58.on inspection of thin walled tubes for transverse and longitudinal flaws by guided ultrasonic...

8/12/2019 58.on Inspection of Thin Walled Tubes for Transverse and Longitudinal Flaws by Guided Ultrasonic Waves

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IEEERANSACTIONS ON SONICS AND ULTRASON ICS, VOL SU-23, NO. S , SEPTEMBER 1976 369

viously, more comprehensive wo rk, unde r closely mon itoredconditions where the ndividual parameters are independentlycontro lled, is necessary.REFERENCES

[ l ] A. Korpel, L. W. Kessler, and P. R. Palermo, An Acoustic Micro-scope Operating at 100 MHz, Nature, 232,110-1 11 (1971).[21 L. W. Kessler, A. Korpel, and P. R. Palermo, Simultaneous Acous-tic and Optical M~croscopy f Biological Specimens, Nature 239,111-112, 1972 (Sept. 8).[31 L. W. Kessler, P. R. Palermo, and A. Korpel, Practical High Reso-lution Acoustic Microscopy, Acoustical Holograph y, Vol. 4,Plenum Press, New York (1972), G Wade, Ed., pp. 51-71.[4] R. Adler, A. Korpel, and P. Desmares, A n Instrument for MakingSurface Waves Visible, IEEE Trans. S on ia and Ultrasonics,SU-15,157-161 (1968).

[S] L. W. Kessler, A Review of Progress and Applications in AcousticMicroscopy, (A) J. Acoust . Soc . Amer . , 53, 246, 1973, Manu-script version J Acoust . Soc. A m e r . , May 1974.[6] L. W. Kessler, P. R. Palermo, and A. Korpel, Recent Develop-ments with the Scanning Laser Acoustic Microscope, in Acoust icHolography, Vol. 5, P. Green, Ed., Plenum Press, New York(1 974).[7] L. W. Kessler, Acoustic Microscopy-A New Dimension in Ultra-sonic Visualizat ion, in Proc. of Ultrasonics International 1973(London), March 1973, p. 173, IPC Science and Technology PressLtd., London (1973).Acoustic Microscopy, in Proceedings of the Second World Con-gress on Ultrasonics in Medicine, Rotterdam, June 1973,M. de-Vlieger, Ed., Excerpts Medica Foundation (1974).posium (1974), IEEE Cat. 74-CHO-896-1S.

[8] L. W essler, High Resolution Visualization of Tissue with

[g] L W. Kessler, The Sonomicroscope,Proc. of Ultrasonics Sym-

On Inspection of Thin-Walled Tubes for Transverseand Longitudinal Flaws by GuidedUltrasonic WavesWOLFGANG MOHR AND PAUL HOLLER

Abst racr-A method to test thin-walled tubes with diameters ofa fewcentimeters by guided ultrasonic waves is reported. The principle isthe application of two types of axially symmetric ultrasonic tubemodes: longitudinal modes with particle displacements coupled inaxial and radial directions for ransverse failures and torsional modes,d a t i n g n the circumferential direc tion only, for ongitudinalfailures. Both types of modes propagate alongthe tube in the axialdirection. A pulse-echo technique, therefore, is possible. The pulsesare excited and received a t one endof the tubeby contactless electro-dynamic transducers. As soon as the tube is put into a transducer coilat one end, testing of the whole tube can be accomplished in a fewmilliseconds. Transportation or rotation of the tubes is not necessaryduring the test.

TI. DISPERSIONHARACTERISTICSOF T H E TUBEMODESHE COMPLETE system of tube modes was introduced in1959by Gazis in solving the three-dim ension alwave equa-tion for the corresponding bound ary conditions l ] , [2] . Thetube mode s can be classified int o tw o classes: axially sym-metric and nonaxially sym metric modes. Th e axially sym -metric types then aresubdivided into longitudinal modesL ( 0 , m) m 1 , 2 , 3 , . and torsional modes T 0,m) m0 , 1 , 2 , 3 , . . .

Manuscript received April 19, 1976. This work was supported bythe Bundesministerium fur Forschung und Technologie under ContractThe authors re with the Institut furzerstorungsfreie PriifverfahrenRS 102-18.der FraunhoferGesellschaft, Saarbrucken, Germany.

Nomenclature has been chosen according to Z emanek [3]and Meitzler [ 4 ] . As to the modes L ( 0 , m ) , he radial andaxial particle motions are coupled . The re do not occur anyazimu thal displacements, whereas in the case of torsion almodes only azimutha l displacements occur. The lowest tor-sional mode T( 0 ,O ) s free of dispersion: phase and groupvelocities are equal to th e shear velocity in an unlimitedmedium. In the case of nonaxially symmetric modes F ( n , m),all three particle mo tions are coupled to o ne another. Theparameter n ( n = 1 , 2 , 3 , . a indicates the number of thesymmetric diameters of the tube, hereas m indicates thenumber of the modeas in the axially symmetric type of wave.One can imagine the modes F( n , m ) as composed of tw ocount er-rotating screw waves.have been used. Fig. 1 shows the dispersion curve for modeL 0 , 2 ) together with some measured points) for a fwedratio of wall thickness h to mean tube radius R in a tripleway, showing frequen cy, phase velocity, and group velocity.For higher values of the normalized wave number h/X, hismode approaches asymptotically the symmetric late modeso ; hat means the circumferentialoupling by curvature ofthe tube wall progressively loses its effect. Fig. 2 showsschematically the particle motion in modes L ( 0 , l ) and L 0,2)in a long itudina l section of the tube. For higher h/h thesemodes are indentical with the plate mo des a and s o respec-tively. In Fig. 3 the azimu thal displacements are shown in

For the experiments, mainly the modes L 0, 2) and T(0,O)


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IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, SEPTEMBER 197670L

3

l

Fig. 1 . Tube mode L 0,2 : theoretic al dispe rsion curves and valuesmeasured (h = wall thickness, R = mean radius, h = wavelength ofguided wave, c = phase velocity, cg = group veloci@, c? = velocity oftransverse waves , v = Poisson number). c / c t , cg /c t , a: auste-nite DIN 1.4541). / c t , cg/ci , S : austenite DIN 1.4 550 )./ c t , A g / c t , A n: ferrite.

Fig. 2. Particle motion in two lowes t axially symmetric tube modesschematic, longitudinal section of tube).

cross section of the tub e for the tw o owest torsional modesT(0,O)and T(0, 1) corresponding to plate modes Yo andS H , ) . The dispersion curve for mode T( 0 ,O ) s trivial: phaseand groupvelocity are cons tant and equa l to thehear velocity,and the frequen cy curve is a straight line with a slope of 2.0.11. EXCITATION ND RECEPTION F ULTRASONIC ULSES

Fig. 4shows the principle of electrodynamic excitation oftube waves [5] -[7] . This arrangement is suitable for testin gausten itic and ferritic tubes. Tra nsmit ting and receiving trans-ducers consist of coils which are arranged at on e en d of thetube and whichare adjusted in position axially near the endof the tube in order to produce constructive interference ofthe pulses running in both directions. RF currents of suitablefrequency are driven through the transmitter coil, and they

Fig. 3 . Particle motion in two lowest torsional modes (h/R= 1 h/h -0, cross section of tube).

transmitter CO /

t ransducer c o l ir os s sect ion

Fig. 4. Principle of electrodynamic excitation of guided mode s inaustenitic tubes.

induc e eddy currents in the tube surface. The direction ofthe current alternates long the transducer with aperiodcorresponding to the wavelength of the ultrasonic wave tobe generated. A force results from the interaction of an ap-plied static magnetic field 15-22 k c ) transverse t o the trans -ducer and the induced eddy currents. This force is transferredto the ion la tt ice , andhis leads t o particle displacements,i.e., it generates an ultrasonic wave. This wave propa gatesalong the tube and s reflected partially by flaws and totallyat the end of the tube, The flaw echoes are recorded in areceiving transdu cer by means of the reciprocal effe ct. Thereceiving transducer is located within the tube immediatelybelow the transmitting transducer. By this, better localresolution of flaw echoes is possible tha n if the transm ittingand receiving coil were arranged outside or inside the tub enex t to each other. It is possible, too, to use one transducercoil as a trans mitter a nd receiver at the same time. However,this leads to a loss of sensitivitv of betwe en 10 and 20 dB


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MOHR AND HOLLER: INSPECTION OF THIN-WALLED TUBES FOR FLAWS 37

Fig. 5 . Electrodynamic transducers for excitation and receptionoftube modes.

Fig. 6 . Relative particle amplitudes for L ( 0 , 2 ) ,h / R = 0.1.

m c g n e t i c y o k ec y l ~ n d r ~ c o la) I

b ) transducerCOl S f e r r l t c

le

m o g n e t l c y o k eho rseshoe\

CGl/magner tz log

Fig. 7. Electrodynamic transducers for ferritic tubes (longitudinalsections).

com pare d to the se of two separate coils which are individuallyelectrically optimize d. The design of the transduc er is shownmore clearly in Fig. 5, where an outside transduc er transm it-ter) and an inside transduc er receiver) are seen. The wiresare wound around a ridge that exte nds over half the circum-

ference of the transduc er. The direction o f winding alternate sfrom ridge t o ridge, so that two ridge widths and two trenchwidths make u p one wavelength of the guided wave.If the trans duc er s arranged inside the magnetic field in amanner such that the sect ions of theinding running in thedirection of the circumference are covered by the pole shoes,then an axial force acts on the tube , and it is possible to e xcitelongitudinal modes, for example,L(0,2) . If, however, thesmall axially running winding section s are covered b y the poleshoes, an azim uthal force acts on the tube that will lead totorsional modes. The trans ducer coils have been optim ized bymeans of an equivalent circuit of a transfo rmer [8]. Thetransm itting transdu cer s used in series resonance, the receiverin parallel resonance.

Fig. demonstrates in w h c h range of the dispersion curvemode L ( 0 , 2 ) can be most easily excited. T h s is the range oflow wave numbers h/h , where displacements in the tube sur-face are mainly in the axial direction. In the case of the ar-rangement shown in Fig. 4 used primarily for austenitic tubes,it is difficult to exer t radial forces. This can be done moreeasily for ferritic tu bes. A magnetic yoke shown in Fig. 7can be used; the magnetic field is guided by the tube wall. Inaddition to this, transduc er design is easier, since the wiresare woun d circumferentially not over half the circumferenceas in Fig. 5 .

111. MEASUREMENTSA block diagram of the measuring equ ipm ent can be seen in

Fig. 8. A pulse genera tor max imum pulse power 22 kW)generates rectangular pulse train s by mean s of a digital drivingunit, where the numb er and the w idth of the single pulses aswell as pulse intervals can be adjusted . Th e tran smitting trans-ducer a cts as a resonance circuit Q value about 3) and limitsthe band wid th of the transm itting signal, so that i t leads to analmost sinusoidal c urrent. The receiving transduc er trans-form s the ultrasonic pulse into an electrical receiving voltagethat can be displayed on an oscilloscope. In a hete rody nereceiver spectrum analyzer), the signal may alternatively beselectively filtered, dem odulated , andinearly or logarithmicallyamplified. Moreover, it is possible to digitize the receivedsignal and to process it by mea ns of c ompu ter.The received amplitudes of the first echo from the endofthe tube lie in the volt range. The refo re, t he signal-to-noiseratio, in general, is higher th an 100 dB. The dynam ic range,however, is limited by a coherent background. A maximumdynamic range of a bout 70 dB has been obtained. Only somecomponents of the coherent background have been identifiedto date . These are other modes, higher harmonics, and spatialsidelobes. The ex act nature of the b ackgrou nd ill have tobe studied more closely by theoretical and experimentalinvestigations.

The dete ction of four transverse tang ential notche s in anaustenitic tube with anarrangement shown in Fig. 4 modeL(0,2)) s given in Fig. 9 see also [9] , [101 for wavelengthsof 15,8, and 4 mm. The signal is demodulated and linearlyamplified arbitrary scale of ordinate ). The notches are


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37 IEEE TRANSACTI ONS ON SONICS AND ULTRASONICS, SEPTEMBER 1976

Fig.

CRIVER

osMoscoFGENERATOR

PROBE

T R A N S W l D iEL OYN (ELASTIC LBE pRaeE SECTRUM ~W A V E ) ANALYZERK T I V E

FKjNALFig. 8. Block diagram of measuring apparatus.

DEMOW L ATEOSIGNAL

d d c t - ho s l echo f r om d l ~0.05~151 no1 f rom t r a n s v e r s e tube e n d+e cctr ,col no fch ess t r a y

9. Detect ion of series of four transverse notches wi th tube modeL ( 0 , 2 ) (austenitic tube: length 5 m , diameter 22 mm, thickness 1mm).

tangential saw cuts at distances of 1 m from each other. Theirdepths are 3 0 , 2 0 , 10, and 5 percen t o f th e wall thickness.obtained.

The following signal-to-background ratios have been

signal-to-background signal-to-backgroundh ratioorhe 30 per- ratioorhe 5 per-mm A centotch (dB) cent notch (dB)15 0.0678 0.1254 0.250 23

~ ~~

A reduc tion in wavelength will lead to an increasing delay ofthe first echo from the end of the tube, andhis shows thegradual decrease in group velocity of the mod e L ( 0 , 2) seeFig. 1).Longitudinal modes are not suitable to detect longitu dinalflaws because th ey present a very small scatterin g cross sectionto th e propagating wave. It is to be expected that torsionalmod es should be used to detec t longitudin al flaws. Fig. 10demonstrates how mode T (0 ,O) s reflected by three longi-tudinal notches that have been sawed into an austenitic tub ewith a length of 5 m at a distance of 1.25 m to each other .The saw blade had a diameter of 2 cm. These notches arerather deep; thefirst two are one-half of the wall thickness,while the third reaches the inner surface. The flaw echoesare weak: 29 dB below the echo from the tube end for the100percent notch and about 46 dB for the 50 percent notches.The width of the saw cuts 0.2 mm in th e case of the firstnotch, 0.1 mm in the case of th e second) has hardly any effecton the echo amplitu de. T he slightly smaller amplitude of thesecond echo is due to the attenuatio n of the amplitude withincreasing distance covered by the wave. As has also beenproved in ot her measurements, the pulse passing the flaw willbe disturb ed, an d th is can be recognized in a disturban cebehin d the ec ho of th e deepest flaw. This may lead to a lossin the signal-to-noise ratio or, in th e extrem e, even to obscur-ing smaller flaws falling in the s hadow behind bigger ones.When testing the longitudinal notches f Fig. 10by means ofthe tube modeL(0,2) , he flaw echo of the deepest no tchwill be 5 5 dB below the level of the echo from the end of thtube. The detection sensitivity is considerably lower than inthe case of operating in the torsiona l mode . T he flaw echoesof the smaller notc hes will be indic ated almo st as stronglywith a torsional mode as the larger notches are with a longi-tudinal mode.testing for transverse flaws by means of the torsional modeCorrespondingly undefined are the flaw ech oes obt aine d n


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M O H R AND HOLLER: INSPECTION OF THIN-WALLED TUBES FOR FLAWS 37

- , e ch \ o s ' t r o hlongitudmaf notches\direci srgnd st echo+eiecfrrcai stray f rom tube end

Fig. 10. Detection of three longitudinal notches with tube mode T(0,O)(austenitic tube: length 5 m, diameter 22 mm, wall thickness 1mm).T(0,O). Especially in the case of the reflection at the end ofthe tub e w hich can be regarded as an extrem e transverse flaw),there will occur considerable mode conversion. Long itudin almodes generated there will lead to disturbances in the measur-ing range because of their high group veloci ty. By adequ ateoptimization of transducerdesign, wavelength, and frequency,it is possible to largely suppress the ind ication of unw antedmodes, System atic optimization is yet to be carried out .

austenitic tube, 22 X 1 mm2 DIN 1.4550): mode L 0, 2 )Th e following attenua tion values have been measured in an

h,mmttenuation,dB/m)20 0.3715 0.4910 0.648 0.886 1 oo4 1.10

mode T ( 0 , O )h, mm attenuation,dB/m)

2015 0.551 oo~~

By study ing the attenua tion o f multiple reflection at th etube ends, wave paths between 50 and 500 m have been o b-served. The wave path shall be defined as the distance aft erwhich the coherent echoesdisappear into the noise.Whereas Fig. 10 only shows the echoes of very deep longi-tudinal notches, Fig. 1 1 demonstrates those of the smallestlongitudinal flaws detected so far. The signal-to-noise ratiois considerably low er than in the case of transverse flaws ofcomp arable size see Fig. 9 . The low signal-to-noise ratioof the echoes of small longitudi nal notch es ogically leads toa falsification of the flaw amplitude as well as the locatio n ofthe flaw by interferenceof the test flaw echo with the coherentdisturbing background.Fig. 12comp ares the flaw echo levels of transverse andlongitudinal flaw detections. The reference level is that ofthe echo from the end of the tube. Inhis case the attenua-tion of the am plitude w ith ncreasing wave path has beenelim inate d. The transverse flaw echoes are all higher thanthose from longitudinal laws; the difference n level for awavelength of X = 15 mm, as used in bot hcases, is reached atabout 13 dB, independent of the law depth.

dtrect 5rgnal+e/ectr a i tray f rom t u b e enlst echo

Fig. 11. Detection of three longitudinal notches with tube modeT(0,O) (austenitic tube: length 5 m, diameter 18 mm, wall thick-ness 1mm).

-30

-40

50 -

- 6 0 -7 0 1 . . , , , I5 O 20 30 LO 50 60 73 80 Y roon o t c h d e p t h p e r c e n t of w a t l l h r k n e s s l

Fig. 12. Level of test flaw echoes below level of first echo from tubeend (corrected with regard to wave attenuation).

As to the indication of ransverse notches, it seems that forlong wavelengths diffraction chiefly contribute s to th e genera-tion of echoes. A relatively small exponent ial variation ofecho level with notc h dep th can be observed. With decreasingwavelength, first the dete ctio n sensitivity fo r small flaws alsodecreases, but it increases again very stro ngly for growingdispersion X = 8 mm , see Fig. 1). Here, the geometrical crosssection seems to be of growing imp ortance, as can be observedfrom the deviation from the exponential behavior of the echolevel. Furtherm ore, the dynam ic range depends on the wave-length. For h g h detection sensitivity X = 4 mm), the dynamicrange will be lower


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3 7 4 IEEERANSACTIONS O N SONICS AND ULTRASONICS,EPTEMBER 197

use this me thod as, in general, the B . T product has to remainless than 1 in order to avoid strong dispersion. In thi s caseit seems advantageous to use othe r signal processing m etho dsinverse filtering, cepstral analysis) to improve poor local flaw

resolution. These metho ds m ay, however, decrease the signal-to-noise ratio.

IV. CONCLUSIONIt has been shown that tube testing can be accomplished byguided ultrasonic wave pulses withou t moving the tube during

the test. T he m ethod is applicable to bot h ferritic and austen-itic tubes. By the use of electrodynamic transducers, no me-chanical coupling is necessary. A rather high detection sensi-tivity has been ob tai ne d: transverse and longitudinal test flawswith a depth of 5 percent of tube wall thickness have beendetected. Further investigations shall be carried out with theaim of improving the signal-to-noise ratio and the local resolu-tion of flaw echoes, as well as distinguishing differe nt types offlaws.

REFERENCES[ 1 D . C. Gazis, Three-dimensional investigation of the propaga-tion of waves in hollow circular cylinders: I Analytical founda-

tion,J. A c ou s t. S o c . A m . , 31 (1959) 568.11. Numericalresults,J. A c o us t. S oc . A m . , 3 1 (1959) 573A. E. Armenakas, D. C. Gazis, and G Herrmann, Free vibra-tions of circular cylindrical shells, Pergamon Press, Oxford,1969.J. Zemanek, An experimental and theoretical nvestigation ofelastic wave propagation in a cylinder,J. Acoust . Soc. A m . ,

5I (1972) 265.A. H. Meitzler, Mode coupling occurring in the propagation ofelastic puly s in wires,J. Acous t. Soc . A m . , 33 (1961) 435.H. Licht , Uber eine beruhrungslose Anregungs- und Empfangs-moglichkeit von Lamb-Wellen in elektrisch leitfahigen Plattendurch ein modenselektives elektrodynamisches Wandlersystem,Diss. TH Aachen, Verlag J. A . Mayer, Aachen 1973.R. B. Thompson, Electromagnetic, noncontact transducers,IEEE U ltrasonics Symposium P roceedings, 1973, p. 385.E. R. Dobbs, Electromagnetic generation of ultrasonic waves,Chapt. 3 in Physical Acoustics, Vol. X , W. P. Mason and R. N.Thurston, Eds., Academic Press, New York, 1973.H. Licht, Eigenschaften und optimale Konstruktion des elek-trodynamischen Lamb-Weller-Wandlers, IzfP-Bericht, 750204-TW (1975).W. Moh, H. Licht, and P. Holler, Zur Prufung dunnwandigerRohre mit gefuhrten Ultraschallwellen, Materialpriifung, I 7(1975) 7, p. 240.W. Mohr, Erste experimentelle Ergebnisse zur Priifung dunn-wandiger Rohre mit elektrodynamisch angeregten gefuhrtenUltraschallwellen, IzfP-Bericht, No. 750513-TW (1975) .J. Perdijon, Le contrBlc des tubes par ultrasons sans mise enrotation, Revue de MitaNurgie,Jan. 1975, p.73-81.

Cont r ibu to rs

Jerry R Bell was born in Aledo, TX, on July 4,1933. He received the A.A. degree fromWeatherford Junior College in 1952. He re-ceived the B.A. degree with majors in physicsand mathematics n 1954 fromTexas ChristianUniversity. From June 195 4 to June 1955eattended Texas Christian University as a grad-uate student n physics.From 1955 to 1957 hewas an Instructor ofPhysics at Texas Technological College. Hejoined the FortWorth Division of General

Dynamics in 1957 and worked in the Radiation Effects Group until1974. In 1974 hewas transferred to the Materials Research Labo-ratory groupwhere he has been conducting research and developmenwork in nondest ructive testing. His speciality is in computer tech-nology and software development for the acquisition and analysis ofdata-related ultrasonic wave interact ions in materials.

Mr. Bilgutay is a men

Nihat M Bilgutay (S71-M75) was born onMarch 31, 1952, in Ankara, Turkey. He re-ceived the B.S.E.E. degree from Bradley Uni-versity, Peoria, IL, in 1973 and the M.S.E.E.degree from Purdue University, West Lafay-ette, IN, in 1975.He is presently a Research Assistant at Pur-due University where he is working towardsthe Ph.D. degree. His particular interests arein communication theory and ultrasonic flawdetection.nber of Tau Beta Pi and Sigma Tau.

Otto Buck received the Ph.D. degree in physicfrom the University of Stuttgart, Germany, i1961.University of Stuttgart, as a Postdoctoral Fel-low at the Science Center, Rockwell Inte rna-tional, and as a Member of the Technical Stafat Siemens/Germany. Currently, he is theGroup Leader of the Fracture and MetalPhysics Group, the Science Center, RockwellInternational, Thousand Oaks, CA. His main

He has worked as a Research Assistant at th

58.on inspection of thin walled tubes for transverse and longitudinal flaws by guided ultrasonic...

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