Strain Monitoring by Evanescent Wave Spectroscopy
V. Kapilaa, L. Kjerengtroenb, W. M. Crossc, F.J. Johsona, JJ Kellar*c
South Dakota School of Mines & Technology
Science & Engineering Program
bDepament of Mechanical Engineering
cDepament of Materials & Metallurgical Engineering
Abstract
Prior interferometric (strain/temperature) fiber sensors and evanescent wave (chemical) fiber sensors have proven
useful in obtaining information about portions of the lifetime of a composite material. The overall goal of this research is to
develop an infrared (IR) evanescent wave sensor system that can be used to monitor lifetime of a polymer matrix composite.
In this regard, a single fused silica core fiber was placed across a miniature materials tester (MINIMAT), while
simultaneously having the fiber ends attached to an infrared spectrometer. The fiber was strained in increments by the
MLNIMAT, while the IR spectrometer allowed simultaneous determination of the JR spectrum. An increase in baseline
absorbance across the entire JR spectrum occurred as the strain increased. The increase in absorbance is related to au increase
in strain in the fiber. From regression analysis of independent measurements of fiber strain and absorbance, a strong relation
between the change in absorbance and change in strain energy was found. Future work will involve incorporation of the
strain sensing approach with evanescent wave chemical sensing to allow total lifetime monitoring of polymer matrix
composites.
Keywords: Smart materials & structures, fiber-optics, strain sensor, infrared evanescent wave spectroscopy, nondestructive evaluation
Background
Recent years have witnessed a phenomenal growth in the use of advanced composites as structural materials in
aerospace and civil structures, primarily due to their high strength to weight ratio. The performance of the structure is,
therefore, often improved by using these composites. Although gaining popularity, use of composite materials in structural
applications is sometimes prohibited due to the lack of engineering data available on the performance of such structures. This
often leads to overdesign of the engineering material eliminating the benefits of lighter weight and higher strength inherent in
the composite material.'
Many practical applications of composites often necessitate in-service inspection of the composite material for flaws
that might have developed over a period oftime. The conventional techniques for damage inspection, e.g., X-ray, ultrasound,
etc., do not provide useful information for in-service damage detection in the polymeric composite materials (PMCs) and can
only be used for the post damage detection.2 Also, use of these techniques requires disassembly of the component parts of the
structure that results in increased downtime, labor, and cost.3
In recent years, a series of new techniques based upon fiber optic sensing have emerged for nondestructive detection
of flaws in the PMCs. These fiber optic sensors can be embedded in the composite material and can be used for real time
condition-monitoring andlor damage detection in the structures without requiring the disassembly of the components.4
*
Correspondence: E-mail: jkeIlar@silver.sdsmt.edu; Telephone: (605)-394-2423; Fax: (605)-394-3369
Part of the SPIE Conference on Smart Materials Technologies
160
Newport Beach, California • March 1999
SPIE Vol. 3675 • 0277-786X/99/$1O.00
Most of the flaws occurring in the structure depend on the loading conditions and the state of stress and strains in
the material; it is, therefore, useful to monitor the strains in the material. Many different fiber optic sensing techniques have
been developed for monitoring strains in the material while in service. Most popular fiber optic strain sensors, such as
Michelson, Fabry-Perot, Mach-Zehnder, and Dual Mode etc., are based on interferometric techniques. These interferometric
sensors work on the same basic principle and differ in design only. Specifically, a single wavelength laser is passed through
an optical fiber and external loading is applied on a small sensing region. This external loading results in an intensity signal
that is proportional to the cosine of the load induced phase-change. A demodulator is used to decode the intensity signal so as
to get a signal that is directly proportional to the optical phase change. Finally, an optical transfer function is used to relate
the optical phase change provided by the demodulator to the applied load field.5 Though very sensitive, such techniques are
quite sophisticated and expensive. Also, none of the sensor systems based on interferometric techniques can possibly be used
for complete lifetime monitoring of the composite, from fabrication, to use and ultimately failure.
Evanescent Wave Sensor
Another fiber optic sensing technique that has been gaining popularity is evanescent wave spectroscopy (EWS).
EWS have been extensively applied for studying the chemical reactions associated with the polymer curing.69 In contrast to
the development of numerous chemical sensors, relatively few attempts have been made to develop temperature and stress
sensors based on infrared spectroscopy.'°'2 In addition, evanescent wave sensing for smart structure applications has
received scant attention in the literature.
Next, we give a brief overview of the fundamental operating principles of EWS. Figure 1 shows a schematic of the
EWS technique. Evanescent wave sensors function by passing a light beam through the length of the fiber such that internal
reflections occur at the fiber core-cladding interface. At each reflection, a small amount of the electric field associated with
the light beam (called evanescent wave) actually goes beyond the fiber and penetrates the cladding surrounding the fiber. The
strength of this evanescent electric field depends on the refractive indices of the core and cladding, wavelength of the light
beam, and the angle of incidence of the light beam at the core-cladding interface.13 The fibers used in many fiber optic
sensors utilize transparent cladding. However, if the cladding is thin enough or a portion of the cladding is removed, the
evanescent wave can be used to sense the medium in which the fiber is embedded.6 If the vibrational frequency of the
molecules in the surrounding medium matches with the frequency of the light wave penetrating the medium then this wave is
absorbed. When infrared (IR) radiation is used as the light beam, this absorption is manifested as absorption bands in the JR
spectrum. The peak intensity of the absorption band depends on the attenuation of the light beam and position of the
absorption band depends on the molecular vibrational frequency of the absorbing species. Often time such evanescent wave
sensing occurs in the JR region (500-12000 cm') here vibrational spectra can be generated.
Evanescent wave sensors have been shown to be quite successful in cure monitoring of the polymers, e.g., shown in
Figure 2 are mfrared spectra showing the curing of epoxy generated in our labarotory. The focus of this research is to
develop a sensor system that is capable of monitoring the complete lifetime of polymer matrix composite systems. Such a
sensor system would be able to monitor the curing behavior of the polymer, and also allow measurement of composite strain
after curing. This paper focuses on an infrared multi-wavelength fiber optic strain sensing system developed in recent
research that characterizes the strain behavior of single fibers using infrared spectroscopy.
Experimental
The fiber optic evanescent wave strain sensor was made by FTP 100/120/140 fibers provided by Polymicro
Technologies Inc. These fibers consisted of a 100 .tm diameter fused silica core, a 10 jtm fluorine doped fused silica
cladding, and a 10 tm polyimide buffer. A miniature materials tester (MINIMAT) from Rheometric Scientific was used to
strain the fibers. The mechanical stage of the MINIMAT was installed with a 200 N load beam, and a programmable
displacement. Two support shafts are required to operate the MINIMAT. One is attached to the moving stage and the other is
attached to the load beam. For tensile tests tensile clamps are placed on the ends of the support shafts. A stepper motor
provides the driving force for the mechanical stage. The stepper motor causes the moving crosshead to move towards or
away from the load beam at speeds ranging from 0.01 mm to 99.9 mm/mm. The maximum allowed travel of the moving
stage is 100 mm. The MINIMAT control software was used to fix the testing parameters e.g. strain rate, maximum
displacement etc. JR spectra were collected with a BioRad Digilab FTS-40A Fourier transform infrared (FT-JR) spectrometer
161
using a quartz beamsplitter operating in the near-infrared (4000-12000 cm'). The BioRad fiber optic sampling accessory
consisted of a three-axis fiber holder/positioner and a liquid nitrogen cooled, narrow band, mercury-cadmium-telluride
(MCT) detector was used for all the experiments. The fiber ends were cemented into SMA connectors and polished with 12,
3, 1, 0.3 m polishing papers obtained from Fiber Instruments Sales Inc.
The length of the fibers used in all the experiments was approximately 1 10 cm. A midportion of fiber,
approximately 80 mm long, was glued to the top of the MINIMAT grips with a 5-minute epoxy. The fiber ends were then
attached to the fiber-optic sampling accessory. Once the sensor was connected to the sample accessory, a background
spectrum of 1024 scans at 16 cm' resolution was taken of the unstrained fibers in air. The fiber was then strained by
operating the MINIMAT at a speed of 0.05 mmlmin. After every 0.1 mm increment in the fiber length an absorption
spectrum of 256 scans at 1 6 cm' was obtained.
To achieve good bonding between fiber and MINIMAT grips, the polyimide buffer on the fiber was removed by
dipping the sensing region in hot sulfuric acid and treating the bare fiber with the silane coupling agent (3aminopropylethoxy-silane from Huls America Inc.) prior to cementing the fiber on the MINIMAT grips. To ensure that the
load was applied uniaxially and to obtain a consistent gauge length, an aluminium template was made. This template was 80
mm in length with a thin ime inscribed on its top surface. Two removable grip pieces were situated in the template and the
fiber was glued to the grip pieces with 5-minute epoxy. Following curing of the epoxy the fiber and grip pieces were
removed from the template and connected to the M1NIMAT and the fiber was strained according to the procedure given
above.
Results & Discussion
Generation of infrared spectra involved the following three steps,
1.
Collection of a single beam background spectrum of the unstrained fiber
2. Collection of a single beam sample spectrum (sample in this case is a strained fiber)
3 . Ratioing the spectrum in step (2) against the spectrum in step (1) and then taking a negative logarithm of this ratio.
Steps (2) and (3) were performed simultaneously by the FT-JR software. Figure 3 shows a single beam background
spectrum of the fibers used in the present study. The background displays energy distribution of the near infrared light
passing through the fiber. It can be seen that the fiber is highly transparent in the region 5000-8000 cnt'. The intensity of
light outside this region falls rapidly. The absorption spectrum outside this region will, consequently, have more noise.
Therefore, the high intensity region (5000-8000 cm') was chosen for the subsequent study of IR absorption spectra.
Shown in Figure 4 are the absorption spectra of the fiber at different strain levels. Spectra A - F correspond to fiber
stretching of 0 mm, 0. 1 mm, 0.2 mm, 0.3 mm, 0.4 mm, and 0.5 mm, respectively. Spectrum A is obtained by ratioing the
single beam background spectrum of Figure 3 against a similar single beam spectrum. The resultant spectrum is the so-called
100% line, which is a straight line (with some noise associated with it) at approximately zero absorbance level. It was found
that the position of this 100% line can shift up or down if the MCT detector was not brought to thermal equilibrium prior to
the performing strain measurements. Therefore, before taking any absorption spectra of the strained fiber the detector was
allowed to cool for about 2.5 hrs.
Spectrum B in Figure 4 is an absorption spectrum of the fiber after it has been elongated to 0.1 mm. This spectrum
is very similar to spectrum A, however, the average absorbance has increased to 0.0012. In addition, peak-to-peak noise level
has also increased. Both of these factors are associated with a small (— 0.2-0.3%) decrease in the amount of light passing
through the fiber. These same effects are observed for all the subsequent absorption spectra at increasing strain levels (i.e.
absorption spectrum at a particular strain shifts upwards with respect to the position of an absorption spectrum at a lower
strain). As the fiber is strained, the refractive index of the fiber material changes, the core diameter decreases, the density of
fiber material is expected to change. All these changes result in changes in the overall modal distribution of light passing
through the fiber which is accompanied by some light loss. As the increase in absorbance is consistent over the complete
wavelength region, the amount of light lost is relatively constant over the wavelength region studied. Besides the increased
absorbance and noise, non-random noise sometimes developed at higher strains. This can be seen in Figure 4, for spectra C-F
between about 5500 and 6500 cm'. This non-random noise resembles interference fringes. However, the interference fringes
were not obtained consistently during replicate testing, and their occurrence is still under investigation.
162
It is clear from Figure 4 that applied strain causes changes in the JR absorbance spectrum. To help interpret these
results, the average JR absorbance between 5000-8000 cm' was plotted versus the applied strain. A plot of one of the five
replicate tests is shown in Figure 5. Also, the linear, least-square regression line for the data is given in this figure. A good
linear relationship between the JR absorbance and strain is shown by the correlation coefficient R2 =0.976. The slope values
and the corresponding R2 values for the replicate tests is given in Table 1 . It can be seen in Table 1 that the slope of the
absorbance versus strain plot is fairly consistent with a value of approximately 0.5. However, the slope value in test 1 is
found to be markedly different than those in tests 2 to test 4. This discrepancy may be attributed to the errors that might have
occurred in taking the first few strain readings in the very first experiment, but more work is needed to confirm this.
In addition to strain, strain energy is often an important factor in failure analysis. Therefore, the relationship
between JR absorbance and strain energy was also investigated. Strain energy is the increase in energy associated with
deformation (i.e. the area under the stress-strain curve), and is given by,
•i)
Applying Hooke's law for a single fiber in uni-axial tension this will reduce to,
rrr
U=—iiiusdxdvdz=
1
rrLa 2
7-,
8
Where, E = Young's modulus ofthe fiber(in this case 72 Gpa)
L = initial length of the fiber (approximately 80 mm)
d = fiber diameter = 1 1 9 x iO mm
C=
strain in the fiber, defined as
1. —
s=1
10
1 = instantaneous fiber length
l = initial length of the fiber
The strain energy was calculated for the deformations in each of the replicate experiments. A variety of functional
relationships were tested and the best linear relationship was found between the percent change in absorbance and the percent
change in strain energy. A typical plot of this relationship is shown in Figure 6. Similar to the strain, the linear, least squares
regression line is shown in Figure 6. Again, a strong linear relationship is suggested by the correlation coefficient R2 =0.999.
Shown in Table 2, are the slope values and their corresponding R2 values for the replicate tests. The slope values in this case
are more constant (with a value 0.1) than those for the strain vs. average JR absorbance case. This suggests that the linear
relation between % change in strain energy and % change in average JR absorbance could be a better estimate in predicting
the strain values in a fiber.
163
Conclusions
A new, multiple wavelength, infrared fiber optic sensor has been developed. This sensor utilizes an ultra-low OH
fused silica core fiber to pass near-infrared light along its length. The output light is ratioed against the amount of light
passing through the sensor at zero strain to give an absorbance spectrum. The sensor was then subjected to uni-axial tension
and absorbance spectra of the strained fiber were measured. Plotting the mean absorbance between 5000-8000 cm' versus
the strain yielded a linear relationship. Replicate experiments indicated that the slope of the absorbance versus strain plot was
fairly consistent with a value of approximately 0.5. In addition to the relationship between absorbance and strain, a
relationship was sought between absorbance and strain energy. A linear correlatin was found between the change in
absorbance and the change in strain energy. The slope of this linear relationship was found to be 0. 1 .Work is continuing to
blend the type of sensor described in this work with fiber-optic chemical sensing to create a total lifetime fiber-optic sensor
system.
Acknowledgements
This work was supported by the National Science Foundation under Grant #CMS-9453467. In addition, the authors would
like to extend their gratification to Mr. Tormod Sveen, for his help in desigining the aluminium template used in the
experiments.
References:
W.B. Spiliman, Jr., "Fiber Optics & Smart Structures", in Optical Fiber Sensors, John Dakin and Brian Cuishaw, Eds. (Artech House,
1997), pp. 409.
2. J.S. Sirkis, CC. Chang, B.T. Smith, Journal ofComposite Materials, 28, No. 14 (1994), pp.1347.
3. R.M. Measures et al, Optics and Laser Engineering, 16 (1992), pp.127-52.
4. Kausar Talat, Photonics Spectra, April 1991, pp. 85-88.
5. J.S. Sirkis, AD. Kersey, TA. Berkoff, and E.J. Friebele, Experimental Mechanics, June 1996, pp.135-141.
6. F.J. Johnson, ME. Connell, E.F. Duke, W.M. Cross and J.J. Kellar, Applied Spectroscopy, 52 (8), 1998, pp.235-44
7.
S.L. Cossins, ME. Connell, W.M. Cross, R.M. Winter, and J.J. Kellar, Applied Spectroscopy, 50, 1996, pp.900.
8. M.E.Connell, W.M. Cross, T.G. Snyder, R.M. Winter and J.J. Kellar, Composites-Part A, 29 A, 1998, pp.495.
9. S.L. Cossins, ME. Connell, W.M. Cross, R.M. Winter and J.J. Kellar, "Evanescent Wave Spectroscopy for In-Situ Cure Monitoring",
published in SPIE 1 996 International Symposium on Optical Science, Engineering and Instrumentation: Chemical, Biochemical and
Environmental Sensors VIII, SPIE Vol. 2836, R.A. Lieberman, Ed., pp.147-56.
10. Jie Lin and Chris W. Brown, Applied Spectroscopy, vol. 47, No. 1 (1993), pp. 62-68.
11. James W. Rydzak and W. Roger Cannon, J. Am. Ceram. Soc., 72 [8], 1989, pp.1559-61.
12. VI. Vettegren, A. Ya Bashkarev, A.A. Lebedev, Mekhanika Kompozitnykh Meterialov, No. 6, November-December 1990, pp. 978
13. Douglas A. Skoog and James J. Leary, Principles of InstrumentalAnalysis, 4th Edition, 1992, pp. 283-84.
I.
164
Table I (Slope values obtained from replicate tests for strain vs. average
absorbance plots)
Test
Slope
Rz
1
2.382
0.987
2
0.289
0.957
3
0.806
0.999
4
0.605
0.976
Table 2 (Slopes values obtained from replicate tests for % change in strain
energy vs. % change in average absorbance plots)
Test
Slope
R2
1
0.090
0.958
2
0.185
0.971
3
0.164
0.984
4
0.077
0.999
165
///
/
Fiber Sensing Region
(approx. 80 mm.)
To FT-IR
Detector
/
/
Stepper
\
Movable
Stage
To FT—IR Light
Source
Z/ad
poxy on MINIMAT
Grips
Motor
Figure 1. A diagram of the setup for strain measurements using the MINIMAT.
0.07
time
0.05
a)
0
0ICo
0
C,)
0.03
-o
0.01
-0.01
5200
4850
Wavenumber, 1/cm
Figure 2. In situ fiber-optic sensor spectra of the curing of EP0N828 with methylenedianiline.
166
4500
Beam
167
levels.
strain
increasing
at
fiber
silica
single
the
of
spectra
FT-JR
4.
Figure
(cn1)
VteN
&J00
7000
6000
5000
0.0007
0.0017
0.0027
0.0037
A BC 0 E F
-0.0003
C.)C.)
0.0047
fiber.
silica
single
the
of
spectrum
Background
3.
Figure
1)
bers(cm
urn
Waven
10000
7500
5000
2500
-0.1
0.4
09
Cl)
0
1.4
'.
2.4
Cl) C,
2.9
0.0044
0)
0C 0.0035
.0
0 0.0026
Cl)
.0
0)
0.0017
0)
0.0008
0)
>
-0.0001
0
0.001
0.002
0.003
0.004
0.005
0.006
Strain
Figure 5. Linear, least-squares, regression line for strain vs. average IR absorbance.
250
w
y = 0.0765x + 17.176
C.)
= 0.9988
(5
.0
L.
0
Cl)
.0
;ioo
0)
50
CS
0
0
500
1000
1500
2000
2500
3000
% change in Strain Energy
Figure 6. Linear, least squares, regression line for % change in strain energy vs. % change in
average JR absorbance.
168