A novel cement composite containing graphene nanoplatelet (GNP) which can sense its own strain and damage is
introduced in this paper. Piezoresistive strain sensing was investigated for mortar specimens with GNP under both cyclic
and monotonically increasing compressive and tensile strain. Under compression, the electrical resistance decreased with
increasing strain and the normalized resistance can be described by a bilinear curve with a kink at about 400 microstrain.
At low strain, a high gauge factor exceeding 10<sup>3 </sup>in magnitude was obtained and it increased almost linearly with the
GNP content. This can be attributed primarily to the reducing interfacial distance and forming of better contacts between
GNP and cement paste when the composite was initially loaded. At higher compressive strain beyond 400 microstrain,
the gauge factor is consistently about 10<sup>2</sup> for GNP content exceeding the percolation threshold. A different response was
observed for specimens under tension due to the formation and propagation of microcracks even at low tensile strain due
to the brittleness of the material. The initial gauge factor is of the order 10<sup>2</sup> for tensile strain up to 100 microstrain and it
increases exponentially beyond that. The damage self-sensing capability of this conductive cement composites is
explored using electric potential method. Closed form expression for the assessment of damage are derived based on the
mathematical analogy between the electrostatic field and the elastostatic field under anti-plane shear loading. The
derived expression provide a quick and accurate assessment of the damage of this conductive material which is
characterized by its change in compliance.
The piezoresistivity-based strain sensing ability of cementitious composites containing graphite nanoplatelet (GNP) is investigated in this paper. GNP offers the advantages of ease of processing, excellent mechanical and electrical properties at a very low cost compared to carbon nanotubes and carbon nano-fibers. Cement mortar with 0%, 1.2%, 2.4%, 3.6% and 4.8% of GNP (by volume of composite) were cast. The electrical resistance of the specimens was measured by both the two- and four-probe methods using direct current (DC). The effect of polarization was characterized and the percolation threshold was experimentally found to be between 2.4% and 3.6% of GNP based on both accelerated and normal drying specimens. The assumption of Ohmic material was tested with varying current and found to be valid for current < 0.01mA and 0.5mA for four- and two-probe methods respectively. The piezoresistive effect was demonstrated by comparing the gage factors of mortars with GNP vs plain mortar under cyclic loading in compression at 3 strain levels. At low strains, the high gage factor is believed to stem from both the effect of the imperfect interfaces around the GNP and the piezoresistivity of the GNP; at higher strains, the gage factor is likely to be attributed to the piezoresistivity of the GNP and it is still 1-2 orders of magnitude larger than the gage factor arising from geometric changes.
This paper addresses the issue of intermittent data loss during transmission of wireless network sensors and the
application of the reconstruction signal for damage detection with the damage locating vector (DLV) method. The
algorithm makes use of frequencies which contribute significant amount of energy in the signal based on Fourier
transform. As the amplitudes are uncertain due to lost data, the Fourier amplitudes are estimated based on least-square fit
of only the measured portions of the signal. The lost portions are reconstructed through inverse Fourier transform. The
procedure is iterated until the discrepancy between estimated lost portions of two consecutive iterations is below a set
threshold. This threshold and the power spectral threshold to demarcate the significant frequencies are selected based on
results from numerically simulated signals. The reconstructed signals are used with the DLV method for damage
detection to investigate the practicality of this procedure. A cantilever truss structure with a pre-stressed cable was
monitored using six wireless sensors. The pre-stressed cable was cut mid-way during random load application and data
collection. The results obtained support the use of the reconstructed signal within the framework of the DLV method.
The damage locating vector (<i>DLV</i>) method based on dynamic response is (a) modified for structural damage detection
using normalized cumulative energy (instead of stress) indicator, (b) applied to the case where the number of sensors
used is small compared to the structural degrees of freedom, and (c) employed to identify multiple damaged elements of
an existing structure. From the measured structural dynamic response at the reference and damaged states, the change in
flexibility matrix at sensor locations is formulated based on which the <i>DLV</i> is calculated. The <i>DLV</i> is a set of static load
vector which has the property that when applied to the structure at its reference state, no energy is induced in the
damaged elements providing the basis to identify damaged elements. The efficiency and robustness of the proposed
method are examined using both simulated and experimental data.
The use of Frequency Response Functions (FRFs) measured at some points in the accessible area of a structure to detect
damage in the inaccessible area is studied, based on the difference in the normalized FRFs from a previous observed
state. Numerical simulations and experiments were carried out to investigate the effectiveness of the proposed damage
index. Results showed that from the indices at the first flexural resonance mode and in the vicinity of the first resonant
frequency, the damage can be accurately detected and located if there is damage in the accessible area. If the damage is
in the inaccessible area, the existence of the damage can be successfully ascertained although its location may not be
identified accurately. For multi-damage conditions, with damage in both the accessible and inaccessible areas, the
proposed method can accurately detect and locate the damage in the measurable area and detect the damage in the
inaccessible area. The magnitude of the indices provides useful information with regards to the damage severity.
In this paper, packaged fibre Bragg grating (PFBG) sensors were fabricated by embedding them in 70mm x 10mm x 0.3mm carbon-fibre composites which were then surface-bonded to an aluminium beam and a steel I-beam to investigate their strain monitoring capability. Initially, the response of these packaged sensors under tensile loading was compared to bare FBGs and electrical strain gauges located in the vicinity. The effective calibration constant/ coefficient of the PFBG sensor was also compared with the non-packaged version. These PFBG sensors were then attached to an I-section steel beam to monitor their response under flexural loading conditions. These realistic structures provide a platform to assess the potential and reliability of the PFBG sensors when used in harsh environment. The results obtained in this study gave clear experimental evidence of the difference in performance between the coated and uncoated PFBG fabricated for the study. In another experimental set-up, bare FBG and POF vibration sensors were surface-bonded to the side-surface of a CFRPwrapped reinforced concrete beam which was then subjected to cyclic loading to assess their long-term survivability. Plain plastic optical fibre (POF) sensors were also attached to the side of the 2-meter concrete beam to monitor the progression of cracks developed during the cyclic loading. The results showed excellent long-term survivability by the FBG and POF vibration sensors and provided evidence of the potential of the plain POF sensor to detect and monitor the propagation of the crack developed during the test.
In this paper, a finite element method to simulate the overall behavior of ultrasonic motor (USM) is proposed. Firstly, an iterative algorithm using ABAQUS<sup>®</sup> version 6.4 to solve the contact problem with piezoelectric actuation is presented. In each iteration, the dynamic responses of stator actuated by piezoelectric force and updated contact force are solved, from which static (or steady state) contact between deformed stator and rotor are estimated. For the dynamics of stator, three dimensional solid elements are adopted and direct integration method is used because modal-based procedures do not adequately transform the electric loads into modal loads. Rayleigh damping is adopted with the ratio set to 0.5%. For the contact between deformed stator and rotor, Lagrange multiplier method is used to impose the normal and tangential contact constraints between the stator and rotor respectively. Based on the proposed procedure, given the applied torque, axial force, and piezoelectric drive voltages as inputs, the general measures of motor performance are obtained and compared with published numerical and experimental results. The approach presented here provides a more accurate framework with moderate computational cost for modeling USM and serves as a design tool for optimizing prototypes.
The detection of cracks in beams and plates using piezo-actuated Lamb waves has been presented in the last SPIE Symposium. This paper is an extension of the technique to pipes. It has been shown that for a thin-walled pipe, the assumption of Lamb wave propagation is valid. Such waves can be efficiently excited using piezoceramic transducers (PZT) with good control on the pulse characteristics to assess the health of structural components, such as the presence of cracks. In this paper, a systematic methodology to detect and locate cracks in homogenous cylinder/pipe based on the time-of-flight and strength analysis of propagating Lamb wave is proposed. By observing the attenuation in strength of the direct wave incidence at the sensor, the presence of a crack along the propagation path can be determined. At least four actuation positions, two on each end of the pipe segment of interest, are needed to exhaustively interrogate for the presence of cracks. The detailed procedure for locating and tracing the geometry of the crack(s) is described. It is shown experimentally that the detection using circular PZT actuator and sensor, with dimensions of 5.0 mm diameter and 0.5 mm thick, is possible for an aluminum pipe segment of up to at least 4.0 m in length. The proposed methodology is also explored for the aluminum pipe under more practical situations, such as burying it in sand with only the actuator and sensor positions exposed. Experimental results obtained showed the feasibility of detecting the 'concealed' crack on the pipe buried in sand.
This paper describes the design of an extrinsic optical fibre sensors based on poly(methamethycrylate) for structural health monitoring applications. This polymer-based optical fiber sensor relies on the modulation of light intensity and is capable of monitoring the response of the host structure subjected to either static or dynamic load types. A series of mechanical tests have been conducted to assess the response of the plastic optical fiber (POF) sensor. The readings of the sensors attached to an aluminium bar were found to compare well to electrical strain gauge response. The POF sensors were also attached to rebar concrete beams and exhibited encouraging response under flexural loading. Static and cyclic loading tests were also performed and the sensor was shown to exhibit excellent strain linearity and repeatability. Free vibration tests on a cantilever beam set-up in which the POF sensor was surface-bonded to a composite beam were also conducted. The results obtained highlight the capability of the sensor to accurately monitor the dynamic response of the beam. Impulse-type dynamic response of the sensor was also conducted and the POF sensor demonstrated potential for detecting the various modal frequencies of the host structure. POF sensors were also attached to a series of impacted composite beams with varying degree of damage to assess their potential to detect and quantify the damage in the host structure. The results demonstrated the feasibility of using the sensor for structural health monitoring applications.
Plastic optical fibre sensors offer remarkable ease of handling and recent research has shown their potential as a low-cost sensor for damage detection and structural health monitoring applications. This paper presents details of a novel plastic optical fibre sensor and the results of a series of mechanical tests conducted to assess its potential for structural health monitoring. The intensity-based optical fibre sensor proposed in this study relies on the modulation of light intensity as a function of a physical parameter (typically strain) as a means to monitor the response of the host structure to an applied load. Initially, the paper will reveal the design of the sensor and provide an summary of the sensor fabrication procedure followed by an outline of the experimental programme conducted in this study. Two types of sensor design will be evaluated in terms of their strain sensitivity, linearity and signal repeatability. Results from a series of quasi-static tensile tests conducted on aluminium specimen with four surface-attached optical fibre sensors showed that these sensors offer excellent linear strain response over the range of the applied load. Free vibration tests based-on a cantilever beam configuration were also conducted to assess the dynamic response of the sensor. The results demonstrate excellent agreement with conventional sensor readings.
Micropipette aspiration is one of the most widely used techniques for measuring the mechanical properties of single cells. The homogeneous linear elastic half-space model has been frequently applied to characterize the micropipette aspiration of chondrocytes and endothelial cells. However, the linear elastic model is limited to small deformation and the half-space assumption is frequently invalidated when moderately large micropipettes are used. In this work, the linear elastic constitutive model is extended to the neo-Hookean constitutive model and the geometry is simulated more realistically by considering the cell as a sphere. The large-deformation contact mechanics problem is solved using dimensionless axisymmetric finite element analysis. The effects of pipette diameter and fillet radius on the cellular rheological behaviour are also systematically studied. Based on the finite element simulation, empirical relationships have been derived for the direct interpretation of the elastic mechanical parameters from the micropipette aspiration experiments. Micropipette aspiration of late-stage malaria-infected red blood cells (schizonts) is conducted. The infected cells are found to exhibit elastic solid behavior in contrast to the liquid drop behavior of healthy red blood cells. The apparent shear modulus of the schizonts, interpreted from the elastic solid model, is found to be 119±62 Pa.
Lamb waves have attracted great attention in non-destructive evaluation (NDE) due to its efficiency in interrogating a reasonably extensive distance along the plate. Such waves can be efficiently excited using piezoceramic transducers with good control on the pulse charactertistics to assess the health of structural components, such as the presence of cracks. Through selective generation of Lamb waves within a frequency range, linear cracks can be detected via time-of-flight analysis of the wave, by using plain piezoceramics transducers with strategic positioning. Alternatively, using a well-designed inter-digital transducer (IDT), a single Lamb mode can be generated. It is shown that using IDT enhances detection accuracy and robustness in view of its controllability on the duration and direction of the generated wave. It is thus able to locate curved crack accurately as well as trace its geometry. The performance of both actuators are compared experimentally using both plain piezoceramics and IDT to detect different cracks, namely, linear crack, curved crack and multiple cracks, on aluminum plates. Plain piezoceramics provide accurate detection for linear and multiple cracks, and are able to estimate the geometry of a curved crack reasonably well. However, IDT is more efficient and provides accurate results for these three cases.
An analytical model for free vibration analysis of piezoelectric coupled thick circular plate is presented based on Mindlin's plate theory. The distribution of electric potential along the thickness direction is simulated by a sinusoidal function. The differential equations of motion are solved analytically for two boundary conditions of the plate: clamped edge and simply supported edge. Numerical investigations are performed for plates sandwiched in between two surface-bonded piezoelectric layers for various diameter-thickness ratios and the results agree well with those from three-dimensional finite element analyses.
The optimal placement of piezoelectric sensor/actuator (S/A) pairs to maximize the damping effect of a composite plate under a classical control framework using the finite element approach is investigated. Due to the discretization of the spatial domain, the problem falls under the class of discrete optimization. Two optimization performance indices based on modal and system controllability are adopted. The classical direct pattern search method is employed to obtain local optima. It is proposed that the starting point for the pattern search be selected based on the maxima of integrated principal strains consistent with the size of piezoelectric patches used, which would maximize the virtual work done by the equivalent actuation forces along the corresponding mechanical displacements. In this way, the global optimal placement can be efficiently deduced. Numerical simulation using a cantilever composite plate under free vibration shows that the proposed strategy to locate the optimal placement is practical and efficient, with results very close to the global optimal layout from exhaustive search. The speed of convergence is rapid compared to an initial blind discrete pattern search approach. For the specific example used, the S/A pairs positioned close to the support are most effective whereas those near the free end are the least effective, for the first two modes. S/A pairs placed furthest from the center line of the cantilever plate are most effective for torsional vibration control. These findings are in good agreement with the published results.
LQR vibration control of piezoelectric composite plates is investigated via the finite element method. Laminated composite plates with bounded or embedded piezoelectric sensors (PVDFs) and actuators (PZTs) are discretized by an isoparametric element and the governing equations of motion are derived by using the Hamilton's principle. The optimal LQR method is used to couple the discrete distributed actuation and sensing. The Algebraic Riccati Equation (ARE) is solved by MATLAB. To avoid possible numerical difficulty, in the present study only Potter's method rather than MATLAB 's control toolbox functions is used. More emphasis is put on appropriate selection of the weighting matrices of the optimal quadratic objective functions. The present study tried to relate the quadratic functions with some physical meanings to avoid the usual trial and error procedure. The quadratic functions are assumed to consist of independent strain energy, kinetic energy and actuators' input energy. The frequency matrix and the identity matrix are used as the relative weight of the strain energy and the kinetic energy and the actuators' input energy with an adjustable coefficient for each actuator is used as the relative weight of the actuators' input energy. Numerical results show that this method works fairly well for either the output constraint control (0CC) or input constraint control (ICC) and the active damping effect is more sensitive when approaching the breakdown voltages of the actuators.