Embedded passive guided wave tomography. Application to corrosion monitoring in multilayered pipes

Guided wave tomography techniques have been studied in the last decade to quantitatively image defects in waveguides such as plates or pipes. Recent developments based on the use of an autocalibration method, removing the need of a reference acquisition, enabled the absolute imaging of the structure and drastically reducing the sensitivity to environmental and operational conditions. Another interesting feat for the structural health monitoring (SHM) is the use of passive methods relying on the measurement of the ambient noise, related to the use of the structure and its environment, and signal processing based on a cross-correlation to recover signals equivalent to the ones measured when a sensor is emitting. Such system prevents from the need of cumbersome circuitry and energy consumption related to wave emission. It also enables the sole use of sensors that cannot emit waves or only with a poor actuation such as optical fiber sensors. A SHM system relying solely on Fiber Bragg Gratings (FBGs) in optical fibers can then be designed. Several FBGs can be enscribed in a single optical fibers, reducing further the size and weight of the system. The optical fibers being resistant to harsh environments such as extreme temperatures or ionizing environments and having a good integration capability, such a system meets the needs for an industrial application. This talk will hence show the principle of the passive guided wave tomography with embedded sensors in a structure. It will be exemplified in the case of a multi-layered structure, showing the possibility to apply this system in a large panel of cases.