Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention is directed to ultrasonic techniques for detect-
ing flaws on a surface, and in particular, to a me~hod and laser appara-
tus for generating surface acoustic waves. P
Conventional ultrasonic and eddy current techniquea have been
used for detecting flaws in various types of objects. However, these
techniques requir~ contact with the object and, therefore, cannot be used
when non-contact ls desirable or necessary, as with ob~ects at h~gh
temperatures.
Non-contact techniques have been developed in which bulk acous-
tic waves are generated in a material by a laser and the acoustic waves
reflected by the inner flaws are detected by interferometry. The genera- l
tion of bulk acoustic waves is described by W. Kaule et al in United
States Patent 4,121,469 which issued on October 24, 1978; by R.L. Melcher
et al in United States Patent 4,137,991 which issued on February 6, 1979;
and by W. Kaule et al in United States Patent 4,169,662 whlch issued on
October 29 1979. The interferometric method of detecting acoustic waves
in a material is described by C.M. Penney in United States Patent
3,978,713 which issued on September 7, 1976; and by E. Primback in United
States Patent No. 4,180,324~ which issued on December 25, 1979. This
non-contact technique has the advantages of better repetitivity of the
measurement because of the absence o~ a coupling liquid, ease of scan-
nlng, access to concave or irregular surfaces, and large and flat fre~
quency response leading to an improved spatial and temporal resolution.
It has been found, however, that the detection of cracks or
flaws in surfaces may best be carried out by using sur~ace or Rayleigh
acoustic waves, as described in the publication by B.R. Tittman et al,
"Fatigue Lifetime Prediction with the Aid of Surface Acoustic Wave NDE",
Journal N.D.E., 1, 123 (1980). Surface acoustic waves are difficult to
generate by conventional pie~oelectric methods, especially at high re-
quencies, because of precise angular alignment and need for a liquid
couplant which strongly attenuates surface ~aves. The laser generation
of surface acoustic waves has not been very successul to date as out-
lined in the publieation by A.M. Aindow et al "Laser-Generated Ultrasonic
Pulses at Free Metal Surfaces", J. Acoust. Soc. Am., 69, 449, 1981,
-- 2 --
because of low efficiency, low frequency and difficulty to discriminate
against bulk waves.
Summary of the Invention
r
It is therefore an ob;ect of this invention to provide a method
and apparatus for efficiently producing surface acoustic waves using a
laser beam.
This and other ob~ects are achieved ln a method for generating
surface acoustic waves which comprises providing a laser generated
optical beam, and directing the beam to the surface in order to irradiate
it in an arcuate pattern.
In accordance with one a~pect of the invention, the arcuate
pattern may be obtained by focussing the beam into a partial or complete
annulus on the surface by an axicon lens located on the beam path or by
an a~icon reflector for reflecting the beam to the surface.
In accordance with another aspect of the invention, the surface
acoustic waves may be generated by an optical fringe pattern irradiated
on the surface, the fringe pattern being formed from the laser beam which
is split into two beams. In addition, a lens may be placed in the path
oE one of the split beams to form a circular fringe, or a frequency
shifting element may be placed in the path of one of the split beams to
form a moving fringe.
Many other objects and aspects of the invention will be clear
from the detailed description of the drawlngs.
Brief Description of the Drawlngs
In the drawings:
Figure 1 illustrates apparatus for generating surface acoustic
waves;
Figure 2 illustrates the radiated area on the surface to be
inspected;
Figures 3 and ~ illustrate alternate apparatus for generating
the focussed laser beam;
Figure 5 illustrates a fringe method for generating surface
acoustic waves; and
Figure 6 illustra~es a method for producing a moving fringe.
Detailed DescrlpLion
Figure l illustrates one embodiment by which a surface acoustic
wave may be generated efficiently on the surface 1 of a material 2 that
is to be inspected. The apparatus lncludes an infrared laser 3 that
generates a coherent beam. Laser 3, which may be a Q-switched Nd:Y~G
laser, provides a pulsed beam 4 that is focussed by a lens 5 on the sur- -
face 1. In addltion, an axicon 6 or conical lens is used to refract the
beam 4 such that the focussed area has a partial annulus 7 subtending an
angle ~ and havlng a width w, as shown in figure 2.
In this method, a thermal-stress surface wave ls produced by
the absorption in the partial annulus 7 illuminated by the shaped laser
pulse beam 4. This acoustic wave moves off to the right, expanding and
dissipating. At the same time, a wave also moves off to the left where
it converges to a narrow focal region 8, -as illustrated by the dotted
lines. ~ny bulk acoustic waves generated by the heated area 7 also
diverge rapidly, resulting in acoustic echoes of very low power compared
to the converging surface wave. The radius of the converging wave can be
varied by moving the axicon 6 in a vertical direction towards or away
from laser 3. On the other hand, a displacement of the axicon 6 in a
horlzontal direction into or out of the beam 4, will change the converg-
ing angle ~. Large values of ~, i.e. up to 360, ma~ be desirable when
the orientatlon of a surface crack is not known and the surface is being
scanned. Moreover, a large aperture a implies better focussing of the
acoustic wave, because of the diffraction laws. In generating surface
acoustic waves in this manner, the average acoustic wavelength ~ i9 equal
to twice the width w of the heated area. Widtb w for a Q~switched,
single transverse mode Nd:YAG la6er can be made in the order of 100 ~m.
This results in a cross-6ection of the focused surface wave of the order
of 0.2 to 1 mm, dependlng on the value of the angle ~. The increase in
the efficiency, i.e. the increase of the amplitude of the detected signal
with respect to the signal obtained by a conventional unfocussed tech-
nique is thus of the order of 100 if the radius of the converging acous-
tic wave is 1 cm.
The surface wave in the focal region 8 may be detected by con-
ventional interferometric techniques as illustrated in figure 1. Thls
- 4 - .
technique includes the use of a ~ichelson interferometer 10 which pro-
vides a probing beam ll generated by a laser 12. The interferometer 10
further includes a beam splltter 14 whlch allows part of the laser 12
beam to pass through to a lens 15 to produce beam ll. The remaining part
5 of the laser 12 beam is reflected to a mirror 16. The returning beams
are directed to a detector l7 whlch produces a signal for the readout
apparatus 18. Beam 11 i6 deflected by a dichroic mirror 13 to be focus-
sed on the focal region 8 of the surface wave in order to take advantage
of the ~arge signal amplitude produced by the concentration of the acous-
lO tic wave in that region. The presence of a crack 9 ln the path of the
acoustic wave from lts point of orlgln, area 7, will strongly reduce the p
amplitude of the detected signal ln region 8 since the crack 9 would
cause part of the surface wave to be reflected and another portion to be
diffracted. Thus9 cracks or flaws may be detected by scanning the sur-
15 face l of materlal 2. In order to further increase the power of laser 12
and thus the signal to noise ratio of the system, a diode laser 12 that
is pulsed in synchronism with laser 3, may be used.
Figure 3 illustrates an alternate embodlment of the surface J
wave generator in accordance with the pre~ent lnvention. Beam 34 passes
20 through the axlcon 3~ and then through a partially reflecting mirror 33,
a lens 35 and a rotating mirror 32 which reflects the focussed beam 34
onto the surface 31 to ba inspected. The rotating mirror 32 allows the
beam 34 to scan the surface 31. The interfero~etric beam 37 18 reflected
by the mlrror 33 through lens 35 and onto mirror 3~ to scan beam 37
25 across the surface 31 in synchronism with beam 34.
In a ~urther embodiment shown in figure 4, the acoustic wave
generatlng beam 44 i~ directed to a reflecting axicon 46 whlch produces
the curved heated area.
The interferometric signal obtained from the above apparatus
30 may be analysed slmply for the detecting of cracks. On the other hand,
more complex signal processlng may be utili~ed. For instance, a spectro-
scopic analysis of the detected signal may be made taking advantage of
the selective reflectivity by the crack of shorter acoustic wavelengths
as well as time delay of the wave following the crack profile. Such an
35 analysis ls described by C.P. Burger et al, "Rayleigh Wave Spectroscopy
11 2i~ r
to Measure the ~epth of Surface Cracks , 13 Symposium NDE, San Antonio,
April 1981. Similar techniques can be used to analyse dispersive surface
features other than cracks. For example, the thlckness of a coating can
be evaluated from the phase delays of the different spectral components ~,
5 of the detected signal. Other possible application~ are the measurement
of the acoustic velocity and attenuation of the material.
The maximum frequency of the acoustic wave which can be gener-
ated by the method and apparatus described above is limited by the width
w of the laser-heated area 7 to approximately 30 ~UIz. A narrower heated
10 area 7 could be produced by increasing the aperture oE the optical sys-
tem, but this would make scanning more difficult. High-frequency acous-
tic waves may be required in some cases, such as when thln cracks must be
detected, or thin coatings must be inspected. These h~gh frequency sur-
face acoustic waves may be generated efficiently ln the apparatus
15 illustrated in figure 5. A laser 51 generates a beam 52, which is focus- -
sed by a lens 53 onto a beam splitter 54 that produces beams 52' and 52
which are reflected by mirrors 55 and 56 onto the surface 57 of the
material 58 being teste~. The beams 52' and 52 are directed to the same
irradiated region 59 in order to obtain an interference fringe on the
20 surface 57 of the material 58. An optional lens 60 is positioned in the ~,
path of beam 52' such that circular fringes occur which will produce a
converging surface wave travelling towards a probing spot in region 59.
In addition, laser 51 is amplitude modulated with a period TaC following
the resonance condition v = ~aC/Tac where v is the surface-acoustic~wave
phase velocity and ~ac is the surface acoustic wavelength which is equal
to the interfringe of the fringe pattern. Typical values for a mode-
locked laser 51 and a meta11ic surface are TaC ~ 5 nsec and ~ac ~ 15 llm,
which correspond to a frequency of the surface acoustic wave of 200 Mnlz.
The probing syseem cou]d be similar to the one described with
respect to figure 1, if the electronics of the interferometer 10 are suf-
ficiently fast. Another probing technique, which is also well known in
the llterature, ~ay be used as shown sche~atically in figure 5. The
probing laser 61 beam 62 is focussed by a lens 63 onto a probing area 64
which is larger than ~ac~ Beam 62 is diffracted by the surface-wave
train as it mo~es through area 64 towards a detector 65 with its readout
~2~ 2~
66. This probing technique relaxes the electronics speed requirements,
but it has a lower temporal resolution and requires a smoother surface
than the probing technique described with respect to figure 1.
Ihe fringe generating method of generating surface acoustic ;
waves is more complex and more difficult to scan than the earlier de-
scribéd focussing method, however, it is potentially more efficient
because very little power is coupled to bulk acoustic waves. The
resonance conditlon V = ~aC/Tac can be satisfied either for the surface
waves or for the bulk waves but not for both, because these two kinds of
waves have different velocit~es and wavelengths. Thus, nearly 50% of the
acoustic power goes into each of the counter-propagating surface waves,
providing a higher signal together with a lower spurious acoustic noise.
An even larger coupling efficiency is possible by eliminating
the counter-propagating surface wave. This can be done by scanning the
fringe pattern on the surface 57 to be inspected at the same speed a~ the
surface wave velocity. This would require a mirror rotating at very high
speed in order to follow the acoustic wave which travels at a speed in
the order of 3,000 m/sec on the surface. Alternately as shown in fig. 6,
two interfering beams 67 and 68 of different frequencies may be used to
obtain a displacement of the interference fringes within the laser-irra-
diated region 69 without any physically moving part~J The two coherent
beams 67, 68, of frequencies ~1 and ~2~ respectively, are superposed on
the surface 69 so as to produce an interference fringe pattern. If
~2> ~1 the intersection between the wavefronts, which corresponds to the
position of a bright fringe, moves towards the right as the wave pro-
gresses, as can be seen in fig. 6. If the speed of ~he bright fringe ls
the same as the velocity of the acoustic wave, a single acoustic wave
propagating towards the right will be generated. This condition can be
ac/ op (A2 ~ where fac is the frequency of the acoustic
wave and fpp is the frequency of the optical wave. Typlcal values are
fOp~ 3.10 Hz and fac= 5.10 Hz, which gives an interfringe ~ac ~ 60 ~m
and a wavelength shift (~2~ 1.7.10 . This can be obtained, for
example, by shifting the frequency of one of the two beams 52',52 with a
Bragg cell inserted in the path of one of the beams 52', 52 . A typical
Bragg cell with a carrier frequency of 50 M~lz would be suitable.
~2~ 2~ .
The above methods of generating surface acoustic waves provide
many advantages. For example, they provide 1~ an increased optlcal
detectability because of the self-amplificatlon of the convergent surface
acoustic wave as well as highly efficient frlnge generation of the sur-
5 face wave; 2) a reduced acoustic noise because of the enhancement of the ;'
surface wave signal together with a reduction of the coupling efficiency
to spurious bulk acoustic waves; 3) an increased resolution becàuse of
the narrow cross-section of the focussed acoustic wave; and 4) a reduced .
heating of the laser-irradiated surface because of the relatively larg~e
heated surface with respect to cross-section of the focussed acoustic
wave.
Many modifications in the above described embodiments of the
invention can be carried out without departing from the scope thereof and
therefore the scope of the present invention is intended to be limited
only by the appended claims.
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