1987 DuPont manufactured 4560 denier Kevlar/Epoxy Strands were instrumented with nine and three sensors each. Stress tests were performed at 30,45,60,70 and 80% of ultimate strength with dwell times of 10,000 seconds. FBG showed uneven stress levels which is contrary to conventional observation

Arnett, Shawn

Banks, Curtis E.

Grant, Joseph

Russell, Sam

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MSFC-iS;l
Local Strain Measurement of Kevlar Strand with Fiber Optic Bragg Grating
Curtis E. Banks1, Joseph Grane, Sam Russelt3 and Shawn Arnett4
lIntegrated Structure Health Monitoring and Sensors Branch
EV43 MSFCINASA, Huntsville, A135812
2Science & Technology Division
MSFCINASA, Huntsville, Al 35812
3Nondestructive Evaluation Branch
EM20 MSFCINASA, Huntsville, Al35812
4TRIIAustin, Inc.
9063 Bee Caves Rd., Austin, Texas 78733
Abstract
1987 DuPont manufactured 4560 denier Kevlar/Epoxy Strands were instrumented with
nine and three sensors each. Stress tests were performed at 30,45,60,70 and 80% of
ultimate strength with dwell times of 10,000 seconds. FBG showed uneven stress levels
which is contrary to conventional observation.
Introduction
Stress rupture in Composite Overwrapped Pressure Vessel (COPV) is a major concern to
NASA because studies reveled that the probability of stress rupture leading to
catastrophic failure were greater than previously though l . All COpy structure degrades
over time make stress rupture more likely. Kevlar COPVs was used extensively on the
space shuttle because of their weight saving advantages3. The energy of a COpy
structure is held by the overwrap, where as the liner is mainly used to retain the
commodity. DuPont 19874660 denier Kevlar (strand or tow) was used to wrap these
vessels.
To investigate stress rupture a team ofnondestructive evaluation (NDE) experts were
assembled to employ the latest NDE techniques to predict stress rupture. As a means of
developing Fiber optic Bragg Gratings as a viable NDE and Integrated Vehicle Health
Monitoring (IVHM) technique, FBGs were applied to the Kevlar during tensile creep test
to measure local stress alone the tow of the fiber. It is know that localized high stress
field is the likeliest place for stress rupture l .
The Kevlar strand is comprised of several smaller bundles making to 0.1 inch the strand.
The FBG sensor is small compared to the tow making it possible to measure the uneven
strain fields under stress.
All tests were carried out at Texas Research Institute Inc., Austin, Texas.
Experimental Testing and Results
Three ten inch 4650 denier Kevlar epoxy strands were instrumented with FBGs strain
sensors and prepared for tensile creep testing (figure 1). Grip attachment were placed on
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https://ntrs.nasa.gov/search.jsp?R=20080019634 2019-08-30T04:35:03+00:00Z
each end of the strands and affixed to a cardboard backing for handling. One strand was
instrumented with nine sensors. This strand had a total of three FBG cables with three
sensors each. On the other hand, the two remaining strands had three sensors each. Their
spacing was two and three-quarters inch while the first strand spacing was a little over an
inch. At each sensor location the sensor was pulled taunt and glued to the strands with
Vishay M-bond AE-I0 epoxy.
These samples were individually placed inside the instrum for creep testing. Under stress,
a Micron Optic sm125 FBG interrogator was used to track the peak shift ofthe sensors.
The predetermined strain loads were 30, 40, 60, 70, and 80% ofthe ultimate load strength
ofthe strand (212Ibs). The ambient temperature was kept at 34 degrees. At each
increment the dwell time was 10,000 seconds. This allowed the strands to undergo
appreciable creep levels.
Figure 1 - Kevlar/Epoxy Strands with Bonded FBGs
Of most significance in this test was the functioning of the FBGs and the resultant file
creation by the sm125 software. As stress was applied to the strand, peak splitting
occurred at various wavelengths. When the program recognizes additional peaks, their
wavelengths and power levels (dB) were appended to the text file. This would happen
especially when the stress levels were changed. Should a peak stop being recognized, no
appended data was written. This created a significant problem in Excel sorting the data
to align the columns properly. A feature ofthe text file was used to sort the data.
The first column of the file was elapsed time in seconds. The next four show the number
of peaks being identified for each active channel. Therefore, by using logic formulas to
copy data to new columns (based on the exact number of peaks being identified) allowed
all of those with 3, 4, 5, 6 or 7 to be aligned separately. However, where a subset had
blanks, Excel charted them as zeros, hence the row of symbols across the 0 axis. Another
problem became apparent that is shown in Figure 2. Although the sorting based on
2
number of peaks (columns) helped, they were not uniformly consistent regarding the
correct wavelengths. This is visible in the subset of data later in the test around 12,000
microstrain. The microstrains were determined by using a conversion factor of 1.300
picometers per 1,000 microstrain.
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Figure 2 - Extensometer and FBG Microstrain and Load Data for First Strand Test (the FBG data is
plotted in symbols to prevent vertical lines from connecting entries seen as zeroes)
Some peaks appeared and disappeared throughout the test that varied in wavelength.
This data could have been corrected by changing the normalizing wavelength to its
correct assignment, but it is important to illustrate this aspect of the data collection and
operation of the test. Figure 3 shows the peak wavelengths detected throughout the test.
They were "self-sorting" in a manner of speaking in that all were recorded regardless of
which series they were in. Figure 4 shows a portion of the test in higher resolution to
illustrate the range of strains that multiple peaks represent. This spread of around 800
microstrain is not conducive to accurate strain measurement. The curves shapes do,
however, match the extensometer very well, but they indicate that the original gage
length does not apply to them all.
Some ofthese problems could be minimized ifthe operator is present and able to modify
the peak detection parameters in the operating software. This is not practical for long
term tests, and it is problematic in that multiple or attenuated peaks are difficult to sort
properly on a single channel's parameter settings.
It is believed that the cause of the peak splitting has to do with the introduction of either
twisting or bi-axial stresses. Our conclusion is that strands are not easily tested with
FBGs due to surface changes under stress that apparently impart compressive or torsional
stresses on the fiber. As will be discussed later, applying FBGs to a COpy can also be
3
affected by surface changes in the composite under stress and require special bonding
techniques to minimize the effects.
The power level data is shown in Figure 5 below. Compared to peak power level
attenuation on "embedded" FBGs on COPY liners, these are fairly modest losses.
The strand with nine FBGs was then tested; however, installation of an extensometer was
not possible so only crosshead displacement data was collected in addition to the sensors'
data. The crosshead displacement does not provide a very useful method of determining
a tensile specimen's true strain and due to various grip, specimen and load frame
compliances will invariably record higher computed strains than would be measured
locally on the specimen with an extensometer, strain gage or other strain sensor (i.e.,
FBG).
4
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Figure 4 - Close-up View of Part of the Test to Illustrate the Spread in Microstrain (the highest curve is the
extensometer output)
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Figure 5 - Power Levels Show Only Moderate Levels of Attenuation
5
A second strand with a single 3-FBG fiber attached was then tested as illustrated in the
figures 6 below. Fewer spurious peaks developed on this test and less sorting of the data
was required. Again, the extensometer strain is higher than those from the FBGs. The
extensometer had a nominal gage length of 4.4 inches.
30000
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,
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TIME IN SECONDS
--FBG --FBG --FBG -EXTENSOMETER -PRESS
Figure 6 - Load and Microstrain Data for Extensometer and FBGs
Discussions
Although we had many problems with the sm125 interrogation system, quantitative
observation can be made. The strain in a homogenous system under a constant load is the
same throughout. However, in the case of a 4650 denier Kevlar strand this may not be the
case, since the strand or tow is made of smaller bundles. Under strain individual bundles
may have higher strain load than adjacent bundles. Since the FBG is small compared to
the width of the 4650 denier strand it cannot measure the average strain over the Kevlar
strand. But each FBG measures locally the strain of the tow.
For example, in figure 4 there is an 800 microstrain spread. A close inspection will
reviled that many of the sensors were measuring the same strain load. It would appear
that these sensors were either on the same bundle alone the length of the strand or two
bundles were under identical loads.
Similarly, we see a spread over the three FBG instrumented strand in figure 6. Here the
strain difference is smaller. It was also under less twisting which led to peak splitting.
Nonetheless, local strain can be measured with FBG sensors. For composite overwrapped
pressure vessel localized high stress fields can potentially be an area of stress rupture.
6
Conclusion
We have performed tensile creep testing 1987 DuPont manufactured 4560 denier
Kevlar/Epoxy Strands. These stands are from the same stock used to manufacture
composite overwrapped pressure for the Space Shuttle. FBG were used to measure the
localized strain fields. A micron optics sm125 integrator was for peak wavelength
detection which proved to be very difficult to extract useful data.
Quantitatively, the data showed that localized strain field can be observed. The local
strain field is a strong indication of stress rupture in COPY.
References
1. J. Grant, C. Banks, "Strain Measurements Using FBGs on Composite
Overwrapped Pressure Vessels (COPV) in Stress Rupture Test", Proc. ofSPIE
sensor System and Networks: Phenomena, Technology, and Applications for NDE
and Health Monitoring, San Deigo, CA, vol. 6530, pp. 653011-1-653011-10,
2007.
2. J. Derieck, W. De Waele, P. Verleysen, "Monitering of fibre reinforced
composites with embedded optical fibre Bragg sensors, with application to
filament wound pressure vessels" NDT&E International 34 (2001). pp 289-296.
3. G. Kister, D. Winter, R.A. Badcock, Y.M. Gebremichael, W.J.O. Byle, RT.
Meggitt, K.T.V. Grattan, G.F. Fernando, " Structure health monitoring of a
composite bridge using Bragg grating sensors. Part 1: Evaluation of Adhesives
and protection systems for the optical sensors" Engineering Structuring 29 (2007)
440 -448.
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Local Strain Measurement of Kevlar Strand with Fiber Optic Bragg Grating

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