Faculty of Engineering and Built Environment
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Item A comparative analysis of evolutionary algorithms in the design of laminated composite structures(De Gruyter, 2015) Tabakov, Pavel Y.; Moyo, SibusisoAbstract: The increased use of composite materials and structures in many engineering applications led to the need for a more accurate analysis and design optimi-zation. While methods of stress-strain analysis devel-oped faster, optimization techniques have been lagging behind. As a result, many designed structures do not ful-fill their full potential. The present study demonstrates the major achievements in recent years in an application of evolutionary algorithms to the design optimization of fiber-reinforced laminated composite structures. Such structures are of much interest due to high structural design sensitivity to fiber orientations as well as complex multidimensional discrete optimization problems. Using an anisotropic multilayered cylindrical pressure vessel and an exact elasticity solution as an example, we show how the optimum, or near–optimum, solution can be found in a more efficient way.Item Splitting vacancy voids in the grain boundary region by a post-cascade shock wave(Institute of Problems of Mechanical Engineering, 2013) Markidonov, A. V.; Starostenkov, M. D.; Tabakov, Pavel Y.The structural transformation of vacancy voids in the grain boundary region of the bicrystal under the influence of post-cascade shock waves is studied with the aid of molecular dynamics simulations. It is shown that the void may be split into two parts or completely displaced. This effect depends on the relative position of the void and grain boundary dislocations. Also shown is a seamless transfer of vacancy clusters through the tilt grain boundaryItem A computational methodology to select the optimal material combination in laminated composite pressure vessels(2012-12) Tabakov, Pavel Y.; Walker, MarkA methodology to select the best material combination and optimally design laminated composite pres-sure vessels is described. The objective of the optimization is to maximize the critical internal pressure subject to cost constraints. Exact elasticity solutions are obtained using the stress function approach, where the stresses are determined taking into account the closed ends of the cylindrical shell. The approach used here allows us to analyze accurately multilayered pressure vessels with an arbitrary number of orthotropic layers of any thickness and a combination of different materials. The design optimization of the pressure vessel is accomplished using the Big Bang–Big Crunch algorithm,subject to the Tsai-Hill failure criterion.Item Design optimization of anisotropic pressure vessels with manufacturing uncertainties accounted for(Elsevier, 2013-04) Tabakov, Pavel Y.; Walker, MarkAccurate optimal design solutions for most engineering structures present considerable difficulties due to the complexity and multi-modality of the functional design space. The situation is made even more complex when potential manufacturing tolerances must be accounted for in the optimizing process. The present study provides an original in-depth analysis of the problem and then a new technique for determining the optimal design of engineering structures, with manufacturing tolerances accounted for, is proposed and demonstrated. The numerical examples used to demonstrate the technique involve the design optimization of anisotropic fibre-reinforced laminated pressure vessels. It is assumed that the probability of any tolerance value occurring within the tolerance band, compared with any other, is equal, and thus it is a worst-case scenario approach. A genetic algorithm with fitness sharing, including a micro-genetic algorithm, has been found to be very suitable to use, and implemented in the technique.Item A shape control model for piezo-elastic structures based on divergence free electric displacement(Elsevier, 2003) Kekana, Marino; Tabakov, Pavel Y.; Walker, MarkA 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.Item Correlation between elastic and electric properties for cyclically loaded metals(Springer, 2005) 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.Item A technique for stiffness improvement by optimization of fiber steering in composite plates(Springer, 2010) Tabakov, Pavel Y.; Walker, MarkA methodology for stiffness improvement by optimal orientation of fibers placed using fiber steering techniques of composite plates has been developed and is described here. A genetic algorithm is employed to determine the optimal orientation of the tow fibers and, in addition, once the plate has been divided up into cells in order to apply the technique, the orientation gradient between adjacent cells is capped. The finite element method (FEM) is used to determine the fitness of each design candidate. The approach developed also differs from existing ones by having a more sophisticated chromosome string. By relying on the algorithm for the calculation of the fiber orientation in a specific cell, a relatively short and rapid convergence string is assembled. The numerical results obtained show a significant improvement in stiffness when the fiber orientation angle is allowed to vary spatially throughout the ply.Item 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.Item 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.Item 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.