Composite materials are increasingly used in aerospace applications, owing to their high strength-to-mass ratio. Such materials are nevertheless vulnerable to impact damage. It is therefore important to investigate the effects of impacts on composites. Here we embed specialty microstructured optical fiber Bragg grating based sensors inside a carbon fiber reinforced polymer, providing access to the 3D strain evolution within the composite during impact. We measured a maximum strain of -655 με along the direction of impact, and substantially lower values in the two in-plane directions. Such in-situ characterization can trigger insight in the development of impact damage in composites.
Measuring strain at the surface of a structure can help to estimate the dynamical properties of the structure under test. Such a structure can be a fuel assembly of a nuclear reactor consisting of fuel pins. In this paper we demonstrate a method to integrate draw tower gratings (DTGs) in a fuel pin and we subject this pin to conditions close to those encountered in a heavy liquid metal (HLM) reactor. More specifically, we report on the performance of DTGs used as a strain sensor when immersed in HLM during thermal cycles (up to 300_C) for up to 700 hours.
KEYWORDS: Simulation of CCA and DLA aggregates, Signal to noise ratio, Fiber Bragg gratings, Detection and tracking algorithms, Spectral resolution, Sensors, Computer simulations, Algorithm development, Sensing systems, Mechanical sensors
Fiber Bragg grating sensing principle is based on the exact tracking of the peak wavelength location. Several peak detection techniques have already been proposed in literature. Among these, conventional peak detection (CPD) methods such as the maximum detection algorithm (MDA), do not achieve very high precision and accuracy, especially when the Signal to Noise Ratio (SNR) and the wavelength resolution are poor. On the other hand, recently proposed algorithms, like the cross-correlation demodulation algorithm (CCA), are more precise and accurate but require higher computational effort. To overcome these limitations, we developed a novel fast phase correlation algorithm (FPC) which performs as well as the CCA, being at the same time considerably faster. This paper presents the FPC technique and analyzes its performances for different SNR and wavelength resolutions. Using simulations and experiments, we compared the FPC with the MDA and CCA algorithms. The FPC detection capabilities were as precise and accurate as those of the CCA and considerably better than those of the CPD. The FPC computational time was up to 50 times lower than CCA, making the FPC a valid candidate for future implementation in real-time systems.
In recent years, the concept of Nonlinear Structural Intensity (NSI) has been applied to detect fatigue cracks and loose joints in isotropic structures. This paper extends the NSI concept to orthotropic and anisotropic materials and investigates the possibility to use NSI for the localization of a closing delamination in thin laminated plates. When the delamination is excited by a high frequency interrogation signal, the periodic contact occurring between the delaminated plies produces Contact Acoustic Nonlinear (CAN) effects that are associated with the generation of both higher order and fractional harmonics. The closing delamination acts as a mechanism of redistribution of energy from the driving frequency to the nonlinear harmonics. The structural intensity associated with the nonlinear harmonics is an effective metric to identify size and location of the damage. NSI is computed using a combined approach based on a Finite Element (FE) model and a 13 point finite differencing scheme. Using this approach, we performed a numerical investigation on a thin laminated plate to analyze the effect that the material orthotropy has on the propagation of vibration energy and to understand the impact that preferential directions of energy propagation have on the ability to interrogate the damage. Then, the approach is extended for application to an anisotropic symmetric laminated plate.
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