Frequency Spectrum Spatially Resolved AcousticSpectroscopy for Microstructure Imaging

Wenqi Li, Steve D. Sharples, Matt Clark, Mike G. SomekhApplied Optics Group, Electrical Systems & Optics Research Division, Faculty of Engineering

University of NottinghamUniversity Park, Nottingham, NG7 2RD, UK

Email: eexwl6@nottingham.ac.uk, steve.sharples@nottingham.ac.uk

Abstract—The microstructure of a material influences thecharacteristics of a component such as its strength and stiff-ness. A previously described laser ultrasonic technique knownas spatially resolved acoustic spectroscopy (SRAS) can imagesurface microstructure, using the local surface acoustic wave(SAW) velocity as a contrast mechanism. The technique isrobust and tolerant of acoustic aberrations. Compared to otherexisting methods such as electron backscatter diffraction, SRASis completely noncontact, nondestructive (as samples do notneed to be polished and sectioned), fast, and is capable ofinspecting very large components. The SAW velocity, propagatingin multiple directions, can be used to determine the crystal-lographic orientation of grains. Previously, the method used afixed frequency laser and variable grating period (k-vector) todetermine the most efficiently generated surface waves, and hencethe velocity. However, SRAS can also be implemented by usinga fixed grating period with a broadband laser excitation source;the velocity is determined by analyzing the measured frequencyspectrum. In this paper, experimental results acquired using this“frequency spectrum SRAS” (f-SRAS) method are presented forthe first time. The results are illustrated as velocity maps ofmaterial microstructure in two orthogonal directions. The twodifferent ways of performing SRAS measurements—f-SRAS andk-SRAS—are compared, and excellent agreement is observed.Furthermore, f-SRAS is much simpler, and is potentially muchmore rapid than k-SRAS because it can determine the velocityat each sample point in one single shot from the laser ratherthan scanning the grating period.

I. INTRODUCTION

SRAS—spatially resolved acoustic spectroscopy—is a non-destructive testing method which maps surface acoustic wave(SAW) velocity on a material surface. The velocity contrastof SAWs reveals the material’s microstructure and orientationof crystals. This can also be accomplished by other methodssuch as electron backscatter diffraction (EBSD), which gainscrystallographic information by impinging electrons on to thesample and analyzing diffraction patterns [1]. Alternatively, thescanning acoustic microscope (SAM)—a contact technique—analyzes the phase shift of a narrow band SAW signal.Both these methods have disadvantages such as restriction onsample size, or (in the case of the SAM) ambiguity caused by aperiodic phase function. SRAS eliminates those concerns. It isbased on the spectroscopy technique—perhaps more familiarin optics—and uses the spectral characteristics of acousticwaves.

There are two ways to implement the technique. In one—

Fig. 1. Schematic of f-SRAS laser ultrasound system.

named k-vector SRAS—SAWs are generated by a grating pat-tern of pulsed laser light, and the grating period is scanned. Byvarying the grating period (with a fixed temporal modulationfrequency of the laser) a spectrum is built up and the pointwhere the spectrum peaks gives the local velocity, c, throughc = fλ where f is the fixed frequency and λ = 2π/k is thevariable wavelength or k-vector.

Alternatively a fixed period grating can be used in con-junction with a broadband laser source and the peak of thefrequency spectrum of the resulting signal similarly gives,c = fλ, where f is now variable and λ is fixed (by the gratingperiod). This method is termed f-SRAS (frequency spectrumSRAS).

II. FREQUENCY SPECTRUM SPATIALLY RESOLVEDACOUSTIC SPECTROSCOPY (F-SRAS)

A velocity measurement taken using the SRAS techniqueis local to the area where the waves are being generated. Asthe measurement is relatively fast the sample may be scannedand an image built up of the velocity over the sample. Thevelocity at any point is determined by the material elasticparameters at that point and the resulting images clearlyreveal the grain structure. This method is used to measurethe velocity of the SAW over individual grains on severalindustrial materials such as aluminum and titanium alloys.The variations of velocities, which indicate the orientationsof grains, are presented as a color map.

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Fig. 2. Example of optical masks used to generate SAWs. Only one maskis used on each sample.

Fig. 1 shows a schematic of the system setup. In the figure,the pulsed laser is indicated as the excitation source. Alterna-tively, the laser could be one capable of being modulated bya series of sine waves with different frequencies. Both thesemethods are capable of implementing frequency scanning,although the broadband excitation source is generally quickerand is the method used to acquire the experimental results inthis paper.

The pulsed laser beam passes through a mask consistingof a set of stripes, as shown in Fig. 2. The unblocked lightproduces SAWs when it is imaged onto the sample surface.In our instrument there are twenty masks which have differentcurvatures and fringe spacings, to cover a range of differentmaterials and conditions. A smaller distance between thefringes generates SAWs with a shorter wavelength and higherfrequency. A straight (rather than focused) grating was usedwhich, for the majority of the results in this paper, had aneffective period of 55.78μm on the sample.

The laser signal is a short pulse (<10ns) and has a widebandwidth from DC to >100MHz. The grating excitationpattern acts as a filter on the generation of the SAWs andis multiplied with the frequency content of the laser. Theeffective center frequency of the grating is determined by thegrating period and the local velocity (c = fλ again). Sincethe effective bandwidth of the filtering action of the gratinggeneration is much smaller than the bandwidth of the laserpulse we can make the assumption that the frequency contentof the laser is locally flat across the bandwidth of the gratingand hence that the peak of the combined frequency responseis at the same location as the peak of the grating filter; thuswe can determine the local velocity.

Fig. 3 shows a typical SAW signal captured by the system.The spacing of the grating determines the wavelength ofSAWs. The top graph is the waveform in time domain; the FFTis on the bottom along with a Gaussian curve which has beenfitted to the raw FFT data. The frequency where the amplitudeis the largest indicates the closest value corresponding tothe SAWs traveling on the grain. Note that the velocitymeasurement relates to the sample area under the generationpatch, rather than at the detection point or in between thegeneration and detection.

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Fig. 3. Raw data in the time domain (top) and frequency domain (bottom)with Gaussian fit.

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Fig. 4. K-SRAS: amplitude of SAWs with respect to wavelength for a fixedfrequency laser source.

III. K-VECTOR SPATIALLY RESOLVED ACOUSTICSPECTROSCOPY(K-SRAS)

K-SRAS was been described in a previous paper [2]. It isbasically the same theory as f-SRAS but SAWs are generatedby a grating pattern whose spacing is scanned, and a singlefrequency modulated laser beam is used. By varying thegrating spacing, the amplitude of the received signal is thehighest where the wavelength of the SAWs matches the gratingspacing. As the frequency of the laser source is fixed, thevelocity of the waves can be determined.

Fig. 4 shows the amplitude of the received SAWs plottedwith respect to the grating line spacing. As before, the ex-perimental data and the Gaussian curve fitted to this data areshown. Hence, by finding the grating period which generatesthe largest amplitude SAWs, the velocity of a grain at a specifictesting position of the sample material is measured.

IV. COMPARISON OF TWO HIGH RESOLUTION METHODS

In Fig. 5 and Fig. 6, Ti-6246 k-SRAS results are comparedwith f-SRAS results, which represent the velocity of SAWspropagating in two directions. The k-SRAS results were ac-quired at a fixed frequency of 164MHz (typical wavelengths16–19μm) and f-SRAS at a fixed wavelength of 74.4μm (typ-ical frequencies 35–41MHz). Fig. 5 shows SAWs propagating

792 2009 IEEE International Ultrasonics Symposium Proceedings

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Fig. 5. SAW velocity map of Ti-6246: k-SRAS (top) vs f-SRAS (bot-tom), SAWs propagating left to right. Image size 85×36mm, pixel size250×250μm.

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Fig. 6. SAW velocity map of Ti-6246: k-SRAS (top) vs f-SRAS (bot-tom), SAWs propagating bottom to top. Image size 85×36mm, pixel size250×250μm.

from left to right, Fig. 6 shows SAWs propagating from bottomto top. All the SRAS images are scanned at a step size250μm. The spatial resolution of both variations of the SRAStechnique is determined by the size of the grating pattern onthe sample surface. As the two experimental setups (f-SRASand k-SRAS) had very different grating sizes, the k-SRASimages have had a spatial low pass filter applied. This hasbeen done to match the spatial resolution of the two sets ofthe results. Apart from differences in spatial resolution, both ofthe techniques have almost the same velocity distribution. Thecolor bars in the figures define the different SAW velocities,which for this alloy vary from from 2650ms−1 (dark blue) to3050ms−1 (dark red).

The time required to scan a 341×145 pixel image of a85×36mm sample using the k-SRAS technique is 75 minutes,

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Fig. 7. SAW velocity map of Ti-685: k-SRAS (top) vs f-SRAS (bottom),SAWs propagating left to right. Image size 84×36mm, pixel size 250×250μm(top), 125×125μm (bottom).

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Fig. 8. SAW velocity map of Ti-685: k-SRAS (top) vs f-SRAS (bot-tom), SAWs propagating bottom to top. Image size 84×36mm, pixel size250×250μm (top), 125×125μm (bottom).

which gives a scanning speed about 11 pixels/second. Thesame image acquired using f-SRAS takes 100 minutes, whichis 9 pixels/second (including overheads for fly-back duringraster scanning). Although in this case k-SRAS is marginallyfaster than f-SRAS, it should be noted that only two averagesat each testing point were used when the k-SRAS data wascollected, whereas 64 averages were used for f-SRAS.

Fig. 7 and Fig. 8 are velocity maps of a titanium 685 alloy,using k-SRAS (top of each figure) and f-SRAS (bottom ofeach figure) to acquire the results. The image is 84×36mmin size; the pixel size is 250×250μm for the k-SRAS images,and 125×125μm for the f-SRAS images.

This technique can also be applied to a material whichhas smaller grains, such as the 61×41mm aluminum sampleshown in Fig. 9 and Fig. 10. The velocity at each point on the

793 2009 IEEE International Ultrasonics Symposium Proceedings

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Fig. 9. SAW velocity map of aluminum: k-SRAS (top) vs f-SRAS(bottom), SAWs propagating left to right. Image size 61×41mm, pixel size125×125μm.

previous two samples, Ti-685 and Al, is calculated (using thef-SRAS technique) from a single laser shot. This means onlyone waveform has been collected at each sample point, with nodata averaging. The acquisition time of each f-SRAS figure isabout four times less than the time required for the equivalentk-SRAS image. Furthermore, this data rate (40 points persecond) is currently limited by the acquisition hardware (adigital oscilloscope). The Q-switched laser fires at a repetitionrate of 1000–10000 pulses per second: with an appropriatedigital acquisition card, the waveform generated by everylaser pulse could be used to make a velocity measurement,leading to an improvement in acquisition speed of 2–3 ordersof magnitude.

V. SUMMARY

An acoustic spectroscopy technique called SRAS has beendescribed, implemented in two slightly different methods. Bothof them are based on the laser ultrasonics, and in both casesthe microstructure can be imaged by determining the localSAW velocity on the sample surface. The difference betweenk-SRAS and f-SRAS is—as their names imply—the former iseffectuated by varying the fringe spacing of a grating, whichcorresponds to the SAW wavelength at a fixed frequency,building up a spectrum of k-vectors; whereas the latter iseffectuated by a fixed grating, producing SAWs of a fixedwavelength, and a broad frequency laser to determine thefrequency spectrum in a single shot. Unlike existing scanning

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Fig. 10. SAW velocity map of aluminum: k-SRAS (top) vs f-SRAS(bottom), SAWs propagating bottom to top. Image size 61×41mm, pixel size125×125μm.

electron microscope techniques such as EBSD, sample size islimited only by the scanning stages, and so it is a completelynondestructive inspection technique. In the implementationdescribed, the f-SRAS technique can acquire the local velocityof SAWs at 40 points/second (which is nearly four times fasterthan k-SRAS) and furthermore the ultimate limit to acquisitionspeed is the repetition rate of the generation laser, which isseveral thousand pulses per second. The spatial resolution iscurrently of the order of 400μm, which is the size of thegrating on the sample. This can be significantly reduced byusing different masks with fewer lines at shorter wavelengths.Experimentally, the bandwidth of the excitation laser sourceused in our instrument limits the maximum frequency SAWsthat can be excited (hence the minimum size of the grating).Ultimately—with a laser of sufficiently wide bandwidth—theresolving power of the optics used to produce the gratingstructure is the limiting factor on the resolution of SRAS.

ACKNOWLEDGMENT

This work was supported by the UK RCNDE (ResearchCentre in Nondestructive Evaluation).

REFERENCES

[1] V. Randle, Microtexture Determination and Iis Applications. London,UK: Maney Publishing, 2003.

[2] S. D. Sharples, M. Clark, and M. G. Somekh, “Fast noncontact imagingof material microstructure using local surface acoustic wave velocitymapping,” in Ultrasonics Symposium, 2005 IEEE. IEEE, 2005, pp. 886–889.

794 2009 IEEE International Ultrasonics Symposium Proceedings