Multi-Mode and Multi-Frequency Differential Lamb Wave Imaging with in situ Sparse Transducer Arrays

Application of ultrasonic methods to structural health monitoring requires a compromise between coverage and complexity, making it desirable to use as few transducers as possible to cover the largest area. Such a consideration leads to the use of very sparse arrays where transducers are spatially distributed over a large area and are permanently attached to the structure. For plate-like geometries typical of many structural components, Lamb waves are the ultrasonic wave mode of choice because of their ability to propagate long distances while maintaining a reasonable signal-to-noise ratio. Lamb waves can be difficult to analyze due to both their dispersive nature and the presence of multiple modes. Here we consider both broadband and narrowband excitations of attached piezoelectric transducers, and propose analysis methods using signals recorded from all transducer transmitreceive combinations both before and after introduction of damage. Phased signal addition and image fusion algorithms that utilize information from multiple modes and frequencies are applied to the scattered signals, resulting in differential images of the structure. Results are presented for artificial defects in an aluminium plate, and are compared to those obtained using a single narrowband guided wave mode.