Laser generation of convergent acoustic waves for materials inspection

Laser generation of convergent acoustic waves for materials inspection P. CIELO, F. NADEAU and M. LAMONTAGNE A laser technique for generating convergent acoustic waves is described. The optically probed Rayleigh wave in the centre of convergence shows an amplification factor of the order of 20 with respect to a collimated surface wave. Applications to the ultrasonic characterization of layered materials and to crack detection are described. KEYWORDS:

ultrasonic

testing,

lasers, surface

acoustic

waves

to the probed point without spread losses. Such a wave can be generated by focusing the laser to a thin line through the use of a cylindrical lensg. It should be mentioned that this line-source technique is more convenient than the disc-source not only because it avoids spread losses, but also because it produces a higher-frequency surface wave without overheating the surface. Indeed, the wavelength of the surface wave is nearly equal to twice the width of the line if the laser pulse is shorter than the Rayleigh wave transmit time.

Introduction Laser generation of ultrasound’,* is an attractive technique because it requires no contact with the inspected material. Applications of such a technique to non-destructive testin$-s and to high-temperature materials characterization’j*’ have been reported. To take most advantage of the non-contact nature of this method, laser interferometers should also be used for the detection of the generated wave. Laser ultrasound probes have several advantages compared to contact devices: they can be absolutely calibrated have a wide and flat frequency response, do not perturb the ultrasonic wave to be detected, and have a spot size that is much smaller than the ultrasonic wavelength to be probed. The latter consideration is important when a surface wave or a non-normally incident bulk wave must be detected. The main limitation of laser interferometers is their limited sensitivity compared to piezoelectric transducers.

A natural extension of the line-source technique is the converging-wave configuration shown in Fig. 1~‘~. In this case a thin, annular-shaped heated area produces a wave converging towards the detection point. This technique takes full advantage of the very small spot size of the probing interferometer as compared to more sensitive but wider ultrasonic probes such as the piezoelectric or capacitive transducers. A similar concept has been presented in a recent paper” where a water-immersed holographic mask was laser-irradiated to produce a high-intensity focused ultrasonic beam without overheating the laser-irradiated area This approach has some similarity with the modulatedbeam generation of surface waves through a mask’*. Although interesting for medical diagnostics and immersion-bath ultrasonic inspection, this technique appears to be hardly applicable to non-contact fast scanning ndt of solid materials.

Attempts have been made to increase the laserultrasound conversion efficiency to obtain larger ultrasound pulses without reaching the surfacedamaging ablation regime. The use of constraining layer? increases substantially the generation efficiency of bulk waves in the thermoelastic regime. Unfortunately, the requirement for surface layers is not always compatible with quick and non-contact industrial inspection procedures.

In this paper, a simple technique using a focused axicon is described for the generation of a converging surface wave. The interferometrically detected ultrasonic wave at the centre of convergence shows an increase in the signal level by a factor of 20 with respect to a line-source produced wave with equal surface heating. Some applications to the nondestructive inspection of solid materials are also presented.

The other approach to increase the signal level is to control the directional properties of the laser-generated ultrasonic wave. Fig. 1 illustrates this concept in the case of a thermoelastically-generated surface wave. In Fig. la. a disc-shaped laser-heated area produces a surface wave which spreads two-dimensionally over the surface, and is detected by a focused probing beam at a certain distance from the generation area. Fig 1b shows a nearly-collimated surface wave which travels

Experimental The

authors

Research Paper

are at the

Council

recewed

Industrial

of Canada,

26

March

Materials

Boucherville.

1984.

Remed

Research Quebec, 24

Institute, Canada,

October

MARCH

1985

J4B

apparatus

A schematic of the experimental system is shown in Fig 2. A multimode Nd:YAG laser, giving typically

6Y4.

1984.

0041-624X/85/020055X)8/$03.00 ULTRASONICS.

National

0

1985

Butterworth

8 Co (Publishers)

Ltd 55

telescope decreases the beam divergence by an amount equal to its magnification; this was required to obtain a sufficiently narrow annulus with a multimode laser beam and a lens-to-surface focusing distance of almost 1 m. The surface wave is detected in the centre of the annulus by a HeNe interferometer focused on the roughly polished surface of the aluminium sample through a HeNe-reflecting YAG-transmitting dichroic mirror. The interferometer is locked to an average path-difference of (n + 1/4)X by applying a slowly varying correcting voltage on the piezoelectrically controlled reference mirrorr3,r4. In such a case, the detected intensity can be written:

a

(1) Probed

Heated area -

point

where Z,, is the average intensity incident on the detector, I r is the peak-to-peak intensity variation obtained t%r a path-difference variation larger than x/2, and X = 633 nm is the HeNe wavelength. As the ultrasonic surface displacement Ax is much smaller than X, we can write from (1):

/

Ax1:gL,looa” IPP

b

d point

VPP

27

[nml

(2)

where AV and V are the oscilloscope displayed voltages produce “Bby the detected intensities AI and I,, respectively. Such an expression is used to calibrate absolutely the interferometer by measuring the maximum signal fluctuation VPp obtained with a large displacement of the reference mirror. The sensitivity of the interferometer is of approximately 0.02 nm with a bandwidth B = 30 MHz. This is not far from the quantum limit obtained by comparing the photocurrent signal:

Is = rlU = qIpp Ax2nlX Thermoelastic generation of: a - diverging surface acoustic wave; Fig. 1 b - a collimated wave; and c - a converging wave

where n = 0.4 A W-l is the detector shot noise of the photodiode”~‘?

(3) efficiency.

with the

i, = (2rqIc,B)1’Z

(4)

where e = 1.6 X lo-l9 C is the electronic charge. The minimum detectable displacement is obtained by equating (3) and (4):

Ax _

(2 e M9’/*

Tp Fig. 2 Experimental apparatus for the laser generation and detection of converging surface waves. The inset shows a photo of the YAG annulus on the sample plane 15 ns, 0.1 J pulses, is expanded and focused through a positive the sample, where it produces a wide annulus. The diameter of varied by displacing the axicon

56

by a Galilean telescope axicon on the surface of 15 mm diameter, 0.2 mm the annulus can be longitudinally. The

I PP

x

z;;

which gives AX = 0.008 nm for I,, = I,42 = 1 mW. The additional noise that we observe is attributed to laser noise” as well as to a reduction in the Ipr,/l,, ratio caused by the spatial coherence loss of the reflected beamIs. The annular laser pulse produces a convergent as well as a divergent Rayleigh wave, whose vertical displace-

ULTRASONICS.

MARCH

1985

ment on the surface can be represented for r Z 0 by a superposition of continuous annular wavesL9: 5

U, = Ho&r) exp(iwt)

(6)

where H,, is a Hankel function of the first kind for the converging wave, and of the second kind for the diverging wave, k is the wavenumber, x the distance from the centre of the annulus, o the angular frequency and r the time. Except for the small values of r, for which (6) is not valid the convergent waves can be approximated using the asymptotic formulae for the Bessel functions: u,(r -+ -) = (2/nkr)‘h

exp[i(kr + wt - n/4)]

z

0

-5 1

a

(7)

showing that the amplitude of the annular wave is expected to vary as fY2 for large values of r. The amplitude variation of the converging pulse was experimentally recorded over a line passing through the centre of the annulus by angularly scanning the dichroic mirror shown in Fig. 2. The peak-to-peak amplitude of the detected surface wave is plotted in Fig. 3 as a function of the distance r to the centre. As we can see, the amplitude of the detected wave is very large in the centre, while for r > 0.5 mm the curve shows a gradual decay in agreement with (7). The amplitude of the wave near the border of the annulus, r z 7 mm, corresponds to the amplitude of a collimated surface wave as generated by a line-source heating the surface to the same temperature. As such

I

0

-I

b

3

0.5

z

O

-0.5

C

t I

I

I

I

I

2

3

6 Time4

Fig. 4 b--r=0.5

0

4

2

8

6

IO

Distance from centre [mm] Fig. 3

Amplitude

distance

r from

of the detected

the centre

ULTRASONICS.

surface

of convergence

MARCH

1985

wave

as a function

of the

12

Waveforms mm;

of some c-

r=2

signals

reported

I I

I I

I I

[r-s] 5

in Fig. 3: a -

r = 0;

mm

an amplitude is of nearly 0.4 nm, we can conclude that the signal obtained in the centre of a converging wave is amplified by a factor of more than 20 with respect to the one obtained with a line-source of equal power density. The positioning accuracy required to point the HeNe probing beam at the centre of the annulus is of the order of the annulus width, 0.2 mm. Figure 4 shows some typical signals obtained (a) in the centre, (b) at 0.5 mm and (c) at 2 mm from the centre.

57

In the centre, a strong dipolar pulse is obtained with a shape similar to the one previously observed in the thermoelastic regime (see Ref. 2, Fig 19b). Out of the centre, the two pulses coming from the two sides of the annulus are resolved when the distance between the centre of the annulus and the probed point is larger than the pulse duration. As can be appreciated from Fig 4c, the two pulses have different shapes. The converging pulse is unipolar while the diverging pulse is dipolar. Although not fully understood this behaviour appears to be related to the different focusing conditions in front and behind the pulse for the converging and diverging waves. Also, note the presence of a P-wave2 after a time corresponding to the longitudinal wave velocity in the aluminium sample.

Applications

to materials

inspection

Some examples of possible applications of the convergent-wave technique to the non-destructive evaluation of materials are described in this section. The list is by no means exhaustive, while no effort was made in the experimental demonstrations to reach the ultimate sensitivity of such a technique. The convergent-wave technique is well suited to materials characterization through a measure of the acoustic surface-wave velocity on flat or spherical surfaces. A large number of investigations have been reported for the characterization of materials’ properties such as alloy composition, microstructure or residual stress using bulk acoustic waves on parallel plates of known thickness. A much smaller amount of work has been done using surface waves. in spite of the unique suitability of surface waves to measure properties such as hydrogen entry in metalsZo or glass ternpeP. One reason for this is the difficulty to couple and detect surface waves efficiently using liquid-coupled transducers, given the strong attenuation suffered by such waves when a liquid film is present along the surface-wave path. Critical angle reflectometry** has often been used to overcome this problem, but this technique is time-consuming and requires the immersion of the sample. The laser-generated convergent-wave technique would provide a fast and non-contact alternative to such methods. The surface-wave velocity can be obtained in homogeneous materials through time-of-flight measurements from signals of the kind shown in Fig 4a. The resolution is of the order of the 5 ns sampling period leading to an accuracy of the order of + 0.1 %I for an annulus of 30 mm in diameter. An application of such a technique to the characterization of electroplated materials is described in this paper. Ag- and Cr-plated copper samples with different coating thicknesses were inspected using laser-generated convergent surface waves. It is well known23 that materials coated with layers of different acoustic properties are frequency-dispersive when the acoustic wavelength is of the same order of magnitude as the coating thickness. The thickness or the acoustic properties of the coating layer can thus be evaluated by measuring the surface wave velocity at different frequenciesZ4sZ5. Non-contact, single-shot probing of laser-generated surface waves for such an application is an attractive possibility. Some typical waveforms

58

obtained

at the centre of the

laser-generated converging wave on electroplated copper samples with different coating thicknesses are shown in Fig. 5. The dispersive effect of the coating becomes increasingly evident as the coating thickness is increased. The temporal delay of the high-frequency components with respect to the low-frequency main pulse in the Ag-plated sample is apparent in Fig. 5c. while in the thick Cr-plated sample, Fig. Sf. the high frequencies precede the main pulse. This was expected because the short wavelengths tend to propagate mainly within the coating which has an acoustic velocity lower than the substrate in the case of the Agplated sample. and higher than the substrate in the case of the Cr-plated samples. Such a dispersive effect is accompanied by a slight variation in the time-of-flight of the main pulse. as shown in Fig. 6. In this figure, the relative time-of-flight of the main negative pulse is plotted as a function of the coating thickness. A time-of-flight measurement may thus be used as a parameter for the evaluation of the coating thickness, particularly for coatings that arc thin with respect to the main pulse wavelength. Best results should however be obtained by time-resolved spectroscopic techniqueP. which would take full advantage of the flat frequency response of the optical probe and of the wide frequency bandwidth of the laser-generated pulse. Crack inspection is another typical application of ultrasonics. Techniques reported in this field in the past include scattered amplitude methods. bulk-wave and surface-wave timing methods and ultrasonic spectroscopy analysi?‘. Some experiments have been performed to explore the suitability of the convergcntwave technique to crack inspection. A crack was simulated with a I mm deep. 0.1 mm wide eloctrodischarge machined slot across the surface of the sample. Fig. 7 shows the waveforms obtained in the case of an out-of-centre probe and different configurations. As expected, the divergent-wave pulse disappears when the slot is on the other side of the probe with respect to the centre (Fig. 7b) while the convergent-wave disappears when the slot is on the same side as the probe (Fig 7~). Also, note the small pulse reflection from the slot in Fig. 7c. It should be mentioned that while the reflection from a real, irregular crack would be much less visible than with a regular EDM slot, the transmission technique proposed here should not be affected by the shape and orientation of the crack Fig. 7d shows that the converging-wave pulse is barely affected by the preence of a 0.1 mm deep slot. showing that a certain amount of crack depth evaluation is possible by simply observing the signal. Better sensitivity should be obtainable by using more elaborate signal processing methods. such as transmission Rayleigh-wave spectroscopy28,*9 Finally. the advantages of the convergent-wave technique over the spot heating approach in the pulseecho testing of adhesively-bonded plates has been examined. A 4 mm thick aluminium plate epoxybonded to a massive aluminium substrate was inspected using different beam geometries. Fig. 8 shows some typical waveforms. Figs 8a to 8d were obtained by spot heating while Figs Xc and 8f correspond to annular heating. Figs Xa to Xc were obtained on an unbonded region. while Fig. Xf was obtained on a wellbonded region. Figs 8a and 8b correspond to the case

ULTRASONICS.

MARCH

1985

2

Eo

-2 I

C

Fig. 5 Waveforms obtained tn the centre of a 1.5 cm dwneter annulus for an electroplated copper sample: a - bare sample; b coating; c - 60 pm thick Ag coating; d - 7.5 pm thick Cr coating; e - 15 pm thick Cr coating; f - 40 pm thick Cr coating

ULTRASONICS.

MARCH

1985

20 pm thick Ag

59

ultrasonic inspection is presented in this paper. The main advantages of such an approach are the obtention of a large ultrasonic amplitude in the centre of the converging wave, as well as the possibility to obtain short, high-frequency pulses without overheating the surface. Some applications to the detection of surface defects and to the evaluation of layered materials are described.

Acknowledgements The authors wish to thank Prof. G.W. Farnell Drs C.K. Jen and D.A. Hutchins for helpful discussions.

and

References 0.98

White, RM. Surface

0.94 t 0

IO

20

30 Coating

40 thickness

50

when the probing HeNe laser is pointed in the centre of the heated area, with (a) or without (b) plasma breakdown at the irradiated surface. As we can see. the eventual pulse echo is overshadowed by either the plasma turbulence or the thermal expansion in the solid and in the adjacent air layer. It should also be noted that the laser-generation efficiency of longitudinal and shear waves in the thermoelastic regime is maximum in a direction far from the normaP. Figures 8c and d show two waveforms obtained when the HeNe beam is pointed outside of the 1 mm diameter disc-shaped heated area, at a distance of 2 mm (c) and 8 mm (d) from the centre of the heating YAG beam. A weak plasma was produced by the YAG pulse in such cases. The broad Rayleigh (R) wave can clearly be seen in the two waveforms, with a superposed random modulation which is attributed to the presence of ‘hot spots’ in the multimode YAG beam. The air-propagated pressure wave is also apparent in Fig. 8c. As to the longitudinal (L) and shear (S) echoes from the unbonded surface, they are more visible in Fig. Sd, as expected from the angular distribution of the generated bulk waves*, but they are weak and overshadowed by the plasma- and modal structureproduced noise. Figs 8e and f correspond to the annular configuration previously described in this paper. on an unbonded (e) and well-bonded (f) region of the plate. The complete absence of plasma, as well as the narrowness of the acoustic pulses. allow a more consistent evaluation of the reflected longitudinal and shear echoes. It should also be noted that a repetitive Rayleigh pulse provides a reference for the evaluation of the absorptivity-dependent heating energy density.

Conclusion

60

approach

Scruby, C.B., Dewhurst, R.J., Hutchins, D.A., Palmer, S.B. Laser Generation of Ultrasound in Metals. in Research Techniques in NDT, RS. Sharpe (Ed), Vol. 5. Academic Press, London (1982) Krautkramer, J. Unconventional Methods of Generating Coupling and Receiving Ultrasound in NDT, 9th World Conf. NDT. Melbourne (1979)

km]

Fig. 6 Relative time-of-flight of the negative pulse as measured from the waveforms of the kind shown in Fig. 5 as a function of the coatmg thickness, for the Ag and Cr electroplated samples

A converging-wave

Generation of Elastic Waves by Transient Heating, J: Appl. Phys. 12 (1963) 3559

to laser-generated

Probed point \

\ [1

Heated annulus

a

I mmslot

ye--

1’

\

x*

\

b

\

i

,’

‘A__

0’

d Fig. 7 Waveforms obtained on an alumlnlum sample in the presence of an EDM slot, wth an out-of-centre probe; a - no slot; b - 1 mm deep slot on the opposite side of the probe; c - 1 mm deep slot on the same side as the probe; d - 0.1 mm deep slot on the same side as the probe

ULTRASONICS.

MARCH

1985

4mm

:;I YAG F --._ t He-P&? .

__--+

I

I

a

2 %

YAG --__v

. 1

0 -2

I

d

-* _--- _-I I

He-Ne -Qk

5

\

z

/

0

\

%z 4 --_ --_, / __-- _. c

-5

L

Fig. 8 Pulse-echo waveforms obtained with different beam geometries on a partially-bonded 4 mm thick Al plate: a - and b - concentrated YAG beam superposed to the probing beam. with and without plasma respectively; c - and d - concentrated YAG beam, probing beam at 2 mm and 8 mm from the heated area, respectively; e - and f - annular YAG beam, on an unbended and well-bonded area, respectively

4

5 6

7

8 9

10

11

12

13

Wellman, R.L Laser System for the Detection of Flaws in Solids, Harry Diamond Laboratories Rept. HDl-TR-1902, Adelphi, MD (1980) Bar-Cohen, Y. NDT of Microwelds Using Laser-Induced Shock Waves. Brir. J NDT 21 (1979) 76 Pope, LE., Rhode, RW., Percival, C.M. Elevated Temperature Elastic Constants of Fe-30 wt. pet. Ni in Ausenitic and Martensitic Conditions, Metall Trans., 7A (1976) 103 Calder, C.A. Dranex, E.C., Wilcox, W.W. Non-Contact Measurement of the Elastic Constants of Plutonium at Elevated Temperatures. 1 Nucl. Mater. 97 (1981) 126 Von Gutfeld, R.J. Thermoelastic Generation of Elastic Waves for NDT and Medical Diagnostics. Ultrasonics 18 (1980) 175 Aindow, A.M., Dewhurst, R.J., Palmer, S.B. Laser-generation of Directional Surface Acoustic Wave Pulses in Metals. Opt Comm. 42 (1982) 116 Cielo, P., Bussiere, J. Efficient Laser Generation of Surface Acoustic Waves, US Pat. Appln. 454. 094 (1982) Von Gutfeld, RJ., Vigliotti, D.R, Ih, C.S., Scott, W.R. Thermoelastic Hologram for Focused Ultrasound, Appl. Phys Left 42 (1983) 1018 Ash, E.A., Dieulesaint, E., Rakouth, H. Generation of Surface Acoustic Waves by Means of a CW Laser. Electron. Letc 16 (1980) 460 Deferrari, H.A., Darby, R.A., Andrews, F.A. Vibrational displacement and mode-shape measurement by a laser interferometer, J Acoust. SK Am 42 (1967) 982

ULTRASONICS.

MARCH

1985

I4

I5

16

17 18

19 20

21 22

23

24

Kline,RA., Green, R.E., Palmer, C.H. A comparison of optically and piezoelectrically sensed acoustic emission signal, J Acoust. Sot. Am 64 (1978) 1633 Moss, G.E., Miller, LR, Forward, RL Photon-noiselimited Laser Transducer for Gravitational Antenna. Appl Oat 10 (1971) 2495 Palmer, C.H.; Claus, R.O., Fick, S.E. Ultrasonic Wave Measurement bv Differential Interferometry. App.! Opt 16 (1977) 1849 . Cole, J.H. Low-frequency Laser Noise of Several Commercial Lasers, Appl Opt (1980) 1023 Cielo, P. Optical Detection of Acoustic Waves for the Characterization of Materials with Unpolished Surfaces, 102 nd Acoust. Sot. Am. Meetine Miami. (Dec. l-4, 1981) Day, C.K., Koerber, G.G. Annular Piezoelectric Surface Waves. ILEE Tram Sonics Ultrasonics SU-19 (1972) 461 Lumarska, E., Fiore, N. Surface Acoustic Wave Studies of Hydrogen Entry Into a Ni-base Alloy, J Appl. Phys 52 (1981) 2587 Warren, J.M. Non-destructive Measurement of Plate Glass Temper. 12th Symp. NDE. San Antonio (1979) Weston-Bartholomew, W. A Possible Method of Detecting Incipient Creep in Engineering Materials. Int Adv. NDE, 7 (1981) 57 Farnell, G.W., Alder, E.L. Elastic Wave Propagation in Thin Layers in Physical Acoustics, W.P. Mason and RN. Thurston (Eds) Academic Press, New York (1972) Martin, B.G., Becker, F.L. The Effect of Near-Surface

61

25

26

62

Metallic-Property Gradients on Ultrasonic Critical-Angle Reflectivitv . Mater Eval 38 (1980) 92 Flambard, C., Lambert, A. Mesure’ Non-Destructive des Profondeurs de Traitement Thermique, MN: Mare,: Elan 317 (1976) 25 Brown, A.F. Ultrasonic Spectroscopy in NDT. Sci Prog. O_x-f: 65 (1978) 51

27 2X 29

Doyle, P.A., Scala, C.M. Crack Depth Measurement by Ultrasonics: A Review, Ulrrasonin 16 (197X) I64 Silk, M.G. The Determination of Crack Penetration Using Ultrasonic Surface Waves. N/)T Intern. 9 (1976) 290 Burger, C.P., Testa, A. Rayleigh-Wave Spectroscopy to Measure the Depth of Surface Cracks. 13th Symp. on NDE. San Antonio (1981)

ULTRASONICS.

MARCH

1985