Instantaneous Baseline Imaging Using Passive Ultrasonic Guided Waves from Fiber Bragg Grating (FBG) Sensors

The detection and quantification of damage plays a crucial role in several industries. Assessing their severity enables to schedule maintenance appropriately, optimizing costs and reducing risks. Thanks to their long range and high sensitivity to defects, ultrasonic guided waves allow for the Structural Health Monitoring (SHM) of various structures using only a small number of sensors. However, the use of traditional piezoelectric transducers requires extensive wiring installation that weighs on the SHM system’s costs and that may affect its reliability, and is limited to certain temperature and radiation ranges. To that extent, Fiber Bragg Grating (FBG) sensors offer an alternative instrumentation solution that withstands much higher temperatures and radiation levels, with a simplified installation thanks to the multiplexing of several sensors on the same optical fiber. They do not allow for actuation however, but passive guided waves techniques can be used to reconstruct active signals by exploiting the ambient excitation created by Environmental and Operational Conditions (EOCs). Furthermore, these EOCs and especially temperature have a significant impact on ultrasonic signals (independently of the instrumentation used, active or passive), which often exceeds that of the sought-after defects and thus limits the use of a baseline acquired from the structure in its pristine state. The instantaneous baseline technique addresses this issue by using damage free signals of the current state to create a statistical baseline, from which similar paths can be compared. In this study, passive ultrasonic signals acquired experimentally from FBG sensors are used for corrosion imaging through an instantaneous baseline technique. A thick steel plate, which is damaged locally using accelerated corrosion, is instrumented with multiplexed FBGs as well as piezoelectric sensors. Passive ultrasonic signals are acquired by both FBG and piezoelectric sensors using a compressed air jet as the excitation source, and active signals generated with the latter are also used for comparison. The instantaneous baseline method is applied to the signals and coupled with classical imaging techniques such as RAPID. Results obtained from the FBG passive and piezoelectric passive and active setups are then compared and discussed.