structural health monitoring aluminum honeycomb sandwich composite panel

NASA URSP – Internship Final Report

Summer 2012 Session 1

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



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.



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



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:

𝜆𝐵𝑟𝑎𝑔𝑔 = 2𝑛𝑒𝑓𝑓 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−𝝀𝒃)

𝝀𝒃

The above relates the strain 𝜺 on the basis in terms of wavelength with 𝝀𝒃 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.



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



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.



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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structural health monitoring aluminum honeycomb sandwich composite panel

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