Ramani S. Mahendran

Lateral spreading of a fiber bundle via mechanical means

In the current study, the model, previously developed by Wilson, was reviewed and extended to predict the mechanically induced spreading of E-glass fiber bundles. The widths of the as-received E-glass fiber bundles were increased by 200–250% when subjected to a series of reciprocating motions. A 350–450% increase in the widths of the bundles was observed when the tension was released (tension-release mechanism) and the reciprocating motions repeated. The effect of the number of rods, angles and distances between them, and their relative geometry on the extent of fiber spreading was studied. The forces involved in fiber spreading are discussed using micro-mechanics involved in the fiber spreading.

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.

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.

Monitoring and modeling the diffusion profile in a thermosetting resin

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.

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.