Kin-tak Lau

Interfacial bonding characteristics of nanotube/polymer composites

Abstract In this Letter, the stress transfer properties between single-/multi-walled nanotubes and polymer matrix are theoretically studied through the uses of local density approximation, elastic shells and conventional fibre pullout models. Several parameters such as the wall thickness, Young’s modulus, nanotubes’ volume fraction and chiral vectors of the nanotubes were considered in the study. According to the analytical results, it was found that the maximum shear stress, at the bond interface between the nanotubes and matrix, increases with increasing the nanotubes’ wall thickness of nanotube/polymer composites. Besides, the stress transfer length of zigzag nanotube is comparatively shorter than those of the armchair and chiral nanotubes.

Electrochemical performance of a graphene?polypyrrole nanocomposite as a supercapacitor electrode

A unique nanoarchitecture has been established involving polypyrrole (PPy) and graphene nanosheets by in situ polymerization. The structural aspect of the nanocomposite has been determined by Raman spectroscopy. Atomic force microscopy reveals that the thickness of the synthesized graphene is ∼ 2 nm. The dispersion of the nanometer-sized PPy has been demonstrated through transmission electron microscopy and the electrochemical performance of the nanocomposite has been illustrated by cyclic voltammetry measurements. Graphene nanosheet serves as a support material for the electrochemical utilization of PPy and also provides the path for electron transfer. The specific capacitance value of the nanocomposite has been determined to be 267 F g(-1) at a scan rate of 100 mV s(-1) compared to 137 mV s(-1) for PPy, suggesting the possible use of the nanocomposite as a supercapacitor electrode. After 500 cycles, only 10% decrease in specific capacitance as compared to initial value justifies the improved electrochemical cyclic stability of the nanocomposite.

Strain monitoring in FRP laminates and concrete beams using FBG sensors

The fibre-optic Bragg grating (FBG) sensor is broadly accepted as a structural health monitoring device for fibre reinforced plastic (FRP) materials by either embedding into or bonding onto the structures. The accuracy of the strain measured by using the FBG sensor is highly dependent on the bonding characteristics among the bare optical fibre, protective coating, adhesive layer and host material. In general, the signal extracted from the embedded FBG sensor should reflect the straining condition of the host structure. However, due to the existence of an adhesive layer and protective coating, part of the energy would convert into shear deformation. Therefore, the mechanical properties of these materials would affect the resultant strain measured by embedding a FBG sensor into the structure. This paper presents a theoretical model to evaluate the differential strains between the bare fibre and host material with different adhesive thickness and modulus of the protective coating of the embedded FBG sensor. The results are then compared with numerical analysis by using the finite element method (FEM). Experimental work was conducted for both glass fibre composites and FRP strengthened concrete beams with embedded FBG sensors. Externally bonded strain gauges were used to compare the results obtained from the FBG sensors. The theoretical predictions reveal that the axial strain measured at the fibre-core region is lower than the true strain of the host material with increasing thickness of adhesive layer. A thick adhesive layer and low modulus of coating material would enlarge the shear stress concentration area at the bonded end region. An experimental investigation also shows that the FBG sensor can be confidently used with sufficient bond length.

Mechanics of bonds in an FRP bonded concrete beam

Abstract Fibre-reinforced plastic (FRP) materials have been recognised as new innovative materials for concrete rehabilitation and retrofit. Since concrete is poor in tension, a beam without any form of reinforcement will fail when subjected to a relatively small tensile load. Therefore, the use of the FRP to strengthen the concrete is an effective solution to increase the overall strength of the structure. The attractive benefits of using FRP in real-life civil concrete applications include its high strength to weight ratio, its resistance to corrosion, and its ease of moulding into complex shapes without increasing manufacturing costs. The speed of application minimises the time of closure of a structure compared to other strengthening methods. In this paper, a simple theoretical model to estimate shear and peel-off stresses is proposed. Axial stresses in an FRP-strengthened concrete beam are considered, including the variation in FRP plate fibre orientation. The theoretical predictions are compared with solutions from an experimentally validated finite element model. The results from the theory show that maximum shear and peel-off stresses are located in the end region of the FRP plate. The magnitude of the maximum shear stress increases with increases in the amount of fibres aligned in the beams longitudinal axis, the modulus of an adhesive material and the number of laminate layers. However, the maximum peel-off stress decreases with increasing thickness of the adhesive layer.

On polyethylene–polyaniline composites

Mechanical tests (elongation at break and tensile strength), DC electrical conductivity, and electron spin resonance (ESR) investigations on polyethylene – polyaniline blends are reported. While the concentration of the conducting polymer in the blend is raised, the DC electrical conductivity is increased, and the mechanical properties (tensile strength and elongation at break) are depressed. An universal expression for the dependence of mechanical and electrical properties on the concentration of conducting particles is empirically suggested and supported by experimental data. The ESR spectra are single lines, located close to the g ¼ 2:0 value and assigned to the conduction electrons (with uncoupled electronic spins). The reduced asymmetry of the resonance supports the presence of mesoscopic conducting domains. The features of ESR spectra and the connection between ESR parameters and DC conductivity reflects the major role of polarons hopping in the electron transport and rules out the presence of both low and high spin bipolarons. q 2003 Elsevier Ltd. All rights reserved.

Debond induced by strain recovery of an embedded niti wire at a NiTi/epoxy interface: micro-scale observation

Abstract Rapid development in smart structures has enhanced the great efforts in understanding the mechanical and thermo-mechanical behaviour of shape-memory alloy (SMA) materials. SMAs, in the form of wires and strips, have been embedded into advanced composite structures to control the shape and residual stress of the structures. It is well recognised that the mechanical properties of embedded SMA composites are highly dependent on the integrity of the interface, particularly by the existence of high thermal-induced shear stress at the SMA wire/epoxy interface. This paper discusses the debonding failure mechanism of embedded pre-strained SMA wires in an epoxy matrix environment using the scanning electron microscopy (SEM) technique. It was found that debonds occurred at the SMA wire/matrix interface for a wire with a pre-strained level of 8% at a temperature above Af. Several cracks in the matrix were found around the wire-end region and arrayed along the circumferential direction. A sharp crack, which was propagated towards the radial direction of the wire, was induced in a matrix with high air-bubble content during the wire/matrix debonding process. The effect of high-temperature treatment of SMAs was also studied. It was found that the surface layer of the wire peeled off when the wire was heated at 773 K for 10 min.

Vibration characteristics of SMA composite beams with different boundary conditions

Abstract Recently, the development of shape memory alloy (SMA) actuators, in the forms of wire, thin film and stent have been found increasingly in the fields of materials science and smart structures and engineering. The increase in attraction for using these materials is due to their many unique materials, mechanical, thermal and thermal-mechanical properties, which in turn, evolve their subsequent shape memory, pseudo-elasticity and super-elasticity properties. In this paper, a common type of SMA actuator, Nitinol wires, were embedded into advanced composite structures to modulate the structural dynamic responses, in terms of natural frequency and damping ratio by using its shape memory and pseudo-elastic properties. A simple theoretical model is introduced to estimate the natural frequency of the structures before and after actuating the embedded SMA wires. The damping ratios of different SMA composite beams were measured through experimental approaches. The natural frequencies changed slightly at a temperature above the austenite finish temperature of composite beams with embedded non-prestrained SMA wires. However, the increase of the natural frequencies of the beams with embedded prestrained SMA wires were found in both the theoretical prediction and experimental measurements. The damping ratios of SMA composite beams increased with increasing the temperature of the embedded wires with and without being pre-strained. Compressive and local failures of the beams with high wire content are a possible explanation.

Control of natural frequencies of a clamped-clamped composite beam with embedded shape memory alloy wires

In this study, an analytical model for the evaluation of natural frequencies of glass fibre composite beams with embedded shape memory alloy (SMA) wires, called “SMA composites” is presented. The beams were clamped at both ends and different numbers of SMA actuators were embedded at an interlayer of the composite beams. The changes of tensile modulus, internal recovery stress and strain, and stresses due to the thermal expansion of the beams and wires were considered in the study. The natural frequencies evaluated from the analytical model of the SMA composite beams compared well with experimental measurements. In the study, it was found that the natural frequencies of composite beams decreased with increasing the number of embedded SMA actuators at a temperature below martensite finish temperature, Mf. Although the modulus of the SMA material was relatively higher than glass fibre composites, the decrease of the natural frequencies was due to the increase of the overall density of the SMA composite beams. However, at a temperature above austenite finish temperature, Af, the natural frequencies of the beams with low SMA wire fraction were initially decreased. The frequencies were then increased with continuously increasing the number of SMA wires. These phenomena agree well with the experimental observations.

Micro-hardness and Flexural Properties of Randomly-oriented Carbon Nanotube Composites:

The carbon nanotubes possess many unique mechanical and electrical properties, and have been appreciated as new advanced materials for nanocomposite structures, particularly for the development of nanocomposite films. Nanotubes may also be used as nano-reinforcements for matrix system for fibre-reinforced plastic structures in order to improve out-of-plane properties, thus increasing the delamination resistance. However, those properties are highly relied on the structural integrity and homogeneity of the nanotube composites. Unfortunately, only a little works have paid much attention on these issues recently. It has been obviously proved that the atomic architecture on the nanotube’s surface may be affected after the nanotubes were chemically reacted with polymer matrix. The weak bonding force among the different layers (bonded by a weak Van Der Waals attractive force) of multiwalled nanotubes may also cause a discontinuous stress transfer from the outer-shell to the inner of the composites. This paper reports the micro-hardness and flexural properties of nanotube composites with different amounts of nanotubes content. Experimental measurements and microscopic observations of the nanotube-epoxy composites before and after the tests are discussed in detail. The results show that the hardness of the nanotube composites varied with different nanotube weight fractions. The flexural strength decreased by 10% for a nanotube composite beam with 2 wt.% of nanotubes. The SEM images also revealed that all nanotubes were completely pulled out after the flexural strength test due to a weak-bonding strength between the nanotube and matrix.

Strain monitoring in composite-strengthened concrete structures using optical fibre sensors

In this paper, the mechanical behaviour of the composite-strengthened concrete structures is addressed. Optical fibre sensor presents a great deal of potential in monitoring the structural health condition of civil infrastructure elements after strengthening by externally bonded composite materials. The use of embedded optical fibre sensor for strain and temperature monitoring enables to reveal the status of the composite-strengthened structure in real-time remotely. In this paper, an experimental investigation on the composite-strengthened concrete structures with the embedment of fibre-optic Bragg grating (FBG) sensors is presented. Single- and multiplexed-point strain measuring techniques were used to measure strains of the structures. Frequency modulated continuous wave (FMCW) method was used to measure strains in different points of the structure with using only one single optical fibre. All strains measured from the sensors were compared to conventional surface mounted strain gauges. Experimental results show that the use of the embedded FBG sensor can measure strain accurately and provide information to the operator that the structure is subjected to debond or micro-crack failure. Multiplexed FBG strain sensors enable to measure strain in different locations by occupying only one tiny optical fibre. Reduction of strength in composite laminate is resulted if the embedded optical fibre is aligned perpendicular to the load-bearing direction of the structure.