Laser Air Hybrid Ultrasonic

ABSTRACTLaser generation and air coupled detection of ultrasound were com-

bined to produce a hybrid noncontact and remote ultrasonic technique. Thetechnique is suitable for general testing of a number of structural materi-als. Using this hybrid technique, guided surface waves were generated topropagate along the flange, tread and rim edge of a railroad wheel. Thewaves complete one revolution around the circumference of the railroadwheel in 1 ms and interact with surface breaking discontinuities that arepresent on the various parts of the wheel. The flexibility and remote natureof this technique suggest that wayside testing of wheels on a moving traincan be made possible.Keywords: laser ultrasound, air coupled ultrasound, railroad wheel,cracks.

INTRODUCTIONThe laser/air hybrid ultrasonic technique is a noncontact and re-

mote technique that combines laser generation with air coupled de-tection of ultrasound. The noncontact and remote nature of thistechnique provides it with the flexibility to detect discontinuities inunfavorable positions and orientations. It has been shown that thistechnique can be an effective and easy to use method for rail tracktesting (Kenderian et al., 2001; Kenderian et al., 2002). For decades,optical methods have been widely popular as noncontact and re-mote ultrasonic detection techniques. However, their efficiency re-lies heavily on the amount of light reflected back from the surface.As a result, the curvature, roughness and cleanliness of the reflect-ing surface all have a negative influence on the amount of light re-flecting back to the optical detector. This, unfortunately, rendersthese techniques ineffective for many industrial applications in-cluding the railroad industry. A comparative study between aircoupled and interferometric detection (Kenderian et al., 2002) orelectromotive force (EMF) detection (Cerniglia and Djordjevic, inreview) of ultrasound demonstrates the superiority of the laser/airhybrid ultrasonic technique over a purely laser based ultrasonictechnique, within the operating frequencies of the air coupled de-tector (that is, below 2.25 MHz).

The present paper presents a new technique that would enablethe railroad industry to perform noncontact and remote testing ofrailroad wheels while in motion. No method is currently availableto the railroad industry to perform dynamic tests on railroadwheels.

EXPERIMENTFor the generation of ultrasound, a Q-switched Nd:YAG laser

operating at 1064 nm (4.2 10-5 in.) wavelength with pulse dura-tions of 4 to 7 ns and maximum energy of 400 mJ/pulse was used.

With a single pulse, a multimode signal was generated. The propa-gation properties of the signal are a function of the laser pulsewidth and intensity, point of impact, surface condition and shape ofthe pulse illuminating the surface, but are not affected by the angleat which a pulse is delivered.

For the detection of ultrasound, a capacitive air coupled trans-ducer (Schindel and Hutchins, 1994; Schindel and Hutchins, 1995)capable of detecting frequencies between 50 kHz and 2 MHz wasused. The frequency range of this capacitive air coupled transducerwas adequate for the type of discontinuities that cause concern forthe railroad industry. These air coupled transducers are capable ofoperating at remote distances exceeding 150 mm (5.9 in.) (Kender-ian, 2002). Naturally, higher frequency components attenuate se-verely in air. Therefore, as the standoff distance between the air cou-pled transducer and the specimen is increased, the upper limit ofthe frequencies retained by the detector is lowered.

Overall signal strength as a function of standoff distance followsan exponential behavior. Good signals were readily available up to40 mm (1.6 in.) and useful measurements were possible past 80 mm(3.2 in.), as shown in Figure 1a. Figure 1b shows that the optimumdetection angle for a rayleigh wave propagating in steel is 6.5 de-grees and angular variation of 2 degrees still retains good signalintensity. The test configuration is that shown in Figure 1c. Usingthe acoustic velocity in air Cair = 0.33 mm/s (0.01 in./s), therayleigh wave velocity in steel CR = 2.9 mm/s (0.1 in./s) and90 degree propagation angle of the rayleigh wave in steel, Snellslaw calculations confirmed our experimental observation of thecritical refraction angle at 6.5 degrees in air. This is presented inEquations 1 and 2.

(1)

(2)

RESULTSPreliminary experiments performed on a small 32 kg (70 lb) sec-

tion of the wheel showed that an acoustic signal generated with alaser line source was more effective than that generated by a laserpoint source for the detection of saw cuts. Similar results were re-ported for surface breaking cracks on rail head (Kenderian et al.,2001; Kenderian et al., 2002), with a detailed discussion explainingthe underlying cause for the enhanced sensitivity of a line source tosurface discontinuities given in a separate paper (Kenderian et al.,in publication). Based on these results, the laser source used in thispaper was focused to a line parallel to the direction of the slots,which is the transverse direction on the wheel.

air degrees= =sin

..

.10 332 9

6 5

sin

air

steel

air

steelsin=

CC

Materials Evaluation/April 2003 505

Laser/Air Hybrid Ultrasonic Technique forRailroad Wheel Testing

by Shant Kenderian,* B. Boro Djordjevic* and Robert E. Green, Jr.

Submitted October 2002

* Center for Nondestructive Evaluation, The Johns Hopkins University,810 Wyman Park Drive, Baltimore, MD 21211; (410) 516-0818; fax (410)516-7249; e-mail .

Materials Science and Engineering Department, The Johns HopkinsUniversity, 3400 N. Charles St., Maryland Hall 102, Baltimore, MD21218.

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 505

From Materials Evaluation, Vol. 61, No. 4, pp: 505-511. Copyright 2003 The American Society for Nondestructive Testing, Inc.

A calibration railroad wheel, 910 mm (36 in.) in diameter andweighing approximately 365 kg (800 lb), was used for test purpos-es. The wheel contained four manufactured discontinuities markedwith the letters A, B, C and D. A, B and C were slots made to thewheel flange, tread and rim, respectively, and D was a hole drilledin the side of the wheel.

The wheel was divided into 360 degrees. The location of 0 de-grees was determined from the point where the acoustic signal wasgenerated, that is, the point at which the generation laser illuminat-ed the surface of the wheel. The positions of the detector and thediscontinuity were referred to in terms of their location, in degrees,along the wheel circumference with respect to the acoustic source.The detector position was designated with the letter , while that ofthe discontinuity was designated with . Figure 2 depicts the gener-al setup for the wheel experiments performed in this study.

Surface Slots on Wheel FlangeIn the detection of slot A, a 16 mm (0.6 in.) long, 1.5 mm (0.06 in.)

wide and 5.3 mm (0.2 in.) deep saw cut was made to represent asurface breaking crack on the wheel flange. The detector was keptat the = 90 degrees position. The wheel was rotated such that theslot fell in various positions between = 10 and 180 degrees. Asnoted previously, the optimum detection angle for a steel to airleaky rayleigh wave is 6.5 degrees. Therefore, a detector positioned

between 0 and 180 degrees, say at 90 degrees, and inclined at 6.5 de-grees from the surface normal, optimally detected a signal travelingthe short path, that is counterclockwise according to Figure 2, trav-eling one quarter of the wheel circumference. Similarly, when thisdetector was inclined at 353.5 degrees from the surface normal, itoptimally detects a signal traveling the long path, that is clockwiseaccording to Figure 2, traveling three quarters of the circumference.Short path and long path inclinations were denoted with the lettersS and L, respectively.

A 17 mm (0.7 in.) long laser line source was used to generate theacoustic signal used for the detection of the 16 mm (0.6 in.) slot. De-tector standoff distance was kept at 8 mm (0.3 in.) for long path de-tector inclination and 16 mm (0.6 in.) for short path inclination. Fig-ure 3 shows a broad view, 0 to 1000 s, of a signal generated at0 degrees and detected at 90 degrees with the detector in the longpath L inclination and slot A located at = 150 degrees. Interesting-ly, the wheel geometry is such that a rayleigh wave completes onerevolution around the wheel in approximately 1 ms. This is obviousfrom the 3.1 and 2.9 m (122 and 115 in.) circumference of the wheelflange and tread, respectively, and the nearly 3 mm/s (0.1 in./s)rayleigh wave velocity in rail steel (Bray and Vezina, 1991). Direct,reflected and transmitted waves are observed.

The first arrival, at 285 s, was a direct wave traveling 90 de-grees counterclockwise to the detector, propagating 775 mm (31 in.)

506 Materials Evaluation/April 2003

Figure 1 Signal strength of capacitive air coupled detector: (a) as a function of standoff distance with optimum 6.5 degrees inclination to thesurface normal; (b) inclination to the surface normal; (c) test configuration under which measurements were taken.

(a)

(c)

(b)

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 506

in steel (or one quarter the circumference of the flange) and 8 mm(0.3 in.) in air. Although the detector was not optimized for shortpath signal detection or counterclockwise propagation in Figure 2,a fraction of the strong direct signal was still detected. This was dueto the large 10 mm (0.4 in.) aperture of the detector and small 8 mm(0.3 in.) standoff distance of the detector. Other factors also playedan important role in the detection mechanism of the capacitive aircoupled transducer. Some of these factors include the direction ofparticle displacement of the acoustic signal in steel and air and at-tenuation through air and impedance between air and the detectormembrane.

The second arrival, at 630 s, was a reflected wave travelingfrom 0 degrees past the detector at = 90 degrees counterclockwise,redirected upon reflection from slot A at = 150 degrees and prop-agating back to the detector at = 90 degrees in a clockwise direc-tion. The total propagation distance is 1.8 m (71 in.) in steel and8 mm (0.3 in.) in air. The amplitude of the reflected wave is greaterthan that of the direct wave. The reason for this is that with its L in-clination, the detector was optimized for the detection of signalspropagating in the clockwise rather than the counterclockwise di-rection, according to the setup shown in Figure 2.

The third arrival, observed at 800 s, was a transmitted wavetraveling clockwise from 0 degrees to slot A at = 150 degrees,transmitting through the slot and continuing in a clockwise direc-tion to the detector at = 90 degrees. The total propagation distanceis 2.3 m (92 in.) in steel, traveling three quarters of the circumferenceof the flange, and 8 mm (0.3 in.) in air. Considering rayleigh wavevelocity CR 3 mm/s (0.1 in./s) in rail steel and wave velocityCA = 330 m/s in air (0.01 in./s), the calculated arrival times ofall three waves agree well with the observed signals. The amplitudeof the transmitted wave was smaller than the direct and reflectedwaves mainly because of the substantially longer propagation pathin steel, which would effectively increase signal attenuation.

While the acoustic source and receiver were stationary, thewheel was rotated so that slot A changed its position with respect togeneration and detection points. The direct and the transmittedwaves did not change their position in the time domain. This is be-cause their propagation distances did not change with the wheelsrotation. However, the arrival time of the reflected wave variedwith the rotation of the wheel for obvious reasons. Using this tech-nique, successful detection of slot A was possible for positions be-tween 0 and 180 degrees. Detection capacity was not limited to thisrange, except that for positions between 180 and 360 degrees, a bet-ter signal was obtained using a detector in the short path S inclina-tion to detect a reflected wave propagating in the counterclockwisedirection. Figure 4 shows results obtained with the detector in the Sinclination positioned at = 90 degrees and slot A positioned at = 13 degrees.

Frequency analysis of the direct, reflected and transmitted ultra-sonic waves is presented in Figure 5. Figure 5a shows that theupper and lower boundaries of the frequency of the direct wave areset by the 0.3 MHz high pass filter used in this experiment and the2 MHz detection limit of the capacitive air couple transducer. As ageneral rule, wavelengths smaller than the depth of a crack reflectback from the crack while those with larger wavelengths transmitthrough. At its deepest point, slot A is 5.3 mm (0.2 in.) deep. Arayleigh wavelength of R = 5.3 mm (0.2 in.) corresponds with a fre-quency of 0.57 MHz in steel. Accordingly, frequencies higher than0.57 MHz are expected to reflect back from slot A and those lowerthan 0.57 MHz to transmit through. Figures 5b and 5c show the fre-quency spectrum of the reflected and transmitted waves, respec-tively. For the reflected wave, a sharp decline is observed at fre-quencies lower than 0.6 MHz, while the frequency of thetransmitted wave is confined between 0.3 MHz and 0.75 MHz, asexpected.

Surface Slots on Wheel TreadA similar test configuration was used in the detection of slot B, a

26 mm (1.02 in.) long, 1.5 mm (0.06 in.) wide and 8 mm (0.3 in.)deep saw cut made to represent a surface breaking crack on the cen-ter of the wheel tread. A 25 mm (0.98 in.) long laser line source wasused to generate the acoustic signal for the detection of the 26 mm

Materials Evaluation/April 2003 507

Figure 2 General configuration and nomenclature used in thisstudy: generation of acoustic source determines the reference point of0 degrees ( = detector position angle; = crack position angle;L = detector inclination for clockwise, or long path, signal; S = detector inclination for counterclockwise, or short path, signal.

Figure 3 Broad view of a signal generated at 0 degrees and detectedat 90 degrees, with the detector in the long path inclination and amachined slot located at = 150 degrees along the flange of a rail wheel.

Figure 4 Close up of a signal generated at 0 degrees and detected at90 degrees, with the detector in the short path inclination and amachined slot A located at = 13 degrees along the flange of a railwheel.

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 507

(1.02 in.) slot. Detector standoff distance was kept at 25 mm(0.98 in.) throughout this part of the experiment. Figure 6 shows abroad view of a signal generated at 0 degrees with the detector in thelong path inclination positioned at = 90 degrees and slot B at = 10,100 and 120 degrees. Similar to the results previously shown in Fig-ure 3, three main arrivals are also observed in Figure 6. The directand transmitted waves travel one and three quarters of the circum-ference of the wheel tread, respectively. Therefore, their arrival time

can be predicted and was unchanged with the rotation of thewheel. While these waves were stationary in the time domain, asshown in Figure 6, the arrival time of the reflected wave wouldchange with slot position . Because the transducer was oriented inthe long path direction, it was not optimized for the detection ofany waves approaching from the short path direction. Accordingly,the counterclockwise waves, which are the transmitted waves inFigure 6a and the direct waves in Figures 6b and 6c, were absent orweak. Note that the transmitted and direct wave determination isinterchangeable, based on the position of the slot with respect to thegeneration and detection points. In Figure 6a, the arrival time of thedirect wave, approaching the detector from the long path direction,is 833 s. The reflected wave from slot B at = 10 degrees propa-gates an additional distance of 163 mm (6.4 in.) to arrive 56 s later.

508 Materials Evaluation/April 2003

Figure 5 Frequency analysis with respect to a 5.3 mm (0.2 in.) slotalong a rail wheel: (a) direct waves; (b) reflected waves; (c) transmittedwaves.

(a)

(b)

(c)Figure 6 Broad view of a signal generated at 0 degrees with thedetector in the long path inclination positioned at = 90 degrees andslot B positioned at: (a) = 10 degrees; (b) = 100 degrees;(c) = 120 degrees.

(a)

(b)

(c)

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 508

Similarly, when the slot is positioned at = 100 degrees, the reflect-ed wave propagates an additional distance of 163 mm (6.4 in.) to ar-rive 56 s behind the direct wave, as shown in Figure 6b. In Figure6c, the corresponding delay associated with the reflected wavefrom a slot positioned at = 120 degrees is 168 s, so that the arrivaltime of the reflected wave would be 496 s.

Surface Slots on Wheel RimA similar procedure was followed for the detection of slot C, a

15 mm (0.6 in.) long, 2 mm wide (0.08 in.) and 15 mm (0.6 in.) deepsaw cut, which was made at the rim edge of the wheel tread. Aschematic drawing of the detector position with respect to the rimedge, shown in Figure 7, demonstrates that, due to the roundness ofthe rim edge and curvature of the wheel tread, the area of the wheelsurface available to the detector is limited to a large extent. Should acontact transducer be used, this interface would be reduced to apoint. However, due to the noncontact nature of the capacitive aircoupled transducer and its 10 mm (0.4 in.) aperture, the transducerwas capable of detecting acoustic signals propagating along the rimedge of the wheel tread, although some limitations were imposed.That is, reflected and transmitted signals were not as distinct asthose presented earlier for the wheel tread and flange. In spite of the

Materials Evaluation/April 2003 509

Figure 7 Front and side views of transducer position with respect tothe rim edge of the wheel tread showing a limited interface between thedetector and the wheel.

Figure 8 The direct wave propagates counterclockwise along the rim edge of the wheel tread, detected with the air coupled transducer positioned at = 90 degrees in the short path inclination, with 20 mm (0.8 in.) liftoff and crack positions at: (a) 10 degrees; (b) 5 degrees; (c) 10 degrees;(d) 45 degrees; (e) 80 degrees; (f) 100 degrees.

(a)

(c)

(e)

(b)

(d)

(f)

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 509

fact that the wheel flange had a round edge similar to that of therim, the acoustic signal was somewhat confined to the body of theflange and interacted more pronouncedly with slot A, which ex-tended through most of the flange width. In contrast, along the rimedge the acoustic signal was free to spread upward towards thetread and downward towards the hub. In addition, due to thegeometry of the rim edge, slot C was deep in the center but wasquickly reduced to a shallow cut along the edges. The length of slotC did not extend to a significant portion along the rim width theway slot A did along the flange. These factors, acting collectively, re-duced the detectability of slot C in comparison to the detectabilityof slots A and B.

The results shown in Figure 8 demonstrate that the laser/air hy-brid ultrasonic technique is capable of detecting saw cuts along therim edge of the wheel tread. However, the detected signals werenot as pronounced as those obtained for the wheel flange and tread,as shown in Figures 4 and 6.

Small Hole on the Side of the Wheel FlangeA 3.3 mm (0.1 in.) hole was drilled in the side of the flange and

marked with the letter D. The hole was too small to produce a con-siderable effect on an acoustic signal transmitting through thehole. With the detector kept at = 10 degrees and standoff distanceof 21 mm (0.8 in.), the wheel was rotated so that the hole wassometimes positioned between generation and detection, that is at0 degrees < < 10 degrees, and sometimes outside this region, thatis at 10 degrees < < 0 degrees. When the hole was inside this re-gion, between generation and detection the amplitude of the trans-mitted signal (Figure 9a) was not considerably different from thatof the direct signal (Figure 9b). However, Figure 9b shows thatwhen the hole was outside this region and with proper inclinationof the air coupled detector, a reflected component of the acousticsignal could be detected to make possible the detection of a smallhole such as that represented by D. Figure 10 shows similar resultsobtained with the hole positioned at = 8 and 13 degrees. Due to

510 Materials Evaluation/April 2003

Figure 9 Transmitted, direct and reflected waves resulting from alaser generated acoustic signal interacting with a 3.3 mm (0.1 in.) holeand detected with an air coupled transducer positioned at = 10 degrees: (a) hole positioned at = 3 degrees; (b) hole positioned at = 3 degrees.

(a)

(b)

Figure 10 Direct and reflected waves resulting from a laser generatedacoustic signal interacting with a 3.3 mm (0.1 in.) hole and detectedwith an air coupled transducer positioned at = 10 degrees: (a) holepositioned at = 8 degrees; (b) hole positioned at = 13 degrees.

(a)

(b)

Table 1 Propagation distances and expected arrival times for the waveforms pertaining to Figures 9 and 10

Hole Position from Figure Detector Direct Transmitted ReflectedAcoustic Source Standoff

25 mm (1.0 in.) 9 21 mm (0.8 in.) 107 s25 mm (1.0 in.) 9 21 mm (0.8 in.) 107 s 123 s65 mm (2.6 in.) 10 16 mm (0.6 in.) 92 s 135 s

100 mm (3.9 in.) 10 16 mm (0.6 in.) 92 s 158 s

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 510

Materials Evaluation/April 2003 511

the curvature of the wheel, a detector fixed at = 10 degrees was ca-pable of detecting the hole as far as = 15 degrees. Beyond thisrange, the hole, generation source and detector fell out of alignmentand the hole was no longer within detection range of the current ex-perimental setup. Table 1 gives signal propagation distances in steeland air and expected arrival times for the waveforms shown in Fig-ures 9 and 10.

CONCLUSIONThe main characteristic of the laser/air hybrid ultrasonic tech-

nique is its noncontact and remote method of operation. The tech-nique can be applied on many structural materials in their industri-al field condition. That is, although thin layers of dirt, oxides, greaseand other contaminants can slightly dampen the signal detected bythe air coupled transducer, they have a more significant effect onenhancing the laser generated signal. As a result, a stronger signal isdetected with the presence of such contaminants. The techniquecan tolerate surface roughness reasonably well. However, pitting,spalling and porosities open to the surface can attenuate the surfacewave to a great extent. Surface waves were generated on various re-gions of the railroad wheel and detected successfully without theneed for modifying the surface through cleaning or polishing. Fur-thermore, all signals shown in this paper represent single events,that is averaging techniques were not necessary as a way to im-prove the signal to noise ratio.

The use of the laser/air hybrid ultrasonic technique introducesa degree of flexibility to railroad wheel testing that would enable itto be performed without the need to establish contact between theprobe and the tested part and at relatively remote distances. It wasdemonstrated in this paper that 360 degree coverage is possible forthe flange and tread of the railroad wheel with an approximatetime of 1 ms elapsed between generation and detection. For therim edge, 90 degree coverage was possible with a single laserpulse. These results strongly suggest that this technique may be

engineered to perform wayside ultrasonic testing of wheels on amoving train. No such technique is available to the railroad indus-try today. In light of the increasing reports of wheel and track relat-ed rail accidents, the development of a wayside field testing tech-nique has become increasingly critical.

ACKNOWLEDGMENTSThe authors thank Transportation Technology Center, Inc., for

providing wheel specimens and financial support.

REFERENCESBray, D.E. and G. Vezina, Ultrasonic Applications in the Railroad Industry,

Nondestructive Testing Handbook, second edition: Volume 7, Ultrasonic Test-ing, P. McIntire, ed., Columbus, Ohio, ASNT, 1991, pp. 594-634.

Cerniglia, D. and B.B. Djordjevic, Ultrasonic Detection by Laser-based Sen-sors and by Wideband Air-coupled Transducer, Research in Nondestruc-tive Evaluation, in review.

Kenderian, S., B.B. Djordjevic and R.E. Green, Jr., Point and Line SourceLaser Generation of Ultrasound for Inspection of Internal and SurfaceFlaws in Rail and Structural Materials, Research in Nondestructive Evalua-tion, Vol. 13, 2001, pp. 189-200.

Kenderian, S., Advanced Ultrasonic Techniques to Determine the Structur-al Integrity of Rail Steel, The Johns Hopkins University, Department ofMaterials Science and Engineering, PhD dissertation, 2002.

Kenderian, S., B.B. Djordjevic and R.E. Green, Jr., Laser Based and Air-coupled Ultrasound as Noncontact and Remote Techniques for Testingof Railroad Track, Materials Evaluation, Vol. 60, 2002, pp. 65-70.

Kenderian, S., B.B. Djordjevic and R.E. Green, Jr., Sensitivity of Point andLine Source Laser-generated Acoustic Wave to Surface Flaws, IEEETransactions on Ultrasonics, Ferroelectrics, and Frequency Control, IEEE, inpublication.

Schindel, D.W. and D.A. Hutchins, Air Coupled Ultrasonic Transducer,US Patent 5,287,331, 1994.

Schindel, D.W. and D.A. Hutchins, The Design and Characterization of Mi-cromachined Air-coupled Capacitance Transducers, IEEE Transactions onUltrasonics, Ferroelectrics, and Frequency Control, Vol. 42, 1995, pp. 42-50.

06_505_522TPs_18pgs 3/10/03 7:31 AM Page 511