Edmon Chehura

Optical fibre grating refractometers for resin cure monitoring

The use of fibre grating refractometers as a means of monitoring the cure of a UV-cured epoxy resin is presented. The wavelength shift of the attenuation bands of a long period grating and the spectral response of a tilted fibre Bragg grating sensor were measured simultaneously during the cure of the resin and compared with measurements made using a fibre optic Fresnel-based refractometer. The results showed a good correlation (6 × 10−3 rius) and illustrate the potential of the techniques for non-invasive composite material cure monitoring.

Surface strain measurement: a comparison of speckle shearing interferometry and optical fibre Bragg gratings with resistance foil strain gauges

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry (shearography) and fibre Bragg grating (FBG) sensors, is compared with that of resistance foil strain gauges (RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.

AUTOMATED MANUFACTURE OF 3D REINFORCED AEROSPACE COMPOSITE STRUCTURES

Purpose – The purpose of this paper is to focus on exploring an innovative combination of cutting‐edge technologies to be implemented within automated processes for composite parts manufacturing. The objective is the design of a production route for components with tailored fibre orientation and ply lay‐up, with improved damage tolerance thanks to through‐the‐thickness reinforcement and integrated health monitoring systems based on optical fibres technology. This study is part of the FP7 project ADVITAC.Design/methodology/approach – The proposed technologies are described in detail and their compatibility and potential for integration are discussed.A set up for on‐line monitoring of infusion and curing processes of carbon/epoxy laminates preformed by dry fibre placement technology is proposed, and a preliminary study of their mechanical performance is presented. The possibility of reinforcing through‐the‐thickness preforms manufactured with dry slit tapes automatically laid‐up and consolidated by laser he...

A pressure sensor based upon the transverse loading of a sub-section of an optical fibre Bragg grating

An experimental and theoretical study of locally transverse loaded fibre Bragg gratings (FBGs) for pressure sensing purposes is presented. When a load is applied to a short section of an FBG, a spectral hole is generated in the reflection spectrum, which exhibits a redshift in wavelength as the load is increased. This effect has been modelled using Rouards method and has been characterized experimentally. Two techniques for analysing the spectrum for pressure measurements at constant temperature are considered, one based on measurement of the wavelength shift of the spectral hole, and the other on measurement of peak reflectivities either side of the hole. Normalized pressure sensitivities of 3.30 ± 7.93 × 10−2 MPa−1 and 4.97 × 10−4 ± 1.37 × 10−5 MPa−1 were obtained for the reflectivity and wavelength measurement, respectively. A pressure resolution of 1.3 kPa, over a range of 745 kPa, was achieved when using a tunable laser of 1 pm wavelength resolution to interrogate the sensor. For pressure measurements in the presence of temperature changes, a technique based on the measurement of the position of the spectral hole within the Bragg envelope is presented. This technique allows the opportunity to discriminate pressure and temperature using a single FBG sensor element.

Multi-component strain development in superconducting magnet coils monitored using fibre Bragg grating sensors fabricated in highly linearly birefringent fibre

The commissioning of superconducting magnet coils was monitored using embedded optical fibre Bragg grating sensors (FBG) fabricated in highly linearly birefringent (HiBi) fibre. The HiBi FBG sensors monitored the internal strain developed in the coils during the energization of the coils. The development of multiple components of strain in the coils when the magnet was energized and quenched was monitored, revealing phenomena that it had not been previously possible to measure using other sensor technologies.

A simple and wavelength-flexible procedure for fabricating phase-shifted fibre Bragg gratings

A simple procedure for fabricating phase-shifted fibre Bragg gratings (PSFBGs) that does not require the use of a phase-shifted phase mask is presented. Two standard fibre Bragg gratings (FBGs) of equal length and with the same Bragg wavelength are inscribed sequentially in a single mode optical fibre, such that the FBGs overlap physically by one grating period. This procedure induces a spectral hole in the middle of the reflection spectrum, equivalent to a π-phase-shifted FBG. PSFBGs have been fabricated in the 1300 nm and 1550 nm wavelength bandwidths, demonstrating flexibility in the choice of the centre wavelength. The PSFBGs fabricated are compared with a model based on a modified Rouards method, which is subsequently used to investigate the influence of translation stage errors on the fabrication of the PSFBGs. The model demonstrates that the strength/reflectivity of the PSFBG influences the spectral-hole linewidth as well as the finesse of the device while the length of the PSFBG affects only the linewidth of the spectral hole but not the finesse of the device. The PSFBG devices produced by this technique are reproducible and the fabrication process is fast and is well suited for the fabrication of WDM PSFBGs.

Pressure measurements on aircraft wing using phase-shifted fibre Bragg grating sensors

Experimental results of static pressure measurements from specially packaged π-phase shifted optical fibre Bragg gratings (FBGs) that were installed into a 2D, 3 element high lift wing are presented. The static pressure measurements from the fibre sensors are found to be in good agreement with theory. Further pressure calibrations are currently under investigation.

Fibre Bragg grating based effective soil pressure sensor for geotechnical applications

An effective-soil-pressure sensor for geotechnical applications based on Fibre Bragg Gratings is presented. The sensor simultaneous measures total soil pressure and pore pressure, allowing the calculation of the effective stress of soil. Calibration of the sensor using pressurised air demonstrated a pressure sensitivity of 2.02x10-3 ± 2.84x10-5nm/kPa and 1.87x10-3 ± 6.88x10-5nm/kPa for the total and pore pressure respectively. This corresponds to a pressure resolution of 4.95x10-1kPa and 5.46x10-1kPa for total and pore pressure using a 1pm interrogation system. Measurements undertaken in two types of soil demonstrated dependence of the total pressure sensitivity on soil density/stiffness. Pore pressure measurements agreed well with the preliminary calibration.

Enhanced sensitivity fibre Bragg grating (FBG) load sensor

The characterization of a load sensor based on the transverse loading of a subsection of a fibre Bragg grating (FBG) embedded within a cube of epoxy resin is presented. When the epoxy resin cube is loaded transverse to the axis of the fibre, its deformation transduces the load to a strain along the axis of the optical fibre, which changes the period of the embedded section of the FBG. This creates a spectral dropout within the bandwidth of the FBG, with an absolute wavelength that is linearly dependent on the applied load. This technique enhances the sensitivity of the FBG to transverse loading by a factor of 15, to 2.9 × 10−2 ± 0.01 nm N−1, when compared to the direct transverse loading of a bare fibre, and also protects the fibre from mechanical damage at the loading point.

Rouard's method as a modelling tool for the sensing characteristics of complex fibre Fabry-Perot interferometers formed between chirped fibre Bragg gratings.

Phase and spectral response characteristics of fibre Fabry-Perot interferometers (FFPI), formed between linearly chirped fibre Bragg gratings (FBG), were theoretically modelled using Rouards method. The method models accurately the phase shift through a complex grating structure which the transfer matrix model is unable to achieve. The FBG FFPI cavities were modelled with variable free spectral range that was tailored further to provide strain insensitivity and strain enhancement by making the chirp parameter and cavity length satisfy the desired conditions at a given wavelength of illumination.