Quantification of Surface Wetting in Plate-like Structures via Guided Waves

Ultrasonic guided wave methods are being widely explored for inspection of large structural areas using a sparse array of surface-mounted transducers. However, structural health monitoring (SHM) with guided waves can be problematic because variable operational conditions can significantly affect the measured ultrasonic signals. Although homogeneous temperature variations have been well-researched and successful compensation algorithms developed, few studies have considered distributed surface wetting. This paper reports results from an experimental investigation of the effects of surface wetting on guided wave signals recorded from a spatially distributed array affixed to a stiffened aluminum plate. Droplets of water were applied to the plate in a random and increasing manner until almost forty percent of the plate was wet, and the percent surface area covered at each increment of wetting was independently measured via digital photographs of the plate surface. Ultrasonic guided wave signals resulting from a broadband chirp excitation were recorded from all transducer pairs in between increments of wetting. Root mean squared errors as compared to the dry plate are calculated from these signals at multiple frequencies to investigate the efficacy of quantifying the degree of surface wetting. This study is a first step in the process of assessing and addressing the effects of distributed surface wetting on the performance of a guided wave SHM system.