Structural Damage Location with Fiber Bragg Grating Rosettes and Lamb Waves

The aim of this study is to present the results of testing a damage detection and damage localization system based on fiber Bragg grating sensors. The objective of the system is to detect and locate damage in structures such as those found in aerospace applications. The damage identification system involves Bragg gratings for sensing ultrasound by detecting the linear strain component produced by Lamb waves. A tuneable laser is used for the interrogation of the Bragg gratings to achieve high sensitivity detection of ultrasound. The interaction of Lamb waves with damage, e.g., the reflection of the waves at defects, allows the detection of damage in structures by monitoring the Lamb wave propagation characteristics. As the reflected waves produce additional components within the original signal, most of the information about the damage can be found in the differential signal of the reference and the damage signal. Making use of the directional properties of the Bragg grating the direction of the reflected acoustic waves can be determined by mounting three of the gratings in a rosette configuration. Two suitably spaced rosettes are used to locate the source of the reflection, i.e., the damage, by taking the intersection of the directions given by each rosette. A genetic algorithm (GA) can be used to calculate that intersection and to account for any ambiguities from the Lamb wave measurements. The performance of the GA has been studied and optimized with respect to the localization task. Initial experiments are carried out on an aluminum structure, where holes were drilled to simulate the presence of damage. The results show very good agreement between the calculated and actual positions of the damage.