Enhancing the defect localization capability of a guided wave SHM system applied to a complex structure

Abstract

Sparse-array structural health monitoring systems based on guided waves have been proposed by many authors, current signals being compared with a baseline obtained when the structure was known to be defect free. An image of the structure in the form of a ‘C-scan’ map showing likely defect locations can be produced by combining information from different sensor pairs in the array. It is generally recognized that temperature compensation is essential for the method to work and various compensation methods have been proposed with good results. However, artifacts are commonly seen in the images, making reliable defect location difficult. This is because, as well as the first reflection from the defect that maps to the correct defect location in the image, shadowing effects occur later in the signal and these combine to produce artifacts. This effect can be reduced by appropriate gating of signals, although at a cost in area coverage. If images are formed with multiple different gate locations, the artifact positions and intensities change, but the defect always produces a strong indication. Therefore by combining multiple images, the artifacts can be suppressed and the defect is located more reliably. This strategy has been successfully demonstrated for defects both within the transducer array and close to an edge of both a simple plate and a shipping container door.