9 - Sparse array imaging with guided waves under variable environmental conditions

Abstract

Sparse arrays of piezoelectric transducers are being considered for guided wave structural health monitoring of plate-like aerospace components because of their ability to efficiently interrogate large areas for small defects with a relatively small number of sensors. Many imaging methods have been developed to localize damage based upon changes in received signals from multiple transducer pairs relative to baseline data acquired from the undamaged structure. Most of these methods perform well when baselines are acquired under the same environmental conditions as current signals. Here the performance of both delay-and-sum (DAS) and minimum variance (MV) imaging is considered in the context of changing environmental conditions. The well-known optimal baseline selection method is applied along with the proposed baseline stretch, shift, and scale (BS3) method. Both temperature and loading variations are examined experimentally for a variety of damage types (holes, notches, fatigue cracks, and impact damage) and specimens (aluminum and composite plates). It is shown that the efficacy of the compensation method depends upon its specific implementation as well as the imaging method and type of environmental variation. For MV imaging, which is sensitive to the relative amplitudes of scattered signals, it is important to compensate all array signals consistently, whereas for the DAS method, compensation by individual signals in the array is almost as effective. It is also shown that applying the BS3 compensation method in a variable loading environment can significantly degrade imaging results unless loads are matched between baselines and current signals.