Optical-Fiber Sensors for the Detection of Acoustic Emission
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Optical-Fiber Sensors for the Detection of Acoustic Emission
William B. Spillman Jr. and Richard O. Claus Abstract Acoustic emission (AE) is used as a means to anticipate the mechanical failure of critical materials and structures by detecting the release of energy caused by material rearrangement at the microlevel. Optical-fiber sensors have potential advantages over conventional tuned piezoelectric transducers and signal-processing methods for the detection of such types of ultrasonic acoustic wave events. A number of fiber Bragg grating techniques are presented, which in particular offer the potential to provide the high-speed signal processing and ability to multiplex numbers of AE sensors necessary to detect, quantify, and locate AE sources and thereby determine material properties and damage. Keywords: acoustic emission, optical-fiber sensors, signal processing.
Nature and Significance of the Problem: Acoustic Emission Detection Advanced vehicles, structures, and systems depend upon the long-term integrity of a large number of material components in order to function safely and effectively. To maximize long-term performance, large numbers of material samples from which critical components are fabricated are typically loaded to failure, and statistical data are obtained concerning their inherent failure behavior. Based on that data, structures that incorporate such materials are designed with anticipated safety factors and probabilistic lifetimes. In addition, nondestructive evaluation (NDE) techniques are used to periodically inspect materials for critical flaws induced by damage or that lie outside of the typical distribution of material properties.1 Parts are either replaced or repaired on schedule, or in response to such NDE analysis. The fundamental problem is that this process is largely interpolative, in that material failure cannot, in general, be predicted. The detection of acoustic emission (AE) events in materials offers one method whereby some types of material failure may in some cases be anticipated. AE is associated with a local release of energy within a material due to structural rearrangement at the molecular level, typically caused by
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mechanical stress. The release of energy generates an acoustic stress wave within the material, hence the term “acoustic emission.” Because the release of energy occurs quickly, the temporal rise time of the stress wave is short, so the associated bandwidth of the AE signals is large. Conventional piezoelectric crystal-based AE transducers have a center frequency of 2.25 MHz, in order to detect such events. Depending upon the nature of the material and the energy release mechanism (fiber breakage or matrix cracking in advanced polymerbased composites, or slip, dislocation, or twinning in metals), high-frequency information may be used to determine the nature of the mechanism and thus indicate how the material is beginning to fail at the microlevel. By simply counting AE events and noting the frequency of occurrence of events during material loading, the
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