Surya D. Pandita

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.

The design and optimisation of a rig to enable the lateral spreading of fibre bundles

Fibre-reinforced composites consist of three key components: the reinforcing fibres, the matrix and the interface between the fibre and the matrix. The efficient impregnation of the reinforcing fibre bundle by the matrix is a primary prerequisite for the production of advanced fibre-reinforced composites. This process can be significantly enhanced by spreading the filaments in the reinforcing fibre bundle. The authors previously reported on a manual technique for spreading the filaments in a bundle. This involved subjecting a fibre bundle to a series of reciprocating motions over a rod. The effect of releasing the tension on the bundle was also considered. On the basis of the observations made in the previous study, a mechanised rig was designed, manufactured and optimised to enable the lateral spreading of the filaments in a bundle of E-glass fibres. A Taguchi-based approach was used to optimise the variables on the rig such as the number and configuration of rollers, haul-off speed of the fibre bundle, pre-tension in the bundle and the rotational speed of the roller carrier hub. The maximum degree of fibre spreading achieved for a commercially available 2400 tex E-glass fibre bundle was 250%.

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.

Chemical Process Monitoring and the Detection of Moisture Ingress in Composites

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.

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.

A modified pultrusion process

This paper reports on a modified pultrusion process where the conventional resin bath was replaced with a custom-designed, enclosed resin impregnation unit. A feature of this modified production process is that the rovings were spread, prior to impregnation, using a compact fibre spreading unit. The resin impregnator was designed to accommodate 60 rovings of 2400 tex E-glass. The design features enabled specified modes of impregnation to be enacted including, resin-injection, pin-impregnation, capillary-impregnation and compaction. The impregnator was designed to accept pre-mixed resin from a pneumatically activated pressure-pot or a custom-designed resin delivery system. Pultrusion trials were conducted on a commercial machine using a conventional resin bath, the pressure-pot and the impregnation unit. The physical, mechanical and thermo-mechanical properties of the composites pultruded using the modified technique were marginally superior to those manufactured using the conventional resin bath. However, the environmental benefits of the modified pultrusion process were demonstrated to be significant.

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.