Ronald C. Spooncer

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.

Comparative study of optical fiber cure-monitoring methods

This paper reports on a comparative study undertaken for different types of optical fiber sensor developed to monitor the cure of an epoxy resin system. The optical fiber 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 fibers within a specially prepared sleeve with a gap between the optical fiber end-faces. During cure, resin from the specimen flowed into the gap between the optical fibers 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 fiber. 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 fiber 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 fiber. 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 over the range 1450 - 1700 nm during the cure reaction. Consequently these techniques required tunable 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 fiber would transmit light. The advantages and disadvantages of these three methods are discussed.

Optical fiber displacement sensors for process and manufacturing applications

We describe three novel optical fiber probes, all based on the focused-beam reflective principle, to measure displacement, form, and surface topography, respectively. Each depends on deriving twin displacement/optical output characteristics that may be resolved by difference/sum referencing. The displacement sensor adopts twowavelength operation using a zone plate to give opposing displacement/output characteristics. The device is noncontacting, and a resolution of better than 0.1% of span is anticipated. A 3-D optical touchprobe has been built for use with coordinate measuring machines. A mathematical model has been generated that relates output to stylus movement, and the model has been verified experimentally. A resolution of 0.1 μm in the x-y plane and 1.0 μm in the z direction is achievable. A further noncontacting probe has been developed for the measurement of surface topography whose output shows low dependency on surface reflectance. A mathematical model has shown good correspondence against a wide range of surface compositions and textures; a resolution of better than 1 μm is foreseen. Proposals are made for industrial implementation of all three probes.

An Optical Fibre Pressure Sensor Using A Holographic Shutter Modulator With Two-Wavelength Intensity Referencing

An optical fibre pressure sensor utilizes displacement of a holographic grating to intensity modulate a signal wavelength; a second wavelength passes through the grating unmodulated and its transmittance is used to reference the signal wavelength intensity.

Two-wavelength referencing of an optical fibre intensity-modulated sensor

This communication describes a method of referencing out adventitious intensity variations in an optical fibre intensity-modulated sensor by measuring the transmitted light intensity at two wavelengths, one of which carries the signal information and the other being used to normalise the intensity.

In situ cure monitoring of advanced fiber reinforced composites

This paper describes a comparative study of in-situ cure monitoring and cure modelling by three methods: (a) evanescent wave spectroscopy, (b) refractive index change, (c) near- infrared spectroscopy. Optical fibers were embedded into aerospace epoxy resins during the manufacturing process of the composite. The cure characteristics were then tracked in real- time during the processing of the material via evanescent wave interaction. This technique is based upon monitoring of characteristic infrared absorption bands of the resin system to find the concentration of the epoxy and amine hardener as a function of cure time. Hence this technique is suitable for on-line process monitoring and optimization. Results obtained from the optical fiber sensors were used to model the curing behavior of the resin system. The results were compared with near-infrared spectroscopy and differential scanning calorimetry experiments carried out under similar conditions. The feasibility of utilizing refractive index changes to monitor the extent of cure has also been demonstrated.

An optical fibre displacement sensor with extended range using two-wavelength referencing

Abstract A twin-channel optical fibre displacement sensor has been demonstrated which has a near-linear span of 40 mm. The sensor incorporates a zone-plate and uses two wavelengths to generate separate displacement-output characteristics for each channel. Repeatability is inadequate for industrial implementation, but improvements are proposed to bring the performance up to commercial standards.

Environmentally Compensated Photoelastic Pressure Sensors With Optical Fibre Links

The accuracy of photoelastic modulated optical fibre sensors suffer from temperature-dependency. Two designs have been developed, aimed at eliminating this deficiency. The evolution of the designs is explained and preliminary results reported.

Optical fiber evanescent wave cure monitoring of epoxy resins

Two types of optical fiber sensors (OFS) were investigated for use in monitoring the cure of an epoxy-amine resin system: (1) an evanescent wave sensor and (2) a refractive index sensor. The evanescent wave sensor was used to detect changes in concentration of the active chemical species involved in the cure reaction via evanescent wave near-infrared spectroscopy. By using the optical fiber as an attenuated total reflection waveguide, spectra were collected over the range 1490 - 1570 nm at regular time intervals during the cure. This technique enabled the depletion of amine to be monitored. Results obtained via this method were fitted to kinetic models which allowed prediction of the reaction rate at different cure temperatures and conversions. The optical fiber evanescent wave sensor results were compared with data obtained using an established cure monitoring technique (FT-IR spectroscopy). A theoretical model of the evanescent sensor has been used which describes the relationship between evanescent absorption as a function of absorber concentration and refractive index. Predictions of sensor response were undertaken using absorption data from FT-IR spectroscopy and refractive index results as a function of cure time. The predicted sensor response was then compared with experimentally obtained sensor data. An optical fiber sensor which monitored the cure process via refractive index change was also investigated. Sensors were set up to allow simultaneous collection of data during cure from the OFS, together with data from transmission near-infrared spectroscopy and Abbe refractometry. In this way the response of the sensor to changes in the cure state of the resin, refractive index and temperature was compared.