Extracting guided waves from cross-correlations of elastic diffuse fields: Applications to remote structural health monitoring

Due to their apparent complexity, diffuse fields are not commonly used by conventional structural health monitoring methodologies. However, recent theoretical and experimental studies have demonstrated that local Green's functions (GFs) can be estimated from the cross-correlation of elastic diffuse fields (CDFs) recorded between any points of a sensor grid and generated by sources located remotely from the monitoring area. Hence, this CDF technique effectively transforming each measurement point into a virtual elastic source, thus allowing to measure the local structural response (or GF). Theoretical and experimental studies are conducted to assess the broadband performance of the CDF technique for the extraction of low-frequency coherent Lamb waves (4-90 kHz) propagating in a thin aluminum plate of complex geometry using a scanning laser Doppler vibrometer. The coherent signal-to-noise ratio and the accuracy of the phase and group velocity measurements of the coherent A(0) mode obtained from CDF are also measured to quantify the influence of the spatio-temporal characteristics of the recorded diffuse field, the number and spatial configuration of the remote ultrasonic sources, and the influence of incoherent measurement noise (e.g., sensor noise). In addition, a criterion to assess the emergence rate of late coherent coda arrivals from CDF is also formulated. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3257602]