Shoaib A. Malik

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 Composites: In-Situ Detection of Fibre Fracture

The primary load-bearing component in a composite material is the reinforcing fibres. This paper reports on a technique to study the fracture of individual reinforcing fibres or filaments in real-time. Custom-made small-diameter optical fibres with a diameter of 12 (±2) micrometres were used to detect the fracture of individual filaments during tensile loading of unreinforced bundles and composites. The unimpregnated bundles were end-tabbed and tensile tested to failure. A simple technique based on resin-infusion was developed to manufacture composites with a negligible void content. In both cases, optical fibre connectors were attached to the ends of the small-diameter optical fibre bundles to enable light to be coupled into the bundle via one end whilst the opposite end was photographed using a high-speed camera. The feasibility of detecting the fracture of each of the filaments in the bundle and composite was demonstrated. The in-situ damage detection technique was also applied to E-glass bundles and composites; this will be reported in a subsequent publication.

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.

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.