Ser Tong Quek

Design of hybrid controller for dynamic positioning from calm to extreme sea conditions

High-level control of dynamic positioning systems on marine vessels using hybrid controller is developed to extend the operational weather window for marine operations to harsh environments. The types of hybrid controller considered are multi-output PID controllers with position measurements, and multi-output PID and acceleration feedback controllers with position and acceleration measurements. Numerical simulations and experiments in addition to stability proof are conducted to verify the proposed hybrid-controller dynamic positioning system in varying environmental conditions from calm to extreme seas.

Probabilistic Analysis of Soil-Water Characteristic Curves

Direct measurement of the soil-water characteristic curve (SWCC) is costly and time consuming. A first-order estimate from statistical generalization of experimental data belonging to soils with similar textural and structural properties is useful. A simple approach is to fit the data with a nonlinear function and to construct an appropriate probability model of the curve-fitting parameters. This approach is illustrated using sandy clay loam, loam, loamy sand, clay, and silty clay data in Unsaturated Soil Database. This paper demonstrates that a lognormal random vector is suitable to model the curve-fitting parameters of the SWCC. Other probability models using normal, gamma, Johnson, and other distributions do not provide better fit than the proposed lognormal model. The engineering impact of adopting a probabilistic SWCC is briefly discussed by studying the uncertainty of unsaturated shear strength due to the uncertainty of SWCC.

Finite Element Simulation of the Micropipette Aspiration of a Living Cell Undergoing Large Viscoelastic Deformation

Experiments and computational modeling in single cell mechanics are increasingly being used to evaluate the mechanical properties of living cells such as neutrophils, erythrocytes and fibroblasts. Here, we perform modeling of the micropipette aspiration experiment which has been widely used for measuring the viscoelastic properties of single cells. The commonly used standard linear solid model is extended into the standard neo-Hookean solid model and the large deformation of anchorage-dependent cells in response to micropipette aspiration is simulated. The effects of pipette radius and fillet radius on the rheological behaviour of the cell are also systematically studied. Based on the finite element results, three relationships are derived for the interpretation of the mechanical parameters from the micropipette aspiration of cytoskeleton-rich eukaryotic cells.

Assessment of an extrinsic polymer-based optical fibre sensor for structural health monitoring

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 extrinsic polymer-based 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 outline of the sensor fabrication procedure followed by a brief description of its basic measurement principle. Two types of sensor design (fluid type and air type) will be evaluated in terms of their strain sensitivity, linearity and signal repeatability. Results from a series of quasi-static tensile tests conducted on an 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. A comparison of the strain response of these sensors highlights the significant improvement in strain sensitivity of the liquid-filled-type sensor over the air-filled-type sensor. The specimens were also loaded repeatedly over a number of cycles and the findings exhibited a high degree of repeatability in all the sensors. Free vibration tests based on a cantilever beam configuration (where the optical fibre sensor was surface bonded) were also conducted to assess the dynamic response of the sensor. The results demonstrate excellent agreement with electrical strain gauge readings. An impulse-type loading test was also performed to assess the ability of the POF sensor to detect the various modes of vibration. The results of the sensor were compared and validated by a collocated piezofilm sensor highlighting the potential of the POF sensor in detecting the various eigen-frequencies of the vibration. Finally, preliminary results of a loading–unloading test of the same sensor design encased within a metal tube will be presented. The results obtained were encouraging offering the possibilities of employing the proposed device as an embedded sensor for damage detection in concrete beams.

A Model for the Analysis of Beams with Embedded Piezoelectric Layers

This paper provides a basic mechanics model for the flexural analysis of beams with embedded piezoelectric layers. The Euler model for a long and thin beam is employed and two different ways of connecting the electrodes on the surfaces of the piezoelectric layers, namely, closed circuit (Case I) and open circuit (Case II) are considered. The distribution of the piezoelectric potential in the longitudinal direction is derived by including Maxwell equation in the formulation and assuming a half-cosine distribution for the potential in the thickness direction of the piezoelectric layer for Case I. The validity of this assumption is investigated first theoretically and then numerically by FEM for a pure piezoelectric beam subjected to uniform moment. A semi-analytical analysis is carried out for Case II where an electric potential function is assumed. The resonant frequencies of the structure for the models presented are first validated by the FEM software, ABAQUS, for simply-supported and propped cantilever boundary conditions. Based on the results of vibration analysis, it is shown that the dynamic characteristics of the entire structure can be significantly affected by piezoelectric effects, especially for the open circuit case. More importantly, the mode shapes of the electric potential in the piezoelectric layer are different for Cases I and II. For the closed circuit case, the potential shape function is related to the transverse displacement, or more accurately the curvature of the beam. For the open circuit case, the potentials at the boundaries in the longitudinal direction are directly related to the slope of the deflection of the beam. Hence, the commonly adopted assumption of uniform electric potential needs to be carefully re-examined.

Smart multifunctional cement mortar containing graphite nanoplatelet

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.

Multi-Operational Controller Structure for Station Keeping and Transit Operations of Marine Vessels

In this brief, a novel control system structure for integrated dynamic positioning, maneuvering, and transit operation of marine vessels is proposed. The proposed structure is based on supervisory switching control (SSC) using switching logics in conjunction with operator initiated commands. The SSC is a hybrid system consisting of continuous state multi-controllers and discrete state logics that allow switching among the various controllers for the particular operations. The switching between appropriately designed controllers facilitates operations from normal conditions to extreme situations such as severe seas and possible failure situations. Through this, it will be possible to extend the vessel operability under harsh environments, and increase the safety and performance in marine operations with greater fault-tolerance. One demonstrating example of the integrated marine control system verified by experiments is demonstrated.

A power-law rheology-based finite element model for single cell deformation

Physical forces can elicit complex time- and space-dependent deformations in living cells. These deformations at the subcellular level are difficult to measure but can be estimated using computational approaches such as finite element (FE) simulation. Existing FE models predominantly treat cells as spring-dashpot viscoelastic materials, while broad experimental data are now lending support to the power-law rheology (PLR) model. Here, we developed a large deformation FE model that incorporated PLR and experimentally verified this model by performing micropipette aspiration on fibroblasts under various mechanical loadings. With a single set of rheological properties, this model recapitulated the diverse micropipette aspiration data obtained using three protocols and with a range of micropipette sizes. More intriguingly, our analysis revealed that decreased pipette size leads to increased pressure gradient, potentially explaining our previous counterintuitive finding that decreased pipette size leads to increased incidence of cell blebbing and injury. Taken together, our work leads to more accurate rheological interpretation of micropipette aspiration experiments than previous models and suggests pressure gradient as a potential determinant of cell injury.

Plastic Optical Fiber Sensors for Measurement of Large Strain in Geotextile Materials

A high-strain optical fiber sensor system has been developed to provide a cost-effective solution for measurement of very large strain. The measurement of large strain in the order of a few tens of % strain (up to 40 % strain) in geotextile materials has been achieved using an extrinsic plastic optical fiber sensor (POF). Based on this design, the sensor is not limited to operating within the elastic region of the plastic fiber proposed by other workers in the field. The present design allows for compression strains to be measured even after exceeding strain levels of 5% (typical plastic strain values of POFs based on fiber stretching.) The POF were initially calibrated using a linear variable displacement transducer and the results based on tensile tests of a series of geotextile fabrics have shown to compare well with other reference measurements.

Damage Quantification of Flexurally Loaded RC Slab Using Frequency Response Data

A one-way reinforced concrete (RC) slab was subjected to short-duration concentrated impact load and its dynamic characteristic for the virgin and damaged conditions were studied using two signal processing techniques. The recorded strain and acceleration signals were analyzed using the Fast Fourier Transform (FFT) and the Hilbert Huang Transform (HHT). From these analyses, the percentage reductions in the modal frequency for varying degrees of damage (or magnitude of applied load) were obtained. Based on the eigen-solution of a 3-element partitioned beam model, the frequency–damage relationship was also estimated using the observed initial flexural stiffness of the half-cycle hysteresis path associated with each stage of applied load. Both semi-empirical and experimental results showed close agreement and a 30% frequency reduction was observed between the virgin state and yield. Three quarters of the total frequency reduction from virgin-to-yield occurred within an applied load range of 30% of the yield load.