VISUALIZATION OF SURFACE-BREAKING TIGHT CRACKS
BY LASER-ULTRASONIC F-SAFT
M. Ochiai1, D. Levesque2, R. Talbot2, A. Blouin2, A. Fukumoto1 and J.-P. Monchalin2
1
Power and Industrial Systems R&D Center, Toshiba Corp., Yokohama 235-8523 Japan
Industrial Materials Institute, National Research Council Canada, Boucherville, PQ,
J4B 6Y4 Canada
ABSTRACT. A new method to obtain a detailed image of surface-breaking tight cracks by
using laser-ultrasonics is proposed. The surface opposite the cracking is scanned by the
generation and detection lasers. Detected signals are processed by the Fourier-domain Synthetic
Aperture Focusing Technique (F-SAFT). The point of this study is to use the laser-generated
shear waves, which interacts very effectively with the crack roots. A detailed image of the stress
corrosion cracks, having depths of typically 0.5 mm and widths of less than 0.05 mm, is
successfully reconstructed.
INTRODUCTION
Laser-ultrasonics has brought practical solutions to a variety of nondestructive
evaluation problems that cannot be solved by using conventional ultrasonic techniques
based on piezoelectric transduction [1,2]. Laser-ultrasonics uses two lasers, one with a
short pulse for the generation of ultrasound and another one, long pulse or continuous,
coupled to an optical interferometer for detection. Laser-ultrasonics allows for testing at a
long standoff distance and inspection of moving parts on production lines. The technique
features also a large detection bandwidth, which is important for numerous applications,
particularly involving small crack detection and material characterization.
For defect detection, the spatial resolution in laser-ultrasonics is dependent
upon the spot sizes of the generation and detection beams at the surface of the inspected
specimen, and could be inadequate for detecting small and deep flaws. If the laser beam is
focused onto a small spot, a strongly diverging wave is obtained, leading also to poor
resolution for the detection of deep flaws. In conventional ultrasonics, the detection of
small buried defects is achieved by focusing the acoustic field with lenses or curved
transducers or by using a computational technique that basically consists in performing the
focusing numerically. This method is known as the Synthetic Aperture Focusing Technique
(SAFT) [3].
SAFT is traditionally implemented by scanning a focused piezoelectric
transducer over the surface of the specimen and then processing the collected data array. In
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/$20.00
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addition, the coherent summation performed by SAFT yields an improved signal-to-noise
ratio (SNR). Originally developed in the time domain, SAFT can be advantageously
implemented in the frequency domain (F-SAFT). F-SAFT is based on the angular
spectrum approach [4-6], which allows a significant reduction in processing time as
compared to time-domain SAFT.
In this paper we are reporting the use of laser-ultrasonics for collecting the
ultrasonic data. We are presenting a highly sensitive laser-ultrasonic system combined with
F-SAFT. The technique is then applied to data collected on an stainless steel plate with
stress corrosion cracking (SCC). It will be shown that SAFT data processing improves the
ability to detect and size defects.
PRINCIPLE OF SAFT
We assume that the generation and detection beams are focused at the same
location onto the surface. By scanning the beams or moving the part fixed on an X-Y
translation table with discrete and equal steps, a 2-D mesh at the surface of the specimen is
obtained. As shown in figure 1, if a pointlike flaw is present at point C located at a depth z
within the sample, this flaw re-radiates the acoustic field originating at point MI. The
acoustic signal S(Mi,t) received at any point MI in the measurement mesh exhibits a peak at
time t = 2di / v, where v is the longitudinal acoustic velocity in the material and di the
distance CM;. Consequently, the summation:
S(C)=
X
Mj e mesh
s(M,,t = —— )
^
V
(1)
'
separates the points C where superpositions build up and flaws are present, from the points
C where no coherent superposition occurs and the material is sound. Moreover, the
function Z(C) increases the signal-to-noise ratio for the detection of flaws by the factor >/N ,
where N is the number of points MI in the measurement mesh aperture (the synthetic
aperture).
This simple reconstruction algorithm is a phase correction technique since it
performs the summation of signals shifted in time. The time shift of each signal is a
function of the point MI where the signal is collected and of the point C where one wants to
check the presence of a flaw. It can be shown that the lateral and depth resolutions obtained
from the SAFT algorithm are respectively:
z
vA t
Ax^vAt- ,
A z ^ ——
(2)
where At is the ultrasonic pulse duration and a is the dimension of the synthetic aperture.
While maintaining the depth resolution of the conventional pulse-echo method, the SAFT
processing leads to improved lateral resolution. In practice, the strength of ultrasonic wave
emission and the detection sensitivity both decrease away from the normal to the surface,
so that the total opening angle of the synthetic aperture is limited to roughly 60° , which
means a ~ z.
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a
d
FIGURE 1. Principle of SAFT.
This data processing approach, while straightforward in its principle and
implementation, is not very efficient and is very computation intensive. An alternate and
better approach is to perform the data processing in the Fourier domain (F-SAFT) and to
benefit from the Fast Fourier Transform algorithm to reduce the processing time. The FSAFT is a backpropagation technique which is based on the angular spectrum method of
the scalar diffraction theory. Starting from the acoustic field S(x,y,z = 0,t) at the sample
surface of the measurement mesh (axes x and y), a 3-D Fourier transformation is performed
with respect to variables (x, y, t) into a 3-D Fourier space represented by variables
(a x ,a y ,f). Physically this consists in representing each acoustic field at f by a superposition
of plane waves with wavenumbers of components ox, ay. Then, assuming a flaw located at
depth z, the transformed field S(a x ,c y ,z = 0,f) is backpropagated to the source plane as
follows:
f2
(v/2) 2
(3)
The exponential function in equation (3) is called the backpropagator and is
actually a phase shifter for the propagating waves. Note that the backpropagator is the same
for all points at the same depth z. For simultaneously scanned generation and detection (see
figure 1), the acoustic propagation distances are doubled and this is accounted for in the
backpropagator by a velocity reduced by the factor of 2. After backpropagation to the
source plane, a summation over temporal frequency components f is performed as follows:
(4)
where Q is the actual frequency bandwidth of the system. Finally, an inverse 2-D Fourier
transformation of E(o x ,a y ,z) with respect to variables (a x ,a y ) into (x,y) yields the function
Z(x,y,z). A point C (at position x, y, z) will correspond to a flaw if the summation £ at this
point exhibits a peak. This processing method is also presented in more details in
references [3-6].
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FIGURE 2. Experimental set up for laser ultrasonic SAFT.
DESCRIPTION OF THE SETUP
An experimental configuration of laser ultrasonic SAFT is outlined in Fig.2.
Ultrasonic waves were generated by Q-switched Nd: YAG laser pulses having a wavelength
of 532nm, a pulse duration of about 10ns and a repetition rate of lOHz. Pulse energies of
75mJ were delivered to the sample surface via some optics in air. These energies are quite
intense but are still below the threshold for optical fibre delivery [7]. A lens was used to
focus the laser pulses onto the sample surface with a diameter of about lOOjam. The fluence
at the surface was estimated about IkJ/cm2 and this produced ablation plasma at each
irradiated spots. This method of excitation simultaneously generated a various ultrasonic
modes; surface-skimming longitudinal waves (P), Rayleigh waves (R), bulk longitudinal
waves (L), bulk shear waves (S) in the sample medium and shock waves (A) originated
from the expansion of ablation plasma in air. These waves except the shock wave had
frequencies of, typically, 0.5MHz to 20MHz and respective particular directivities.
Ultrasonic waves generated by the incident laser pulse and travelling in the
medium were detected using an optical detection system based on a confocal Fabry-Perot
interferometer [8]. Two multimode optical fibres having a core diameter of 400jum were
employed; one fibre was used to deliver light from the detection laser to the sample surface,
while the other one was used to bring scattered light back to the interferometer. The
detection laser was a fundamental NdiYAG laser having a wavelength of l,064nm. This
detection laser emitted long pulses (typically, 70|is) with highly stabilized frequency and
high peak power (typically, IkW). The confocal Fabry-Perot is 1m long and has an optical
bandwidth of from 2MHz to 12MHz, approximately. A PIN photodiode detected the light
demodulated by the Fabry-Perot. The detected
signal was high-pass-filtered with a cut off
frequency of 1MHz and amplified by 40dB.
The signals were sampled with a sampling
rate of lOOMsample/s and stored for the
signal processing in a personal computer. This
computer also controlled the synchronization
between the emissions of two laser sources
and the scan of the sample.
A sample to be tested was fixed on a 3dimensional scanning stage (x-, y-, zdirections as shown in Fig.2). This stage FIGURE 3. SCC visualized by PT.
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enabled to adjust focus (z-direction), to scan the sample 2-dimensionally. The step size of
the scan was 0.2 mm and the scanned area was about 40 x 40 mm.
The tested samples were made from 100mm(L) x 50mm(W) x 10mm(D)
stainless steel plates and contained a few stress corrosion cracks. These SCCs, typically
each are less than O.lmm wide, was introduced by boiling in 42% MgCl2 solution with
some extent of loaded stress. Typical surface image obtained by liquid penetrant testing
(PT) is shown in Fig.3.
The samples were interrogated from a surface opposite to the cracking. For
each node MI of the measurement mesh, an A-scan was collected, digitized and stored in
the computer memory. The data was processed by specially developed software for the FSAFT algorithm.
Generation
1 st bottom echo
2nd bottom echo
EXPERIMENTAL RESULTS
I
Typical A-scan obtained on
the sample is shown in Fig.4. This signal
was filtered with the high-pass filter
(/c=lMHz) and temporally averaged 4
times. The large perturbation observed at
Time (lus/div)
the beginning of the waveform was
initiated by the arrival of S-wave and A- FIGURE 4. Typical A-scan obtained on SCC sample.
wave. It continued for about Ijis. The
detection system will not sense any other signal during the perturbation; the duration can be
however minimized by optimizing the generation laser energies and the separation between
generation and detection.
The data were processed using the F-SAFT algorithm. C-scans acquired on the
two different samples and reconstructed by using both L-wave velocity and S-wave velocity
are compared to the images by PT in Fig.4. Each C-scan was obtained by selecting the
peak-to-peak value from each A-scan in a narrow gate at depths corresponding to the
cracking surface. It is found that F-SAFT provide very fine surface images containing SCCs.
The detectability of detailed structures of F-SAFT can exceed that of PT, which is well
known as the most sensitive method to visualize fine cracks. When compared to the FSAFT processed images using L-wave (F-SAFT(L)) and S-wave(F-SAFT(S)), signal-tonoise ratio (SNR) of F-SAFT(L) is slightly better than one of F-SAFT(S). Here, SNR was
estimated as the ration between the peak height of the crack indication and of its neighbor
noises. However, the spatial resolution of F-SAFT(S) is higher than of F-SAFT(L) as
shown in Table 1. The spatial resolution of 0.2mm is limited by the step size of the
scanning, not by the signal processing.
TABLE 1. Summary of laser-ultrasonic SAFT.
F-SAFT(L)
F-SAFT(S)
SNR
(dB)
Half width
(mm)
SNR
(dB)
Half width
(mm)
Sample#54
25
0.8
16
0.2
Sample#56
24
1.0
19
0.2
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F-SAFT(L)
F-SAFT(S)
PT
Sample
#54
Sample
#56
FIGURE 5. Results of laser ultrasonic F-SAFT on SCC samples.
It should be mentioned that the SAFT processing cannot be performed in
real-time since it requires all data from the measurement mesh to operate. As an example,
the F-SAFT processing time for this experiment was about 7 min on a PC for an initial
data set of 401 x 401 A-scans of 3200 datapoints and for reconstruction at 200 depths.
CONCLUSION
We have reported on the use of SAFT data processing to improve the spatial
resolution and the detectability of laser-ultrasonics. The laser-ultrasonic technology is well
suited for SAFT data acquisition since laser beams are easily scanned. Moreover, laserultrasonics is a non-contact inspection technique that can be used on parts of complex
shapes. In our setup, the two laser beams, which act as the acoustic source and receiver,
were focused onto the surface at about the same location and the parts were moved. Other
configurations where the two laser beams are offset from each other or are not
simultaneously scanned can also be easily implemented. This work has shown that SAFT
data processing results in an improvement of the lateral resolution of laser-ultrasonics. In
addition, the averaging performed by SAFT processing increases the capability of defect
detection. We have also shown that SAFT processing can be used to resolve the contour of
a non-planar back surface. With prior knowledge of the part shape, the laser-ultrasonic
SAFT technique can also be applied to samples with non-planar front surfaces.
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REFERENCES
1. Scruby, C. et al., Laser-ultrasonics: techniques and applications, Adam Hilger, Bristol,
UK (1990).
2. Monchalin, J. -P., Review of Progress in Quantitative Nondestructive Evaluation 12A,
495, Plenum, NY (1993).
3. Lorraine, P.W. et al., "High resolution laser ultrasound detection of metal defects,"
Review of Progress in Quantitative Nondestructive Evaluation 16, 555 (1997).
4. Mayer, K. et al., Ultrasonics 28, 241 (1990).
5. Busse, L.J., "Three-dimensional imaging using a frequency-domain synthetic aperture
focusing technique," IEEE Transactions UFFC 39, 174 (1992).
6. Teo, TJ. et al., Ultrasonic Imaging 8, 213 (1986).
7. Schmidt-Uhlig, T. et al., "Laser shock processing with 20 MW laser pulses delivered by
optical fibers," Eur. Phys. J. AP., 9 (2000) 235
8. Monchalin, J.P., et al., IEEE Trans. Ultrason. Ferroelectr. Frequency Contr., UFFC-33,
(1986) 485-499
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