wide range f applicatins acrss numerus industries, and there is a trend f increased use f cmpsites in the aircraft industry. The Being 787 Dreamliner has fully half f its airframe materials created using cmpsites 4. In many f the applicatins, failure f the cmpsite in service can be critical. This was clearly evident with the American Airlines flight 587 disaster which resulted in 260 passenger casualties and 5 grund casualties. The failed carbn fiber vertical stabilizer can be seen in Figure 1. Thus, there is a need t rapidly assess the structural health r integrity f a cmpsite. Figure 1: Failed carbn fiber and epxy resin vertical stabilizer f American Airlines flight 587. Sensrs and Smart Structures Technlgies fr Civil, Mechanical, and Aerspace Systems 2010, edited by Masayshi Tmizuka, Chung-Bang Yun, Victr Giurgiutiu, Jerme P. Lynch, Prc. f SPIE Vl. 7647, 764744 2010 SPIE CCC cde: 0277-786X/10/$18 di: 10.1117/12.847720 Prc. f SPIE Vl. 7647 764744-1
2. INITIAL APPROACH TO DEVELOPMENT Initially, this wrk sught t add prperties t the cmpsite itself that are infrmatin-based withut cmprmising the structural integrity f the hst cmpsite material. This infrmatin tk the frm f structurally integrated micrsensrs that culd mnitr and reprt n the lcal structural envirnment n request r in real-time as necessary. The envisined sensrs culd read such structural health related parameters as temperature, lad, strain, acceleratin, r acustic emissins. This develpment apprach included: Embedding micrsensrs int cmpsite materials Adding prcessing pwer t frm addressable sensr ndes Frming netwrks with efficient prcessing strategies t lcate and assess damage The increased knwledge f the structure, based n actual data rather than statistical assumptins and extraplatins, wuld allw fr greater flexibility in cmpsite design. The develpment f a reliable means f ascertaining the status f the cmpsite structure wuld allw fr mre high perfrmance designs while minimizing ver designing and building in redundancy. 3. FEASIBILITY STUDY Preliminary wrk t demnstrate the feasibility and survivability f embedding sensrs in cmpsite materials was cnducted. A ten by ten array f digital thermmeter sensrs was fabricated n cnventinal printed circuit bard material with micrprcessrs prgrammed t cmmunicate ver a netwrk. The substrate and sensrs were embedded in an aramid and epxy cmpsite. The resulting panel and its successful temperature sensing ability are shwn belw. Figure 2: Aramid/epxy cmpsite panel with embedded netwrk cnsisting f a 10 by 10 array f individually addressable thermal sensrs. A hand is placed n the panel (left) generating a thermal image (right). 4. MECHANICAL INVESTIGATION Essential t the applicatin f smart cmpsites is the issue f the mechanical cupling f the sensr t the hst cmpsite material. The thrust f the research presented here was t characterize the effects f embedding sensrs n the mechanical prperties f the hst structural cmpsite material in rder t select a sensr and embedding cnfiguratin that culd seamlessly be integrated int the hst cmpsite withut cmprmising the integrity f the structure. The testing cnfiguratins included quasi-static tensin, tensin-tensin fatigue, quasi-static shrt beam shear, and shrt beam shear fatigue. Further testing details and cmplete results can be referenced in the related publicatin3. Prc. f SPIE Vl. 7647 764744-2
Figure 3: Materials utilized in mechanical investigatin fllwing the initial develpment apprach. Figure 4: Graph f average fatigue shrt beam shear life f cmpsite materials with embedded sensrs. Figure 5: (Left) Typical failure f [±45] 10 samples with embedded simulated sensr. (Right) Micr-crack initiatin at sensr and resin pcket eyelet in [0] 4 at 5% strain. Prc. f SPIE Vl. 7647 764744-3
5. COMPUTATIONAL INVESTIGATION Bth 2D plane strain and 3D FEA mdels have been develped t analyze the stress/strain state surrunding the embedded micrsensrs within a unidirectinal cmpsite laminate. The bjective f the numerical effrt was t take int accunt the bserved resin-rich areas caused by embedment and t determine their effects n the lcal stress field arund the embedment and the crrespnding ptential failure mdes. Figure 6: Micrgraph f a sectin f S2-glass/epxy cmpsite laminate with embedded simulated micrsensr with lcal finite element mesh f 2D FEA mdel verlaid. The anistrpic nature f cmpsite materials causes significant attenuatin and dispersin f acustic waves, which brings several difficulties in lcating the surce f a damage event utilizing acustic emissins (AE). The applicatin f a triangulatin methd typically requires a cnsiderable amunt f cmputatin and a great deal f intermediate data strage. A lk-up table methdlgy fr AE surce lcatin was therefre implemented as alternative t the triangulatin methd. The idea behind f this methd is t cmpare the acquired data while the structure is in service with the pre-calibrated AE behavir f the studied structure. Cmplete details and results can be referenced in the related publicatin 2. Figure 7: Schematic diagram f cmpsite panel with PVDF sensrs and lk-up table calibratin grid. Prc. f SPIE Vl. 7647 764744-4
6. CURRENT APPROACH TO DEVELOPMENT The basis f the current develpment apprach is an active mnitring technique that utilizes acustic detectin f damage. The fundamental strategy is t fabricate a netwrk f wireless sensing mdules that are capable f bth sending and receiving acustic signals. In practice, each mdule will send a signal t the nearby mdules. Then, by cmparing the current wavefrm t an established baseline wavefrm, determine if there have been changes indicative f damage t the structure. This apprach is essentially a permanent wireless acustic nndestructive testing (NDT) system. Figure 8: Active in situ nndestructive testing system diagram. The advantages f the in situ NDT system include: Figure 9: Self-cntained wireless sensr mdule (wsm). The pwer requirements are greatly reduced because the wireless sensr mdules need nly be active fr the shrt duratin when the lcal regin is being tested The wireless bandwidth requirements are greatly reduced because the cmmunicatin need nly be between the few active wireless sensr mdules The active in situ NDT system ffers the ability t bserve lng term deteriratin r damage withut the need fr cnstant mnitring There are sme increased demands this apprach places n the system, including: The transducers and assciated electrnics must be stable ver a lng perid f time since it is a cmparisn t an established baseline value that is imprtant T make accurate measurements, accurate timing between neighbring wireless sensr mdules must be maintained, and there are a variety f schemes t achieve this, with the chice depending upn the actual wireless prtcl implemented Prc. f SPIE Vl. 7647 764744-5
7. CONCLUDING REMARKS The gal fr the first phase f the current develpment apprach fr this prject is t create a 4 by 4 array f wireless sensr mdules n an actual cmpsite panel frm an aircraft. Currently, tw wireless sensr mdules, shwn in Figure 9, alng with radial shear sensrs n each mdule have been used t successfully send and receive ultrasnic signals. A third cntrl mdule cnnected t a PC is used t cnfigure the mdule t either be in transmit r receive mde, trigger a test, and cllect the received data frm the mdule. Additinal mdules have been manufactured t scale up the experiment s that an array f mdules can be used t cllect data. ACKNOWLEDGEMENTS This research has been cnducted at the Center f Excellence fr Advanced Materials (CEAM) at the University f Califrnia, San Dieg. This wrk is supprted by bth the Natinal Science Fundatin CMS grant number 0330450 t the University f Califrnia, San Dieg and Spirit AerSystems. REFERENCES [1] Daniel, I. M. 1993. Cmpsite Materials. Handbk n Experimental Mechanics: Secnd Editin, ed. Albert S.Kbayashi. Sciety fr Experimental Mechanics, Bethel, CT. [2] Huang, Yi. Develpment f Self-Mnitring Structural Cmpsites with Integrated Sensing Netwrk. Diss. University f Califrnia, San Dieg, 2008. San Dieg: PrQuest, 2008. (AAT 3307585) [3] Schaaf, Kristin. Cmpsite materials with integrated embedded sensing netwrk. Diss. University f Califrnia, San Dieg, 2008. San Dieg: PrQuest, 2008. (AAT 3307537) [4] Wallace, Bruce. 2008. Cmpsites in demand fr the aircraft industry. Deseret News. http://www.deseretnews.cm/article/1,5143,695270738,00.html (accessed May 8, 2008). Prc. f SPIE Vl. 7647 764744-6