System and Method for Ultrasonic Image Reconstruction Using
Mode-Converted Rayleigh Wave Technique
Dong-Man Suh1, 2 , Young-Hwan Ryu2 , Young-Kwun Chang3
Je-Jung Sung4
1
Kunjang College, Sungsan-myun Kunsan, Chonbuk 573-709, Korea
Raynar, HTVC 5114 KAIST Yusung-Ku Daejon, 305-701,Korea
3
KTL, Seoul,Korea, 4 Sae-An Eng., Seoul, Korea
2
ABSTRACT. In conventional ultrasonic imaging methods, the specular reflections are usually used
for image reconstruction. However, these techniques are not efficient in some cases due to multiple
specular reflections, and Mode-Converted acoustic waves. In this study, Mode-Converted Rayleigh
wave (MCRW) technique for ultrasonic characterization of defect is presented, where Rayleigh waves
traveling along the surface of strong scatters in the materials are used for reconstruction of inclusions
with pitch-catch method. To show the validity of the proposed method, test specimens with various
kinds of artificial reflectors, void-like defects, and flat-bottom holes are used.
INTRODUCTION
The size, shape, and orientation of a crack in materials are important for the failure
prediction in nondestructive evaluation (NDE). However, it is still a difficult problem to
determine the size and nature of flaws. Recently, a small crack detection and sizing technique
using MCRW was proposed [1-2]. By the technique, it is possible to detect a crack as small as
1-2mm. In [3], it was shown that ultrasonic image reconstruction can be performed
successfully by MCRW. In conventional ultrasonic imaging methods, the specular reflections
are usually used for image reconstruction. However, these techniques are not efficient in some
cases due to multiple specular reflections, and Mode-Converted acoustic waves [2]. In
ultrasonic imaging, the resolution is sometimes poor as the wavelength of inclusion
approaches the dimensions of the object. This situation occurs when the inclusion to be
characterized is small. There are many techniques for image reconstruction. The Born
inversion technique is still under debate. The major problem in the Born inversion appears to
be in matching the bandwidth of the transducer used to the flaw being examined. This study
describes a method of defect detection and sizing by MCRW in the back-scattered direction in
the circularly arrayed pitch-catch method. When a beam is directed to a crack, there are
reflection, refraction and diffraction from the crack. If the crack surface is smooth, the
specular echo and the MCRW are measured since the time-delay between the two waves is
relatively large. This large time-delay is due to the fact that the MCRW is slower than the
incident wave. In addition, deconvolution method [2, 4] is used to remove the specular
reflections from a crack. Section II describes inclusion sizing by MCRW traveling along the
surface. Image reconstruction using MCRW technique is proposed in section III. Finally, brief
conclusions are given in section IV.
Mode-converted Rayleigh Wave Technique
Fig. 1 schematically shows Rayleigh wave method for crack sizing in the test block. It is
assumed that the distance from the transducer to the crack is large compared with the height
(or diameter) of the flat bottom hole (FBH), and that the beam width of the transducer is
narrow. Also, the incident beam is assumed to be perpendicular to the crack surface. Then, the
signals received at the transducer T is: the specular reflection ① and the trailing Rayleigh
wave ②. In addition, the signals received at the transducer R is: the specular reflection ④ and
the trailing Rayleigh wave ③.
FIGURE 1. A Schematic showing the MCRW
The pulse/echo method uses one transducer (transducer T), but the pitch/catch method uses
two transducers (transducers T and R). As can be seen from Fig. 2, the signal from a crack
(hole) by pulse/echo is mainly specular wave because the Rayleigh wave is weak and close to
the specular reflection.
FIGURE 2. Specular reflection signals from 3.2 mm FBH by pulse/echo.
FIGURE 3. Specular and Rayleigh wave signals by pitch/catch.
On the other hand, as can be seen from Fig. 3, Specular and Rayleigh wave signals can be
differentiated in pitch/catch method. In pulse/echo method, the amplitude of Rayleigh waves
is much smaller than the specular reflections. However, in pitch/catch method, the amplitude
difference between the two waves is not large. Actually, the amplitude difference depends on
the experimental situations and it is important to keep the difference between the two waves as
small as possible for better ultrasonic imaging.
In pulse/echo method, the time of flight can be expressed as follows (Fig. 4):
I) Beam width < crack size: Specular: 2l / VL, Rayleigh: l / VL + (1/ cosΘ) / VL + (h/2)/VR
II) Beam width > crack size: Specular: 2l / VL, Rayleigh: l / VL + (1/ cosΘ) / VL + h / VR
where VL and VR means the velocity of specular wave and the velocity of Rayleigh wave
respectively.
FIGURE 4. Pulse/echo method: (a) beam width < crack size h, and (b) beam width > crack size h.
In pitch/catch method, the time of flight can be expressed as follows (Fig. 5):
I) Beam width < crack size: Specular: 2l / VL, Rayleigh: l / VL + (1/ cosΘ) / VL + (h/2) / VR
II) Beam width > crack size: Specular: l / VL + (1/ cosΘ1) / VL , Rayleigh: l / VL + (1/ cosΘ2) /
VL + h / VR
FIGURE 5. Pitch/catch method: (a) beam width < crack size h, and (b) beam width > crack size h.
Ultrasonic reconstruction by MCRW
To obtain a good MCRW image, it is important to minimize the effect of specular echo
on MCRW. In this paper, we use deconvolution technique to suppress the effect of specular
echo on MCRW [5]. The ultrasonic imaging method by MCRW is proposed as follows:
1. The highest specular echo from the center of the reflector surface is arranged at the image
center.
2. Each Rayleigh wave from the circularly located receiver R is processed by deconvolution.
3. Each Rayleigh wave from step 2 is arranged to the image center (MCRW is orthogonal to
the axial beam direction).
To show the validity of the proposed method, test specimens with flat-bottom holes with the
following dimensions are tested using the testing system shown in Fig. 6: 4×10×5, 2×10×5,
1×10×5, 0.5×10×7.5, 0.5×10×5 and 0.5×10×2.5 mm. The experimental conditions are
summarized as follows:
Pulse/echo mode set-up
·Transducer: 10Mhz, 1/4 Inches Round
·Single cycle burst step, Cycle per burst:1
·Burst width: 0.1 µ sec, Gated Amp control: 1.28V
·Diplexer Preamp gain: 20dB, RA-30 Attenuator 24dB
T/R Mode Set-up
·Transducers: 10Mhz, 1/4 Inches Round
·Single cycle burst step, Cycle per burst: 1
·Burst width: 0.1 µ sec, Gated Amp control: 1.28V
·High-impedance Preamp: 20dB, RA-30 Attenuator: 0dB
Fig. 7 shows the sizing of 4×10×5 mm FBH and it can be seen that the slit of 4 mm is pretty
accurately measured as 3.8 mm (length = time × VR). Fig. 8 shows the sizing of 2×10×5 mm
FBH and it can be seen that the slit of 2 mm is also pretty accurately measured as 1.74 mm.
Fig. 9 shows that the slit of 1 mm is measured as 0.87 mm.
FIGURE 6. Testing system (Ritec Ram 1000 system)
FIGURE 7. The sizing of 4×10×5 mm FBH. (P/E:pulse/echo, T/R:pitch/catch method)
FIGURE 8. The sizing of 2×10×5 mm FBH. (P/E:pulse/echo, T/R:pitch/catch method)
FIGURE 9. The sizing of 1×10×5 mm FBH. (P/E:pulse/echo, T/R:pitch/catch method)
FIGURE 10. Reconstructed image of 4.0 mm cylinder (P/E:pulse/echo, T/R:pitch/catch method)
FIGURE 11. Reconstructed image of FBH with 2×10×5.
To obtain the MCRW image, transducers are scanned circularly. Fig. 10 shows the restored
image of a FBH of 4.0 mm cylinder. It can be seen from Fig. 10 that the proposed method
performs pretty well for the cylinder-like FBH. The Fig. 11 shows the reconstructed image of
FBH with 2×10×5 mm.
In our method, specular reflection signal by the pulser/echo method is used as reference signal.
By applying a deconvolution operator to the signal, the restored signal is obtained [5]. Then,
3-dimensinal FBH image by MCRW image method is obtained.
From this method, the defect size can be successfully predicted. In the restored image, the
Rayleigh wave signals are arranged linearly by 1mm increment.
CONCLUSIONS
An image reconstruction method based on MCRW technique was proposed. By the
proposed method, each Rayleigh wave signals can be reconstructed from the specular
reflection signals included Rayleigh wave by pitch/catch although the Rayleigh wave signal
amplitude is much weaker than the pulser/echo signal. By experiments, it was shown that an
image could be successfully restored by MCRW image method. This method also provides
accurate prediction of the defect size.
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