Showing papers by "William H. Prosser published in 2003"

Simultaneous Temperature and Strain Sensing for Cryogenic Applications Using Dual-Wavelength Fiber Bragg Gratings

Abstract: A new technique has been developed for sensing both temperature and strain simultaneously by using dual-wavelength fiber-optic Bragg gratings. Two Bragg gratings with different wavelengths were inscribed at the same location in an optical fiber to form a sensor. By measuring the wavelength shifts that resulted from the fiber being subjected to different temperatures and strains, the wavelength-dependent thermo-optic coefficients and photoelastic coefficients of the fiber were determined. This enables the simultaneous measurement of temperature and strain. In this study, measurements were made over the temperature range from room temperature down to about 10 K, addressing much of the low temperature range of cryogenic tanks. A structure transition of the optical fiber during the temperature change was found. This transition caused splitting of the waveforms characterizing the Bragg gratings, and the determination of wavelength shifts was consequently complicated. The effectiveness and sensitivities of these measurements in different temperature ranges are also discussed.

Sensor Technology for Integrated Vehicle Health Management of Aerospace Vehicles

Abstract: NASA is focusing considerable efforts on technology development for Integrated Vehicle Health Management systems. The research in this area is targeted toward increasing aerospace vehicle safety and reliability, while reducing vehicle operating and maintenance costs. On‐board, real‐time sensing technologies that can provide detailed information on structural integrity are central to such a health management system. This paper describes a number of sensor technologies currently under development for integrated vehicle health management. The capabilities, current limitations, and future research needs of these technologies are addressed.

Concepts and Development of Bio-Inspired Distributed Embedded Wired/Wireless Sensor Array Architectures for Acoustic Wave Sensing in Integrated Aerospace Vehicles

Abstract: This paper discusses the modeling of acoustic emissions in plate structures and their sensing by embedded or surface bonded piezoelectric sensor arrays. Three different modeling efforts for acoustic emission (AE) wave generation and propagation are discussed briefly along with their advantages and disadvantages. Continuous sensors placed at right angles on a plate are being discussed as a new approach to measure and locate the source of acoustic waves. Evolutionary novel signal processing algorithms and bio-inspired distributed sensor array systems are used on large structures and integrated aerospace vehicles for AE source localization and preliminary results are presented. These systems allow for a great reduction in the amount of data that needs to be processed and also reduce the chances of false alarms from ambient noises. It is envisioned that these biomimetic sensor arrays and signal processing techniques will be useful for both wireless and wired sensor arrays for real time health monitoring of large integrated aerospace vehicles and earth fixed civil structures. The sensor array architectures can also be used with other types of sensors and for other applications.

Development of embedded sensor models in composite laminates for structural health monitoring

Abstract: A new improved nonlinear transient generalized layerwise theory for modeling embedded discrete and continuous sensor(s) outputs in laminated composite plates with acoustic emission from cracks and embedded delaminations is developed. The computational modeling involves development of a finite element scheme using an improved layerwise laminate theory for a composite laminate plate with embedded discrete and continuous sensors and embedded discrete delaminations. The simulated cases studied included cantilever plates with embedded sensors and embedded delamination under low frequency vibration and square plates with discrete embedded sensors and continuous embedded sensor architecture and embedded discrete delaminations under high frequency acoustic emission. The effect on sensor outputs due to scattering of the acoustic emission due to the presence of delamination is also investigated. It is expected that this analytical model would be a useful tool for numerical simulation of composite laminated structures with embedded delaminations and embedded sensor architecture, particularly since experimental investigation could often be prohibitive to simulate different conditions.

Multiplexing Technology for Acoustic Emission Monitoring of Aerospace Vehicles

Abstract: The initiation and propagation of damage mechanisms such as cracks and delaminations generate acoustic waves, which propagate through a structure These waves can be detected and analyzed to provide the location and severity of damage as part of a structural health monitoring (SHM) system This methodology of damage detection is commonly known as acoustic emission (AE) monitoring, and is widely used on a variety of applications on civil structures AE has been widely considered for SHM of aerospace vehicles Numerous successful ground and flight test demonstrations have been performed, which show the viability of the technology for damage monitoring in aerospace structures However, one significant current limitation for application of AE techniques on aerospace vehicles is the large size, mass, and power requirements for the necessary monitoring instrumentation To address this issue, a prototype multiplexing approach has been developed and demonstrated in this study, which reduces the amount of AE monitoring instrumentation required Typical time division multiplexing techniques that are commonly used to monitor strain, pressure and temperature sensors are not applicable to AE monitoring because of the asynchronous and widely varying rates of AE signal occurrence Thus, an event based multiplexing technique was developed In the initial prototype circuit, inputs from eight sensors in a linear array were multiplexed into two data acquisition channels The multiplexer rapidly switches, in less than one microsecond, allowing the signals from two sensors to be acquired by a digitizer The two acquired signals are from the sensors on either side of the trigger sensor This enables the capture of the first arrival of the waves, which cannot be accomplished with the signal from the trigger sensor The propagation delay to the slightly more distant neighboring sensors makes this possible The arrival time from this first arrival provides a more accurate source location determination The multiplexer also identifies which channels are acquired by encoding TTL logic pulses onto the latter portion of the signals This prototype system was demonstrated using pencil lead break (Hsu-Neilsen) sources on an aluminum plate It performed as designed providing rapid low noise trigger based switching with encoded channel identification this multiplexing approach is not limited to linear arrays, but can be easily extended to monitor sensors in planar ot three dimensional arrays A 32 channel multiplexing system is under development that will allow arbitrary sensor placement Another benefit of this multiplexing system is the reduction in the expense of data acquisition hardware In addition, the reduced weight and power requirements are of extreme importance for proposed AE systems on aerospace vehicles

Ultrasonic lamb waves in a thin plate

Abstract: The behavior of the wave field produced in a thin unidirectional graphite/epoxy composite plate by a dynamic point load is studied using an approximate shear deformation plate theory (S.D.P.T) and a finite element analysis (F.E.A). Comparisons are made for propagation at 0 o , 45 , and 90 directions relative to the fibers showing excellent agreement between the two model approaches. The approximate method is then used to calculate the response of a composite plate as well as of an aluminum plate to a uniform dynamic surface load distributed in a circular region. A periodic reversal in the phase of the signal with propagation distance is observed. It is found that this is caused by the strong dispersion of the first antisymmetric waves at low frequencies. For clarification, the steepest descent method is applied to obtain a closed form analytical expression for the far field response in the aluminum plate for a Dirac delta source. It is shown that the waveform carries a singularity that reverses its phase at regular intervals. The present work should be helpful in understanding the nature of waveform signals produced by impact loads and in the detection and characterization of impact damage in composite structures. Keywords: Thin plate, unidirectional graphite/epoxy composite, periodic phase reversal, low frequency, first antisymmetric mode, the steepest descent method