On-line measurement of material properties is a goal of many manufacturers to improve production and quality. The elastic stiffness of paper is important for the paper industry. Currently, the elastic constants of paper are measured offline with contact ultrasonic methods [1-4]. Piezoelectric transducers are placed in contact with the paper surface to generate and detect plate wave modes, known as Lamb wave modes [5-7]. At low frequencies, where the wavelength of the elastic wave is larger than the paper thickness, two wave modes dominate in the paper, an anti-symmetric or flexural mode and a symmetric or thickness mode. Measurements of the phase velocities of these modes along both the machine direction (MD) and the perpendicular cross direction (CD) of the paper web provide an important parameter revealing the increased stiffness in the paper along the MD direction

Brodeur, P. H.

Gerhardstein, J. P.

Habeger, C. C

Lafond, E. M.

Pufahl, B. M.

Telschow, Kenneth Louis

Walter, John Bradley

UNT Digital Library

INEEL/CON-99-00406PREPRINTFabry-Perot Laser Ultrasonic Elastic AnisotropyMeasurements on a Moving Paper WebJ. B. WalterK. L. TelschowJ. P. GerhardsteinB. M. PufahlC. C. HabegerE. M. LafondP. H. BrodeurJuly 25, 1999 – July 30, 199926th Annual Review of Progress in QuantitativeNondestructive Evaluation571.03 - 03/23/98 - Rev. 01This is a preprint of a paper intended for publication in ajournal or proceedings. Since changes may be madebefore publication, this preprint should not be cited orreproduced without permission of the author.This document was prepared as a account of worksponsored by an agency of the United States Government.Neither the United States Government nor any agencythereof, or any of their employees, makes any warranty,expressed or implied, or assumes any legal liability orresponsibility for any third party's use, or the results ofsuch use, of any information, apparatus, product orprocess disclosed in this report, or represents that itsuse by such third party would not infringe privatelyowned rights. The views expressed in this paper arenot necessarily those of the U.S. Government or thesponsoring agency.FABRY-PEROT LASER ULTRASONIC ELASTIC ANISOTROPY MEASUREMENTSON A MOVING PAPER WEBJ. B. Walter and K.L. TelschowIdaho National Engineering and Environmental LaboratoryLockheed Martin Idaho Technologies Co.Idaho Falls, ID 83415-2209J.P. Gerhardstein, B.M. Pufahl, C. C. Habeger, E.M. Lafond, andP.H. BrodeurInstitute of Paper Science and Technology500 10th St. NW, Atlanta, GA 30318INTRODUCTIONOn-line measurement of material properties is a goal of many manufacturers toimprove production and quality. The elastic stiffness of paper is important for the paperindustry.  Currently, the elastic constants of paper are measured offline with contactultrasonic methods [1-4].  Piezoelectric transducers are placed in contact with the papersurface to generate and detect plate wave modes, known as Lamb wave modes [5-7].  Atlow frequencies, where the wavelength of the elastic wave is larger than the paperthickness, two wave modes dominate in the paper, an anti-symmetric or flexural mode anda symmetric or thickness mode.  Measurements of the phase velocities of these modesalong both the machine direction (MD) and the perpendicular cross direction (CD) of thepaper web provide an important parameter revealing the increased stiffness in the paperalong the MD direction.Laser ultrasonic techniques offer the possibility of making these measurementsonline, while the paper is moving on the paper web, since physical contact with the paper iseliminated.  These techniques both generate and detect elastic wave motion with lasers.Generation is provided by a pulsed laser, which delivers a very short but high intensityimpulse to the paper through absorption of the light at the surface.  Either thermoelasticexpansion, at low energies, or ablation of material, at high energies, convert the energyimpulse into elastic waves in paper.  The first is non-damaging and preferred for paper butis much less efficient, producing smaller wave motions.  An optical interferometer is usedto demodulate the small phase modulation that the elastic wave produces on a CW probelaser beam reflected from the detection spot.Ultrasonic motion of a surface can be measured with many optical techniques [8].These methods are based on optical interferometry and have similar sensitivities.  Thedetection system signal-to-noise ratio, in the shot noise limited regime, is controlled by theamount of light entering the interferometer.  Because diffuse surfaces scatter incident lightinto a hemisphere, only a very small portion of the incident light contributes to the detectedsignal.  For a standoff distance of a few centimeters, optical collection efficiencies aretypically in the 0.1% - 1% range.  There is also a limit to how much light can be focussedonto the paper without damaging it, the limit depending on the paper, the wavelength of thelight and the size of the detection spot.  For detection techniques that can use the light frommultiple speckles, a larger detection spot can be used.A confocal Fabry-Perot interferometer is particularly suited to this situation [9].The technique is self-referencing in that the photodiode signal results from the interferenceof light scattered from the paper surface and delayed portions of that light generated frommultiple reflections within the Fabry-Perot cavity.  As all of the optical wave fronts arenearly identical to that scattered directly from the paper surface, light from a relativelylarge surface area (1-4 mm2) including many speckles is collected and interferedefficiently.  This gives the Fabry-Perot technique the optical efficiency needed to providesufficient sensitivity for detection from diffuse material surfaces such as those of mosttypes of paper without utilizing light levels that damage the paper.Measurements off-line, on stationary paper, have been successfully made using aMichelson interferometer [10-12].  Motion of the paper surface, such as on-line duringpaper manufacture on a paper web, introduces additional problems.  Laser ultrasonicsoffers great potential for implementation in the field, and even in situ during thepapermaking process.  This paper describes the challenges of applying laser ultrasonics to amoving surface and reports on successful measurements on a moving sheet of paper using aFabry-Perot detector.MEASUREMENTS ON STATIONARY PAPERInitial measurements were made on stationary samples.  The experiment is depictedin Figure 1.  The incident detection light, an argon ion laser capable of providing 1.5 wattsat 514 nm, was focussed onto the surface with optics not shown.  Scattered light wascollected by lens L1, irrespective to polarization, and processed by the Fabry-Perotinterferometer.  Generation was provided by a pulsed Nd:YAG laser with a pulse width ofabout 15 ns and pulse energy up to 400 mJ at 1064 nm.  The generation pulse was focussedonto the surface by lens L2, which was either a cylindrical lens for a line focus, or aspherical lens for a spot focus.Figure 2a presents the lowest order symmetric (S0) and antisymmetric (A0)waveforms on a stationary 0.1 mm thick stainless steel sheet.  It is evident that the uniformline source employed produced little or no signal between the S0 wave at 2.5 µs and theonset of the A0 wave at 3.5 µs.  Figure 2b shows equivalent waveforms recorded forSampleIncident detection lightPhotodiodeFabry-PerotGeneration LaserL1L2Figure 1.  Experimental arrangement for Lamb wave mode measurements on a stationarysheet.0.1 mm copier paper, commercial paper used in photocopiers.  Taking note of the longertime scale for Figure 2b, it can be seen that the general frequency content is lower in paperthan for stainless steel.  This is due to larger attenuation of the Lamb waves in paper.  Alsoevident is the complex signal from the S0 wave, extending from about 3 µs until the onsetof the A0 wave at 15 µs.  This is believed to be due to the heterogeneity of the paper at thefiber level, resulting in non-uniform generation along the generation line and also scatteringof the wave between generation and detection.Measurements of the S0 and A0 mode wave velocities were taken as a function ofpropagation direction on 59# raw stock paper.  Single shot waveforms were recorded withslight ablation, using a spot focus about 6 mm from the detection spot. The paper wasrotated keeping the source and detector geometry fixed. Wave speeds were determinedfrom the earliest signal arrival time. The S0 wave velocity in the MD was 3.75 km/s asdetermined by measurements taken over multiple distances for a signal with a bandwidth ofa0 5 10−0.500.5Response (V)Time (µs)S0 waveA0 wave  b0 10 20 30 40 50−3−2−10123Response (V)Time (µs)S0waveA0waveFigure 2.  Lowest order Lamb waves generated with a line source and measured with aFabry-Perot detector for a) 0.1 mm thick stainless steel and b) 0.1 mm thick copier paper.The waveforms are clearly separated in stainless steel, with little or no signal between theS0 and A0 waveforms.4 km/s90°270°180° 0°Figure 3. The speed of the S0 waveform for a sample of 59# raw stock paper at varyingangles.  The machine direction (MD) corresponds to 90° and 270°.  The velocity ratiobetween the MD and the perpendicular cross direction (CD) was 1.48 ± .11 at 1 MHzwhich agrees well with contact measurement results for this paper.about 1 MHz. The recorded velocity ratio between the MD and the CD was 1.48 ± .11 at1 MHz which agrees with the contact piezoelectric result 1.41 ± 0.02 for this paper [13].This demonstrated that the Fabry-Perot detector is useful for detection on paper.  Iteffectively accommodated the diffusely reflecting surface of paper, was readily scannedover the paper surface to make measurements at different locations and in differentdirections, and automatically eliminated low frequency interfering signals due to vibrationand laser phase noise.  The frequency response of the detector at the Idaho NationalEngineering and Environmental Laboratory, optimized for operation above 1 MHz, is notideal for measurements on paper. Attenuation in the paper limits the primary wavecomponents in the A0 wave to below about 1 MHz, while the S0 mode shows dispersionabove 1 MHz.  Both of these facts contributed to the complex waveforms recorded inpaper.MEASUREMENTS ON MOVING PAPERA machine designed and constructed at the Institute for Paper Science andTechnology provided simulation of a moving paper web.  It was capable of moving a stripof paper attached to a revolving wheel at speeds up to 400 meters/minute (m/min), about25% of the speed of paper during manufacture.  The simulator allowed for contact freeoptical access to the moving surface.Velocity Dependent NoiseInitial measurements on moving paper revealed significant additional noise in thedetection process.  This noise was due to phase fluctuations that appeared to derive fromseveral sources. Shot to shot variation resulted from fine scale heterogeneity.  Localvariation in the optical reflectivity produced deviations in both the generation and detectionprocesses, contributing to fluctuations in the strength of the S0 and A0 modes from onelocation to another.Figure 4.  Average waveforms for 42# liner board at 10, 100 and 400 m/min. Averagingreveals the A0 mode, but at speeds faster than about 10 m/min the motion noise is too largefor the weaker S0 mode.The motion noise can be suppressed by averaging.  Figure 4 displays waveformsaveraged 128 times for 42# liner board moving at 10, 100 and 400 m/min. The A0 mode isclearly seen even at 400m/min, but the S0 mode is not discernable at speeds faster thanabout 10 m/min due to the large amplitude of the motion noise.  The noise can also bereduced by high pass filtering the signal, but this filters out much of the A0 mode also, andextends the upper speed for the S0 mode to only about 200 m/minThe noise due to motion was examined more closely by recording the Fabry-Perotsignal without generating the wave modes with the pulsed laser.  Figure 5 presents noisespectra at various speeds for three cases.  Figures 5a and 5b show data for 0.04 mm and0.26 mm detection spots, respectively, on 59# raw stock paper, while Figure 5c shows datafor 0.06 mm detection spot on a relatively smooth surface, the plastic wheel of thesimulator.  Each contains a large low frequency component that is present even when thesimulator is not moving and increases with the simulator speed.  Each also contains aplateau noise component that increases in bandwidth but not amplitude as the simulatorspeed is increased.  For a non-specular surface, increasing the spot size reduces thebandwidth of the plateau component.  Also, reducing the number of speckles, from themany speckles of the paper to just a few speckles from the plastic wheel, significantlyreduces the amplitude of the plateau component.  The low frequency component waslargely unaffected by either change.The noise observed includes contributions from several sources.  The noise presentfor a stationary target include contributions from laboratory vibrations, laser phase noiseinherent in the laser, electronic interferometer control fluctuations, electronic signalamplification noise, and shot noise in the detection process.  These components determinethe maximum sensitivity of the Fabry-Perot system and are present in all of themeasurements.0.1110100,150,200,250,300,350,400 m/minFabry-Perot Noise0.11100,10,25,50,60,75,100,150,200,250,300,350,400 m/min0 2 4 6 8 100.1110Frequency (MHz)0,25,50,100,150,200,250,300,350,400 m/minabcFigure 5.  Noise spectra at various web simulator speeds for 3 situations, a) 0.04 mm andb) 0.26 mm detection spots on 59# raw stock paper, and c) 0.06 mm detection spot on thespecular surface of the web simulator wheel.The additional noise from the moving surface is attributed to a) flutter noise due tomisalignment and vibrations in the web simulator, and b) speckle noise due to the movingspeckle pattern at the detector. The flutter noise increases with web speed and accounts forthe low frequency, web speed dependent component.  The speckle noise has fixedmaximum amplitude, since the optical phase shift is limited to modulo 2pi.  The frequencyat which these speckles move across the detector aperture increases with web speedextending the bandwidth observed and accounts for the plateau component.Noise AvoidanceThe speckle noise can be avoided, for the most part, by making measurements withthe setup depicted in Figure 6. The tracking mirror M2 was rapidly swept through severaldegrees to match the speed of the detection spot to the speed of the paper at the time of themeasurement.  The polarized beam splitter cube BSC and wave plates λ/2 and λ/4 wereadded to provide coaxial illumination and detection beams during the sweep. Thus thesame spot on the moving paper was illuminated during the measurement time and thePDRotatingWheelIncident Detection LightFabry-PerotGeneration LaserCollection OpticsL2M1M2RotatingWheelCollection OpticsL1λ/2λ/4BSCFigure 6.  Optics setup to minimize speckle noise for moving sample.  Mirror M1 is fixedwhile M2 rotates in sync with generation laser pulse.Figure 7.  Comparison of waveforms detected a) without and b) with tracking for 59# rawstock paper moving at 100, 200, 300 and 400 m/min.  The traces at 10 m/min are withouttracking, showing that it is only necessary to match the speed to within about 10 m/min.speckles were nominally stationary at the detector.  Inclusion of the BSC sacrificed half ofthe light collected due to depolarization of the reflected light.  Inclusion of M2 also limitedthe light collection efficiency, as it was kept small, about 12 mm across, to minimize itsinertia.  The trigger signals for the generation laser were derived from the position outputof the control for M2, so the position of the generation spot could be selected relative to theposition of the detection spot, and the ultrasonic measurement could be controlled andrepeated.Dramatic improvement is possible with tracking. The success of thisreconfiguration is illustrated in Figure 7, which shows data recorded with M2 positioned29 mm from the surface of the wheel. To compensate for the reduced light collectionefficiency, the detection laser provided 2 W illumination at 532 nm.  Moving papersustained much greater illumination than stationary paper due to the reduced time for localheating. The detection spot was focussed to about 0.5 mm, which avoided damage to thepaper.  The plate waves were generated by an 11 mJ pulse at 1064 nm focussed to aroughly 1 mm diameter spot, displaced 8 mm laterally from the detection spot.  All of thetraces in Figure 7 are single shot, with frequency content limited to greater than 100 kHzand less than 10 MHz.  While there is little evidence in Figure 7a of the A0 waveform forthe mirror not tracking, the A0 waveform is clear at all speeds in Figure 7b for the mirrortracking.  Even the S0 waveform can be identified at 3 µs in Figure 7b for the paper movingat 300 m/min.CONCLUSIONUltrasonic measurements have been made on paper moving at up to 400 m/minusing a Fabry-Perot detector.  The principle obstacle for measurements on a movingsurface was speckle noise, due to the changing speckle pattern at the detector.  This wasovercome by moving the detection spot with the sample surface using a tracking mirror.The ultrasonic waveforms were recorded without averaging and with only minimalfiltering, and showed the S0 Lamb wave mode at up to 300 m/min, and the A0 Lamb wavemode at up to 400 m/min.  These measurements could be extended to paper mill speeds,1600 m/min, with suitable changes in the tracking mirror.ACKNOWLEDGMENTSThis work was sponsored by the U.S. Department of Energy, Office of EnergyEfficiency and Renewable Energy, Office of Industrial Technologies under DOE IdahoOperations Office Contract DE-AC07-94ID13223.REFERENCES1. R. W. Mann, G. A. Baum and C. C. Habeger, Tappi 63, 163 (1980).2. G. A. Baum, D. C. Brennan and C. C. Habeger, Tappi 64, 97 (1981).3. C. C. Habeger and W. A. Wink, J. Appl. Polymer Sci.32, 4503 (1986).4. P. H. Brodeur, M. S. Hall and C. Esworthy, J. Acoust. Soc. Am. 94, 2215 (1993).5. M. Van Zummeran, D. Young, C. Habeger, G. Baum and R. Treleven, Ultrasonics 25,288 (1987).6. G. A. Baum, Appita 40, 288 (1985).7. C. C. Habeger, M. Van Zummeran, W. A. Wink, B. M. Pankonin and R. S. Goodlin,Tappi 72, 171 (1989).8. J.-P. Monchalin, IEEE Trans. UFFC 33, 485 (1986).9. K. L. Telschow, J. B. Walter and G. V. Garcia, J. Appl. Phys. 68, 6077 (1990)10. M. A. Johnson, Y. H. Berthelot, P. H. Brodeur and L. A. Jacobs, Ultrasonics 34, 703(1996).11. J. C. Cheng and Y. H. Berthelot,J. Phys. D. 29, 1857 (1996).12. P. H. Brodeur, M. A. Johnson, Y. H. Berthelot and J. P. Gerhardstein, J. Pulp andPaper Science 23, J238 (1997).13. B. Pufahl, private communication, Jan 9, 1998.

Fabry-Perot Laser Ultrasonic Elastic Anisotropy Measurements on a Moving Paper Web

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Fabry-Perot Laser Ultrasonic Elastic Anisotropy Measurements on a Moving Paper Web

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