Gerard Franklyn Fernando

In-situ process and condition monitoring of advanced fibre-reinforced composite materials using optical fibre sensors

This paper presents a general overview of a number of optical fibre sensor systems which have been developed and used in advanced fibre-reinforced composites for in-situ process and condition monitoring. The in-situ process monitoring techniques were optical-fibre-based evanescent wave spectroscopy, transmission near-infrared spectroscopy and refractive index monitoring. The optical fibre sensors were successful in tracking the cure reaction. The condition monitoring of advanced fibre-reinforced composites was carried out using two intensity-based optical fibre sensor systems: an extrinsic multi-mode Fabry-Perot sensor and Bragg gratings. In addition to this, the feasibility of using the reinforcing fibre as a light guide was demonstrated. These sensor systems were evaluated under quasi-static, impact and fatigue loading. The test specimens consisted of prepreg-based carbon-fibre-reinforced epoxy and glass-fibre-reinforced epoxy filament-wound tubes. Excellent correlation was obtained between surface-mounted strain gauges and the embedded optical fibre sensors. The feasibility of using these sensor systems for the detection of impact damage and stiffness reduction in the composite due to fatigue damage was successfully demonstrated.

A novel ultrasound fibre optic sensor based on a fused-tapered optical fibre coupler

This paper describes the design and development of a novel fibre optic sensor to detect ultrasound. It is based on a fused-tapered optical fibre coupler, incorporating mechanical strain amplification in the sensor design. Strain amplification allows the sensor to be used for detecting low acoustic energy, such as acoustic emission emanating from composite materials and structures. This fibre optic ultrasonic sensor has a frequency response range from several tens of kHz to several hundred kHz. The sensitivity of the sensor system was found to be 0.56 V mbar−1 with an acoustic noise floor of 18 µbar at 155 kHz when evaluated in a water bath. The bandwidth of the electrical circuit was in the range of 10 kHz up to 1 MHz.

Simultaneous strain and temperature measurement of advanced 3-D braided composite materials using an improved EFPI/FBG system

Simultaneous strain and temperature measurement for advanced 3-D braided composite materials using fibre-optic sensor technology is demonstrated, for the first time. These advanced 3-D braided composites can virtually eliminate the most serious problem of delamination for conventional composites. A tandem in-fibre Bragg-grating (FBG)/extrinsic Fabry-Perot interferometric sensor (EFPI) system with improved accuracy has been used to facilitate simultaneous temperature and strain measurement in this work. The non-symmetric distortion of the optical spectrum of the FBG, due to the combination of the FBG and the EFPI, is observed for the first time. Experimental and theoretical studies indicate that this type of distortion can affect the measurement accuracy seriously and it is mainly caused by the modulation of the periodic output of the EFPI. A simple method has been demonstrated to improve the accuracy for detection of the wavelength-shift of the FBG induced by temperature change. A strain accuracy of ∼ ±20 μe and a temperature accuracy of ∼ ±1 °C have been achieved, which can meet the requirements for practical applications of 3-D braided composites.

An intensity-based optical fibre sensor for fatigue damage detection in advanced fibre-reinforced composites

The detection of fatigue-induced damage within fibre-reinforced composite materials is of vital importance in areas where these materials are used for critical load-bearing applications. Embedded fibre-optic sensors are suitable for damage detection in composite materials because they can be easily integrated into the structure with minimal interference with the bulk properties of the composite. In this current study, a novel intensity-based optical fibre sensor was designed and evaluated under tension/tension and tension/compression fatigue loading conditions. This intensity sensor has the ability to monitor rapid strain changes without loss of the reference level. Furthermore, this sensor utilizes relatively simple and inexpensive instrumentation. The fibre-optic strain sensor was embedded within a 16-ply 0/90 carbon/epoxy laminate at the manufacturing stage. The embedded optical fibre sensor was not found to affect the fatigue life of the composite both under tension/tension and tension/compression loading. The feasibility of using the embedded sensor for fatigue damage detection was demonstrated. Excellent correlation was also obtained between the strain data from a surface mounted optical fibre sensor and a surface-mounted extensometer under fatigue loading conditions.

In situ cure monitoring of epoxy resins using optical fibre sensors

This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy. The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy. During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

In-situ cure monitoring using optical fibre sensors - a comparative study

This paper reports a comparative study undertaken for different types of optical fibre sensor developed to monitor the cure of an epoxy resin system. The optical fibre sensors used to monitor the cure process were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. The transmission sensor was prepared by aligning two optical fibres within a specially prepared sleeve with a gap between the optical fibre end faces. During cure, resin from the specimen flowed into the gap between the optical fibres allowing transmission spectra of the resin to be obtained. The evanescent wave sensor was prepared by stripping the cladding from a high refractive index core optical fibre. The prepared sensor was embedded in the sample and attenuated total reflectance spectra recorded from the resin/core boundary. Refractive index monitoring was undertaken using a high refractive index core optical fibre which had a small portion of its cladding removed. The prepared sensor was embedded in the resin specimen and light from a single wavelength source was launched into the fibre. Changes in the guiding characteristics of the sensor due to refractive index changes at the resin/core boundary were used to monitor the progress of the cure reaction. The transmission and evanescent wave spectroscopy sensors were used to follow changes in characteristic near-infrared absorption bands of the resin during the cure reaction over the range 1450-1700 nm for the evanescent sensor and 2100-2250 nm for the transmission sensor. Consequently, these techniques required tuneable wavelength sources covering specific wavelength ranges. However, the refractive index based sensor used a single wavelength source. Therefore the equipment costs for this type of sensor were considerably less. Additionally, the refractive index sensor did not require a single wavelength source at any particular wavelength and could be applied to any spectral region in which the optical fibre would transmit light. The advantages and disadvantages of these three methods are discussed.

Simultaneous strain and temperature measurements in composites using extrinsic Fabry–Perot interferometric and intrinsic rare-earth doped fiber sensors

Abstract This paper reports on a novel optical fiber-based sensing system for conducting simultaneous strain and temperature measurements. The sensor design involves the use for the first time of an extrinsic Fabry–Perot interferometric (EFPI) strain sensor in conjunction with a rare-earth doped fiber fluorescence decay-time based temperature sensor. The combined sensors were embedded in a carbon fiber reinforced composite system and evaluated. The feasibility of using this type of embedded sensor configuration for simultaneous strain and temperature measurements was demonstrated.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

Development of a microwave calorimeter for simultaneous thermal analysis, infrared spectroscopy and dielectric measurements

An instrument has been developed for monitoring cure processes under microwave heating conditions. The main function of the instrument was a calorimeter for performing microwave thermal analysis. A single model resonant cavity was used as the heating cell in the microwave calorimeter. Thermal analysis measurements were obtained by monitoring the variation in the microwave power that was required to maintain controlled heating of the sample. The microwave thermal analysis data were analogous to conventional differential scanning calorimetry measurements. The dielectric properties of the sample, as a function of the extent of cure, have been obtained using perturbation theory from the changes in resonant frequency and quality factor of the microwave cavity during heating. Additionally, remote sensing fibre-optic probes have been employed to measure real time in situ infrared spectra of the sample during the cure reaction. In this paper, we describe the design and operation of the microwave calorimeter. Examples of experimental results are also presented.

A multi-mode extrinsic Fabry - Pérot interferometric strain sensor

This paper reports on the fabrication and evaluation of a multi-mode extrinsic Fabry - Perot interferometric (EFPI) sensor which is capable of measuring both tensile and compressive strains. A scanning monochromator was used to measure the absolute cavity length of the EFPI sensor. Sensors of this type were embedded within a 16-ply carbon-fibre-reinforced epoxy composite and tested under quasi-static tensile and compressive loading conditions. Excellent correlation was observed between the EFPI sensor and a surface-mounted extensometer. The sensor system can operate in the strain range from -1 to 1% with an accuracy of better than 30 micro-strain. Preliminary results indicated that the sensor design was relatively insensitive to temperature in the range 38 - 180 . An analysis of the relationship between the insensitivity and the sensor geometry is also presented.