Dee Harris

Clean wet-filament winding - Part 1: Design concept and simulations

This is a two-part paper where part 1 presents details of a modified wet-filament winding process. Here, the resin bath was replaced with a resin injection system that impregnated the fibres prior to winding them onto a rotating mandrel. The resin and hardener were stored in separate containers and pumped on-demand via a pair of precision gear-pumps to a static mixer. The mixed resin system was then supplied to a custom-designed resin impregnation unit. The theoretical basis for the design of the resin impregnation unit is presented along with simulations of the various parameters that influence the impregnation time and the degree of impregnation. Part 2 of this series papers presents the experimental data on the performance of the resin impregnation unit and a comparison of the physical and mechanical properties of the tubes manufactured using the conventional and modified wet-filament winding techniques.

A novel multifunctional fibre optic sensor

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.

In-situ damage detection using self-sensing composites

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.

A comparison of cure monitoring techniques

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.

Self-sensing, self-healing, and crack-arrestor composites

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 composite.

Self-sensing composites: in-situ cure monitoring

The term self-sensing composites is used to describe the case where the reinforcing glass fibres in advanced fibre reinforced composites are used as the sensors for chemical process monitoring (cure monitoring). This paper presents conclusive evidence to demonstrate that reinforcing E-glass fibres can be used for in-situ cure monitoring. The cure behaviour of an epoxy/amine resin system was compared using evanescent wave spectroscopy via the reinforcing E-glass fibre and conventional Fourier transform infrared spectroscopy. This paper also reports for the first time that evanescent wave spectroscopy via E-glass fibres can be used to detect the presence of silane coupling agents. Preliminary results indicated that the cure kinetics on the E-glass fibre surface, as observed using evanescent wave spectroscopy, were influenced by the silane coupling agent.

Clean and environmentally friendly wet-filament winding

Abstract: This chapter reports on a modified wet-filament winding technique where the components of the resin system were stored in individual reservoirs and pumped on-demand through a static-mixer; the mixed resin system was then fed to a custom-designed fibre impregnation unit. The quality of the components produced using the conventional and modified techniques were evaluated using image analysis and the split-ring test. The fibre volume and the void fractions were also measured. It was found that the modified wet-filament winding technique yielded tubes with equivalent properties in comparison with conventionally wound tubes. However, the volume of solvents consumed was reduced significantly.

Process monitoring and damage detection using optical fibre sensors

Abstract: The aim of this chapter is to provide an overview of the use of optical fibre sensors for process monitoring and structural integrity assessment of composites. The term ‘process monitoring’ is used here to describe the various chemical reactions that take place during the cross-linking of thermosetting resins; the term ‘curing’ is sometimes used to describe the cross-linking process. Apart from monitoring the depletion and formation of specified functional groups during the cross-linking reactions, other parameters of interest include the temperature and residual fabrication strain at specified locations within the preforms. Whilst the focus of this chapter is on thermoset-based composites, the fibre optic sensor systems described here are equally applicable to thermoplastic reinforced composites.

Finite element modelling of fibre Bragg grating strain sensors and experimental validation

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.

Fiber optic sensor design for chemical process and environmental monitoring

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 resin system.