School of Technology
http://repository.aust.edu.ng/xmlui/handle/123456789/2728
2019-06-21T00:37:50ZMultistable and morphing corrugated shell structures
http://repository.aust.edu.ng/xmlui/handle/123456789/3096
Multistable and morphing corrugated shell structures
'Compliant' structures, i. e., structures that undergo large deformations as part of their
normal behaviour, can perform the function of a complex mechanism with just a single,
or very few, components. When multistability is introduced into compliant structures, an even greater simplification can be made, with actuators and/ or locking mechanisms
becoming redundant. Corrugated shells are commonly encountered at all scales of engineering, providing shell structures of a dramatically increased stiffness at little extra
cost or weight. What is historically less understood is the compliant behaviour of corrugated shells. The topic of this thesis, therefore,' is the development and analysis of new categories of structure incorporating corrugations, utilising their compliancy to achieve morphing behaviours that are not otherwise possible. Three distinct categories of shell are studied: (i) 'flat' corrugated sheets, which are multistable under the action of internal stresses, 'snapping' between their flat state and a cylindrically coiled state: these shells are also capable of developing 'twisted' stable states. An algebraic model based on internal strain energy determines the conditions for the various modes of multistability for uniform changes in curvature throughout the shell, and a plastic flow model describes the formation of the internal stresses; (ii) 'curved' corrugated sheets, 'which can morph between shapes of differing Gaussian curvature without plastic deformation. Again, a simplified algebraic model describes their behaviour, as does a numerical model. These models consider the strong coupling that occurs between bending and stretching in corrugated shells; and (iii) 'doubly-corrugated ' sheets, similar to an eggbox in concept, which , again, can morph between shapes of different Gaussian curvature, and also exhibit synclastic bending behaviour. Some of the analyses performed on the curved corrugated sheets are also applied to these shells.
The qualitative behaviour of the analytical models matches, in every detail considered, the behaviour observed in prototypes. Natural-mode analyses on the category (ii) and
(iii) shells demonstrate that the interesting behaviour occurs at relatively low stiffnesses, implying that t his behaviour is potentially useful. Quantitatively, however, the analyses for categories (i) and (iii) above do not prove simple to verify. Physical tests compare the internal stresses of the category (i) shells to those predicted by the plastic flow analysis: the predicted stresses exceed those measured by, typically, 50 %, but it is not established whether the discrepancy is due to the simplifications in the model, inaccuracies in the crude test method or a flmv in om understanding. On its prediction of the curvatures created, the model is more successful. A test is more easily devised for the category (ii) shells, and measurements of the shape change are within 4 % of both the finite-element simulations and the simplified algebraic models over 90 % of the test range. In measuring forces, to test the constitutive relations produced; the accuracy is lower, largely due to the difference between our material model and the real properties of the polymer used: there is strong agreement between the finite element
model and the algebraic model. In conclusion, this thesis is successful in generating new varieties of structure, which have a a wide potential for application. A broad range of designs and analyses are presented, describing the behaviour of these structures well enough that our understanding of them seems justified. Nonetheless, this work merely scratches the surface of what may be achieved in this field, demonstrating some of the potential of such shells and leaving much to be done in their further evaluation and development.
2009-06-09T00:00:00ZScale effects in tests on footings
http://repository.aust.edu.ng/xmlui/handle/123456789/3097
Scale effects in tests on footings
This dissertation presents an investigation of the effects of stress, and of absolute and
relative particle size, in tests on vertically loaded footings. Two granular materials, namely, a silica rock flour and a Chatelet flint grit, which differed in nominal diameter by a factor of 50 but were otherwise practically similar in all other grain characteristics
were used in this work. A comprehensive series of triaxial tests under a wide range of
cell pressures was carried out to quantify the stress and absolute particle size effects.
Model footing tests were also performed by pushing a rigid circular punch
axisymmetrically into the flat surface of a cylindrical soil model either under 1-g (gravity) with surcharge or under elevated g in a centrifuge. The 1-g and centrifuge test series were used to study the scale effects on the surcharge term Ng and the self-weight term Nγ of the Terzaghi bearing capacity equation, respectively. Parameters varied were
punch diameter, particle size and surcharge or g level. Two theoretical analyses were
attempted based on the finite element method and the method of characteristics. Using
the Schofield Soil Model, the finite element analysis can give a reasonable order of
magnitude prediction for the settlement of the footing under working load conditions.
When the effect of reducing angle of shearing with increasing stress was taken into
account together with the change of geometry due to footing penetration, the angles of
shearing inferred from the method of characteristics fall within ±20 of those measured in
triaxial compression tests. Distortion due to violating the scaling law by not conserving
the ratio of particle size to model dimension was not considered to be significant. Distortion due to violating the constitutive soil behaviour by varying the absolute particle size was found to be significant due to differences in grain crushing, but this can be accounted for effectively by the new style of calculations developed in the thesis.
1988-02-21T00:00:00ZAnalysis of concrete beams with partially bonded composite reinforcement
http://repository.aust.edu.ng/xmlui/handle/123456789/3095
Analysis of concrete beams with partially bonded composite reinforcement
Beams prestressed with partially bonded fiber-reinforced plastic (FRP) tendons have high strength and rotation capacity but cannot be modeled by conventional techniques. Herein, it is assumed that all deformation takes place at cracks between rigid bodies. By setting up appropriate compatibility and equilibrium equations, the behavior at a single crack can be modeled, which then allows predictions to be made as to which of four possible events will occur next. These lead either to beam failure, or to changes in the geometry that can be analyzed using the same techniques. Comparisons are made with test results, and reasonable agreement is shown.
2000-01-01T00:00:00ZAdaptive techniques in signal processing and connectionist models
http://repository.aust.edu.ng/xmlui/handle/123456789/3092
Adaptive techniques in signal processing and connectionist models
This thesis covers the development of a series of new methods and the application of
adaptive filter theory which are combined to produce a generalised adaptive filter system
which may be used to perform such tasks as pattern recognition. Firstly, the relevant
background adaptive filter theory is discussed in Chapter 1 and methods and results which are important to the rest of the thesis are derived or referenced. Chapter 2 of this thesis covers the development of a new adaptive algorithm which is designed to give faster convergence than the LMS algorithm but unlike the Recursive Least Squares family of algorithms it does not require storage of a matrix with n2 elements, where n is the number of filter taps. In Chapter 3 a new extension of the LMS adaptive notch filter is derived and applied which gives an adaptive notch filter the ability to lock and track signals of varying pitch without sacrificing notch depth. This application of the LMS filter is of interest as it demonstrates a time varying filter solution to a stationary problem. The
LMS filter is next extended to the multidimensional case which allows the application
of LMS filters to image processing. The multidimensional filter is then applied to the
problem of image registration and this new application of the LMS filter is shown to have significant advantages over current image registration methods. A consideration of
the multidimensional LMS filter as a template matcher and pattern recogniser is given.
In Chapter 5 a brief review of statistical pattern recognition is given, and in Chapter 6 a review of relevant connectionist models. In Chapter 7 the generalised adaptive filter is derived. This is an adaptive filter with the ability to model non-linear input-output
relationships. The Volterra functional analysis of non-linear systems is given and this is
combined with adaptive filter methods to give a generalised non-linear adaptive digital
filter. This filter is then considered as a linear adaptive filter operating in a non-linearly
extended vector space. This new filter is shown to have desirable properties as a pattern
recognition system. The performance and properties of the new filter is compared with current connectionist models and results demonstrated in Chapter 8. In Chapter 9 further mathematical analysis of the networks leads to suggested methods to greatly
reduce network complexity for a given problem by choosing suitable pattern classification indices and allowing it to define its own internal structure. In Chapter 10 robustness of the network to imperfections in its implementation is considered. Chapter 11 finishes the thesis with some conclusions and suggestions for future work.
1990-11-13T00:00:00Z