Analysis and Modeling of Diffuse Ultrasonic Signals for Structural Health Modeling

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

Analysis and Modeling of Diffuse Ultrasonic Signals for Structural Health Monitoring The 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 as temperature, moisture and strain become smaller and more robust. However, these devices are limited to 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 send ultrasonic energy throughout the entire structural volume and thus have the potential to detect critical changes. Before this goal can become a reality, revolutionary advances must be made in the analysis and processing of the received ultrasonic signals so that structural changes that may lead to catastrophic failure can be reliably detected with an acceptably low false alarm rate. A significant complication is that benign environmental effects such as changes in temperature and surface conditions can cause larger changes in the ultrasonic signals than actual flaws. Intellectual Merit: The research proposed here is a combination of experiments and waveform modeling, and considers the development and verification of signal processing, classification and data fusion methods based upon quantitative changes in the ultrasonic signals. The testing mode considered is that of diffuse ultrasonic waves whereby a point-like impulsive excitation is used to generate multi-modal elastic waves that fill the structure with sound. Ultrasonic diffuse wave theory models the rate of energy decay but does not predict the details of the complex time domain signals. The proposed research will combine time-dependent analysis of coherence with feature extraction, classification methods, signal modeling and simulation, and data fusion to tackle the challenging problem of detecting and characterizing damage. One inherent problem in using classification methods such as neural networks for this application is the need to have a large set of signals from a wide variety of flaws; this is not practical for structures outside the laboratory. A key aspect of the proposed research is to develop methods for perturbing the baseline signal from the undamaged structure in order to emulate a wide variety of structural and environmental changes. Four tasks are defined as follows: 1. Signal Processing and Classification Methods. Development of quantitative differential methods to determine if signal changes are due to structural or environmental effects, and to characterize structural changes as to location, severity, type, etc. 2. Modeling and Simulation of Diffuse Ultrasonic Signals. Modeling of diffuse ultrasonic signals, and simulation of environmental and structural changes. 3. Transducer Placement and Data Fusion. Analysis of optimum transducer organization and placement, and fusing of data from multiple transducers. 4. Design and Implementation of Experimental Measurements. Ultrasonic diffuse wave measurements using metallic, composite and cement-based structures. Broader Impact: This multidisciplinary research program will lead to effective use of ultrasonic sensors for continuously monitoring the health of critical structures, which will enable appropriate action to take place prior to catastrophic failure. Furthermore, the methodologies developed will have broad application to other disciplines such as sonar, radar and biomedical signal processing. A key part of the educational impact is the participation of undergraduates as well as graduate students, and a concerted effort will be made to recruit women and underrepresented minorities. Also proposed is the development of a graduate course in ultrasonic wave propagation and signal processing that will combine the fundamentals of acoustic and elastic wave propagation with signal processing methods as applied 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.