Health Monitoring of Submerged Plates Using Ultrasonic Guided Waves

Abstract

Several offshore and marine infrastructural systems involve plate like members in submerged condition and are often subjected to extreme service and environmental conditions leading to deterioration in the form of corrosion loss, fatigue cracking and other mechanical degradations. Submerged condition of such structures often makes them inaccessible for various established non-destructive technologies. Most of these conventional techniques approaches require interruption of normal service and removal of the subject structure from submerged condition. The prohibitive out-of-service inspection costs restrict the frequency of such investigations, thus exposing them to a great risk of unnoticed fatal damages. Hence, there is a need to supplement or replace these methods with an efficient, reliable, in-situ, non-contact and non-destructive monitoring technique for submerged plate assemblies. The present work reports a laboratory study using a pair a mobile non-contact probes arranged in pitch-catch orientation employing immersion coupling utilizing Leaky Lamb waves for assessing and characterizing notch and corrosion damages in submerged plates. The propagation characteristics of the different Lamb wave modes in a submerged plate are studied. The longitudinal wave excited by cylindrical transducer falls obliquely on the submerged plate using surrounding water as natural couplant. The transmitted signal is received by other transducer after it has traversed through the length of the plate. Use of water coupled transducers makes the system non-contact and non-invasive. Interaction of propagating waves with the simulated damages in the form of machined notches has been studied. By comparing the transmitted signals of the healthy plate with that of the notched plate, damage monitoring technique is developed. Specific Lamb wave modes sensitive to near surface and sub-surface damages have been identified.