The demand for a successful flaw analysis is that the test equipment produces no distortion on the echos no noise. I _... I i.j J...

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1 SYSTEM ANALYSIS FOR WIDE BAND ULTRASONIC TEST SET-UPS Ulrich Opara Krautkramer GmbH Clgne, West Germany INTRODUCTION In the last years, the discussins abut ultrasnic test equipment fcussed n amplifier band width and n different types f transmitters, withut leading t a clear recmmendatin. One nw tries t reach a better understanding by a system analysis f the test equipment. As pinted ut in Fig. I., a typical test set-up fr a ultrasnic inspectin cnsists f a transmitter which drives the electrical part f the prbe. Via the piezelectric effect, the mechanical part f the prbe generates an ultrasnic wave which is reflected by the flaw, received by the prbe again and amplified in the receiver. Therefre, the ech displayed n the CRT is affected nt nly by the type and size f the flaw, as it shuld be, but als by all nn-ideal parts in this transmissin chain. The demand fr a successful flaw analysis is that the test equipment prduces n distrtin n the echs n nise. r----- r----, I _... I i.j J... j-c, I jl))) ~ I)))j ~ i c, I i "AW.!-'.--I t -4 L.. I I!... w,.c,.. i ~ I I - r r i j '---l.-...l...+--'-----f ~ L ~ Fig. 1. Schematics f an ultrasnic test set-up. 189

2 190 U.OPARA In the fllwing, we will investigate these demands in three different regins f the test sample: in the middle f the test piece, near the entrance surface and very far away frm it. Of curse, the first case is the easy ne. NDT far away frm any Surface The mst natural transmitter fr wide band ultrasnic test equipment is the s-called spike pulser which generates a needle-like electrical pulse fr the prbe excitatin. Such a transmitter has a flat tp frequency spectrum, as it is knwn frm its similarity t the a-functin. Of curse, this transmitter needs a flat tp receiver t achieve a flat tp verall frequency respnse in the test equipment. Principally, there are n limitatins t bandwidth, if we exclude the limited bandwidth f the prbe. A secnd type f transmitter is the s-called step-pulser which excites the prbe by a sharp vltage step, as shwn in Fig. 2. If this pulse is differentiated, a a-functin is the result. A frequency spectrum as l/f, where f is the frequency is, therefre, expected. Nw, is we need an verall flat tp frequency respnse, a receiver with an "inverse" frequency behaviur, as shwn in Fig. 3, is needed t cmpensate tj-.e TIME PULSE FREQUENCY DOMAIN Amplltud. (ltnearl [dbl 0) 1,0 10 Fig. 2. Step excitatin, time and frequency dmain. (dbl f [101Hz) Fig. 3. Inverse receiver fr the step-pulser. frequency dmain.

3 WIDE BAND ULTRASONIC TEST SET-UPS 191 respnse f the transmitter. Again, n band limitatins are expected with this cncept. The third type ~ transmitter which is used, is the s-called square wave pu1ser. The square wave pu1ser generates a vltage step and a perid T later, a secnd step t the base line. The frequency spectrum is knwn as the sinc-functin which, at lw frequencies, shws a flat respnse and at higher frequencies a l/f decay with zeres (Fig. 4). The crrespnding "inverse" amplifier t give a flat verall frequency respnse, is nt easy t design. But bth, the flat tp amplifier and the amplifier fr the step-pu1ser, give a reasnable bandwidth. The advantage f the step-pu1ser is the dubled amplitude in the frequency spectrum in a limited regin, which gives up t 6 db gain in the eches if using band limited prbes, cmpared t the ther pu1sers. T this pint neither the spike-pu1ser cncept nr the step-pu1ser cncept has a measurable advantage when cmpared. The prblem arises frm the cmmn practice t misuse a step-pu1ser fr wrking with needle-pulses. With the help f the damping resistr and the prbe capacity (Fig. 1) the step-pulse f the nrmal transmitter is differentiated, thus having a sharp needle-like transmitter pulse with a flat tp frequency spectrum. Nw numerus prblems arise: Since the damping resistr wrks in the transmitter and in the amplifier input stage simultaneusly, nt nly the l/f decay is cmpensated, but an increasing frequency respnse ging with f is effective at lwer frequencies (see Fig. 5). The net effect is a limited band width, which depends n the prbe and n the flaw detectr. Each prbe has t be tuned individually with the damping resistr t mve the flat respnse f the transmitter n the flat plateau f the prbe. As shwn in Fig. 6 a distrtin in the mean frequency up t a factr f tw t lwer frequencies is pssible in this tuning prcess. The signa1-t-nise rati gets wrse. The pint at which the signa1-t-nise rati is determined is where the lwest amplitude in the transmissin chain is present. This is, f curse, the prbe utput t the amplifier. Fr a gd signal-t-nise rati, IdBl [steel] 0) 1,0 10 Fig. 4. Frequency spectrum f a square-wave pu1ser.

4 192 U.OPARA (db) ~ R 4000 O : RO: 1000 RO: 50 n ,1 10 f[mhz) Fig. 5. Frequency spectrum f the spike-pulser fr different lw cut-ff frequencies. (db) 0,1 10 f (MHz) Fig. 6. Cnvlutin f a spike-pulser with a wide band prbe, fr different damping values. the amplitude at this pint shuld be as high as pssible. All filters shuld wrk after this critical pint. But, as indicated in Fig. 5, the tuning prcess with the damping resistr des just the reverse and remves the lw frequency part f the spectrum just befre this critical pint. A summary fr NDT far away frm any surface: the step-pulser, tgether with.its inverse amplifier prvides the best band width and the best signal-t-nise rati cmpared t the square-wave-pulser r t the spike-pulser, when it is the previusly mentined simple type. NDT near the Entrance Surface Measuring near the transmitter-pulse r near the interface ech can change the bjectives, because reslutin prblems can lead t distrtins f the riginal ech in the same way as incrrect amplifier passbands. Fr a systematic investigatin f this prblem, the effect f the interference f tw echs has t be analyzed. As a measure f interference, the fllwing definitin seems t be suitable:

5 WIDE BAND ULTRASONIC TEST SET-UPS 193 t is the time between the tw pulses xl(t) and x2(t). (See Fig. 7.) If I(t) is zer, bth pulses d nt interfere, when their distance in time is t. Fr simplicity, we assume that xl(t) a = x2(t), meaning that the shape f the interface ech is the same as the shape f the flaw ech beside the amplitude, which is a times higher. Therefre: I(t) = a.jr xl(,-t).x l (,)d, = a.acf(xl(t» _00 where AFC(t) is the s-called autcrrelatin functin f x(t). All flaw detectrs have a transmissin band with a lw cut-ff frequency and a high cut-ff frequency. The autcrrelatin functin f such a limited passband is sketched typically in Fig. 8, fr tw values f the lw cut-ff frequency. It shws that the lwer cut-ff frequency determines the range f the influence zne fr very lw interference values, whereas the high frequency cut-ff determines the amunt f interference, when high interference ccurs. In ther wrds: if we have x I-- t 1 Fig. 7. Interference f tw eches. [linearj ~ q-f ~ T J, ttmm steel) Fig. 8. Autcrrelatin functin fr tw passbands with different lw cut-ff frequencies.

6 194 U.OPARA reslutin prblems with tw pulses f almst equal size, that is a ~ l, then the amunt f interference is determined dwn t very lw values by the high cut-ff frequency. Otherwise, and that is the typical NDT prblem, if we have reslutin prblems with a large pulse like the interface ech, and a small pulse like the flaw ech, it fllws that because a»l the lw cut-ff frequency f the passband determines the amunt f interference. In this case it is favrable t reduce the band width t lwer frequency values fr a faster decay f the interference zne (Fig. 9). The result f this analysis is that near bundaries where reslutin prblems might distrt the flaw ech, the peratr has t make a tradeff between band width and reslutin. This trade-ff depends n the amplitude rati f the interfering pulses and has t be adjusted individually. With the spike-pu1ser cncept the cntrl f the lwer cut-ff frequency is pssible t a certain amunt with the adjustment f the damping resistr. If the ne interfering pulse is the transmitter pulse, the pulse rati a is simultaneusly affected, leading t a saturatin effect in the reslutin cntrl [1). As said befre, the mean frequency will be shifted in this tuning prcess simultaneusly. Due t a decrease in the ech amplitude, the SiN rati will becme wrse. With quency is frequency affect ed. the step-pu1ser cncept the cntrl f the lwer cut-ff frepssible thrugh the damping resistr. Neither the mean nr the pulse rati "a" in the interference functin is The lw cut-ff frequency can be fully cntrlled (Fig. 10). The same applies t the square-wave-pu1ser which needs an additinal cntrl, fr example an adjustable damping resistr. t cntrl the lw cut-ff frequency. NDT at Large Distances At large distances the ech amplitude will decrease. due diffractin and attenuatin. In this case, there is n means t enhance the SiN rati, if a spike-pu1ser r step-pu1ser cncept is used, but the increase f the transmitter pulse. This square-wave-pu1ser. r, better a ''burst'' pu1ser, at this pint ffers the pssibility t increase the amplitude by a multi-excitatin. [linear) Q»<1 - t t[mm steelj Fig. 9. Interference zne fr tw lw cut-ff frequencies.

7 WIDE BAND ULTRASONIC TEST SET-UPS 195 [db) HMHzl Fig. 10. Band width cntrl with the step-pu1ser cncept. The nly prblem is that this principle f multi-excitatin nly wrks with prbes f high Q. because the gain in pwer is prprtinal t Q r. in ther wrds: the band width must be limited. SUMMARY We can summarize as fllws: under equal measuring cnditins. and using an apprpriate transmitter cncept. the usable band width in NDT with ultrasund depends nly n the existing nise. either statistical nise r interfering eches. Fr mst applicatins the step-pu1ser with an inverse amplifier has the mst favrable features. REFERENCE 1. U. Opara. H. Jacbs. H. Sch1emm. "Optimized Frequency Respnse in Instruments". 10th Wrld Cnference n Nn-destructive Testing. Mscw DISCUSSION Frm the Flr: I'm curius. are yu cvering these srts f systems nw with different ptins pu1sers? U. Opara: Yes. we nw nrmally ffer the step-pu1ser cncept in ur tp instruments. In the USDI. either a step-pu1ser r a square-wavepu1ser may be selected.