Composite-Overwrapped Pressure Vessels:Composite-Overwrapped Pressure Vessels:Towards High Resolution In Situ Structural HealthTowards High Resolution In Situ Structural HealthMonitoring Using Optical Fiber Bragg Grating SensorsMonitoring Using Optical Fiber Bragg Grating Sensors1. Background1. Background

What is an FBG?What is an FBG?An FBG sensor is a specialized structure placed inside an optical fi ber to refl ect a specifi c wavelength of light, i.e. λ = 1500 nm. When the fi ber is stretched or compressed, the grating refl ection wavelength shifts up or down, respectively.

What does a FBG measure?What does a FBG measure?The FBG measures strain, which is a unit-less ratio representing the change in length experienced by a fi ber. The strain value is recovered based on the shift in refl ected wavelength.

How are FBGs used?How are FBGs used?FBGs are embedded into objects, creating “smart structures.” Common applications are bridges and airplanes. They measure structural health in a non-destructive and real-time manner.

What is a COPV?What is a COPV?A COPV starts with a thin shell of metal, i.e. aluminum or titanium, and is overwrapped with fi ber composite, i.e. Kevlar or carbon fi ber. This technique reduces weight while providing maximum pressurization capability; having high effi cacy to the space program where minimizing weight is critically important.

2. Experiment2. ExperimentThis project analyzed data from an experiment conducted July 2006 at the NASA White Sands Test Facility. The experiment took a 40” spherical COPV to burst and employed 36 FBG sensors measuring strain; 29 measuring latitudinal strain on 25 planes and 7 measuring longitudinal strain on 1 plane (please see fi gure).

3. Mission & Method3. Mission & MethodThe goal set by the student was to model COPV deformation using only FBG sensor data.

Step One: Setup DataStep One: Setup DataThe fi rst step involves basic data processing--setting up the experiment confi guration, combining and sequencing data from multiple interrogator fi les, exponential data smoothing, and rule-based removal of failed sensors.

Step Two: InterpolationStep Two: InterpolationSensor data is scattered across the sphere. However, there is insuffi cient data to immeditately develop a rigorous mathematical model. The concept of a virtual sensor array was devised, providing adequate granularity. Data from the real sensors was interpolated.

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A Delaunay triangulation was used to partition the scattered sensors along the surface of the sphere. Every virtual sensor is triangulated to and interpolated from three real sensors and their values.

Step Three:Step Three:Surface ModelingSurface ModelingBased on the positions of 16 neighboring virtual sensors, 48 independent equations can be formulated. Solving this system fi nds the constants for a cubic interpolating parametric polynomial in two dimensions.

Green andyellow correspond to latitudinal and longitudinal sensors, respectively.

Red corresponds to the Delaunay triangulation, computed based on real sensor positions.

Abstract: A framework of mathematical analysis and model visualization was developed in order to assess the structural health of a Composite-Overwrapped Pressure Vessel (COPV) based on data acquired from multiple embedded Fiber Bragg Grating (FBG) sensors during pressurization from ambient to burst.

The vesselThe vessel

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This provides ample degrees of freedom to model the sphere surface, subdivided into patches of 4x4 virtual sensors with overlap.

Source: Angel (2009)

The strain for every virtual sensor is computed based on its position using limits and arc length quadrature via the derived cubic polynomial.

Step Four: OptimizationStep Four: OptimizationDenote by the function f(x) the sum of the absolute difference between calculated strain (using our cubic polynomial) and

sampled strain (the interpolated values) for all virtual sensors in our experiment. With sensor positions given in x, we can formulate the nonlinear optimization problem min f(x) to adjust these positions so that calculated and sampled strain coincide. The result, a model of the deformed sphere.

4. Conclusion4. ConclusionA novel framework using FBG sensor data has been developed enabling sophisticated volumetric and surface analysis of pressurized vessels.

Presenter:Derrick Tobias BabbIndustrial and Systems EngineeringUniversity of FloridaMentors:Dr. Benjamin PennDr. Curtis BanksEV 43, NASA Marshall

Purple corresponds to virtual sensors, whose positions are adjusted through nonlinear optimization to match the interpolated strain.