High speed/High capacity distributed Fiber Bragg Grating sensing technique for Structural Health Monitoring (SHM) applications

Schedule-driven Non-Destructive Evaluations (NDE) are carried out during structure/equipment’ life to detect major degradations endangering safety and impairing service availability. In addition to NDE, Structural Health Monitoring (SHM) involves the use of in-situ Fiber Bragg Grating (FBG) sensing systems and algorithms to evaluate structure worthiness. FBGs are mostly used as strain/temperature sensors but are also used for acoustic sensing, as substitutes to piezoelectric actuators. The SHM of large structures or acoustic measurements for passive/active tomographic techniques simultaneously require a high capacity and readout rate. However, commercially available FBG readout units rely upon Wavelength-Division Multiplexing (WDM) or Optically Frequency-Domain Reflectometry (OFDR) techniques. WDM-based units are limited in capacity (several tens of sensors) but may reach high scan rate (MHz or beyond MHz). Conversely, OFDR-based units are limited in scan rate (typically several tens of Hz) but may accommodate large number of sensors (typically up to 2000). Tomography with acoustic techniques requires both high capacity and high scan rate with the aim to improve quality of image reconstruction. Optical Time-Stretch (OTS) is a time-domain technique that has potential to improve both capacity and scan rate and to open the way to efficient tomography reconstruction processes. The basics of OTS is to use a pulsed laser, a highly dispersive medium and a high bandpass photodetector in order to convert a Bragg wavelength shift into a time delay. The doctoral candidate will investigate several ways to implement OTS to SHM. Draw-Tower Gratings (DTG) and chirped gratings will be used for the measurement of strain profiles and acoustic field emission on metallic and carbon fiber-reinforced plastics (CFRP) composite structures. The candidate will first assess the performance of the OTS technique in laboratory (LSPM) with piezoelectric actuators and laser-ultrasonics (if available, with CNRS/PIMM). Then, the OTS device will be tested onto several demonstrators provided by partners within the MSCA USES 2 doctoral network: civil engineering structure (BAM, Berlin), hydrogen storage canister (Faber, Cividale del Friuli) and CEA DAM (Le Ripault) and finally onto a metallic pipeline for fluid transport (ENI, Milano). The doctoral candidate will move onto those test sites during three 2-month periods. He will implement the OTS technique and gather experimental feedback.