(a) on-board sensors evaluat ion, (b) damage signature and background noise characterization, and (c) optimizat ion of noise filtering techniques.

Transcription

1 REAL-TIME AIRCRAFT STRUCTURAL MONITORING USING ACOUSTIC EMISSION S. Y. Chung Generl Dynmis Fort Worth, TX INTRODUCTION Advned design of irrft strutures my inorporte res inessible for in-servie inspetion. It is importnt to monitor high stress intensity substrutures on irrft to ssure the integrity of irrft strutures nd to derese life-yle ost of dvned militry irrft. Aousti emission (AE) provides n idel mens for rel-time struturl monitoring. Aousti emission is the stress wve generted by rpid relese of energy from lolized soures within stressed mteril. Aousti sensors ombined with n effetive signl proessor n be used to detet ousti emission events relted to struturl dmges nd serve s rel-time nondestrutive evlut ion (NDE) tool. In-flight AE monitoring of irfrme omponents hd been onduted on Air Fore C-SA nd C-13S trnsport irrft [1,2]; RCAF nd RAAF re urrently investigt ing pplitions of AE monitoring for their inventories of triner/fighter irrft [3-S]. AE ws lso onsidered by USAF for monitoring F-10S ftigue-ritil res, but ws not implemented due to short remining opertion servie life of F-10 fleet [6]. The fesibility of AE monitoring for modern high performne fighter irrft is the subjet of this investigtion. SCOPE AE monitoring system for use in high performne fighters must be light-weight nd smll in size. It must be pble of filtering out inflight noise nd must be ble to monitor omposite s well s metlli strutures. In order to develop n effetive system tht fits the requirements, reserh efforts hve been direted towrd the following res: () on-bord sensors evlut ion, (b) dmge signture nd bkground noise hrteriztion, nd () optimizt ion of noise filtering tehniques. 371

2 The progrm strted with evlution of on-oord sensors. Stte-ofthe-rt ousti sensors n be onstruted from piezoeletri ermi, polymer film, or fiber optis. Polymer film, suh s polyvinylidene fluoride (PVDF), hs reently been used s trnsduers for mny sound pplitions (7). Flexibility is mjor dvntge of polymer film sensors. Pled in ritil lotions on the irfrme, they n withstnd flexing without rking. PVDF performed s well s onventionl PZT sensors in deteting AE events in omposite strutures, but ws not s sensitive for use in luminum omponents [8]. Chrteriztion of AE signls due to vrious dmge modes in omposites s well s in luminum lloys were lso investigted by performing stti tension tests nd ftigue tests [91. In this report, reent resuits from in-flight struturl noise mesurements of fighter irrft nd omponent durbility test re presented. IN-FLIGHT STRUCTURAL NOISE MEASUREMENTS Aousti sensors were instlled t severl struturl lotions of fighter irrft for investigting in-flight struturl noise. Configurt ion of AE sensors on the irrft is shown in Fig. 1. Hrdmn fst setting Double/Bubble epoxy ws used to mount the sensors. Aousti Emission Tehnology (AET) Corportion's AC-17L resonnt response sensors were used. The sensors re of differentil type onstrution nd re ompletely shielded. A premplifier (AET Model 140B) ws mounted lose to eh sensor to provide 40 db mplifition of output signls from the sensor. An nlog irborne mgneti tpe reorder (AR-700) is used to reord the output signls from the premplifier diretly. FS REF ORIENTATION VIEW WllUI - () SECT A A looking AFT. FS,"., o (1110 SCAlE' -\- ACOUSTIC SENSORS," el VENTRAL fin REF _ WllI.10 IL 0.00 REF. I Fig. 1. (b) Il , SECTI-B lookllig FWD. FH33.G0 (1110 SCAlE' I IL 24.2 IL 0.00 REF l ZU : 1 :. ACOUSTIC SEIISORS - -\. -. " ". "\ - (e) SECT C'C lookllig FWO. FS446. (1110 SCAlE' Aousti Sensor Configurt ion for In-Flight Struturl Noise Eesurement 372

3 Prior to flight test, the irrft ws subjeted to n extensive flight lods librtion ground test. During the flight lods librtion, ousti emission ws rel-time monitored by using the onbord sensors nd premplifiers. AE signls were nlyzed using AET-SDDD ousti emission system. The system is 8-hnnel miroomputerbsed AE system. It n proess signls from eh sensor hnnel independently ording to the following AE prmeters: ringdown ounts, event ounts, event durtion, mplitude, rise-time, nd energy. Distribution displys of pek mplitude, event durtion, rise-time, energy, nd slope Cmplitude/rise-time) versus events re lso provided. The system n be used for rel-time AE monitoring with extensive pbility for dt disrimintion bsed upon hrteristi of AE prmeters. AE dt mesured by AET-SDOO were reorded in ls M-bytes hrd disk for post-proessing dt nlysis. A Sngmo mgneti tpe reorder ws lso used in onjuntion with AET-SOOO for reording nlog AE wveforms. Thirty-three test onditions were investigted. For eh test ondition, the pplied lod ws rmped up from zero to 100% nd returned to zero lod, nd it ws performed for four times. AE dt were reorded dur ing the entire test proedure. AII observed ousti noise ws trnsient in nture, s observed in the typil "ringdown" ptterns shown in Fig. 2 C), Cb), nd (). Eletril trnsients usully hve only ouple of ringdown ounts per event, s shown in Fig. 2(d). Only very few eletril trnsients were observed. By gting the ringdown ount prmeter in AET- SOOO, noise due to eletril trnsients n be esily disriminted. AE dt were hrterized ording to event rte, pek mplitude, nd event durt ion prmeters. '" w '" GAIN) O ) '" O IlO OI GAIN) Ibl '" w '" ::; IlO OI GAIN) rime (MICROSECOND) 1" ;::.2 TIME (M ICROSECONO) 60 DB GAIN Id) il lr -., TIME IMICROSECOIIO) TlME IMICROSECOND) 2000 Fig, 2. Typil Aousti Trnsients (, b, nd ) nd El etril Trnsient Wveform (d) During Ground Test. 373

4 Most of AE events ourred durîng tne first run of tne test. There is qulittive orreltion between the totl number of AE events reorded by sensor nd the mximum mirostrin mesured t the lotion of the sensor dur ing the test. Those Ai' events nve mplitudes rnged from 3 to 7 db nd hve durtions less thn 300 miroseonds. Obviously, those AE events re stress indued struturl noises, inluding struturl rubbing nd fretting. Flight tests of the irrft re urrently in progress. In-flight AE.mesurements re performed onurrently with other dedited flight tests. Two hnnels of AR-700 mgneti tpe reorder with frequeny response from 10 KHz up to 20 KHz re vilble for AE mesurements. A CEC VR-3600 tpe drive is used to ply bk AE signls into the AET-SOOO AE system for dt nlysis. A wveform nlyzer, ANALOGIC DATA-6000, is lso used for AE wveform hrteriztion. A flight test tpe ontining one flight hour of AE dt hs been nlyzed. Preliminry results show tht most of ousti trnsient wve forms re similr to those observed during ground tests. In Fig. 3, AE event rte (A) whih ws reorded by the sensor loted t bulkhed FS 446 is ompred with vrious flight prmeters: the stress mesured ne r the lotion of the sensor (B), ltitude (C), Mh number (D), nd norml elertion (E). It is lerly shown tht AE event rte is ssoited with struturl loding, s is produed during nigh-g mneuvers or rpid hnges in irrft speed, ltitude nd stress. fa)!! l o II: 1;; o E ;: 41 Q :::>!: Zl C % 2 fi) fc) (O) fel ii 4 IZ TIME fmim.) Fig. 3. Correltion of AE Event Rte with Vrious Flight Prmeters. 374

5 COMPONENT DURABILITY TEST A bulkhed test omponent ws ftigue loded with F-16 flight lod spetrum for 16,000 equivlent flight hours of durbility test. Two AE sensors were mounted on the test omponent t high stress lotions s shown in Fig. 4. The AET-SOOO system ws used for rel-time monitoring. It strted monitoring t 3,000 flight hours. The test dt were nlyzed on-line. The energy prmeter, whih is funtion of mplitude nd durtion prmeters, provides onvenient mens to identify hnges in AE hrteristi during the ftigue test. The energy distribution grphs fter 00 equivlent flight hours of test were printed out for omprison. Before 8,000 flight hours, energy distribution of AE events showed very little hnge. It hs shrp pek t bout 82 db (reltive energy unit). After 8,000 flight hours, the AE spetrum strted to hnge grdully s the test progressed. It hs mjor brod distribution entered t 82 db nd lso high energy pek t 12 db. The energy distribution grphs for the period between 7,00 to 8,000 flight hours nd the period between 14,00 to 1,000 re shown in Fig. (A) nd (B) respetively. Fig. 4. Aousti Sensor Configurtion On A Bulkhed Test Component. 37

6 20 (A) ENERGV DISTRIBUTION OF AE EVENTS DURING DURABILITV TEST OF BH446 (7,00 8,000 FH) l- '" Z > <C O l- '" Z > <C (O) ENERGV DISTRIOUTION OF AE EVENTS DURING DURAOILITV TEST OF OH446 ( ,000 FH) O 0 Fig ENERGV (RELATIVE UNIT) Energy Distributions of AE Events During Durbility Test of A Bulkhed Component: (A) 7,00-8,000 Flight Hours; nd (B) 14,000-1,000 Flight hours. A liner soure lotion (time-of-flight) progrm ws used in lter stge of the test in n ttempt to determine the lotion of AE soures. NDE tehniques, inluding eddy urrent, ultrsoni, fluoresent penetrnt, surf e replition, nd x-rdiogrphy were used to inspet the test omponent fter 16,000 equivlent flight hours of durbility test. Two ftigue rks were found inside fstener hole (#301 hole shown in Fig. 4) lose t the rmpit. The lrgest rk length ws bout 0.17 inh. The lotion of the fstener hole whereftigue rks were found oinided with the high AE tivity zone deteted by the time-of-flight soure lotion tehnique. By viewing the AE monitoring dt, the initition of the ftigue rks ws estimted to begin t bout 8,00 flight hours of test. CONCLUSION In onlusion, the results of ground test nd flight test hve shown tht irfrme struturl noise omprises both ousti nd eletril trnsients. The eletril trnsient n be filtered out by gting ringdown ount prmeter. Aousti trnsient noise is most likely due to struturl rubbing nd fretting. Aousti trnsient event rte is orrelted with struturl loding nd is not too high for prtil opertion of onventionl AE systems. The result of the bulkhed omponent test hs shown tht AE energy distribution is onvenient prmeter to exmine sudden hnges in AE hrteristi during ftigue loding. The time-of-flight soure 376

7 lotion tehnique is useful for monitoring lolized re in omplex struture. However, fretting signls nd rk growth signls n not be distinguished by simple AE prmetri nlysis nd sptil filtering. Additionl dvned filtering tehniques re needed to seprte fretting signls from rk growth signls in high fretting noise environment. REFERENCE 1. C. D. Biley, "Aousti Emission for In-Flight Moitoring of Airrft Struture", Mter. Evl. 34, (1976). 2. M.E. Mizell nd W. T. Lundy, Jr., "In-Flight Crk Detetion System for the C-13 Lower Center Wing Skin", ISA Pro. 22nd Int. Instr. Symposium, Sn Diego, CA., p.29f, S. L. MBride, "Cndin Fores In-Flight Aousti Emission Monitoring Progrm", Pro. of the ARPA/AFML Rev. of Progress in Quntittive NDE, July 17-21, I. G. Sott, "In-Flight Ftigue Crk Monitoring Using Aousti Emission", Presented t the 13th Symposium on Nondestrutive Evlution, Sn Antonio, P. H. Hutton nd J. R. Skorpi, "In-night Crk Monitoring Using Aousti Emission", 26th Int. Instr. Symposium, Settle, WA., My J. Rodgers, "PRAM Projet-The F-10 Aousti Monitoring", Finl Report, USAF PRAM Progrm, July H. Sussner, "The Piezoeletri Polymer PVF2 nd Its Applitions", 1979 Ultrsoni Symposium Proeedings, 1079, pp ; Institute of Eletril nd Eletroni Engineers, New York, N.Y. 8. S. Y. Chung nd F. H. Chng, "On-bord Aousti Sensor Development", Engineering Reserh Report No. 2379, Generl Dynmis, Fort Worth, Texs, S. Y. Chung nd F. H. Chng, "Chrteriztion of Aousti Emission from Gr/Ep Composites", Engineering Reserh Report No. 244, Generl Dynmis, Fort Worth, Texs,