At the previous SPIE conference in San Diego (2008), the authors presented and compared a range of low-cost optical fibre sensors for monitoring the cross-linking process of a thermosetting resin. The same sensor was used subsequently
to monitor and quantify the diffusion of water in the cross-linked polymer.
The current paper presents recent data on the deployment of an array of low-cost fibre-optic sensors to monitor the water
diffusion front. The data obtained from the sensors are compared with conventional gravimetric measurements and
theoretical predictions for the diffusion profile for water ingress in a cross-linked epoxy/amine resin system.
The authors have demonstrated previously that reinforcing glass fibres can be used as light-guides to facilitate chemical
process monitoring and structural integrity assessment of fibre reinforced composites. In the current paper, the authors
explore concepts for the development of self-sensing, self-healing and crack-arrestor composites.
The first part of the papers presents a brief overview of previously reported technologies for self-sensing, self-healing
and crack-arrestor; the advantages and disadvantages of the various technologies are discussed. The second part of this
paper present the design concept and performance requirements for the self-sensing, self-healing and crack-arrestor
composites. The final part of the paper presents preliminary results on the manufacture and evaluation of this class of
Whilst considerable progress continues to be made on the design and deployment of fibre optic sensors for chemical
process monitoring and structural integrity assessment, the majority of these sensor designs can only impart information
on one or two relevant measurands. For example, in the case of chemical process monitoring of advanced fibrereinforced
composites involving thermosetting resins, it is generally appreciated that cross-linking kinetics can be
influenced by a number of factors including the following: the stoichiometry of the reagents, temperature, surface
chemistry of the substrate and presence or absence of contaminants. Thermosetting resins also shrink during the crosslinking
process. When thermosets are used and processed above room temperature during the production of fibrereinforced
composites, upon cooling back to ambient temperature, residual stress can develop due to the mismatch in
thermal expansions between the reinforcing fibres and the matrix.
This paper reports on recent progress on the design and demonstration of a novel multi-functional fibre optic sensor that
can provide data on (i) temperature, (ii) strain, (iii) refractive index, (iv) transmission infrared spectroscopy and (v)
evanescent wave spectroscopy. A unique and attractive feature of this sensor is that a conventional commercially
available Fourier transform infrared spectrometer is used to interrogate the sensor. The sensor design is based on an
extrinsic fibre Fabry-Perot interferometer.
Fibre Bragg grating (FBG) sensors continue to be used extensively for monitoring strain and temperature in and on
engineering materials and structures. Previous researchers have also developed analytical models to predict the loadtransfer
characteristics of FBG sensors as a function of applied strain. The general properties of the coating or adhesive
that is used to surface-bond the FBG sensor to the substrate has also been modelled using finite element analysis.
In this current paper, a technique was developed to surface-mount FBG sensors with a known volume and thickness of
adhesive. The substrates used were aluminium dog-bone tensile test specimens. The FBG sensors were tensile tested in
a series of ramp-hold sequences until failure. The reflected FBG spectra were recorded using a commercial instrument.
Finite element analysis was performed to model the response of the surface-mounted FBG sensors. In the first instance,
the effect of the mechanical properties of the adhesive and substrate were modelled. This was followed by modelling the
volume of adhesive used to bond the FBG sensor to the substrate. Finally, the predicted values obtained via finite
element modelling were correlated to the experimental results. In addition to the FBG sensors, the tensile test specimens
were instrumented with surface-mounted electrical resistance strain gauges.
Significant progress has been made in recent years on the design and deployment of optical fibre-based sensors to
monitor the cross-linking (cure) reactions in thermosetting resins. In the current study, the following sensor designs
were used to study cross-linking reactions of an epoxy/amine resin system: (i) intensity-based Fresnel sensors, (ii)
extrinsic fibre Fabry-Perot interferometic (EFPI) sensors, (iii) fibre Bragg grating (FBG) sensors and (iv) sensor designs
to enable transmission, reflection and evanescent wave spectroscopy.
This paper presents a detailed study on a comparison of the above-mentioned techniques for a commercially available
epoxy/amine resin system. Conventional Fourier transform infrared spectroscopy was used as the reference method for
obtaining quantitative data on the cross-linking kinetics. The shrinkage of the resin during cross-linking was monitored
using EFPI and FBG sensors. This paper also discusses the cross-linking data obtained using optical fibre-based
evanescent wave spectroscopy.
The focus of this paper is on real-time damage detection in reinforcing fiber bundles and composites using high-speed
photography and image analysis. In other words, the end of a reinforcing fiber bundle or composite is imaged and the
sequence of fiber fracture is monitored using a high-speed camera. These studies were undertaken using as-received and
silane-treated custom-made optical fibers of around 12 μm diameter and E-glass fibers of 15 (±3) μm diameter.
The first part of this paper reports on the techniques that were developed to produce void-free test specimens and the
procedures used for imaging the end of the fiber bundle and composite during tensile loading. Evanescent wave
spectroscopy was used to study the effect of silane treatment on the cross-linking kinetics of an epoxy/amine resin
system. Conventional piezo-electric acoustic emission (AE) transducers were used to monitor the acoustic events
occurring during the tensile test. The signals from the AE transducers were used to trigger the high-speed camera.
The second part of this paper presents details of the image analysis routines that were developed to track the light
intensity transmitted through individual fibers during tensile loading. Good correlation was observed between the
transmitted light intensity and the AE signals.
It is generally appreciated that the ingress of moisture in composites can have adverse effects on matrix-dominated
properties such as the glass transition temperature and compressive mechanical properties. Moisture ingress in
composites can also lead to swelling and blistering. A number of excellent studies have been reported on the detection,
modelling and effects of moisture ingress on the properties of thermosetting resins (matrix) and composites. However, it
is generally taken for granted that the quality of the resin and the processing conditions used to cross-link the resin are
identical. Given the recent advances in the design and deployment of optical-fibre sensors in composites, it is now
possible to use the same sensor to facilitate in-situ cure monitoring and structural health monitoring (after processing).
This paper will present recent developments in the design of
low-cost fibre-optic sensor systems for in-situ chemical
process monitoring and the detection of moisture ingress after curing. The cure kinetics derived from three fibre optic
sensor designs is presented as well as those obtained from evanescent-wave spectroscopy using E-glass fibres.
After conducting the in-situ cure monitoring experiments, one of the fibre-optic sensor designs was selected and the
samples (with the embedded sensors) were dried to constant mass at 50°C then transferred to water baths maintained at
70, 50, and 30 °C. The diffusion kinetics for the samples was determined using samples without and with embedded
optical-fibre sensors. The effect of moisture ingress in the resin was also assessed using dynamic mechanical thermal
analysis (DMTA), transmission infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Preliminary
results are also presented to demonstrate that the reinforcing fibres in E-glass composites can be used to track the cross-linking
kinetics of a commercial epoxy/amine resin is presented.
Cure monitoring is a term that is used to describe the cross-linking reactions in a thermosetting resin system. Advanced
fiber reinforced composites are being used increasingly in a number of industrial sectors including aerospace, marine,
sport, automotive and civil engineering. There is a general realization that the processing conditions that are used to
manufacture the composites can have a major influence on its hot-wet mechanical properties. This paper is concerned
with the design and demonstration of a number of sensor designs for in-situ cure monitoring of a model thermosetting
resin system. Simple fixtures were constructed to enable a pair of cleaved optical fibers with a defined gap between the
end-faces to be held in position. The resin system was introduced into this gap and the cure kinetics were followed by
transmission infrared spectroscopy. A semi-empirical model was used to describe the cure process using the data
obtained at different cure temperatures. The same sensor system was used to detect the ingress of moisture in the cured