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Detecting damage in structures that are in service and operating is difficult using conventional non-destructive evaluation techniques. This thesis examines the use of acoustic emission and resulting waves in the structure to determine damage in the structure. In order to detect and measure the waves generated continuous sensors are used. Continuous sensors contain multiple interconnected sensor nodes that form an array of sensors covering the whole structure. A new concept of active fiber composite sensor is added to the continuous sensor. The use of active fiber sensor brings the possibility of unidirectional sensing in continuous sensor. The advantage of this passive health monitoring approach is that the sensors are highly distributed and uses parallel processing allowing large structures to be monitored for damage using a small number of channels of data acquisition. In the thesis, the continuous sensor is modeled and simulated by solving the elastic response of a plate and the coupled piezoelectric constitutive equations. The model and simulation allow the sensor to be optimized for a particular material and plate size. The simulation predicts that acoustic waves representative of damage growth can be detected using continuous sensors. The simulation results show the possibility of unidirectional sensing and give some insight into the sensor response. Based on the simulation results the unidirectional sensor are constructed and tested. To improve the sensitivity of the continuous sensor, unidirectional active fiber composite sensors were built from piezoceramic ribbon preforms. Different designs and sensor configurations are examined and advantages are discussed. The sensor design proposed is manufactured in Smart Structures and Bio-Nanotechnology Laboratory. Step by step manufacturing of the active fiber composite sensors is also discussed in the thesis. The continuous sensors constructed in the lab are evaluated in a realistic test to show their ability to detect acoustic emissions caused by damage to a composite material. The sensors were mounted on narrow glass fiber plates and tested to failure in a mechanical test machine. The sensors were able to detect the propagation of crack when the structure containing the damage was still intact and was in service. Results from the experiments are presented and explained in relevant section of this thesis. Use of active fiber sensor as continuous sensors can potentially reduce the cost, complexity, and number of channels of data acquisition so that this technique becomes practical to perform structural health monitoring. |
