NASA evaluated using fiber Bragg gratings (FBG) embedded in honeycomb sandwich carbon composite panels as impact sensors to detect damage. An FBG sensor was embedded in a panel between the face-sheet and core. Impacts of 1 ft-lb were made at one-inch intervals and data was collected from the FBG and acoustic emission sensors. Results showed that FBG sensors could successfully detect impacts and have benefits over traditional sensors by enabling strain measurement and eliminating needs for additional systems.

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Structural Health Monitoring Aluminum
Honeycomb Sandwich Composite Panel (SHM)
Dawid M. Yhisreal-Rivas1
NASA Marshall Space Flight Center, Huntsville, AL, 35812
NASA evaluated Fiber Bragg Gratings as a potential impact sensor to detect the impact
damage of a honeycomb sandwich carbon composite panel. The sensor was embedded between
the eight ply face-sheet and impact of 1 ft-lb was taken in one-inch intervals from the sensor’s
location. As of this writing the project has picked up where the use of AE (Acoustic Emissions)
along with FBG’s (Fiber Bragg Gratings) are to be used in the analysis of impact on composite
materials. The AE sensor is placed a number of controlled distances away from the embedded FBG
sensor and AE sensor and impacting would allow for data gathering from both devices for
comparison. The benefit of FBG’s in Structural Health Monitoring (SHM) Aluminum Honeycomb
Sandwich Composite Panels came from being able to use signal delays that occur from impact to
triangulate position but also the fact that strain can also be measured with the same system thus
effectively eliminating the need for another system to allow for strain measurement.
Nomenclature
AE = Acoustic Emissions
FBG = Fiber Brag Grating
SHM = Structural Health Monitoring
NDE = Nondestructive Evaluation
I. Introduction
ASA’s use of strain sensors to monitor a structures health has been a part of the process for some time, but with
new emerging technologies the use of Fiber Brag gratings as an impact sensor for composite materials proved
to be a step in the right direction as the new sensor would enable the use of composite materials with fiber bragg
gratings embedded within. The use of this sensor for real-time analysis of structural health monitoring would allow
for weight reduction, reduced electromagnetic interference, and reduce the amount of sensors and costs that are
usually associated with the evaluation of a structures health. The area of research that involves fiber optics and fiber
brag gratings has long been developed for fifteen years and much research has been documented in the process of
embedding FBG’s into materials for a variety of different of applications. Although this seems like a long time the
technology is still within its infancy when compared with other sensors. Yet once compared with sensors that have
been established for longer periods of time one would see the advantages that this type of sensor would have over
others because of costs and flexibility to an application. The time and costs that it takes to perform nondestructive
evaluation on vessels are an issue if the vessel is to stay in use or storage over a period of time. The faster and more
effectively accurate a vessel can be monitored strips away the time and costs of it being out of service, and the use of
FBG’s embedded within the composite allow for real time monitoring the vessel regardless of its service status.
Research outlined within this article deals with prototyping composite aluminum sandwich boards with fiber optics
inlayed with FBG’s so that effective monitoring and data gathering could be done to acutely determine the
characteristics of the sensor over varying distances with a constant impact force within a controlled environment
alongside AE sensors to contrast accuracy of readings gathered by National Instruments 6800 Data Acquisition
Systems.
1
NASA Science and Technology Institute for Minority Institutions (NTSI) Intern, Marshall Space Flight Center,
Space Sciences, The University of Texas at El Paso.
N

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II. General Guidelines
To begin the data analysis the composite boards comprised of an aluminum medium sandwiched between 16 ply
(16 individual sheets) carbon fiber laminate along with a fiber optic with a FBG tuned at 1550nm
Figure1. 15 x 15 aluminum carbon fiber composite sandwich (this particular pic is not 16 ply it is merely a
representation of the finished prototype of the composite panel).
With an embedded fiber optic FBG sensor in the composite we determined that placing this at the center would
allow us to outline a grid so that we could understand the maximum sensitivity based on the angle and distance out
of an impact. The goal is have a sensor that can be placed out a distance of eight feet from one another that would be
able to detect an impact of at the very minimum of one foot pound at any angle within the given parameters of and
eight by eight square foot coverage. The need to set up a test bench to allow for this required an impacting system
(Figure 4.), tunable laser, signal converter (Figures 2 & 3.), AE system (Figure 5.), and data acquisitioning capable
system and software for both the acoustic emission and FGB signal.

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Figure 2. TUNICS-Plus (Yenista) Tunable External Cavity Laser
Figure 3. Optic Signal Converter
Figure 4. Impacting System
Similar tools are used within research of determining impact damage and data gathering. In the case of NASA the
use of an acoustic emission system along with an acoustic emission sensor placed within the vicinity with the fiber
bragg grating optic fiber allows for comparison of the two signals for clarity and further analysis of signal
propagation can be determined with another signal. If there are small fluctuations within one signal an not the other

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allows for comparison of the signals at a given time to determine why in this case the fiber bragg grating was not
detecting the small change that the other signal. Once determined the change can then be noted and adjustment of
the system is applied. The use of an acoustic emission sensor is chosen with NASA’s application because of the
want for the system to detect where the impact happened is a goal that is to be achieved with a fiber bragg grating
optics system embedded within the structure. Acoustic emission sensors are further along within the development of
placing sensors along the surface and detecting impacts by the use of triangulation and the process of delays and
intensity of the signal. The problem noticed with AE sensors are that of is the sensor capable of measuring intensity.
If so will this still be possible in the event of electromagnetic interference and noise. The reliability of fiber optics is
that this is not an issue, and if affected the signal can easily compensate with noise and electromagnetic interference
is very little with the FBG. Tunable laser was set to run through a sweep of ranges set by us. The range used was
1530nm to 1570 so that the half max peak can be determined. Half max peak is the area that the FGB operates in a
linear fashion which eases the complication of shifting. Next the use of an impacting system to cause a controlled
impact. After the impact a system capable of recording the data is used to gather the information so that later
analysis can be done. The two systems in our case were the use of National Instruments 6250 data acquisitioning
cards along with Physical Acoustics Corporations Micro II Digital acoustic emission system.
III. Procedure
The First step to procuring data that would allow for scrutiny was to get our tunable laser and set this to a
range that would allow us to find the half max peak of the particular fiber that we would be testing as not all fibers
will have the same ranges although they have been specified in a particular wavelength mode. This process is to
ensure that the data collected is fine tuned to a particular fiber thus eliminating and errors that could possibly set off
our results. Once the tuning of a fiber that will be tested has been completed then the next step is to provide the fiber
optic with an optic signal that is tuned via tunable laser. Our choice of wavelength was 1550nm. Finally the impact
system was set up to 1 foot pound per square inch and the impact was done sending a signal that would propagate
through the structure would cause the sensors to pick up the small changes in the material by the use of the FBG;s.
The signal can be seen because the FBG acts will filter out certain wavelengths and others are reflected depending
on how much of change is created within the spacing of each grating. This effect is measured down to the micro
strain. The wavelength that is reflected is determined by the following equation:
𝜆 𝐵𝑟𝑎𝑔𝑔 = 2𝑛 𝑒𝑓𝑓 Λ
Where 𝜆Bragg is the Bragg resonant wavelength, neff is the effective refraction index, and Λ is the periodic variation
(spacing) of the FBG.
𝜺 =
(𝝀−𝝀 𝒃)
𝝀 𝒃
The above relates the strain 𝜺 on the basis in terms of wavelength with 𝝀 𝒃 being the base frequency of the fiber
bragg grating. The base frequencies used in our studies were 1550nm as mentioned earlier. Using the two equations
above assist with turning the data that is read back into strain via the read back wavelengths
IV. Results
Impact data was analyzed with software to determine the peaks of the signals so that the frequencies that were
show during impact could be shown along with their amplitudes and intensity plots.

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Figure 5. Embedded Fiber Composite Board
Figure 5 shows a board that was tested and the results gathered are shown in figure 6 where the board has been
impacted within 1 inch of each other.
Figure 6 Sample Impact Signal

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Figure 7 Sampled Signal
The use of a process named Shearography use the method of exposing the panels to heat and a diffused laser so that
the small changes down to the Nano scale are shown due to the change that will exist between the impacted area and
the panel. The impacted area will absorb and dissipate at a different rate than the rest of the panel due to the fact that
its shape is now different than that of the panel (Figure 8 & 9.).
Figure 8 (diffused laser) Figure 9 (Impact Damage)
V. Conclusion
The use of fiber optics as a sensor is currently in its infancy but can and will be advanced with research.
The current uses of fiber brag gratings and fiber optics are, structural health monitoring, humidity sensors,
temperature sensors, and also strain sensors. The application of FBG’s as a sensor for real time monitoring of
structural health is the emphasis so that weight and systems needed to monitor the structure throughout the duration
of its life can be reduced to simply one portable integrated system.

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Acknowledgments
Dawid M. Yhisreal-Rivas thanks… NASA Marshall Space Flight Center, Dr. Curtis Banks, Dr.Benjamin Penn,
Dr. Virgilio Gonzalez, and finally The University of Texas at El Paso for the opportunity that has etched itself into
the memories of so many.
.
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