This paper reports the testing results of radiation resistant fiber Bragg grating (FBG) in random air-line (RAL) fibers in comparison with FBGs in other radiation-hardened fibers. FBGs in RAL fibers were fabricated by 80 fs ultrafast laser pulse using a phase mask approach. The fiber Bragg gratings tests were carried out in the core region of a 6 MW MIT research reactor (MITR) at a steady temperature above 600°C and an average fast neutron (>1 MeV) flux >1.2 × 10 n/cm/s. Fifty five-day tests of FBG sensors showed less than 5 dB reduction in FBG peak strength after over 1 × 10 n/cm of accumulated fast neutron dose. The radiation-induced compaction of FBG sensors produced less than 5.5 nm FBG wavelength shift toward shorter wavelength. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal sensitivity and temporal responses of FBGs were determined at different accumulated doses of neutron flux. Results presented in this paper reveal that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers can survive harsh in-pile conditions. Despite large parameter drift induced by strong nuclear radiation, further engineering and innovation on both optical fibers and fiber devices could lead to useful fiber sensors for various in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

In this paper are presented imaging results of defects in composite plates using guided wavebased algorithms such as Delay and Sum and Excitelet. Those algorithms are applied to passive data, for which the signal corresponding to each emitter-receiver couple is recovered thanks to the cross-correlation of the ambient noise measured simultaneously by the two sensors. The transition to passive imaging allow the use of lighter sensors unable to emit ultrasonic waves such as Fiber Bragg Gratings sensors on optical fibers, which are used in this study. The imaging results presented here show the feasibility of active and passive imaging in composite plates using Fiber Bragg Gratings as receivers, reducing the impact of the acquisition system on the structure in the context of Structural Health Monitoring.

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Isentropic compression of metallic samples under High Pulse Powers (HPPs) are performed at CEA-Gramat. Large strains and high strain rates are obtained under intense magnetic field-driven Laplace forces. On account on electromagnetic immunity, a dedicated Fiber Bragg Grating (FBG) mainframe was designed by CEA LIST. Ring aluminum samples with crossed FBGs bonded onto the external surface were tested on the CYCLOPE HPP facility. Dynamic strain measurements were performed along with Photonic-Doppler Velocimetry (PDV). FBG strain data compare well to strain data derived from PDV displacements, leading the way to direct dynamic extensometry in the purpose of improving Magneto-Hydro-Dynamics (MHD) codes.

Structural health monitoring (SHM) consists in embedding sensors in a structure like aircraft fuselages, pipes or ship hulls in order to detect defects (for example cracks or corrosion in metallic materials or delamination in composite materials) before a serious fault occurs in the structure. Guided elastic waves emitted by a sensor and propagating to another one are often used as the physical way of detecting the defect. However, the implementation of SHM systems is restricted in many situations by the necessity to store or to harvest the electric energy necessary to emit the waves and also by the intrusiveness of the sensors. Guided wave tomography imaging is able to localize and quantify the severity of the defect when it comes to loss of thickness such as corrosion or erosion. However, it needs many sensors (generally piezoelectric -PZT -transducers) and it has a cost, particularly in terms of intrusiveness. The idea in this paper is to use less intrusive sensors such as fiber Bragg Gratings (FBG) to perform guided wave tomography. The inspected area of a pipe is surrounded by two rings, one with PZT and the other one with FBG.