Hang-yin Ling

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

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.

Low velocity impact on shape memory alloy stitched composite plates

Delamination of advanced composite materials due to various scenarios such as low velocity and ballistic impacts and high strain rate is one of the major problems for aerospace and automotive structural applications. The low velocity impact does not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures decrease dramatically. Shape memory alloy (SMA) materials possess many unique mechanical, thermal and thermal–mechanical properties compared with other conventional materials. Many studies have reported that the superelastic and hysteresis properties of the SMA materials can absorb energies coming from external excitations or sudden impacts. Stitching is well recognized as a promising technique to enhance the through-the-thickness reinforcement, in order to improve the delamination properties of composite structures. By stitching SMA wires into the composite structures one is theoretically able to reduce the risk of delamination of the structures during impact. In this paper, the damage resistance properties of SMA stitched glass/epoxy composites after low velocity impact are experimentally and theoretically studied. The results show that the tensile strength of composite plates increased and the number of translaminar cracks decreased after being stitched by SMA wires. Theoretical study also proves that the delamination energy of composite plates after stitching by superelastic SMA wires is smaller than that of an unstitched composite plate because of the energy absorbed by the SMA wires.

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

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.

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

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

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.

Simultaneous estimation of Poisson's ratio and Young's modulus using a single indentation: a finite element study

Indentation is commonly used to determine the mechanical properties of different kinds of biological tissues and engineering materials. With the force–deformation data obtained from an indentation test, Youngs modulus of the tissue can be calculated using a linear elastic indentation model with a known Poissons ratio. A novel method for simultaneous estimation of Youngs modulus and Poissons ratio of the tissue using a single indentation was proposed in this study. Finite element (FE) analysis using 3D models was first used to establish the relationship between Poissons ratio and the deformation-dependent indentation stiffness for different aspect ratios (indentor radius/tissue original thickness) in the indentation test. From the FE results, it was found that the deformation-dependent indentation stiffness linearly increased with the deformation. Poissons ratio could be extracted based on the deformation-dependent indentation stiffness obtained from the force–deformation data. Youngs modulus was then further calculated with the estimated Poissons ratio. The feasibility of this method was demonstrated in virtue of using the indentation models with different material properties in the FE analysis. The numerical results showed that the percentage errors of the estimated Poissons ratios and the corresponding Youngs moduli ranged from −1.7% to −3.2% and 3.0% to 7.2%, respectively, with the aspect ratio (indentor radius/tissue thickness) larger than 1. It is expected that this novel method can be potentially used for quantitative assessment of various kinds of engineering materials and biological tissues, such as articular cartilage.

Development of an ultrasound platform for the evaluation of plantar soft tissue properties: A feasibility study on silicone phantom feet

A novel ultrasound (US) platform, consisting of an embedded US transducer connected in series with a circular silicone layer, was developed to evaluate the mechanical properties of human plantar tissues in this study. Force exerted by the foot was determined based on the deformation of the silicone layer, which was measured by US. The platform could capture the deformations of both the silicone layer and plantar tissues simultaneously. The stiffness of the plantar tissue was then extracted from the force-deformation curve. To test the feasibility of the US platform, eight phantom feet with different stiffnesses were tested using this new system. The moduli of the phantom feet were also measured by the tissue ultrasound palpation system (TUPS). The results showed that the phantom stiffness determined using the platform was in linear correlation with the corresponding modulus measured by the TUPS (R2 = 0.8914). The current system can be improved by using several US transducers to perform multiple measurements at the same time for reliable assessment of human plantar tissues in a non-invasive, convenient, and cost-effective way.

Fiber Bragg Grating Sensor/Piezoelectric Actuator Hybrid System for Damage Detection in Composite Laminates

In this paper, fiber Bragg grating (FBG) sensor and piezoelectric (PZT) actuator are used to develop a hybrid system for the evaluation of delamination in glass fiber-reinforced epoxy (GF/EP) composite laminates. The surface-bonded PZT actuator generates ultrasonic Lamb wave in the composite laminates, while the FBG sensor, which is embedded in the composite laminates, captures the Lamb wave signal. Wavelet analysis is introduced to extract signal spectrographic characteristics in the time-scale domain appropriately. Since the propagation characteristics of Lamb wave is altered by the existence of damage in the composite laminates, delamination information can be obtained from the received signal. With the assistance of a signal generation and an acquisition system, this methodology enables active sensing and non-destructive evaluation of delamination in the composite laminates. Experiments have been carried out with GF/EP composite beams to examine the feasibility of the proposed detection technique. The acquired and processed Lamb wave signals corresponding to different delamination sizes are compared.

Residual Stress in a Polymer-Glued FBG Temperature-Compensated Sensor for Civil Engineering Applications

In recent years, embedded fibre-optic sensors as structural health monitoring devices have been widely used in both civil and aerospace engineering applications. Their small physical size and ability to immunize electromagnetic interference make them ideal sensing devices, which provide highly accurate and reliable strain and temperature measurements for structures. This paper presents a new designed temperature-compensated fibre-optic Bragg grating (TCS) strain sensor for imbedding into cement-based materials to measure their mechanical and thermal strains individually or simultaneously. However, the residual stress generated due to the constrained boundary of a steel tube that is used to protect the sensor, will influence the accuracy of measurement. Therefore, a theoretical model that is used to estimate this stress at different temperature conditions is discussed.

An embedded FBG sensor for dynamic strain measurement for a clamped-clamped composite structure

A comparison of strain measurement results, from an embedded fibre-optic Bragg grating (FBG) sensor and surface mounted strain gauge, at different vibration frequency ranges and using a clamped-clamped glass fibre composite beam, is presented. It is shown that the FBG sensor is able to precisely measure the peaks at the first-two natural frequency modes compared with the spectrum captured from the strain gauge. The results also demonstrate that the strains measured from the FBG sensor agreed well with the strain gauge at frequencies below 100 Hz. Beyond this value, the actual strain on the beam surface was less than 3µe, and the data extracted from the strain gauge are no longer valid. For a clamped-clamped structure, the longitudinal strain of the beam correlates to its vibration amplitude and excitation frequency. Increasing the frequency results in decreasing the longitudinal strain of the beam and erroneous measurements from the strain gauge resulted. This study provides important information on the feasibility of using embedded FBG sensors as vibration monitoring devices to measure mechanical performance of composite structures.