DAMAGE INSPECTION AND EVALUATION IN THE WHOLE VIEW FIELD
USING LASER
A. Kato and T. A. Moe
Department of Mechanical Engineering
Chubu University
Kasugai, Aichi 487-8501, Japan
ABSTRACT
In this study, we investigated a method to evaluate fatigue damage of steels in the whole view field without contact using laser.
In the earlier stage of fatigue, slipbands are produced on a metal surface and the slipband density increases with progress of
fatigue damage. If a laser light illuminates the specimen surface, diffusion pattern of the reflected light from the specimen
surface changes due to surface change. Using this method, we are able to detect fatigue damage based on diffusion pattern
change due to surface property change caused by slipbands. We tried three types of optical setups to detect fatigue damaged
area in the whole view field of the observing area with CCD camera and evaluate fatigue damage. The diffusion pattern
change accompanied by the loading cycles was observed under constant stress amplitude with the three optical setups. We
investigated the possibility of detecting fatigue area and evaluating fatigue damage using the optical setups. The results
showed that there is a possibility of detecting fatigue damaged zone and evaluating fatigue damage by the optical setup using
slit beam of laser light
Introduction
In the case of fatigue in mild steels, slipband firstly appears on the surface of the test specimen and surface condition changes
as slipband density intensifies accompanied by increase of number of loading cycles. In our laboratory, we investigated the
methods on evaluation of fatigue damage as well as fatigue life estimation by detecting change in surface condition, which can
be seen by observing change in diffusion pattern while laser spot beam is applied to the place where fatigue damage occurred.
We can detect and evaluate fatigue damage at the early stage of fatigue before initiation of small cracks. This method can be
applied if we know fatigue damage zone but it is firstly necessary to find out the fatigue damage zone unless we know that
zone. It is necessary to do damage evaluation after detecting the fatigue damage spot.
In our study, we investigated about the evaluation methods and detection of fatigue damage zone of the whole observatory
part concerning fatigue of the same steel specimen. It is the most convenient making both detection of damage spot and
evaluation with one method, then we also tried about the possibility of this method. We observed the change in diffusion
pattern from the surface of the test specimen in the case of receiving fatigue damage with CCD camera by using the optical
system that spot beam, slit beam and collimated beam of laser are applied to the surface of the test specimen and scattered
light is observed. We tried to investigate fatigue damage detection with evaluation method in the whole view field of a camera.
Experimental Procedure
The material used in this experiment is a structural steel, JIS SS330. It is indicated chemical composition of the material in
Table 1 and mechanical properties in Table 2. The specimen used
is a strip with thickness of 3.1mm and slight round cut with the
Table 1 Chemical composition of the material (wt.%)
radius of 20mm at the central part as shown in Fig.1. The rolling
C
Si
Mn
P
S
direction of the material is longitudinal direction of the specimen.
0.05
0.01
0.29
0.012
0.007
Machine hardening and residual stress caused by machining
process were removed by normalizing (air cool after holding 30
minutes at 600℃). Then specimen surface was polished with
Table 2 Mechanical properties
emery paper its particle number is #800 as the final finish. We
Yield Strength
Tensile Strength
Elongation
made the experiment using a fluid servo system fatigue test
(MPa)
(MPa)
(%)
machine with the repeated stress speed of 20Hz and the sine
331
426
40
wave stress cycle maintaining maximum stress fixed and
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16
86
184
The experimental setups used in our study are 3 types
as shown in Fig.2. All light sources used are He-Ne
laser (output wave length = 632.8nm, beam diameter is
about 1mm, output power is 8mW). First, Fig.2(a)
shows the same optical system as the one we
previously used. Laser spot beam illuminates the
specimen surface and a diffusion pattern due to the
reflected light from the specimen surface is formed on
the ground glass placed in front of the specimen as a
screen. An image of the diffusion pattern is obtained by
a CCD camera and input into PC. The point applied by
spot beam is the point on the surface edge close to
notch root.
The optical system shown in Fig.2(b) shows laser slit
beam illuminates the specimen surface at longitudinal
direction and a diffusion pattern due to the reflected
light from the specimen surface is observed with a CCD
camera. We tried the fatigue damage detection in the
observatory area by scanning slit beam on the central
part of the specimen. Details will be explained later.
R20
t =3.1
26
minimum stress 0. At suitable intervals of repeated
stress cycles, stop the test machine and then
observation due to the optical system expressed later
and measurement of surface roughness were made for
the specimen. Surface roughness was measured to
longitudinal and perpendicular direction of the
specimen at the surface nearby central round cut edge.
Fig. 1 Test specimen
Collimated
beam
Slit beam
scan
(a) Spot beam
Specimen
(b) Slit beam
Specimen
(c) Collimated beam
Fig. 2 Detection method of fatigue damage
The optical system shown in Fig.2(c) shows the parallel light (beam diameter of about 70mm) due to spatial filter and
collimator lens illuminates the specimen surface and a speckle pattern image of the specimen surface is obtained by a CCD
camera and input into PC. Speckle pattern changes due to unevenness distribution of the surface profile and we can consider
that initial surface polished by emery paper and the surface, slipbands broken out due to fatigue have a great difference in
unevenness distribution and sense of direction.
Surface Condition and Change in Diffusion Pattern Due to Fatigue
In the case of stress amplitude σa=150MPa, change in surface profile diagrams and observatory results of each optical system
of Fig.2 due to number of loading cycles N are shown in Fig.3(a), (b), (c) and (d). N=0 is the initial condition. About the surface
profile diagrams of (a), we find that as increase in number of loading cycles N and surface roughness becomes gradually
larger due to increase in slipband density. (b) is the diffusion pattern of the reflected light from the specimen surface
illuminated by spot beam same as to the one used as in the past in our lab[1]. As increase in the number of loading cycles,
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point of the reflected light broadens from initial stage. In the case of N=15×10 , light intensity at the center declines at the
moment and diffusion pattern broadens. (c) is the diffusion light distribution of slit beam. It is the diffusion pattern in the case of
slit beam applied to the bottom part of the notch root. (d) is an illumination of collimated beam. The part, a dark stripe breaks
out in central part in horizontal direction is seemed to occur slipbands, and it can be seen that width of them broadens as
increase in number of loading cycles. According to this method, we can consider that fatigue damage part can be detected by
observing difference in brightness. If we compare this result to that of (c) of slit beam, it can be thought of that slipbands
appear at the central part of the straight-line shape reflected light of the slit beam. In that part, light is scattered and brightness
is declined. The length of the part where light is scattered becomes longer with increasing numbers of loading cycles and
corresponds to the broadness of slipbands in (d).
From these results, reflected light pattern from a spot beam broadens as N increases and fatigue damage progresses as
shown in Fig. 3 (b). This shows that fatigue damage evaluation at a point on the specimen is possible. It was found in our
previous study that evaluation of fatigue damage and fatigue life estimation is possible with this method. In (d), speckle pattern
on the specimen surface illuminated by a collimated beam differs in the areas where slipbands appeared and not appeared
and fatigue damaged area is visualized clearly on the camera image. Detection of initiation and expansion of the fatigue
damaged zone is possible by this method. It is not easy to evaluate amount of fatigue damage from only these images,
however. In (c), reflected light from a slit beam has a cut at the center where slipbands appear, comparing the images in (d).
This is because that that diffusion of reflected light from the specimen surface occurs at position where slipbands appear and
Z μm
(1) N = 0
1.25
1
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
0
0.25
0.5
0.75
1
1.25
0.75
1
1.25
0.75
1
1.25
10mm
10mm
5mm
(2) N = 3× 10 4
Z μm
l mm
1.25
1
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
0
0.25
0.5
(3) N = 15 ×10 4
Z μm
l mm
1.25
1
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
0
0.25
0.5
l mm
(a) Surface profile diagrams
(c) Slit beam
(b) Spot beam
(d) Collimated beam
Fig. 3 Change in surface profile and diffusion pattern by fatigue (σa=150MPa)
light distribution broadens and the maximum brightness of the reflected light decreases. Thus, brightness at the center
decreases by diffusion and line cut appears shown in (c). Diffusion at the center with slipband on the surface seems to differ at
different stage of fatigue. Analytical method for evaluating light intensity distribution was considered. We investigated this slit
beam method for detection and evaluation of fatigue damage next.
Fatigue Damage Evaluation by Slit Beam
We observed diffusion pattern change by slit beam of
laser. The experimental setup used is shown in Fig. 4. The
spot beam is changed to slit beam through a cylindrical
lens. Slit beam illuminates the specimen surface at seven
locations scanning the slit beam shown in Fig. 2 (a).
Reflected light from the specimen forms reflection pattern
on the ground glass. Reflected light intensities are
captured by a CCD camera as digital images and
analysed the image on a PC. Figure 5 shows examples of
the observed diffusion pattern. The figures are images
after superimposing the each image from seven different
slit beam. In the figure, (b) is the initial diffusion pattern.
There is no line cut in each reflected light line in (b). In (c),
the reflected light lines are cut very slightly at center.
Slipband appears firstly at the center from the notch root
as very thin line in the specimen. Surface property
changes by slipband. Reflected light diffuses at the
position where surface change occurred by slipband
caused by fatigue. The slight cut of the line in (c) shows
the location where slipband started to appear. As number
of loading cycles increases, the light cuts become longer
Specimen
CCD camera
Ground glass
Mirror
To PC
Cylindrical lens
Mirror
ND filter
He-Ne laser
Fig. 4 Experimental system
Micro-rotary stage
Fig. 4 Optical setup for slit beam illumination
(c) N = 0.5×104
(b) N = 0
(a) Position of slit beam
(d) N = 1×104
(e) N = 10×104
Fig. 5 Reflected light pattern of slit beam (σa=150MPa)
shown in (d) and (e) and we can know the fatigue damaged zone expands. In Fig. 5, it is found that line cut of the reflected
light of slit beam expresses diffusion of laser light reflected from the specimen surface and the location of the light line cut
corresponds to the location of slipband caused by fatigue damage on the specimen. We can find very clearly initiation and
location of fatigue damage of the specimen from the images shown in Fig. 5 and these images are very useful to visualize
fatigue damage on the specimen. Detection of the fatigue damaged zone is possible observing this kind of image. It is more
useful, if it is possible to evaluate fatigue damage based on these observations. We considered a method to evaluate amount
of fatigue damage from this observation result next
Figure 6 shows gray level distribution of reflected light from the specimen surface on horizontal direction vertical to reflected
light line as shown in the figure. Gray level distribution at the fatigue damaged zone is shown for different number of loading
cycles. The maximum gray level is high and the distribution has very peaky distribution at the initial situation (N=0). The
maximum gray level lowers and gray level distribution expands as N increases. It is clear from the figure that gray level
distribution of the reflected light at fatigue damaged zone changes as fatigue damage increases. We considered a method to
evaluate characteristics of the gray level distribution.
An example of the gray level distribution at diffused location of the reflected light is shown in Fig. 7 (a). Light intensity far from
the center should be zero, but the image from the CCD camera has small value. And gray levels at right and left side far from
the center are slightly different. This seems to be because that the laser slit beam does not illuminate on the specimen
perfectly vertical on the surface, but has a small angle from the vertical and reflection is slightly different at right and left side
of the illuminated position. The base gray level was obtained by linear approximation using gray levels far from the center and
moves base line to zero position (c). Horizontal coordinate, xc, of the centroid of the gray level distribution was obtained and
gray level distribution is approximated with the following equation,
g = ae
−b x − x c
(1)
wh =
ln 2
b
(2)
The parameter wh expresses width of the gray level
distribution. Relation between wh and N is shown in Fig.
9. From the figure, it is found that wh increases clearly
as N increases. The specimen fractured soon after the
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last observation at N=25.8 × 10 . The parameter wh
increases with N until fracture. We can conclude that wh
changes with progress of fatigue and it is possible to
Grey level g
where the coefficients a and b in the equation are obtained from the gray level values with the least squares method. Obtained
curve from the experimental data is shown in the figure (c). The figure shows the curve fit very well with the experimental data.
The coefficients a and b express characteristics of the
gray level distribution. Relation between the coefficients
250
a and b with number of loading cycles is shown in
Figure 9 (a). The figure shows that both coefficients a
and b decrease as number of loading cycles increases.
200
N=0
In Eq. (1), coefficient a is the maximum value of g at the
center. Width wh where g takes half of the maximum
N=5×104
150
value is expressed with the following equation （Fig. 8）.
N=0
N=15×104
N=25×104
100
N=5万回
N=１５万回
N=２５万回
Diffusion pattern
by slit beam
50
0
0
100
200
300
400
Position ( x pixels)
500
600
Fig. 6 Comparison of grey level distribution
use this parameter to express amount of fatigue damage.
Conclusions
120
100
G re y level g
In this study, we investigated a method to detect and evaluate
fatigue damage for steel specimen using laser. We tried three
kinds of optical setups illuminating laser light with (a) spot
beam, (b) slit beam and (c) collimated beam. It was found that
the method using slit beam has a possibility of both detection
and evaluation of fatigue damage in the whole view field
observing with a CCD camera In these optical setups,
We observed reflection light pattern of laser slit beam
illuminated on the specimen surface. We can find fatigue
damaged zone in the whole view field scanning the slit beam
on the specimen surface and succeeded to visualize fatigue
damaged zone with an image of the reflected light pattern. It
was clarified that gray level distribution of reflected light at
fatigue damaged zone changes with number of loading cycles.
We considered a method to evaluate the characteristics of the
gray level distribution. We derived a parameter to characterize
the gray level distribution of the laser diffusion pattern and it
was found that the parameter changes clearly with progress of
fatigue damage. We can use the parameter to evaluate fatigue
damage.
60
40
20
0
0
200
400
600
Position ( x pixels)
800
1000
(a) Grey level distribution
120
100
G re y le v e l g
From this study, it was found that we can use the method of
laser light reflection from specimen surface with slit beam to
visualize the fatigue damaged zone and detect easily fatigue
damage initiation and expansion of the area. There is a
possibility to use the method we developed in this study to
detect fatigue damaged position and also evaluate amount of
fatigue damage with one optical setup. This seams to be very
effective for actual application.
80
80
60
40
20
0
Acknowledgments
0
This research was supported by a grant from the High-Tech
Research Center Establishment Project from Ministry of
Education, Culture, Sports, Science and Technology, Japan.
200
400
600
Position ( x pixels)
800
1000
(b) Base gray level adjustment
100
References
2.
Kato, A. and Sano, M., “Fatigue Life Estimation Using
Diffusion of Laser”, Advances in Nondestructive
Evaluation, Trans Tech Publications, 793-799 (2004)
Kato, A., Kawamura, M., and Nakaya, I., Damage
Monitoring of Metal Materials by Laser Speckle Assisted
by Image Processing Techniques, JSME Int. J., Ser. A, 38
(2), 249-257 (1995).
80
G re y level g
1.
g = ae
60
− b x − xc
40
20
0
0
200
xc
400
600
Position ( x pixels)
800
1000
(c) Curve fitting with an exponential function
Fig. 7 Procedure for evaluation of gray level distribution
250
200
a
−b x−xc
g = ae
0.5a
0.08
a
b
g
150
0.06
Fractured
at N=25.8×104
100
0.04
50
wh
x − xc
0.02
0
0
0
Fig. 8 Calculation of parameter wh
5
10
15
20
N (×104)
25
30
(a) Coefficient a and b
160
140
120
wh
0
b
a
0.1
100
80
Fractured
at N=25.8×104
60
40
20
0
0
5
10
15
20
25
N (×104)
(b) Change of the parameter wh
Fig. 9 Change of parameters evaluating gray level
distribution in diffusion pattern
30