Showing papers by "Kin-tak Lau published in 2005"

Cluster size effect in hardness of nanoclay/epoxy composites

Abstract: The mechanical and thermal properties of nanoclay polymer composites have been experimentally investigated over the last decade. Most of the research has been focused mainly on the control of their interplanar structures, which govern the global properties of the composites. In reality, these structures (both exfoliated and intercalated patterns) are hardly achieved through the use of conventional manufacturing process for plastic products. Different sizes of clusters mixed by nanoclays and matrix would be easily formed, particularly in the extrusion of polymer-based components. This paper experimentally studied the hardness and interlaminar shear properties of nanoclay/epoxy composites with different amount of nanoclay content, which formed different sizes of nanoclay/epoxy clusters after mixing in an extruder. The results showed that the micro-hardness of the composites could be enhanced when a small amount of nanoclay was added into the epoxy. However, there was an optimal limit in which the hardness was dropped by continuously increasing the nanoclay content. Microscopic observation on the fracture surfaces showed that the size of the clusters varied with the amount of nanoclays used in the composites. Although previous literatures have reported that the use of nanoclays in polymer-based composites could enhance their mechanical properties, the interlaminar shear test indicated that the short beam shear strength of the composites decreased after adding the nanoclays into the matrix.

Design of pull-out stresses for prestrained SMA wire/polymer hybrid composites

Abstract: Shape memory alloy (SMA) materials possess many unique mechanical, thermal and thermal-mechanical properties that govern their temperature-dependent shape memory, superelastic and pseudoelastic effects. SMA actuators, in the forms of wire or strip embedded into polymer-based composite structures are able to control the structures' shape and stiffness in order to accommodate any environment impacts. Due to an intrinsic shape memory effect (SME) of prestrained SMA wires, additional recovery forces, which are able to alternate the dynamic and buckling properties of the structures, are generated upon heating. This heat is usually induced by a means of externally applied current. To fully use the benefit of the SME in SMA composite structures, understanding the interfacial bonding behaviour and stress transfer properties between the embedded SMA wires and matrix is crucial. In this paper, the wire pull-out properties of prestrained SMA wire/polymer composites subjected to different temperatures were experimentally studied. The results indicated that the critical debonding stress, that initiates fully debond at a bond interface between the SMA wire and matrix increased with increasing the externally applied current, and thus changing the phase condition in the wire. This phenomenon was due to the SME of the prestrained wires and can be interpreted by using a newly developed phase–stress–displacement diagram.

Embedded fibre Bragg grating sensors for non-uniform strain sensing in composite structures

Abstract: A methodology for evaluating the response of embedded fibre Bragg grating (FBG) sensors in composite structures based on the strain in a host material is introduced. In applications of embedded FBG sensors as strain sensing devices, it is generally assumed that the strain experienced in a fibre core is the same as the one measured in the host material. The FBG sensor is usually calibrated by a strain gauge through a tensile test, centred on obtaining the relationship between the surface strain in the host material and the corresponding Bragg wavelength shift obtained from the FBG sensor. However, such a calibration result can only be valid for uniform strain measurement. When the strain distribution along a grating is non-uniform, average strain measured by the strain gauge cannot truly reflect the in-fibre strain of the FBG sensor. Indeed, the peak in the reflection spectrum becomes broad, may even split into multiple peaks, in sharp contrast with a single sharp peak found in the case of the uniform strain measurement. In this paper, a strain transfer mechanism of optical fibre embedded composite structure is employed to estimate the non-uniform strain distribution in the fibre core. This in-fibre strain distribution is then utilized to simulate the response of the FBG sensor based on a transfer-matrix formulation. Validation of the proposed method is preceded by comparing the reflection spectra obtained from the simulations with those obtained from experiments.

Fibre optic sensors for delamination identification in composite beams using a genetic algorithm

Abstract: Fibre Bragg grating (FBG) sensors associated with a genetic algorithm (GA) were used to detect and identify the size and location of delaminations in composite beams. A theoretical beam model is implemented into the GA for on-line delamination parameter searching. The objective function of this vibration-based delamination detection problem in the GA is defined as the sum of squared ratios of the differences between the shifts of eigenvalues of a delaminated beam measured by the FBG sensors and calculated from the theoretical beam model to the eigenvalues of an intact beam measured by the FBG sensors in the first three vibration modes. The principle of the FBG sensors for vibration detection is briefly discussed in this paper. A laser vibrometer and an accelerometer are utilized to compare the results measured from the FBG sensors. The reliability of using the FBG sensors for delamination detection is highlighted. Different delamination sizes and locations in spanwise and thickness-wise directions of the beams are simulated to demonstrate the feasibility of using the GA for the detection of delamination in the composite beams.

Efficiency of genetic algorithms and artificial neural networks for evaluating delamination in composite structures using fibre Bragg grating sensors

Abstract: The efficiency of genetic algorithms (GAs) and artificial neural networks (ANNs) in the quantitative assessment of delamination in glass fibre-reinforced epoxy (GF/EP) composite laminates was evaluated comparatively. For GA-based identification, a theoretical model and a vibration-based objective function were established to relate the delamination parameters to the shift in structural eigenvalues. For the ANN-based approach, feedforward artificial neural networks were configured and trained using the structural eigenvalues obtained from different damage groups, under the supervision of an error-backpropagation neural algorithm. By way of validation, dynamic responses of selected GF/EP laminate beams containing various delaminations were captured using embedded fibre Bragg grating sensors, from which the structural eigenvalues were extracted and used inversely to implement the damage assessment via the GA and the ANN. The performances of the two algorithms were addressed as regards the prediction precision and computational cost.

Utilization of embedded optical fibre sensors for delamination characterization in composite laminates using a static strain method

Abstract: Embedded fibre Bragg grating (FBG) sensors are utilized to characterize a delamination in glass fibre-reinforced epoxy (GF/EP) composite laminates using a static strain method. Composite beams with different edge delaminations in the thickness-wise direction are monitored by the FBG sensors, whose centres are located at delamination tips of the beams, under a three-point bending test. A surface bonded strain gauge is employed to calibrate the FBG sensor using the bending test. Moreover, strain distribution within the sensing region of the delaminated composite beams is numerically calculated by the finite element method (FEM). FEM results establish the relationship between the strain distribution of the beams and the shape of reflection spectra from the FBG sensors. A correlation between delamination location in the thickness-wise direction of the beams and the shape of the reflection spectra is then highlighted.

Effects of embedded optical fibre on mode II fracture behaviours of woven composite laminates

Abstract: Effects of embedded optical fibre on mode II fracture behaviours of woven composite laminates are theoretically and experimentally investigated. An analytical model of End Notched Flexure (ENF) specimens with different delaminations in the thickness-wise direction is studied. Mode II energy release rates (GII) are calculated based on the analytical model associated with critical loads obtained from ENF tests. Comparisons between the ENF specimens with and without embedded optical fibre on the critical load, the GII, fracture surface and microscopic fracture behaviours are studied. The influence on the mode II fracture behaviours of the ENF specimens is studied on the basis of the relationship of the separation and an interaction, between the delamination and the embedded optical fibre.

Conducting and Antistatic Composites for Space Applications

Abstract: The percolative dependence of the DC conductivity on the volume concentration of fillers for composites obtained by dispersing conducting particles into polymeric matrices is studied in detail. An empirical Boltzmann like dependence is proposed for the modeling of the dependence of DC conductivity versus filler concentration. This expression allows for a more accurate determination of the percolation threshold in the case of broad percolations. It is shown that the loading of the polymeric matrices with conducting fillers produces percolative-like changes of various physical properties (such as the reciprocal of the tensile strength and the reciprocal of the double integral of the resonance spectrum). Experimental mechanical, electrical, and electron spin resonance data on polyvinylchloride-carbon, polyvinylchloride-polyaniline, and polyethylene-polyaniline composites are reported.