Monitoring and characterizing corrosion in aluminum using Lamb waves and attached sensors [6532-52]

Corrosion is detrimental to the structural integrity of many critical components, and ultrasonic methods are routinely used in the field to make thickness measurements at points of interest. However, is often difficult to assess the true extent of corrosion damage because of the likelihood of missing small corroded areas and the difficulty in mapping the extent of large corroded areas without an extensive number of time consuming measurements. Guided ultrasonic waves have the potential to both detect corrosion as early as possible and reduce the subsequent inspection time. This paper presents results from a study using Lamb waves to quantify the area extent of corrosion in an aluminum plate specimen. A sparse array of ultrasonic transducers was attached to an aluminum plate, and broadband excitation methods were used to generate both symmetric and anti-symmetric Lamb wave modes. As has been demonstrated in previous studies, the through transmission response recorded from each transmit-receive pair may be analyzed to determine if a defects exists and approximately determine its location. This paper presents a method to determine the exact location and quantify the extent of the corroded area using an acoustic wavefield imaging method. Lamb waves are generated using one of the permanently attached transducers as a source, and the acoustic wavefield is captured on the surface of the plate using an air-coupled transducer as a receiver. Full wavefield data are recorded as the receiver is scanned over the specimen surface, and wavefield images are processed to remove the strong incident wave and enhance the weaker scattered waves. The amplitude at the crest of the leading Lamb mode (S0) is analyzed to produce spatial images of defective areas. Measured length and area results from these images compare very favorably with actual defect sizes. Results are also presented for scattering from a through hole with a simulated crack.