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This paper presents test results and performance characterization of the first fixed-wing unmanned vehicle capable of full cross-domain operation in both the aerial and underwater environments with repeated transition and low-energy loitering capabilities. This vehicle concept combines the speed and range of an aircraft with the persistence, diving capabilities, and stealth of a submersible. The paper describes the proof-of-concept vehicle including its concept of operations, the approaches employed to achieve the required functions, and the main components and subsystems. Key subsystems include a passively flooding and draining wing, a single motor and propeller combination for propulsion in both domains, and aerodynamic–hydrodynamic control surfaces. Experiments to quantify the vehicle performance, control responses, and energy consumption in underwater, surface, and flight operation are presented and analyzed. Results of several full-cycle tests are presented to characterize and illustrate each stage of operation including surface locomotion, underwater locomotion, water egress, flight, and water ingress. In total, the proof-of-concept vehicle demonstrated 12 full-cycle cross-domain missions including both manually controlled and autonomous operation. [ABSTRACT FROM AUTHOR]

Fiber Bragg grating (FBG) sensors can be multiplexed in large numbers to monitor the performance of large structures. This paper addresses the collection of FBG reflection spectra from wavelength division multiplexed sensors at fast acquisition rates. The spectral and temporal resolution is first derived as a function of the tunable filter and measurement system properties. The method is applied to impact loading investigations of a stiffened composite skin panel. The reflected spectrum of each FBG in an array, embedded in the panel, is collected at 100 kHz during the impact events with a spectral resolution down to 40 pm. Visualization of the FBG responses to these impact events, including the presence of spectral distortion in some FBG spectra, is presented. Future analyses based on the full-spectral data sets can enable the assessment of the localized progression of internal damage in such structures. [ABSTRACT FROM PUBLISHER]

We experimentally subject a fiber Bragg grating to an unknown, variable temperature gradient. We use the full-spectral response of the grating to determine the magnitude of the gradient over the length of the grating via the full width at quarter maximum bandwidth. The experimental bandwidth and spectrum deformation were compared with a numerical model consisting of an analytical heat transfer model, a finite element analysis model, and the transfer matrix (T-matrix) method. The numerical model showed excellent agreement with the experimental results when the T-matrix method was modified to include the slope of the gradient in addition to the magnitude of the gradient. [ABSTRACT FROM AUTHOR]

To achieve better designs for spacecraft heat shields for missions requiring atmospheric aero-capture or entry/reentry, reliable thermal protection system (TPS) sensors are needed. Such sensors will provide both risk reduction and heat-shield mass minimization, which will facilitate more missions and enable increased payloads and returns. This paper discusses TPS thermal measurements provided by a temperature monitoring system involving lightweight, electromagnetic interference-immune, high-temperature resistant fiber Bragg grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. Such fiber-optic sensing technology is highly sensitive and accurate, as well as suitable for high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. Experimental results are provided to demonstrate the temperature monitoring system using multisensor FBG arrays embedded in a small-size super-light ablator (SLA) coupon which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high-temperature FBG array was fabricated and tested for 1000°C operation, and the temperature dependence considered over the full range (cryogenic to high temperature) for which silica fiber FBGs have been subjected. [ABSTRACT FROM AUTHOR]

Although the practice of repairing or strengthening concrete infrastructure with externally bonded carbon-fiber-reinforced polymer (CFRP) has become common, the development of robust guidelines and techniques for its quality assurance, inspection, and monitoring is lagging significantly. Whereas common inspection methods (e.g., visual inspection or acoustic sounding) are sufficient to broadly identify deboned regions, it is difficult to reliably and accurately define the boundaries of bond defects, as is often required by acceptance criteria. Infrared thermography is a more sophisticated alternative, but the results of its application are influenced by ambient environmental conditions and operator interpretation of the data. Further, for applications in which multiple layers of CFRP are required, none of these techniques are capable of fully characterizing existing bond defects, including the depth through the thickness. To this end, pulse phase thermography (PPT) has the potential to more reliably and accurately detect the location and size of bond defects. By observing images in the frequency domain, one is able to identify a specific "blind frequency" at which a certain defect is no longer visible, and thereby also predict the defect's depth through the thickness. This paper presents a procedure using PPT to develop a calibrated equation based on blind frequency, specific to the strengthening system, which allows for one to predict the interface at which a bond defect is present for externally bonded CFRP-to-concrete systems. With the development of more robust techniques enhancing quality assurance and reliability, and the facilitation of regular monitoring and inspection, infrastructure owners will gain more confidence in this repair or strengthening technology, ultimately leading to its widespread acceptance and adoption. [ABSTRACT FROM AUTHOR]

Describes the balance technique for monitoring the structural integrity of prismatic structures. Composition of prismatic structures; Utilization of finite-length fiber optic displacement sensors; Sensitivity to the presence of damage; Extension of method to anisotropic and nonhomogeneous materials.