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field test results of vertical split head and rail base crack detectionperformed at the Rail Defecì Test Facility at the Federal RailroadAdministration's Transportation Technology Center located inPueblg Colorado.

EXPERIMENT_ For the generation of ultrasound, an infrared Nd:YAG pulselaser is used operating with a 4 to 7 ns pulse width and a maximumenergy of 800 mj per pulse. The laser beam is delivered through aset of beam steering mirror assembìies and shaped through a sèt oflenses before illuminating the surface of the rail. The lasei source isfocused to a point or a line, depending on the type of discontinuitytesting to be perforrned. The tóting of"intemal éàcks, such as rrerti-cal split head cracks, requires bulk waves, where a point source ismore effective (Kenderian et aL,2002a). For the testing of surfacecracks, such as rail base cracks, a line source is used for better direc-tivity of the acoustic wave (Aindow et aI., 1982) and sensitivity tosurface discontinuities (Kenderian et a1.,2007; Kenderian et al.,2003c). The laser source operates in the ablative regime with waterbeing added to rail surface as a constraining layer to increase theamplitude of the acoustic signal. To examine the damage caused byablatiory three rail specimens marked with the letters A, B and C areablated with 1, 10 and 100 pulses each. The nine ablated regions, inaddition to those outside ablatioru are labeled alphanumeriiallv. in-dicating the rail specimen and the number of lasèr pulses, as sh-ownin Table 1. The laser operates with a 10 Hz repetitión rate, while thebeam is focused to a 1 mm (0.04 in.) point and optimized for a max-imum energy output of 800 mJ per pulse.

Tqble I Tested region nomencloture for roil specimens A. B ond C

Optical micrographs of the nine ablated areas, compared withthose outside the ablated region, show no trace of phaie transfor-mation, brittle microstructure, thermal microcracking or any appar-ent metallurgical change caused by ablation. ObÉervationJaremade with 50x, 100x and 50px magnification power. Figure 1shows digital images of the miirostru"cture of regions AO A"1, A10and 4.100 with a magnification power of 100x.

Table 2 shows average microhardness test results perfonned onrail specimens A and B, using the Vickers scale undefO.s kg (1.1 lb)of force. The hardness of the regions of specimens A and B-that en-dured 1, 10 and 100 ablative laser pulses does not differ significant-ly from the hardness of the regions outside ablatioru A0 and B0.This is hue for measurements at the surface and subsurface, 6.5 mm(0.25 in.) deep. However, the hardness at the surface is lower thanthat below thè surface due to decarburization at the surface. If brit-tle microstmcture is produced, such as bainite and martensite, as adirect result of the heat introduced by the ablative laser pulse, thehardness at lhe abìated regions is expected to be much higher. Thehardness of bainite in rail steel is typically higher than 400 and thatof untempered martensite can approach 800 on the Vickers scale.

For detection, single or multiple capacitive air coupled hansduc-ers are used. These detectors operate with a broadband frequencyspanning between 50 kHz and2.25 MHz. A multichannel '250kJJz

Tqble 2 lvicrohordness test results in Vickers scole of the surfoceond subsurfoce, ó.5 mm (0,25 in,) deep

SpecimenAOA14104100BOB1B108100

Laser Pulses Specimen A Specimen BO A O B O1 4 1 8 1

10 410 810100 4100 8100

Specimen CCOC1c10c100

Surface228244228235247229250258

Subsurface309318311J I Z

299284308308

ligurg J - Optical.milograph images ot' rail cp:cimen Awith regions enduring ablatioe laser pulses, shozuing lack of damage due to ablation:

(a) raith no pulses; (b) after one pulse; (c) after 10 pulses; (d) after100 pulses.

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crack, the surface wave is una,ffected" while the bulkwaves interactwith the crack. They reflect and mode convert from, transmitthrouglU diffract around and resonate between the boundaries ofthe crack. As a result the direct bulk waves, which arrive before thesurface wave in the absence of the crack, attenuate due to transmis-sion through the crackwhen present.

_The spherical front of a point source laser generated acoustic sig-nal provides the condition that results in a iride range of angulírscattering upon reflection from an irregular surface, such as that ofa vertical split head crack. The same condition promotes diffractionaround the crack tip. Mode converted and reónating waves radi-ate from the crack tip toward the surface of the railheid. Collective-ly, a substantial amount of a delayed signal (arriving after the sur-face wave) is received by the detéctoq, thus indicatin! the presenceof an intemal crack sudr as a vertical split head. The time of flight ofthe rayteigh wave serves as a guideline for the no crack cond"itiorywhere bulk modes arrive before the rayleigh wave and the crackconditiory where the delayed modes arrive after. Using this tech-nique, 89 measurements are collected, with a 100% success rate offirdi.g a vertical split head crack.

RoilBose FroclureRail base fractures are progressive fractures that develop in a

transverse plane of the rail base (Sperry Rail Service, 1999).-Theyoriginate from the outer edge of the base, usually caused by a nickor blow resulting in an indentation or step. Initially, the crack growsrelatively slowly until it has progresseà approximately I2."7 mm(0.5 in.) into thorail. Beyond inis'stuge u ùàd..r ruptti." usuallyoccurs resulting in complete failure.

, Three types of saw cuts are made to represent rail base cracks.The first type extends completely through the 1,2.7 mm (0.5 in.)thick basé.-The second and thlíd typei extend only halfwaythrough the.thickness of the base. They penetrate 6.5 rmn (0.2S in:)deep from the top surface and 6.5 (0.25 in.) high from the bottomsurface. In general, the length of the cracks iibetween 12.7 mm(0.5 in.) and L9 mm (0.75 in.) measured from the edge of the railbase inward towards the web (or center) of the rail. The laser beamis focused to a 12.7 mm (0.5 in.) line on the outer edge of the railbase, parallel to the crack direction. Three transducers are used fordetection: TXL, TX2 and TX3 (Figure 6).

that TX2 may be positioned over a dip, which would partially ob-scure a surface wave from the transduce4 the acoustic wave wouldbe detected by fi3 instead. Each transducer is inclined 6.5 degreesfrom the normal to the surface and optimized to detect u suìfacewave,propagating along the r4il basè in the direction facing thetransducer in accordance with the Snell's law calculations. Thè oo-sition and orientation of the three transducers are confizured iir away that would cover rail segments spanning between tùo consec-utive spikes (or dips) with slight overlap. If a crack is presen! thethree transducers are expected to detect direct, reflected and trans-mitted waves, as shown in Table 3. A position sensor is installed tosense the spikes and trigger the laser accordingly. The laser beamthen illuminates a point on the base, halfway between spikeg andgenerates acoustic waves propagating away from the illuminatedregron. With one laser pulse, a rail base segment between two con-secutive spikes is tested. Other configurations can be designed tocover a distance of several spikes between the three transducers. Úrthe present experiment, the setup is limited to the space availableon the pushcart.

Iqble 3 Crock position ond type of signols detected by eochtronsducer

Crack PositionBetween A and B

Between B and C

Between C and D

TXl TX2transmitted direct and

reflecteddirect andreflected transmitted

direct andreflected direct

TX3direct andreflected

transmitted

transmitted

Figure 7 shows waveforms captured by the three detectors withthe pushcart in motion. A 6.5 mm- (0.25 in.) crack is located befweenpoints A and B, breaking the bottom surface of the rail base. Ascharted in Table 3, the first detector receives a transmitted wave,ryhich iq severelyattenuated by the crack. The remaining detectors,TX2 and fi3, each receive a direct and a reflected wave. Therefore,with one laser pulse, all three detectors indicate that a crack is pre-sent within the rail base segment under test. Surface brealiingcracks on the opposite side of a plate, relative to the ultrasoniisource and receiveq, are not always easily detected; sudr is the casein ft1i-q part of the experiment. Signal processing using wavelettransform can provide complementary information on the natureof these waves and the manner in which they interact with thecrack (Cemiglia and Qordjevic, 2002).

The wavelet toansform breaks down the signal into its frequencycomponents and the time of flight associated lrith each compo.tenl!t then generates a three dimensional plot providing tìme offl ight / frequency / magnitude information of the receivéd signal.The resulting plots are shown in the bottom o îFigweT . The highestfrequenry magnitude is plotted in red, while the magnitude of theremaining freque,ncies is normalized with respect to the highestmagnitude and plotted accordingly. The frequenry content òf thereceived qig"uJs in general is below 1 MHz. Higher frequencies aregenerated with the laser source and the air coupled transducer iscapable of detecting up to2.25 MHz frequenryiignals. Howeve4,attenuation increases with frequency and is more severe in lowdensity materials such as air. As a result, with a 125 mm (6.9 in.)propagation path in ai1, only frequencies below L MHz are avail-able to the detector.

The velocity of longitudinaf shear and rayleigh waves in steel is5.9,3.2 and3 mm/ps (0.23,0.L3 and 0.12 in./frÉ) respectively. Atfrequencies less than L\rftIz, the wavelength of these waves ij lessthan !.% 3.2 and 3 mm (0.23, 0.13 and 0.1tin.) respectively. There-fore, the detected acoustic signal is a complex mix-of raylèigh andlamb waves (surface and plite mode waves) and their ínteLctionwith the crack,

Similar analysis applies to the testing of the remaining two typesof cracks, throúgh ciacks and top surfice cracks, thou[h thefàregenerally easier-to detect. Acousàc signals are found to"propígatealong the rail base unaffected by clipl, spikes, anchorsànd olher

Figure 6 -Test configuration for railbase testing showing aIIcomponents contained in a pushcart and abooe the running surface ofthe rail.

The acoustic wave propagates a distance of BA, BC and BD insteel for the three hansducerg respectively, then 1ZS mm (6.9 in.) inair. The first two detectors, TXl and D{2, are equidistant from thegeneration point B, where BA = BC = 250 mm (9.8 in.). The third de-tector is placed such that BD = 340 mm (13,4 in.). In the likelihood

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