Marcus Perry

Effects of Neutron-Gamma Radiation on Fiber Bragg Grating Sensors: A Review

Fiber Bragg grating (FBG) sensing is currently one of the leading measurement technologies, but has made only minor inroads into nuclear diagnostics due, for the most part, to insufficient proof of survivability in these environments. This paper aims to review the current knowledge in this area, incorporating reviews of radiation-induced darkening of silica, shifts in Bragg peak, and changes to mechanical properties, while taking into account core/cladding chemistry, structure, and coating materials. In doing so, this review not only highlights the achievements of the scientific community up to the present, but more importantly, the key areas in which knowledge is still lacking and further concentrated investigation would be beneficial.

Induction Brazing of Type-I Fiber Bragg Gratings Into Kovar Ferrules Exploiting Curie Transition

While soldering fiber Bragg gratings into ferrules may allow them to overcome their inherent mechanical and chemical weaknesses, the high temperatures required to achieve reliable encapsulation can result in decreased grating reflectivity. In this paper, finite element models and data from on-line temperature measurements of regenerated gratings during induction heating are analyzed. The results are used to design and implement an optimized methodology, which allows standard, type-I gratings to be brazed into ferrules with high-temperature solders, while ensuring low reflectivity losses.

Nanoscale Resolution Interrogation Scheme for Simultaneous Static and Dynamic Fiber Bragg Grating Strain Sensing

A combined interrogation and signal processing technique which facilitates high-speed simultaneous static and dynamic strain demodulation of multiplexed fiber Bragg grating sensors is described. The scheme integrates passive, interferometric wavelength-demodulation and fast optical switching between wavelength division multiplexer channels with signal extraction via a software lock-in amplifier and fast Fourier transform. Static and dynamic strain measurements with noise floors of 1 nε and nε/√Hz, between 5 mHz and 2 kHz were obtained. An inverse analysis applied to a cantilever beam set up was used to characterize and verify strain measurements using finite element modeling. By providing distributed measurements of both ultra-high-resolution static and dynamic strain, the proposed scheme will facilitate advanced structural health monitoring.

High-Speed Interferometric FBG Interrogator With Dynamic and Absolute Wavelength Measurement Capability

A passive, interferometric wavelength demodulation technique has been extended to measure the absolute wavelengths of a multiplexed array of fiber Bragg grating sensors. The scheme retains its original strain resolution of 10 nε/√{Hz}. A proof-of-concept interrogation system was able to determine the absolute wavelength of Bragg peaks to within 20 pm (17 με). Static and dynamic Bragg grating strains were accurately demodulated in both absolute and relative wavelength measurement modes. This demonstration indicates that interferometric techniques are able to provide absolute, static and dynamic measurements of strain within a single platform.

First-time demonstration of measuring concrete prestress levels with metal packaged fibre optic sensors

In this work we present the first large-scale demonstration of metal packaged fibre Bragg grating sensors developed to monitor prestress levels in prestressed concrete. To validate the technology, strain and temperature sensors were mounted on steel prestressing strands in concrete beams and stressed up to 60% of the ultimate tensile strength of the strand. We discuss the methods and calibration procedures used to fabricate and attach the temperature and strain sensors. The use of induction brazing for packaging the fibre Bragg gratings and welding the sensors to prestressing strands eliminates the use of epoxy, making the technique suitable for high-stress monitoring in an irradiated, harsh industrial environment. Initial results based on the first week of data after stressing the beams show the strain sensors are able to monitor prestress levels in ambient conditions.

Tipping point analysis of cracking in reinforced concrete

In this work, we demonstrate that tipping point analysis of strain data can provide reactive and predictive indicators of cracking and structural transitions in a reinforced concrete system. The method is able to detect trend-driven transitions in a short time series of approximately 2000 datapoints, providing a clear indication of when a concrete beam under gradual bending progresses from a linear to a nonlinear strain response. The method is also able to provide an early warning signal of the appearance of bifurcations, such as cracks, with a forewarning of 200–500 datapoints. The method, which was originally developed for applications in geophysics, shows promising results in the area of structural health monitoring, in particular, for real-time observations of civil constructions.

Crack Monitoring of Operational Wind Turbine Foundations

The degradation of onshore, reinforced-concrete wind turbine foundations is usually assessed via above-ground inspections, or through lengthy excavation campaigns that suspend wind power generation. Foundation cracks can and do occur below ground level, and while sustained measurements of crack behaviour could be used to quantify the risk of water ingress and reinforcement corrosion, these cracks have not yet been monitored during turbine operation. Here, we outline the design, fabrication and field installation of subterranean fibre-optic sensors for monitoring the opening and lateral displacements of foundation cracks during wind turbine operation. We detail methods for in situ sensor characterisation, verify sensor responses against theoretical tower strains derived from wind speed data, and then show that measured crack displacements correlate with monitored tower strains. Our results show that foundation crack opening displacements respond linearly to tower strain and do not change by more than ±5 μm. Lateral crack displacements were found to be negligible. We anticipate that the work outlined here will provide a starting point for real-time, long-term and dynamic analyses of crack displacements in future. Our findings could furthermore inform the development of cost-effective monitoring systems for ageing wind turbine foundations.

Deformation monitoring in prestressing tendons using fibre Bragg gratings encapsulated in metallic packages

An investigation into the capability of deformation monitoring using fibre Bragg gratings encapsulated in metallic packages is presented in the paper. The proposed approach relies on a grating inscription into a metal coated fibre and brazing the fibre into a metal capillary using induction heating. A metal rod instrumented with encapsulated FBG strain and temperature sensors is placed in an electromechanical tester and stressed up 80 % of its ultimate tensile strength. It is demonstrated through a 60 h experiment that the sensors are capable of real-time deformation monitoring.

Wireless surface acoustic wave sensors for displacement and crack monitoring in concrete structures

In this work, we demonstrate that wireless surface acoustic wave devices can be used to monitor millimetre displacements in crack opening during the cyclic and static loading of reinforced concrete structures. Sensors were packaged to extend their gauge length and to protect them against brittle fracture, before being surface-mounted onto the tensioned surface of a concrete beam. The accuracy of measurements was verified using computational methods and opticalfibre strain sensors. After packaging, the displacement and temperature resolutions of the surface acoustic wave sensors were 10 μm and 2°C respectively. With some further work, these devices could be retrofitted to existing concrete structures to facilitate wireless structural health monitoring.

Finite difference analysis and experimental validation of 3D photonic crystals for structural health monitoring

In this work, we validate the behavior of 3D Photonic Crystals for Structural Health Monitoring applications. A Finite Difference Time Domain (FDTD) analysis has been performed and compared to experimental data. We demonstrate that the photonic properties of a crystal (comprised of sub-micrometric polystyrene colloidal spheres embedded in a PDMS matrix) change as a function of the axial strain applied to a rubber substrate. The change in the reflected wavelength, detected through our laboratory experiments and equivalent to a visible change in crystal color, is assumed to be caused by changes in the interplanar spacing of the polystyrene beads. This behavior is captured by our full wave 3D FDTD model. This contains different wavelengths in the visible spectrum and the wave amplitudes of the reflected and transmitted secondary beams are then computed. A change in the reflectance or transmittance is observed at every programmed step in which we vary the distance between the spheres. These investigations are an important tool to predict, study and validate our understanding of the behavior of this highly complex physical system. In this context, we have developed a versatile and robust parallelized code, able to numerically model the interaction of light with matter, by directly solving Maxwells equations in their strong form. The ability to describe the physical behavior of such systems is an important and fundamental capability which will aid the design and validation of innovative photonic sensors.