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Faculty of Engineering and Built Environment

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Author: search.filters.author.Walker, MarkHas files: No

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    An Eulerian approach to soil impact analysis for crashworthiness applications
    (Elsevier, 2016) Evans, Wade Robert; Johnson, D.; Walker, Mark
    The primary motivation of this study was the development and implementation of an explicit nonlin-ear dynamic finite element based methodology for investigating the crashworthiness of small lightweight composite aircraft impacting into soft soil. The technique used to characterise and validate a numerical model for soft soil as an impact terrain is the focus of this paper. The technique used was primarily based on the use of a time explicit Eulerian-based finite element analysis code, and this technique was dem-onstrated through the finite element analysis of penetrometer drop tests into soft soil. The Eulerian-based finite element approach was considered rather than the more commonly used Lagrangian-based finite element approach in order to reduce numerical instabilities which often occur with the use of La-grangian solvers when considering problems with large deformations, which is a characteristic of crash analyses. Validation of the numerical model was based on previously published work in which pen-etrometer drop tests into soft soil/clay were performed at the Utah Test and Training Range. Experimental data were presented in this work, as well as a finite element modelling approach based on Lagrangian methods. Based on the results obtained it was concluded that an Eulerian-based approach to soft soil impact analysis, for crashworthiness applications, is valid. Greater model fidelity may possibly be gained by further investigation of the drop test methodology, viscosity effects, variation of soil mechanical prop-erties through the depth range, and strain rate effects.
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    Design optimization of anisotropic pressure vessels with manufacturing uncertainties accounted for
    (Elsevier, 2013-04) Tabakov, Pavel Y.; Walker, Mark
    Accurate 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.
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    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.
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    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.
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    A methodology to design fibre reinforced laminated composite structures for maximum strength
    (Elsevier, 2003) Walker, Mark; Smith, Ryan E.
    A procedure to select the optimal fibre orientations and determine the maximum load carrying capacity of symmetrically laminated fibre reinforced composite structures is described. Cylindrical shells subject to combinations of torque and in-plane forces are used to illustrate the methodology and are optimally designed for maximum strength. Torque tubes are generally used as control mechanisms, for example, in the tail fins of aircraft. The finite element method, based on Mindlin plate and shell theory, is used in this application in conjunction with an optimisation routine in order to obtain the optimal designs. The methodology consists of two stages; the objective of the first is to maximise the strength of the cylindrical shells by determining the fibre orientations optimally while the objective of the second stage is to maximise the in-plane compression loading subject to a failure criterion. The effect of different shell aspect ratios, wall thickness, layer numbers and boundary conditions on the results is investigated.
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    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.
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    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.
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    A procedure to select the best material combinations and optimally design composite sandwich cylindrical shells for minimum mass
    (Elsevier, 2006) Walker, Mark; Smith, Ryan E.
    A methodology to select the best material combination and optimally design composite sandwich cylinders having fibre reinforced skins and low density cores for minimum mass is described. Sandwich constructions generally provide improved stiffness/mass ratios and more tailoring opportunities than monolithics, and thus greater chance of satisfying design constraints. The objective of the optimisation is to minimise the laminate mass by selecting the skin and core material combination, layer thicknesses and skin fibre angles optimally, subject to load and cost 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 procedure and its benefits are demonstrated using Graphite, Glass or Kevlar/Epoxy facings, and Balsa or PVC cores.
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    Optimal design of fibre-reinforced laminated plates accounting for manufacturing uncertainty
    (Springer, 2005) Walker, Mark
    A procedure to design symmetrically laminated plates under buckling loads for minimum weight with manufacturing uncertainty (tolerance) in the ply angle and plate thickness, which are the design variables, is described. A minimum buckling load capacity is the design constraint implemented. It is assumed that the probability of any tolerance value occurring within the tolerance band, compared with any other, is equal, and thus the approach is a worst case scenario approach. The effects of bending–twisting coupling are neglected in implementing the procedure, and the Downhill Simplex method is used as the search technique, but the methodology is flexible and allows any appropriate problem formulation and search algorithm to be substituted. Two different tolerance scenarios are used for the purposes of illustrating the methodology, and plates with varying aspect ratios and loading ratios are optimally designed and compared. The results demonstrate the importance of carrying out design optimisation of composite structures with the effects of manufacturing tolerances included.
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    A technique for optimally designing fibre-reinforced laminated plates under in-plane loads for minimum weight with manufacturing uncertainties accounted for
    (Springer, 2006) Walker, Mark; Hamilton, Ryan Jason
    A procedure to design symmetrically laminated plates under buckling loads for minimum mass with manufacturing uncertainty in the ply angle, which is the design variable, is described. A minimum buckling load capacity is the design constraint implemented. The effects of bending–twisting coupling are neglected in implementing the procedure, and the golden section method is used as the search technique, but the methodology is flexible enough to allow any appropriate problem formulation and search algorithm to be substituted. Three different tolerance scenarios are used for the purposes of illustrating the methodology, and plates with varying aspect ratios and loading ratios are optimally designed and compared.