APROPRIATE ULTRASONIC SYSTEM COMPONENTS FOR NDE OF THICK POLYMER-COMPOSITES*

Transcription

1 APROPRIATE ULTRASONIC SYSTEM COMPONENTS FOR NDE OF THICK POLYMER-COMPOSITES* C.M. Frtunk and D.W. Fitting Natinal Institute f Standards and Technlgy Materials Reliability Divisin Bulder, CO INTRODUCTION In certain marine applicatins, thick plymer-cmpsite materials may have t endure different perating envirnments than thse experienced in traditinal aerspace applicatins. In particular, structures made f such materials may experience very large cmpressive and bending frces. T prevent in-service failure, apprpriate NDE methds and instrumentatin are needed t characterize the state f the material. Specifically, in additin t detecting high-cntrast anmalies (cracks and delaminatins) it may be f interest t determine the pre cntent, measure the fiber vlume, assess the severity f fiber waviness, and the like [1]. The NDE requirements f thick marine cmpsites cannt be met by using traditinal ultrasnic instrumentatin, particularly in the pulse-ech mde. Fr example, cnventinal ultrasnic instruments ften lack features that permit effective pulse-ech peratin belw 1 MHz and have limited dynamic range. In this paper, we describe specific appraches that vercme the abve limitatins. Specifically, we describe a particularly effective transmit/receive circuit cnfiguratin that ffers large dynamic range (greater than 60 db), wide bandwidth (100 khz - 60 MHz), and fast recvery. We als describe the design f a very-lw-nise, high-impedance input preamplifier. We fund that this preamplifier is particularly useful fr cnditining signals generated by very small ("phase insensitive" [2]) transducers used as receivers. SIGNAL-TO-NOISE CONSIDERATIONS The NDE instrumentatin must prvide sensitivity t changes in material prperties (lw-cntrast anmalies) as well as internal discntinuities (high-cntrast discntinuities). Fr this reasn, large dynamic range and brad bandwidth are required simultaneusly. T ptimize the abve characteristics, we have develped analytical tls which accunt fr majr factrs that affect the system respnse as a functin f frequency: (1) transmitter pulse shape, (2) transmitter utput impedance, (3) frequency-dependent material dissipatin, (4) diffractin, (5) transducer insertin lsses, and (6) preamplifier nise factr. * This is a submissin f the U.S. Gvernment, Natinal Institute f Standards and Technlgy, and is nt subject t cpyright. Review f Pr,,., in Quantitative Nndatructive Evaluatin. Vl. lob Edited by D.O. Thmpsn and D.E. Chimenti. Plenum Press, New Yrk,

2 Figure 1 illustrates the applicatin f ur prcedure t finding the ptimum frequency regin fr inspecting a thick-sectin E-glass/epxy material made by a wet filament winding prcess. The signal-t-nise (SIN) capabilities f an ultrasnic inspectin system are defined by the vltage-breakdwn characteristics and utput impedance f the ultrasnic pulser and by the input nise level f the receiver preamplifier. The input nise f ur high-impedance receiver preamplifier is expressed in units f Vlts per rt Hertz (V I Hz). T facilitate a calculatin f the SIN, the transmitter utput-pwer capabilities must be expressed in the same units. This is dne in Figures 1 and 2. Fr simplicity, it is assumed that the transmitter signal and receiver nise levels are cnstant as a functin f frequency. In additin t plts f transmitter signal and receiver preamplifier nise levels, Figs. 1-2 cntain plts f the effects f the frequency-dependent material-prpagatin lsses (diffractin and dissipatin) and transducer-insertin lsses. The plts in Figs. 1-2 cntain all the infrmatin fr a preliminary analysis f the SIN perfrmance f the ultrasnic NDE system as a functin f frequency and material parameters. T calculate the specific plts shwn in Figs. 1-2, we assumed that ur transmitter pulser wuld be capable f prducing a 450 V, 50 ns lng pulse int a 10 a resistive lad. We als assumed that the receiver-preamplifier nise vltage level is 3 nv/ Hz. (The latter assumptin is very cnservative since I nv nise levels can nw be achieved using state-f-the-art FETs [5].) The diffractin lsses were calculated using Lmmel integrals [3], but withut crrecting fr the effects f elastic anistrpy. (Omissin f the effects f anistrpy may result in either an verestimate r underestimate f diffractin lsses, depending n prpagatin directin and material elastic cnstants.) The attenuatin lss estimate was arrived at by extraplating actual measurements btained in the MHz frequency regin int the 1-10 MHz range. The attenuatin lss shwn in Figure 1 demnstrates a frequency-squared dependence while the attenuatin lss in Figure 2 shws a linear frequency dependence. The cmpsite material wall thickness and transducer active diameter were assumed t be 50 and 12.5 mm, respectively. An examinatin f Figs. 1-2 reveals that the ptimum inspectin frequency (in terms f minimizing lsses) is apprximately 400 khz. Belw 400 khz, the prpagatin lsses are dminated by diffractin effects while material-dissipatin lsses tend t dminate at higher frequencies. At 400 khz, the SIN is in excess f 100 db, assuming a 30-dB tw-way transducer insertin lss in this frequency regin. This estimate f the SIN is in gd agreement with the experimental value. Figure 3 shws the blck diagram f ur pulse-ech experimental system. It shuld be nted that this system uses a high-perfrmance lo-bit analg-t-digital (AID) cnverter. The reslutin f this device apprximately matches the greater-than-60-db linear range f ur receiver preamplifier. Als, the radi-frequency (RF) wavefrms are stred in the cmputer fr later prcessing. Althugh the 60-dB-plus linear range f ur receiver circuitry des nt match the l00-db SIN f the received ultrasnic signals, it is significantly greater than the linear ranges f traditinal ultrasnic flaw detectrs. In the fllwing sectins, we describe sme f the underlying circuit tplgies. A HIGH-PERFORMANCE PULSE-ECHO CIRCUIT TOPOLOGY Figure 4 illustrates the principle f peratin f a particularly effective pulse-ech circuit tplgy [3]. In Fig. 4, the square-wave ultrasnic pulser is implemented using a large strage capacitr, C, and tw semicnductr switches, S 1 and S 2. In additin, the 2106

3 r rn "U '-" V--- L !-' ---- r\ \ ' 1 Frequency (MHz) TRANSMmER LEVEL {O.J V/-,IR"z}..- Diffractin Lss Attenuatin Lss Transducer Lss +++++Tlal Lss , USABLE DYNAMIC RANGE al 2 MHz AMPLIFlER NOISE LEVEL (J nv /...,;Hz) Figure 1. Pulse-Ech Ultrasnic System Signal-t-Nise Perfrmance Versus Frequency fr a Filament-Wund Cmpsite. Vltages are referred t a 50 a system. The cmpsite was 50 mm thick with a lngitudinal wave speed f 3 mmlj.ls. A 12.5 mm diameter piezelectric transducer was used. Attenuatin has a frequency-squared dependence in this cmpsite. a II "' " r--. --I--- rn -100 \ "U '-" "' - 1\ TRANSMITTER LEVEL (O.J V /-v'ri) -- Diffractin Lss Attenuatin Lss ***** Transducer Lss Ttal Lss , I USABLE DYNAMIC RANGE at 2 MHz AMPLIfiER NOISE LEVEL (J nv/-v'hz) , 1 Frequency (MHz) 10 Figure 2. Pulse-Ech Ultrasnic System Signal-t-Nise Perfrmance Versus Frequency fr a Unidirectinal Cmpsite. Attenuatin has a linear frequency dependence in this cmpsite. 2107

4 - LOW-IMPEDANCE SQUARE WAVE ULlRASONlC PULSER. BROADBAND ULlRASONlC r, RECEIVER DIPLEXER HIGH ANALOG RESOLUITON OSCUOSCOPE DISPLAY ULlRASONlC mansducer POlYMER COMPOSITE SPECIMEN DIGITAL I.- PERSONAL DATA I--. STORAGE COMPUTER PLOTTER OSCUOSCOPE,., -- Figure 3. Blck Diagram f a High-Perfrmance Pulse-Ech Ultrasund System. 2108

5 HIGH \I r SQUARE WAVE PULSER [-52 r DIPLEXER TRANSMIT/RECEIVE TRANSDUCER,..., I--_I.>+--t-J OUTPUT RECEIVER PREAMPLIFIER Figure 4. Cmpnents f a High-Perfrmance Pulse-Ech System. series resistrs, RV are emplyed t establish the utput impedance. The electrical pulse, which drives the transducer, is generated by first clsing S1' then clsing S2' The switch S2 remains in the clsed psitin until the next ultrasnic pulse. This unique feature f the circuit shwn in Fig. 4 permits the utilizatin f a particularly effective diplexer [transmit/receive (T/R) switch] design and helps t reduce the transducer ringdwn time. The diplexer is implemented using a bradband transfrmer, which increases the transducer input-impedance level. This feature is helpful in minimizing the nise factr (NF) f the receiver preamplifier circuit. Althugh the circuit tplgy illustrated in Fig. 4 is nt efficient in terms f energy utilizatin, it prvides a significant imprvement ver traditinal designs in terms f SIN perfrmance. An Illustrative Pulse-Ech Result Figure S shws an ultrasnic wavefrm btained frm a SO mm thick E-glass/epxy unidirectinal cmpsite sectin that was made using a prepreg-rving fabricatin technique. This material system was fund t exhibit cnsiderably less lss than the material system made using the wet-filament winding prcess. Figure 6 shws sme f the structural details that can be related t specific features in the wavefrm shwn in Fig. S. Specifically, the material made by the prep reg-rving prcess cntains tw thin internal interfaces that exhibit a higher acustic impedance than the adjining material. This difference in acustic impedance results in the tw back-scattered signals that appear in Fig. S between the "main-bang" residue and the back-surface ech. The vltage f the signal back-scattered by the first interface exceeds 1 V after 28-dB f amplificatin. The abslute levels f the signals shwn in Fig. S illustrate the high SIN capabilities f ur ultrasnic system. OPERATION IN TRANSMISSION AND PITCH-CATCH MODES In sme instances, it is advantageus t perfrm measurements n plymer-cmpsite materials using the transmissin r pitch-catch cnfiguratins. In these cnfiguratins, separate transducers are used fr transmitting and receiving the ultrasnic signals. Cnsequently, diplexers and TIR switches are nt needed. Hwever, t achieve ptimum SIN perfrmance attentin must be paid t the selectin f an apprpriate receiver preamplifier design. Our transmissin cnfiguratin uses a preamplifier cnnected directly t the receiving transducer. This cnfiguratin is similar t that shwn in Fig. 3, except fr the absence f the diplexer and additin f a preamplifier. 2109

6 1.50, , (f) (I) (J) > 2nd INTERFACE / BACK SURFACE / \ 1 sl INTERFACE Figure Time (sample perids) Pulse Ech Wavefrm, fr the Sample in Figure 6, Obtained With a High Perfrmance Ultrasund System (square-wave pulser, linear, wide dynamic range, bradband receiver and magnetic diplexer).././././ I N1"EH='ACE 2 Figure 6. Laminated Cmpsite, 50 mm in Thickness. T vercme the effects f randmly ccurring material inhmgeneities, we have been develping small, bradband receiving transducers. We anticipate that the use f such transducers will lead t imprved velcity measurements. In additin, the use f physically small receiver transducers and large transmitter transducers may lead t imprved flaw detectin, particularly in highly inhmgeneus materials [7]. 2110

7 Rll C2 + R6 CJ. I R4 R7 Q4 R8 RJ.3 1 C3+ RJ.4 C4 - C7.12 R2 RS R9 RJ.O OUTPUT INPUT R Figure 7. Schematic f a Lw-Nise, High-Impedance, Lw Input-Capacitance Preamplifier , RECEIVER # ORIENTATION A 0.50 w ::J f ::::i « SAMPLE # Figure 8. Transmissin Wavefrm Obtained Thrugh a 50 mm Thick Glass/Epxy Cmpsite. (1 MHz, 12.5 mm diam. piezceramic transmitter, 0.5 mm diam. PVDF receiver and the receiver preamplifier shwn in Figure 7). 2111

8 Currently, ur receiving transducers are made using the piezelectric plymer PVDF. The transducers are made small in physical size t vercme the randm phase-cancellatin assciated with material inhmgeneities. Hwever, the small size f the PVDF elements results in very small capacitances (apprximately 1 pf). T maximize the sensitivity f these transducers, special preamplifier designs, characterized by lw input capacitance, are needed. In additin, features such as vltage gain (greater than 20 db), large dynamic range (60-70 db), 1 nvi Hz input nise level, and 501l utput impedance are" highly desirable. Figure 7 illustrates the tplgy f ne f ur receiver preamplifier designs that meets r exceeds the abve perfrmance requirements. A typical wavefrm btained in a transmissin experiment n a 50 mm thick E-glass/epxy material is shwn in Figure 8. Nte that the signal exhibits very a wide bandwidth (single-cycle wavefrm) and high SIN. This wavefrm was btained using a I-MHz, 12.5 mm diameter transmitting transducer and a 0.5 mm diameter PVDF receiving transducer. The square-wave pulser design, illustrated in Fig. 4, was used t drive the transmitting transducer. CONCLUSIONS Judicius selectin f cmpnents fr an ultrasnic system and ptimal perating frequency is necessary t facilitate ultrasnic NDE f thick plymer cmpsite materials. This is particularly true when inspecting materials that exhibit high prpagatin lsses due t diffractin and dissipatin effects. The cmpnent perfrmance requirements can be determined and ptimum frequency regins fund if infrmatin n material-prpagatin lsses is available. Special-purpse pulsers, diplexers, preamplifiers, and transducers are required t btain the large signal bandwidths and high signal-t-nise ratis required t characterize the prperties f the materials and detect flaws. Special-purpse transducers may als be needed t vercme the effects f beam distrtin in materials that exhibit large inhmgeneities. REFERENCES 1. C.M. Teller and C.M. Frtunk, "NDE Requirements fr Thick Marine Cmpsites," elsewhere in these Prceedings. 2. J-H M, A.L Rbinsn, D.W. Fitting, F.L Terry, Jr., and P.L. Carsn, "Micrmachining fr Imprvement f Integrated Ultrasnic Transducer Sensitivity," IEEE Trans. Electrn Devices, ED-37 (1), (1990). 3. P.H. Rgers and A.L Van Buren, "An exact expressin fr the Lmmel Diffractin Crrectin Integral," 1. Acust. Sc. Am., 55 (4), (1974). 4. W. Sachse and N.N. Hsu, "Ultrasnic Transducers fr Materials Testing and Their Characterizatin," chapter 4 f Physical Acustics. Vlume XIV, W.P. Masn and R.N. Thurstn, eds. (Academic Press, New Yrk) Dr. Gary Petersen, Ritec, Inc., private cmmunicatin. 6. D.K. Hsu, "Ultrasnic Measurements f Prsity in Wven Graphite Plyimide Cmpsites," in Prceedings f Prgress in Ouantitative Nndestructive Evaluatin, (plenum Press, New Yrk, 1988), Vl. 7b, pp G.1. Psakny and E.R. Green, "Perfrmance f a Bradbanded Ultrasnic Transducer with Caxially-Munted Nndirectinal Receiver," in Prceedings f Prgress in Ouantitative Nndestructive Evaluatin, (plenum Press, New Yrk, 1990), Vl. 9a, pp S.R. Jefferts and F.L. Walls, "A Very Lw Nise FET Input Amplifier," Rev. Sci. Instrum. 60 (6), ,