This paper introduces a new type of fiber-optic acoustic emission (AE) sensor, based on a fused tapered 2x2 coupler. Laser light is injected into one input fiber and variations in the splitting ratio, caused by the passage of an ultrasonic pulse through the coupling region, are measured via a differential detection scheme. The sensor was designed to be directly compatible with an existing AE instrumentation system (the MISTRAS system from Physical Acoustics). Sensors were mounted on the surface of and embedded within glass-reinforced polyester laminates. They were excited with simulated AE signals and their performance was compared to that of a conventional piezoelectric AE transducer. The characteristics of the response of the fiber-optic sensor were similar to those of a commercial piezoelectric transducer. A signal-to noise ratio of 45 dB was achieved.
We present three types of intensity-based fiber-optic accelerometer with two axes of sensitivity. Transmission and reflection, or single-ended sensor configurations are compared and bare fibers or a machined brass element are used a seismic masses. The devices are shown to be capable of measuring accelerations at relatively low frequencies (below 1 kHz). Linear responses to acceleration were observed up to the limit of the testing apparatus and the crosstalk between vibration directions was always -12 dB or less.
The processing of advanced fiber reinforced composites requires accurate data on the temperature and chemical composition of the resin system as a function of time. This paper reports on a preliminary study to evaluate two multi- functional sensor arrangements to facilitate in-situ chemical and temperature monitoring in epoxy resin-based thermosets. Sensor arrangements evaluated for chemical analysis include evanescent wave and transmission-based near-infrared spectroscopy; and for temperature monitoring optical fiber Bragg gratings. A comparative analysis was undertaken to ascertain the relative merits of each sensor configuration.
This paper presents preliminary work on a vibration monitoring system for assessing the condition of engineering structures or materials. It consists of an intensity-based fiber-optic vibration sensor, a fast Fourier transform pre- processing stage and a back-propagation neural network. The response of the vibration sensor to sinusoidal acceleration is compared with that of a piezoelectric accelerometer. The sensor was fixed to carbon-fiber composite panels and its response to acoustic transients was investigated. Signals from breaking a pencil lead or dropping a ball bearing on the panel were compared. The system was then trained to distinguish between the sensor response to dropping a ball bearing on panels with either real or simulated impact damage. The trained network then identified previously unseen examples of these signals with complete accuracy.
This paper presents a general overview of intensity-based optical fiber sensor systems, including those developed at Brunel University for condition and health monitoring of engineering materials. The sensor systems covered are: crack detection, a novel technique for converting glass reinforcing fibers into light guides for sensing applications, vibration monitoring and a new intensity- independent demodulation technique for fiber Fabry-Perot interferometers.
A compact intensity-based fiber-optic vibration sensor, suitable for embedment or surface- mounting has been designed and evaluated. It employs simple and inexpensive instrumentation, and is shown to be responsive to frequencies in the range of 100 - 10,000 Hz. It was mounted onto the surface of specimens of carbon fiber reinforced composite, and proved to be capable of differentiating between the vibrational responses of damaged and undamaged panels. An attempt was made to characterize the sensor's output signal by FFT processing. The sensor also survived embedment in a cementitious composite panel. It is proposed that the device can be used as the sensing element of a real-time mechanical-health- monitoring system.
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