QUANTITATIVE EVALUATION OF A CRACK INSIDE OF
PRESSURE PIPELINE BY SHEAROGRAPHY AND ESPI
Koung-Suk Kim1, Yong-Hun Cha1, Ki-Soo Kang2, Man-Yong Choi3 and Dong-Pyo Hong4
J
Dept. of Mech. Eng., Chosun Univ., 375 Seosuk-dong, Kwangju 501-759, South Korea
Graduate school, Chosun Univ., 375 Seosuk-dong, Kwangju 501-759, South Korea
KRISS, POX 102 Yuseong, Daejeon 305-600, South Korea
4
Dept. of Mech. Eng., Chonbuk National Univ., Duckjin-dong, Jeonju, 561-756 South Korea
2
3
ABSTRACT. Electronic Speckle Pattern Interferometry (ESPI) and Shearography for quantitative
analysis of a crack inside of pipelines are described. Shearography is used widely for non-destructive
inspection because of less sensitivity to environmental disturbance and simple interferometer.
However, it is difficult to determine the defect size quantitatively because there are so many effect
factors- shearing distance, shearing direction, and induced load, which depend on operator's skill and
crack information. These effective factors in Shearography are optimized for quantitative analysis and
the size of crack is determined. And also, ESPI is used for determination of crack size quantitatively.
In ESPI only induced load play an important role and the effective factor is independent on
information of a crack. The displacement of object surface is obtained three-dimensionally and
differentiated numerically, which is same to result of Shearography so that crack size can be
determined quantitatively without any information of crack.
INTRODUCTION
Speckle interferometry allows us to measure surface displacement and vibration
amplitude of an object. Since Butters and Leendertz introduced CCD camera and filtering
techniques into speckle interferometry, a fringe pattern contouring surface displacement
and vibration amplitude can be obtained in quasi-real time, non-contact and whole filed
[1]. This method is known as Electronic Speckle Pattern Interferometry (ESPI) or
Shearography and has been developed by several authors using analog and digital signal
processing techniques. Generally, Shearography is used for non-destructive inspection. It
is less susceptible to environmental disturbance than ESPI or holography because of
directly spatial derivatives measurement of surface displacement and also the method
allows a simple interferometer [2-4]. Also, Y. Y. Hung [4] compared ESPI with
Shearography for non-destructive inspection and the paper presented Shearography is
more practical than ESPI. Although there are many merits for qualitative inspection, so
many effect factors-shearing angle, shearing direction, induced load are existed for the
quantitative analysis of inside defect and also, these factors depend on inspector's skill
positively. However, ESPI is independent on information of a crack and only induced load
(pressure) play an important role. In this paper, in order to detect an inside crack
quantitatively ESPI is used to measure three-dimensional displacement, which is
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
939
differentiated with simple numerical processing, which is equal to result of Shearography.
Experiment reveals how much effective these factors to determine the size of inside crack
is and the crack is determined quantitatively from these factors optimized to artificial crack
inside of pipeline. This paper will give a thorough review of two major optical methods for
quantitative analysis of inside crack: ESPI and Shearography.
BACKGROUND
Electronic Speckle Pattern Interferometry(ESPI)
Holographic interferometry has powerful merit in measurement of surface
displacement. A hologram has fringe pattern, which represent the relative displacement of
the object surface when an object is loaded. However the process of analysis is very
complex. Several speckle techniques have been developed in which recording and
reconstruction processing is fairly simple. ESPI is one of the methods. When two coherent
lights represented by complex are superimposed on the image plane, intensity at a point is
given by [5]
Where, A0, AR: Amplitude of object and reference beam respectively
</)0 , (f)R : Phase of object and reference beam respectively
When the surface is displaced, the phase of all the components scattered is changed by the
same amount A0 so that the intensity is given by
I2=I0+IR+ 2-7/A cos% -<t>0 -A0)
(2)
If the output camera signals Vl and V2 are proportional to the input image intensities, then
the subtracted signal is given by [5]
Where, <p = </>R-<f>0', this signal has negative and positive values. The television monitor
will however, display negative going signals as areas of blackness; to avoid this loss of
signal Vs is rectified before being displayed on the monitor. Out-of -plane displacement of
object d^ is related to the phase difference, which is given by
A
has the general form
940
(4)
Laser
Laser
CCD camera
(a) In-plane displacement sensitive interferometer
Object
(b) Out-of-plane displacement sensitive interferometer
FIGURE 1. Speckle pattern correlation interferometer.
(5)
Where no is Illumination direction vector; n v is Viewing direction vector; so that the
fringe pattern represents in-plane as well as out-of-plane displacement. In-plane and outof-plane arrangement of speckle pattern correlation interferometer is shown in figure 1.
Displacement of each axis ( x , y , z ) is measured by 3D ESPI system within very short time
sequentially. The deformations are merged and imaged 3-dimensionally. Also, strain/stress
analysis through numerical process is done.
Shearography
Shearography measure the first derivative of static displacement gradients, of
which interferometer is shown in figure 2. The object is viewed through the Michelson
interferometer arrangement by CCD camera. The mirror lies parallel to the CCD camera
and shearing mirror is inclined a small angle. Two images are therefore formed in the CCD
plane as a result of the wavefront shearing action of this viewing arrangement. The
intensity at a point in the image plane corresponds to the superposition of the light
scattered from two adjacent points in the original object. When the object is deformed, an
arbitrary point (x, y) on the object surface is displaced to (JC + H, y + v, w) , and a
neighboring point (x + 6x, y) is displaced to (x + &c + u + Su, y + v + Sv,w + Sw) .
Deformation of the object introduces a phase change between two neighboring points. In
an optical set-up designed to make the angle subtended by the imaging lens at the object
surface, the expression for the relative phase change is given by the following: [6]
=
-(l + cos 0 ) ( x , y) + sin 0-(x, y)| &c
ox
dx
941
'J
(6)
Laser
FIGURE 2. A basic arrangement of speckle pattern shearing interferometer.
EXPERIMENT
Figure Pressure piping system with various depths of inside cracks is designed, which
consist of pipeline, pressure valves and pressurizing system as shown in figure 3. The artificial
cracks are parallel to pipe axis with 12 mm length and 1, 2, and 3 mm deep. Pressuring gas (Ni
gas) is regulated by precise valves, values of which are read by pressure gauge. The pipeline to
be inspected is loaded with inner pressure change. The piping will expand only by some
micrometers. In the areas with a weakened wall, e.g. caused by crack or corrosion, the wall of
the pipeline is deformed by the pressure more than in areas without defect. The difference
between a loaded and an unloaded state provides information about the deformation of the wall
of the pipeline. The deformation gradient is obtained by deriving the deformation curve in the
axis of the pipeline with Sheraography system and also, the displacement is obtained directly
with ESPI system. In experiment, the pipeline is inspected on whole-filed, total volume and if
we find any crack, intensively inspected around crack. An image of the pipe is recorded and
stored as a reference image by an image processor. All following images are subtracted in
image processor with real time from the reference image and the result is displayed in a video
monitor. In order to analyze the defect quantitatively, phase shifting and unwrapping method
are used, where four phase shifted speckle interferograms are generated by the difference
between speckle patterns before and after deformation.
Crack depth
1,2,3 mm
FIGURE 3. Pipeline system for the classification of effective factors.
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RESULTS
Quantitative Analysis by Shearography
In experiment the pipeline is inspected on whole-filed, total volume and if we find any
crack, intensively inspected around crack. Figure 4 shows shearogram according to shearing
distance. And figure 5 shows line profile of shearography fringe pattern that is obtained by
phase shifting and unwrapping technique. And the crack size is determined by measuring peak
to peak. The length between two peaks is varied according to change of effective factor. Figure
7(a) shows relation between detected crack size and shearing distance. Known crack size is
12mm. From this result we found that in the case that shearing distance is within crack size
crack, size is detected very well. However, in the case that shearing distance is more than crack
size, the crack is much oversized this result says that shearing distance is important effective
factor. Figure 7(b) shows relation between detected crack size and normalized pressure. It is
optimum in 10% of allowable pressure. Induced pressure has influence on determination of
crack size but its influence is so little. Fortunately, ESPI need jut this effective factor. The
shearing direction must be paralleled to crack direction and the determination of the direction is
very ease.
(a) 5 mm
(b) 10 mm
(c) 12 mm
(d) 15 mm
(e) 18 mm
(f) 20 mm
FIGURE 4. Shearogram according to shearing distance.
FIGURE 5. Determination of a crack size by Shearography.
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24
-oD/tO.4
-n-D/tO.6
21
18
15
12
10
12
15
Shearing distance(mm)
(a) Shearing distance Vs. Crack size
18
20
24
21
I 18
-o-D/tO.4
-o-D/tO.6
1.15
| 12
| 9
6
3
0
10
P/AP(%)
15
20
(b) Normalized pressure Vs. Crack size
FIGURE 6. Determination of crack size according to these factors (shearing distance, induced load) change
(t: Wall thickness, D: Crack depth, P: Induced pressure, YP: Yield Pressure of STS304 (210 kg/cm2)).
Quantitative Analysis by ESPI
If ESPI is used for quantitative analysis of a crack, there is only one factor- induced load.
We need not adjust any optical component. ESPI can measures displacement, and shearography
measure derivative of displacement directly. And these methods have relation in first derivative.
Suppose that ESPI fringe pattern is differentiated numerically, we can get same result to
shearogram. 3D ESPI system used can do measure 3 dimensional displacement (x,y,z)
quickly. The optimized loading (10 % of yield pressure) is induced and each displacements of
the pipeline surface are merged and differentiated by numerical processing. Figure 7 shows line
profile of displacement from ESPI and surface is caved in around crack. If so, we can
determine the crack size from the first derivative of displacement by measuring peak to peak in
same way of shearography. In ESPI, we need just one effective factor induced load and simple
numerical processing. The crack size is evaluated with good agreement.
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0.216_
0.404
0.119 _
0.313 _
0.022 _
0.222 _
-0.076 _
0.131 _
-0.173 _
0.040 _
-0.270 _
-0.051 _
-0.367 _
-0.142 _
-0.464 _
-0.233 _
-0.562 _
-0.324 _
-0.659 _
-0.415 _
-0.756 _
X=9.26
13.38
17.50
21.61
25.54
29.66
33.78
37.89
42.01
46.13
50.62
-0.506 _
X=1 1.13 14.88
18.62
22.36
25.92
29.66
33.40
37.15
40.89
44.63
48.37
6
-2.16
-2.16
-2.16
-2.16
-2.16
-2.16
-2.16
-2.16
-2.16
(a) Deformation profile
(b) The first derivative of (a)
FIGURE 7. Surface profile of ESPI interferogram of inside crack
Comparison of ESPI interferogram and shearogram
Figure 8 shows the line profile of shearogram (Shearography), Integration of
Shearogram, and ESPI interferogram (ESPI). Line profile of shearography is integrated
and compared with that of ESPI, which are almost agreed.
0.5000
0.4000
0.3000
-Shearography
- Integration of shearogram
-ESPI
'E 0.2000
|
•| o.iooo
J o.oooo
ts
I -o.iooo
c2
(2 -0.2000
-0.3000
-0.4000
-0.5000
Position(mm)
FIGURE 8 Comparison of line profile of two methods.
CONCLUSIONS
In this study, Shearogrpahy and ESPI are used for quantitative analysis of a crack
inside of pipeline and all of them are suitable to detect inside crack qualitatively. However
for quantitative analysis, Shearography needs several factors- shearing angle (shearing
distance), shearing direction and induced load. These factors is optimized by experimental
of which conditions are in the cases that crack size is equal to shearing distance and
shearing direction is parallel to crack direction and 10 % of yield pressure is induced.
945
Shearing distance and shearing direction are related to information of crack to be detected.
From these results, we find that in the case of an unknown defect, it is difficult to
determine the shearing distance exactly. However, ESPI is independent on information of
a crack and only induced load (pressure) play an important role. In order to detect an inside
crack quantitatively, ESPI is used to measure three-dimensional displacement and the
information is differentiated with simple numerical processing, which is equal to result of
Shearography, so that crack size can be determined by ESPI quantitatively without any
information of crack.
ACKNOWLEDGMENTS
This study was supported by "New Technology Development and Research Center
of Laser Application" operated by Chosun University and deigned by Regional Research
Center of Korea Science and Engineering Foundation (KOSEF) and Ministry of Science
and Technology (MOST), 2002.
REFERENCES
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2.
3.
4.
5.
6.
Jones, R. and Wykes, C., Holographic and Speckle Interferometry, Cambridge
University Press, Cambridge, Massachusetts, 1983, pp. 165-196.
Ettemeyer, A., Nuclear Engineering and design, 160, 237-240, (1996).
Toh, S. L., Chau, F. S., and Sim, C. W., Journal of Modern Optics, 44, 2, 279~286,
(1997).
Hung, Y. Y., Optics and Lasers Engineering, 26, 421 ~436, (1997).
Cloud, G. L., Optical Methods OF Engineering Analysis, Cambridge University
Press, Cambridge, Massachusetts, 1995, pp. 453-476.
Rastogi, P. K., "Measurement of Static Surface Displacements, Derivatives of
Displacement, and Three-Dimensional Surface Shapes-Examples of Applications to
Non-destructive Testing" in Digital Speckle Pattern Interferometry and Related
Techniques, edited by Rastogi, P. K., JOHN WILEY & SONS LTD, New York,
2001, pp. 141-224.
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