Graphite/epoxy tubes were fabricated with embedded optical fibers to evaluate the feasibility of monitoring strains with a fiber optic technique. Resistance strain gauges were attached to the tubes to measure strain at four locations along the tube for comparison with the fiber optic sensors. Both static and dynamic strain measurements were made with excellent agreement between the embedded fiber optic strain sensor and the strain gauges. Strain measurements of 10(exp -7) can be detected with the optical phase locked loop (OPLL) system using optical fiber. Because of their light weight, compatibility with composites, immunity to electromagnetic interference, and based on the static and dynamic results obtained, fiber optic sensors embedded in composites may be useful as the sensing component of smart structures

Dehart, D. W.

Doederlein, T.

Heyman, J. S.

Holben, M. S., Jr.

Koury, J.

Rogowski, R. S.

English

NASA Technical Reports Server

N90-21090
STRAIN AND DYNAMIC MEASUREMENTS USING
FIBER OPTIC SENSORS EMBEDDED INTO
GRAPHITE/EPOXY TUBES
D. W. DeHart, T. Doederlein, and J. Koury
Air Force Astronautics Laboratory
Edwards AFB, CA
R. S. Rogowski, L S. Heyman, and M. S. Holben, Jr.
NASA Langley Research Center
Hampton, VA
497
https://ntrs.nasa.gov/search.jsp?R=19900011774 2020-03-19T23:21:30+00:00Z
1. INTRODUCTION:
Smart is def'med by The American Heritage Dictionarv as, "Of, relating to, or being a device that imitates
human intelligence."[1] The concept of smart structures meets this definition.
Several planned United States Air Force (USAF) and National Aeronautics and Space Administration
(NASA) space systems such as Space Based Radar (SBR), Space Based Laser (SBL), and Space Station, pose
serious vibration and control issues.. See figure 1. Their low system mass combined with their large size,
extreme precision pointing/shape Controland rapid retargeting requirements will result in an unprecedented
degreeof interaction between the system controller and the modes of vibration of the structure. The re-
suiting structural vibrations and/or those caused by foreign objects impacting the space structure could
seriously degrade system performance, making it Virtually imposslble for passive structural systems to per-
form their missions. There resides a need-i;or creating an-::active vibration control system which will
sense these natural and spurious vibrations, evaluate them and then dampen them out. This active vibra-
tion control system must be impervious to the space environment and electromagnetic interference, have
very low weight, and in essence become part of the structure itself. The concept of smart structures also
meets these criteria. Smart structures is defined as the embedment of sensors, actuators, and possibly
microprocessors in the material which forms the structure, particularly advanced composites, These sensors,
actuators, andmicroprocessors will work interactively to sense, evaluate, and dampen those Vibrations which
pose a threa(-to large flexible space systems (LSS). The sensors Will also be capable of sensing any
degradation to the structure. (Fig. 1.)
CORPORATE FED ARRAY / PHASED ARRAY
TYPICAL SPACECRAFT CONFIGURATION
498
Figure I
•
In conjunction with this problem, the Air Force Astronautics Laboratory (AFAL) and NASA
Langley Research Center (NASA LaRC) have initiated a program to design, fabricate, and experimentally
test composite struts and panels with embedded sensors, actuators, and microprocessors that can be used for
dynamic sensing and controlling vibrations and motions in space structures. The sensors will also be able
to monitor the health of space structures. This program is divided into four tasks. Task one is acquiring
equipment necessary for embedding sensors and actuators into composite material. Baseline structures of
aluminum and composite materials will be fabricated (coupons, flat panels, struts) to demonstrate concept
feasibility. The baseline structures will be tested for stiffness, strength, and vibration for comparison with
data from the structures that have embedded components. Task two will include the processing science
studies required to process the composite components. Task three will include the fabrication of the selected
composite struts with embedded components. Both sub-scale and full-scale struts will be fabricated. Task
four will include non-destructive evaluation, mechanical testing, and vibration damping testing of the Task
three components with the embedded control system for overall concept performance, endurance, reliability
and response.J2] See figure 2.
SMART SPACE STRUCTURES
PROJECT (IN-HOUSE)
Task one encompasses acquiring equipment necessary for embedding
sensors and actuators into composite materials. Baseline structures of
aluminum and composite material will be fabricated.
II. Task two will run parallel to Task I and will encompass studies of
fabrication and processing technologies.
Ill. Task three will encompass the fabrication of subscale and fullscale
composite structures with embedded sensors and actuators.
IV. Task four will encompass experimentation on the fabricated components.
Such experiments will include non-destructive evaluation, mechanical
testing, and vibration testing within a ground dynamic test facility.
Figure 2
499
The AFAL has been working in the area of dynamics and control of LSS for the past five years.
They have had numerous programs, both contractual and in-house, to develop sensors and actuators for
controlling LSS. Presently the AFAL is developing a large scale laboratory, called Advanced Space
Structures Technology Research Experiments (ASTREXr), which will have the capacity of performing large
angle rapid retargeting maneuvers and vibration analysis on LSS. Also they have been fabricating advanced
composite materials for the last four years. However, most of the composite components that were
fabricated were rocket components such as: nozzles, payload shrouds, exit cones, and nose cones. For the
last two years though, the AFAL has been fabricating composite space components such as trusses, tubes,
be_s and flat panels. Research on'_r Optic sensors _ N_A$_-dates back to 1979. Recently an
optical phase locked loop (OPLL) system has been developed that uses fiber optics for sensing. See figure
=
3. Static and dynamic strain measurernents have been demonstrated using this device.[3]
Optical Phase Locked Loop Fiber Optic Sensor
Laser
Oscillator
Counter I Fibe
Filter Mixer
Detector
Figure 3
Environmental
PhaseModulation
5OO
2, FABRICATION OF EXPERIMENTAL STRUTS
The first goal of the program is to demonstrate the feasibility of smart structures. A simple composite
tube structure with a circular cross section was fabricated with a fiber-optic sensor embedded into it. The
tube was chosen because it is a key structural element in most space truss systems. Optical fibers were
chosen as the sensing element because of the demonstrated capability of the OPLL developed at NASA
LaRC. To date, a total of three tubes fabricated at the AFAL have been tested at NASA LaRC. The results
of two of these tubes are presented in this paper. All three tubes were fabricated in a similar manner, by
filament winding on an En-Tec computer filament winding machine. The first two tubes were wound as
one single tube on the same steel mandrel and then cut into two. The composite material used was graphite
epoxy prepreg roving from Fiberite composed of medium strength G-40 graphite fibers from Union Carbide
and 5245 epoxy resin. The single tube prior to cutting was approximately five and one half feet in length,
with an inside diameter of one and one half inches. The single tube consisted of one layer of 90 degree
fiber; two layers of +/- 45 degree fiber; and one layer of 90 degree fiber, with the fiber optics embedded
under the final 90 degree layer. A schematic diagram of the tube lay-up is shown in figure 4. The fiber
optics, made by the Newport Corporation, are F-MSD multimode fiber. The core has a diameter of 49 _m
with a combined core and cladding diameter of 125 _m. They are coated with an acrylic jacket. The fiber
optics were layed in longitudinally up to the center of the tube with both ends of the fiber coming out of
the same end of the tube to prevent spurious vibrations of the fiber optics during the testing of the
composite tube.
90 ply
90 ply
optical fiber
ply
9O ply
Figure 4
501
They were located every 90 degrees throughout the tube with a total of four sets of fiber optics embedded in
the tube. After winding, the single tube was then bagged using standard bagging techniques. Special care
was given in bagging the optical fibers since they are very delicate and fragile and due to the long leads
coming out of the end of the tube. Prior to autoclaving, the optical fibers were coated with a high
temperature silicone (RTV) coating. This coating prevents the flexible acrylic jacket from melting during
the curing process which would then leave only the very brittle bare core and cladding and also acts as a
stress relief where the optical fibers come out of the end of the composite tubes. The single tube was then
placed in a Baron-Blaekslee autoclave and cured at 350 degrees F and 85 psia for four hours. Upon cooling,
the tube was removed from the mandrel and cut in two, giving special care to the optical fibers. Both tubes
were then packaged and delivered to NASA LaRC for strain and vibration testing. The third filament-wound
tube was fabricated very similar to the first two tubes. The only differences Were in the material, size, and
the angle of the composite plies. The composite material used was graphite/epoxy prepreg roving from
Fibe_qte composed of medium strength _VI-6 graphite fibers from -Hercules and 934 epoxy resin. The first
tubes had a fundamental frequency of approximately 33 hertz when cantelivered. The third tube was
designed to have a significantly lower fundamental frequency. A tube with a length of 68 inches and a ply
orientation of 90-(+/-)60-90-90 has a fundamental frequency of 10 hertz. The fiberoptics were embedded in
between the 60 and the second 90 degree layer. In this tube only two sets of fiber optics were embedded at
180 degrees to one another. Also, tlsd fiber bp6csdid not come out of the end of the tube is in the first two
tubes. The fiber optics came out of the circumference of the tube at a distance of four inches from the end
of the tube. This change allowed clamping of the tube at the end without crimping the optical fibers. See
figure 5.
composite tube
0
502
Figure 5
3, STRAIN AND VIBRATION MEASUREMENTS
The vibration and strain experiments were conducted at NASA Langley Research Center using a
modulated diode laser and an optical phase locked loop (OPLL) system which is described in reference [3].
The optical fibers exiting the tube were fusion spliced to the fibers of the optical system. One end of the
graphite/epoxy tube was clamped over a round piece of metal, which was inserted into the tube. For the
first two tubes, this caused a problem with crimping of the optical fibers as they exited from the tube. For
the first few readings, the data taken acted in a reverse fashion than as predicted. For the third tube, this
problem was solved by having the fiber optics exit from the circumference of the tube, not from the end.
The whole assembly was then clamped to an optical table to hold'the end of the tube firmly in a
cantilevered position.Four resistance strain gauges were attached Iongitudimally to the tube for comparison
with the fiber optic sensors. The tube was statically stressed by hanging weights on a weight pan and
attaching it to the free end.Data were taken while the weights were loaded and unloaded. When this force is
applied to the tube, a strain is induced in the composite which in turn induces a strain into the optical fiber
that produces a change in the modulation frequency of the OPLL. The optical fibers can measure both
tension and compressive strain. This stain is governed by the equation change in frequency divided by the
frequency is equal to the negative of the change in length divided by the length (_(::. -.._). Strain on the
surface of the tube was monitored with the strain gauges. The results of these experiments are shown in
figures 6 and 7, which graphically indicate the correlation between the strain gauge measurements and the
changes in modulation frequency. The data indicate very good correlation between the strain gauge readings
and the fiber optic strain measurements. These runs were repeated with excellent reproducibility.
5OO0O
40000
30000
_=
,1=
_" 20000
10000
Correlationof FiberandStrainGauge
!
1000
SIraln -_m
2000
Correlationot FiberandStrain Gauge
¢.
5OOO0
40000
30000
20000
10000
0
0
, m
,. i
1000
S. G. a2
Strain -/=m/m
20O0
Figure 6
503
504
"I"
_=
J=
cn
u=
_=
G9
U.
50000
40000
30000
20000
10000
Correlationof FiberandStrain Gauge
I , I • | • !
S.G. #3
0 i I. i | , ... I i I ,
0 200 400 600 800 1000
50000
40000
0
0
30000
20000
10000
Strain./=m/m
Correlationof Fiberand StrainGauge
I I I I I / _,m
S.G. #4
I I I I I
1 O0 200 300 400 500
Strain - _=m/m
6O0
Figure 7
Strain measurements were also made while the tube was vibrating. The end of the tube was impacted and
the vibration response was analyzed. Figure 8 compares the results obtained by simultaneously monitoring
a strain gauge and the fiber optic signal for the third tube. The time domain and frequency domain data
agree very well, showing a fundamental vibration frequency of 10.4 Hz, which is very close to the design
vibration frequency of I0 Hz. As you can see from the data, the fiber optics can detect the vibration
response significantly better than the strain gauges.
Dynamic Strain Measurements in G/E Tube
_-F,-_F_-_7 t-_'_.o-
Chl .5 V =
At, Ons F _ T/dlv ,2 s Ch2 2DmV "
Trig 1.00dlv÷CHAN I=
- !-t.4_V-Vv_(vt'_F--]-_-"--t4_:-'<''""
I--'
__li
6t 0 ne
Chl .5 V=
F _ T/dlv .2 s Ch2 20mV =
Trlg I .OOdiv * CHAN i =
Figure 8
ORIGINAL PAGE IS
OF POOR QUALITY
505
Graphite/epoxy tubes werefabricated with embedded optical fibers to evaluate the feasibility of monitoring
strains with a fiber optic technique, Resistance strain gauges were attached to the tubes to measure strain at
four locations along the tube for comparison with the fiber opti c sensors. Both static and dynamicstrai n
measurements were made with excellent agreement between the embedded fiber optic strain sensor and the
strain gauges. Strain measurements of 10 -7 can be detected with the OPLL system using optical fiber.
Because of their light weight, compatibility with composites, immunity to electromagnetic interference,
and based on the static and dynamic results obtained, fiber optic sensors embedded in composites may be
useful as the sensing component of smart structures.
I  Y.gF.hrS 
[1] The American Heritage Dictionary, Houghton Mifflin Co, Boston, MA i982.
[2] Capt Ted Doederlein, Lt Douglas DeHart, Mr. Joe Sciabica, Smart Space Structures Project Directive,
Air Force Astronautics Laboratory, Jan 1988.
[3] R.S. Rogowski, L S._Heyman' M. S. Holben, Jr. and P. Sullivan, "A method for monitoring strain
in large structures: Optical and radio frequency devices", Rev. Prog. Quant. NDE, vol 7a, p559 (Plenum
Press, 1988).
? SL
506
z.


Strain and dynamic measurements using fiber optic sensors embedded into graphite/epoxy tubes

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