Research Publications (Engineering and Built Environment)

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Optimal design of symmetric angle-ply laminates subject to nonuniform buckling loads and in-plane restraints
(Elsevier, 1996) Walker, Mark; Adali, Sarp; Verijenko, Viktor E.
Optimal buckling designs of symmetrically laminated rectangular plates under in-plane uniaxial loads" which have a nonuniform distribution along the edges are presented. In particular, point loads, partial uniJorm loads and nonuniform loads" are considered in addition to uniform O' distributed inplane loads" which provide the benchmark solutions. Poisson's effect is" taken into account when in-plane restraints are present along the unloaded edges. Restraints give rise to in-plane loads" at unloaded edges which lead to biaxial loading, and may cause premature instability. The laminate behaviour with respect to fiber orientation changes significantly in the presence of Poisson's eJfi, ct as compared to that o/'a laminate where this" ~Jfect is neglected. This change in behaviour has significant implications Jor design optimisation as the optimal values of design variables with or without restraints differ substantially. In the present study, the design objective is" the maximisation of the uniaxial buckling load by optimally determining the fiber orientations. The )qnite element method, coupled with an optimisation routine, is employed in analysing and optimising the laminates. Numerical results are given for a number of boundary conditions and fi)r uniJormly and non-uniformly distributed buckling loads.
Optimization of symmetric laminates for maximum buckling load including the effects of bending-twisting coupling
(Elsevier, 1996) Walker, Mark; Adali, Sarp; Verijenko, Viktor E.
Finite element solutions are presented for the optimal design of symmetrically laminated rectangular plates subjected to a combination of simply supported, clamped and free boundary conditions. The design objective is the maximization of the biaxial buckling load by determining the fibre orientations optimally, with the effects of bending-twisting coupling taken into account. The finite element method coupled with an optimization routine is employed in analysing and optimizing the laminated plate designs. The effects of boundary conditions, the number of layers and bending-twisting coupling on the optimal ply angles and the buckling load are numerically studied.
Multiobjective optimization of laminated plates for maximum prebuckling, buckling and postbuckling strength using continuous and discrete ply angles
(Elsevier., 1996) Adali, Sarp; Walker, Mark; Verijenko, Viktor E.
The optimal design of uniaxially loaded laminated plates subject to elastic in-plane restraints along the unloaded edges are given for a maximum combination of prebuckling stiffness, postbuckling stiffness and buckling load. The results are also obtained for biaxially loaded plates without elastic restraints. The method of solution involves defining a design index comprising a weighted average of the objective functions and identifying candidate configurations which have to be optimized and compared to determine the best stacking sequence. This multiobjective approach leads to improved prebuckling, buckling and postbuckling performance. A similar approach is adopted in the case of discrete ply angles with the provision that these angles can only take predefined values. From a manufacturing viewpoint, using only certain fibre orientations such as 0, ± 45 and 90 ° is advantageous and cost-effective. The multiobjective design results are compared to single objective ones, and the effect of various problem parameters on the optimal designs are numerically studied. It is observed that the resulting trade-off among the different objectives are not severe leading to well-balanced laminates with regard to the range of loads they are required to carry. A comparison of continuous and discrete optimization indicates that both designs lead to comparable load carrying capacity, with regard to different objectives
Optimal design of symmetrically laminated plates for maximum buckling temperature
(Taylor & Francis, 1997) Walker, Mark; Reiss, Talmon; Adali, Sarp; Verijenko, Viktor E.
The optimal designs of laminated plates subject to nonuniform temperature distributions are givenfor maximum bucklingtemperature. The method ofsolution involves the finite element method based on Mindlin plate theory and numerical optimization: A computational approach is developed that involves successive stages of solution for temperature distribution, buckling temperature, and optimalfiber angle. Three different temperature loadingsare consideredand various combinations of simply supported and clamped boundary conditionsare studied. The effectofplate aspectratioon the optimal fiber angle and the maximum buckling temperature is investigated. The influence of bending-twisting coupling on the optimum design is studied by considering plates with an increasing number of layers.
Optimal design of symmetrically laminated plates for minimum deflection and weight
(Elsevier, 1997) Walker, Mark; Reiss, Talmon; Adali, Sarp
The minimum deflection and weight designs of laminated composite plates are given. The finite element method using Mindlin plate theory is used in conjunction with optimisation routines in order to obtain the optimal designs. Various boundary conditions are considered and results are given for varying aspect ratios and for different loading types. Comparative results are presented for minimum weight priority design as an alternative to minimum deflection/minimum weight priority design to investigate the effect of priority on the deflection and weight.
A procedure to select the best material combinations and optimally design hybrid composite plates for minimum weight and cost
(Taylor & Francis, 1997) Walker, Mark; Reiss, Talmon; Adali, Sarp
The optimal layup with least weight or cost for a symmetrically laminated plate subject to a buckling load is determined using a hybrid composite construction. A hybrid construction provides further tailoring capabilities and can meet the weight, cost and strength constraints while a non-hybrid construction may fail to satisfy the design requirements. The objective of the optimization is to minimize either the weight or cost of the plate using the ply angles, layer thicknesses and material combinations as design variables. As the optimization problem contains a large number of continuous (ply angles and thicknesses) and discrete (material combinations) design variables, a -sequential solution procedure is devised in which the optimal variables are computed in different stages. The proposed design method is illustrated using graphite, kevlar and glass epoxy combinations and the efficiencies of the hybrid designs over the non-hybrid ones are computed.
Minimum weight design of composite hybrid shells via symbolic computation
(Elsevier, 1997) Walker, Mark; Reiss, Talmon; Adali, Sarp
The best layup for a hybrid laminated cylindrical shell subject to a buckling load constraint is determined. The objective of the optimisation is the minimum weight design of these structures. The ply angle is taken as the design variable. Various configurations of graphite and boron epoxy layers are considered in order to determine an optimal stacking sequence. The symbolic computational software package MATHEMATICA is used in the implementation and solution of the problem. This approach simplifies the computational procedure as well as the implementation of the analysisloptimisation routine. Results are given illustrating the dependence of the optimal layup on the cylinder length and radius. It is shown that a general purpose computer algebra system like MATHEMATICA is well suited to solve structural design problems involving composite materials.
Multiobjective optimisation of laminated I-beams for maximum crippling, buckling and postbuckling strength
(Elsevier, 1998) Walker, Mark
The present study deals with the optimal design of uniaxially loaded laminated I-beams for a maximum combination of crippling and buckling load, in the first instance, and a maximum combination of buckling load and postbuckling stiffness, in the second instance. The method of solution involves defining a design index comprising a weighted average of the objective functions and identifying candidate configurations which have to be optimised and compared to determine the best stacking sequence. This multiobjective approach leads to improved crippling, buckling and postbuckling strength. The multiobjective results are compared to single objective results, and the effect of various problem parameters on the optimal designs are numerically studied.
Optimal design of symmetric laminates with cut-outs for maximum buckling load
(Elsevier, 1999) Walker, Mark
Finite element solutions are presented for the optimal design of symmetrically laminated rectangular plates with central circular cut-outs subject to a combination of simply supported, clamped and free boundary conditions. The design objective is the maximisation of the biaxial buckling load by determining the fibre orientations optimally with the effects of bending–twisting coupling taken into account. The finite element method coupled with an optimisation routine is employed in analysing and optimising the laminated plate designs. The effect of plate size and boundary conditions on the optimal ply angles and the buckling load are numerically studied, and these results are compared to those from an article which appeared in this journal in 1996; viz. plates without cut-outs (Walker M, Adali S, Verijenko VE. Optimisation of symmetric laminates for maximum buckling load including the effects of bending–twisting coupling.
Refined theory of laminated anisotropic shells for the solution of thermal stress problems
(Taylor & Francis, 1999) Verijenko, Viktor E.; Tauchert, T. R.; Shaikh, C.; Tabakov, Pavel Y.
A new higher order theory of laminated anisotropic shells for the solution of thermal stress problems that takes into account transverse shear stresses is developed.The theory is based on the kinematic hypotheses that were not assumed a priori but derived on the basis of an iterative technique. The hypotheses take into account the influence of the tangential components of the external loads and the temperature on distributions of the transverse shear stresses through the thickness of the shell. Some analytical solutions are obtained on the basis of the theory developed, and the results are compared with those available in the literature. The theory is also implemented on the basis of the finite element method, and a new triangular finite element is formulated. Some numerical results on the basis of this finite element are also presented.
A method for optimally designing laminated plates subject to fatigue loads for minimum weight using a cumulative damage constraint
(Elsevier, 2000) Walker, Mark
A procedure to optimally design laminated plates for a specific cyclic life using a cumulative damage constraint is described. The objective is minimum weight, and the design variables are the fiber orientation, and the plate thickness. The plates are subjected to cyclic bending loads, and the finite element method, in conjunction with the Golden Section method, is used to determine the design variables optimally. The FE formulation is based on Mindlin theory for moderately thick laminated plates and shells, and the formulation includes bending–twisting coupling. In order to demonstrate the procedure, several plates with differing events, load magnitudes and type, aspect ratios, boundary conditions and cyclic lives are optimised, and compared.
Performance analysis of a shaded-pole linear induction motor using symmetrical components, field analysis, and finite element method
(Institute of Electrical and Electronics Engineers (IEEE), 2000-03) Davidson, Innocent Ewaen; Gieras, Jacek F.
The application of symmetrical components of two-phase asymmetric systems, field theory, and finite element method (FEM) to the performance calculation of the shaded-pole linear induction motor (LIM) is presented. The results compared with measurements are satisfactory though the performance of the shaded-pole single-phase LIM is poor when compared to three-phase LIMs. LIMs generally have low efficiencies due to their open airgap. The maximum efficiency of rotary shaded-pole induction motors with cage-rotors rated at 100 W usually does not exceed 20%.
PWM-VSI inverter-assisted stand-alone dual stator winding induction generator
(Institute of Electrical and Electronics Engineers (IEEE), 2000-11) Ojo, Olorunfemi; Davidson, Innocent Ewean
This paper presents a novel usage of a dual stator winding three-phase induction machine as a stand-alone generator with both controlled output load voltage magnitude and frequency. This generator, with both three-phase power and control windings housed in the stator structure, has the load connected to the power winding and a three-phase pulsewidth modulation (PWM) voltage-source inverter sourcing the control winding. The input to the PWM inverter is either a battery source or a charged dc capacitor. The operational characteristics of these generator schemes with either of the two inverter sources are investigated and shown to have desirable performance. How the load voltage magnitude depends on the various control and design parameters such as rotor speed, compensating capacitance, and load impedance is determined using a detailed mathematical model of the system.
Multi-dimensional design optimisation of laminated structures using an improved genetic algorithm
(2001) Tabakov, Pavel Y.
The present study demonstrates a new variation of the genetic algorithm (GA) technique for engineering applications. This approach is highly efficient for many classes of engineering problems. The proposed selection of the best individuals and localised search makes the search more effective and rapidly improves the fitness value from generation to generation. Both continuous and discrete design variables are considered, and a comparative analysis of the performance of the algorithm is studied. The evaluation of the burst pressure of thick composite pressure vessels based on three-dimensional stress–strain analysis is considered here as an example. Exact elasticity solutions are obtained using the stress function approach where the radial, circumferential and shear stresses are determined taking the closed ends of the cylindrical shell into account.
Stress distribution in continuously heterogeneous thick laminated pressure vessels
(Elsevier, 2001) Verijenko, Viktor E.; Adali, Sarp; Tabakov, Pavel Y.
Stress analysis of multilayered pressure vessels possessing cylindrical anisotropy and under internal, external and interlaminar pressures is given. The special case when the axis of anisotropy coincides with the axis of symmetry Oz and the stresses do not vary along the generator is investigated. In this case there exists a plane of elastic symmetry normal to this axis at every point of the cylinder so that each layer may be considered as orthotropic. However, elastic properties can vary through the thickness of a layer. Exact elasticity solutions are obtained for both open-ended and closed-ended cylinders using a stress function approach. The method of solution allows the forces on the layer interfaces to be taken into account with relative ease. Numerical results are presented for thick cylinders with isotropic and orthotropic layers, and stress distributions across the thickness are shown.
Multiobjective design of laminated plates for maximum stability using the finite element method
(Elsevier, 2001) Walker, Mark
Finite element solutions are presented for the optimal multiobjective design of symmetrically laminated rectangular plates subject to a combination of simply supported, clamped and free boundary conditions. The design objective is the maximisation of the biaxial buckling load and resonance frequency by determining the fibre orientation optimally with the effects of bending–twisting coupling taken into account. The finite element method coupled with an optimisation routine is employed in analysing and optimising the laminated plate designs. The effect of boundary conditions and bending–twisting coupling on the optimal ply angles and the buckling load are numerically studied. In addition, the multiobjective results are also compared to single objective design results
Correlation between elastic and electric properties for cyclically loaded metals
(Springer, 2002) Sevostianov, Igor; Bogarapu, Mahesh; Tabakov, Pavel Y.
A new method of evaluation of the elastic property deterioration due to accumulated damage is suggested and experimentally verified. It is based on the explicit correlations between two groups of anisotropic properties – conductivity and elasticity, recently established for porous/microcracked materials with anistropic microstructures. An experimental study of fatigue has been done to verify the theoretical predictions. The electrical resistance and Young''s modulus are measured as functions of the number of loading cycles in the standard fatigue tests. The agreement between the theoretical predictions and the direct experimental data is better than 10% in all cases. The results allow one to use measurements of the electric resistance to estimate the damage accumulated in methal structures and the decrease of the elastic modulus.
A computational methodology to select the best material combinations and optimally design composite sandwich panels for minimum cost
(Elsevier, 2002) Walker, Mark; Smith, Ryan E.
A procedure to select the best material combination and optimally design sandwich laminates with fibre reinforced skins and low density cores for minimum cost is described. Sandwich constructions generally provide improved stiffness/mass ratios and provide more tailoring opportunities than monolithics, and thus greater chance of satisfying design constraints. The objective of the optimisation is to minimise the laminate cost by selecting the skin and core material combination, layer thicknesses and skin fibre angles optimally, subject to load and mass constraints. As the optimisation problem contains a number of continuous (ply angles and thicknesses) and discrete (material combinations) design variables, a sequential solution procedure is devised in which the optimal variables are computed in different stages. The methodology and its benefits are demonstrated using graphite, glass or kevlar/epoxy facings, and balsa or PVC cores.
Evaluation and effective management of non-technical losses in power networks
(2002-10) Davidson, Innocent Ewean
Market-driven economies and deregulated electricity industry environment have stimulated the minimization of technical and non-technical losses (NTL) even though they do not constitute major operational or quality of supply problems. Their impact is economic and utilities often passed down the costs to consumers. NTL need to be addressed to determine the overall performance of power networks, as these losses are expected to be more dominant at the sub-transmission (132kV-33kV) and reticulation (22kV and 11kV) levels of the electricity supply industry value chain. In some national grid operations, NTL are estimated to account for up to 30% in revenue losses to electric utilities, and overhead expenditure in added maintenance costs. This paper discusses a method for NTL evaluation and an effective management approach to loss minimization and revenue collection. Copyright © 2002 IEEE.
A shape control model for piezo-elastic structures based on divergence free electric displacement
(Elsevier, 2003) Kekana, Marino; Tabakov, Pavel Y.; Walker, Mark
A model simulating the effects of the control potential on the static configuration of a piezo-elastic structure is presented. This model is centred on the electric displacement field, which is shown to be divergence free. Thus, the surface charge effect no control over the configuration of a piezo-elastic structure, save for the control potential derived through passive or active control. Results show that at zero gain the proposed model resembles a structure free from piezoelectric control. Thus, no fictituous stiffness is introduced as is the case with models presented in the literature.