A systematic study of acoustic emission detection using fiber Bragg grating sensors is presented. In this, we attempt to use the fiber Bragg grating to sense the dynamic strain created by a passing ultrasonic wave signal. Our goal is to see if such a sensor is possible, and if so, what the detection sensitivity and limitations will be. To answer these questions, we carried out several experiments involving the detection of simulated acoustic emission events. In the first experiment, we attach a fiber Bragg grating to the surface of a piezoceramic resonator, which is driven by a signal generator. We were able to detect the resulting surface vibration of the resonator up to 2.1 MHz. In the second experiment, we attach a fiber Bragg grating to the surface of an aluminum plate. We excite an acoustic wave using an ultrasonic transducer located at various positions of the aluminum plate. In this way, we demonstrated that the fiber Bragg Grating sensor is capable of picking up the signal coming from a distance (up to 30 cm) for up to 2.5 MHz. In a third experiment, we use the same fiber Bragg grating on aluminum plate set up, but set up an acoustic signal by either a gentle knock on the plate by a pin, or by breaking a pencil lead on the plate. We were able to detect acoustic emission set up by pencil lead breaking up to a frequency of 30 kHz. Higher frequency components were not detected mainly due to the limitation of available electronic equipment at this time (higher frequency band-pass filters and amplifiers). In all the above-mentioned experiments we use a match Bragg grating to demodulate the detected optical signal and use a dual channel scheme for electronic data acquisition and processing (a signal channel and a reference channel).
ABSTRACTmA fiber optic sensor capable of measuring two independent components of transverse strain is described. The sensor consists of a single Bragg grating written into highbirefringent, polarization-maintaining optical fiber. When light from a broadband source is used to illuminate the sensor, the spectra of light reflected from the Bragg grating contain two peaks corresponding to the two orthogonal polarization modes of the fiber. Two independent components of transverse strain in the core of the fiber can be computed from the changes in wavelength of the two peaks if axial strain and temperature changes in the fiber are zero or known. Experiments were performed to determine the response of the sensor by loading an uncoated sensor in diametral compression over a range of fiber orientations relative to the loading. The results of these experiments were used with a finite element model to determine a calibration matrix relating the transverse strain in the sensor to the wavelength shifts of the Bragg peaks. The performance of the sensor was then verified by measuring the transverse strains produced by loading the fiber in a V-groove fixture.KEY WORDSmBragg grating, fiber optic sensor, transverse strain, polarization-maintaining filter Fiber optic sensors can be embedded in polymer-matrix composites and other materials to measure internal strain, temperature and other parameters. These small, highly responsive sensors have been demonstrated in many applications, including manufacturing process monitoring, impact and damage detection and structural health monitoring.1 Fiber optic strain sensors have been the focus of increasing attention as a potentially low-cost, nondestructive means of determining the internal strains and stresses in a material.A schematic of an embedded fiber optic strain sensor is shown in Fig. 1. The axes xl-x2-x3 form the basis for the fiber coordinate system, where Xl is parallel to the axial direction of the fiber and the transverse directions (x2 and x3) are located in the plane of the cross section of the fiber. An embedded sensor may be subjected to an arbitrary strain field, ~, consisting of six components, 8i (i = 1 ..... 6), where the components of strain are presented in contracted notation Original manuscript submitted. " January 14, 1998. Final manuscript received: January 19, 1999 within the fiber; however, in this paper, the terms ei will refer to the average strains in the core of the fiber.Most fiber optic strain sensors are capable of measuring only the axial component of strain in the fiber (e 1). In embedded applications, it is often desirable to measure components of strain transverse to the optical fiber for several reasons. First, for stress measurements, the state of stress within a material will be a function of the complete state of strain, and cannot be computed from a single axial strain measurement. Several individual fiber optic sensors, oriented in different directions, would be required to determine the complete state of strain in these applications.2 A larg...
Fiber-optic sensor technology has experienced tremendous growth since its early beginnings in the 1970s with early laboratory demonstrations of fiber-optic gyros and acoustic sensors and the introduction of the first commercial intensity and spectrally based sensors. These early efforts were followed by a tremendous growth of interest in the 1980s when the number of workers in the field increased from perhaps a few hundred to thousands. The result was the introduction in the 1990s of the first mass produced fiber-optic sensors that are being used to support navigation and medical applications. The number of fiber-optic sensors products can be expected to grow tremendously in the years to come as rapid progress continues to be made in the related optoelectronic and communication fields. This paper provides an overview of some of the technologies being used to support fiber-optic sensor development and how they are being applied.
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