A new method for imaging flaws in plate and shell structures is presented. The method employs twodimensional ultrasonic surface wave data obtained by optical electronic speckle pattern interferometry (ESPI) techniques. In the imaging method, the measured out-of-plane displacement field associated with an externally excited ultrasonic Lamb wave is processed to obtain the spatial frequency domain spectrum of the wavefield. A free space Green's function is then deconvolved from the wavefield to obtain quantitative images of effective scattering sources. Because the strength of these effective sources is directly dependent on local variations in sample thickness and material properties, these images provide a direct map of internal inhomogeneities. Simulation results show that the method accurately images flaws for a wide range of sizes and material contrast ratios. These results also demonstrate that flaw features much smaller than an acoustic wavelength can be imaged, consistent with the theoretical capability of the imaging method to employ scattered evanescent waves. Reconstructions are also obtained from ultrasonic Lamb wave displacement fields recorded by ESPI in a flawed aluminum plate. These reconstructions indicate that the present method has potential for imaging flaws in complex structures for which ESPI wavefield measurements cannot be straightforwardly interpreted.

Abstract-This paper considers the use of combined spatial and temporal filtering to improve the quality of a Structural Health Monitoring (SHM) damage estimate. Many SHM systems produce two-dimensional (2-D) array data that contains structure damage estimate, which is distorted by noise from sensor sources, changing environmental conditions, and other inspection factors. We describe a filtering architecture for processing a sequence of damage estimates generated by a SHM system. We show that the approach can reduce the noise variability and enhance the damage estimate. The filter is designed as an Infinite Impulse Response (IIR) filter in time and space and is predicated on an understanding of the point spread function for the ultrasonic interrogation. We discuss the basis for using a spatially invariant blurring operator to represent this interrogation process as well as the design of the filter. The filtering is performed in space by rejecting component of the input signal outside of the spatial frequency pass band. The filtering is also performed in time by rejecting the components of the signal outside of the dynamical pass band.

Measurement of surface displacements due to ultrasonic wave propagation in elastic solids has traditionally been studied using single-point measurement techniques such as ultrasonic transducers or interferometers. Full-field methods, excluding scanning techniques, are uncommon; examples include holographic interferometry and Schlieren imaging. In many cases, these techniques have been used to yield qualitative results due to inherent difficulties in processing the data. Laser-modulated phase-stepping digital shearography is a full-field, common-path, interferometric method which can be used to quickly visualize ultrasonic wave fields. A shearography system will be described that provides not only whole-field images of ultrasonic waves, but quantitative displacement data within these fields. Images are shown of propagating plate and bar waves, and mathematics relating the calculated optical phase to the ultrasonic wave parameters are presented.

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Ultrasonic elastic waves have traditionally been examined using single point measurement techniques. in this paper, we present a technique capable of recording out-of-plane ultrasonic displacement data over wide areas. The method employs two-dimensional surface vibration data collected via electronic speckle pattern interferometry used in combination with laser modulation and optical phase stepping. Using this rapid, wide-area measurement method, images of ultrasonic Lamb waves on ideal and flawed plate structures are presented. For a single ultrasonic excitation frequency, multiple data records are collected as an externally excited traveling plate wave moves through the sample. When an optical phase shift is introduced between the data records, the data can be processed to yield quantitative displacement fields. The resulting processed images demonstrate evidence of Lamb wave reflection, transmission and scattering. Following the full field data collection, a novel inverse scattering algorithm was applied to reconstruct images of the scattering sources responsible for the measured displacement field data. The results of these investigations will be presented and discussed in the context of detecting hidden flaws in aging aircraft.