Analysis and Modeling of Diffuse Ultrasonic Signals for Structural Health Modeling

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Analysis and Modeling of Diffuse Ultrasonic Signalsfor Structural Health MonitoringThe use of permanently mounted sensors to monitor the health of critical structures such as airplanes,bridges and buildings is quickly becoming a reality as sensors for measuring such physical quantities astemperature, moisture and strain become smaller and more robust. However, these devices are limitedto point, or local, measurements and thus do not truly interrogate the volumetric state of the structure.Sparse arrays of permanently mounted ultrasonic sensors, acting as both sources and receivers, can sendultrasonic energy throughout the entire structural volume and thus have the potential to detect criticalchanges. Before this goal can become a reality, revolutionary advances must be made in the analysisand processing of the received ultrasonic signals so that structural changes that may lead to catastrophicfailure can be reliably detected with an acceptably low false alarm rate. A significant complication isthat benign environmental effects such as changes in temperature and surface conditions can causelarger changes in the ultrasonic signals than actual flaws.Intellectual Merit: The research proposed here is a combination of experiments and waveformmodeling, and considers the development and verification of signal processing, classification and datafusion methods based upon quantitative changes in the ultrasonic signals. The testing mode consideredis that of diffuse ultrasonic waves whereby a point-like impulsive excitation is used to generate multi-modalelastic waves that fill the structure with sound. Ultrasonic diffuse wave theory models the rateof energy decay but does not predict the details of the complex time domain signals. The proposedresearch will combine time-dependent analysis of coherence with feature extraction, classificationmethods, signal modeling and simulation, and data fusion to tackle the challenging problem of detectingand characterizing damage. One inherent problem in using classification methods such as neuralnetworks for this application is the need to have a large set of signals from a wide variety of flaws; thisis not practical for structures outside the laboratory. A key aspect of the proposed research is todevelop methods for perturbing the baseline signal from the undamaged structure in order to emulate awide variety of structural and environmental changes.Four tasks are defined as follows:1. Signal Processing and Classification Methods. Development of quantitative differential methods todetermine if signal changes are due to structural or environmental effects, and to characterizestructural changes as to location, severity, type, etc.2. Modeling and Simulation of Diffuse Ultrasonic Signals. Modeling of diffuse ultrasonic signals, andsimulation of environmental and structural changes.3. Transducer Placement and Data Fusion. Analysis of optimum transducer organization andplacement, and fusing of data from multiple transducers.4. Design and Implementation of Experimental Measurements. Ultrasonic diffuse wavemeasurements using metallic, composite and cement-based structures.Broader Impact: This multidisciplinary research program will lead to effective use of ultrasonicsensors for continuously monitoring the health of critical structures, which will enable appropriateaction to take place prior to catastrophic failure. Furthermore, the methodologies developed will havebroad application to other disciplines such as sonar, radar and biomedical signal processing. A key partof the educational impact is the participation of undergraduates as well as graduate students, and aconcerted effort will be made to recruit women and underrepresented minorities. Also proposed is thedevelopment of a graduate course in ultrasonic wave propagation and signal processing that willcombine the fundamentals of acoustic and elastic wave propagation with signal processing methods asapplied to ultrasonics. This research program will also complement other efforts at Georgia Tech,effectively creating a critical mass of research in ultrasonics for structural health monitoring.