Browsing by Author "Nalitolela, Noel Gerald"

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    Detecting and Localising Damage in Bridge Structures from Vibration Measurements
    (ESRSA Publications, 2013-06-29) Nalitolela, Noel Gerald
    Vibration characteristics of a structure correlate to the distribution of mass, stiffness and damping in the structure. Therefore, a change in any one of these three parameters will result in a change of the vibration characteristics of the structure. It is a fact that a structural damage results in a local reduction of stiffness of the structure and therefore a reduction of the natural frequencies. Methods to detect and possibly localise damage in structures using vibration measurements have been a subject of research interest among the structural dynamics community. The use of a vibration based methodology in detecting and localising damages in big structures like bridges will provide an efficient monitoring and diagnostic tool and overcome accessibility difficulties associated with physical inspection. The paper discusses a methodology proposed for the detecting and localising damage in structures with specific application to structural damage in bridges using vibration data.
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    Development of a Genetic Algorithm Based Search Strategy Suited For Design Optimisation of Internal Combustion Engines
    (Tanzania Journal of Engineering and Technology, 2008) Nalitolela, Noel Gerald; Mshana, J. S.
    Engine design optimisation is a multi-objective, multi-domain problem in a discontinuous design space. The state of the art of optimisation techniques shows that only methods of direct and adaptive search are appropriate for this type of problem. These include, adaptive random search, simulated annealing, evolution strategies and genetic algorithms. Of these methods, the genetic algorithms have been shown to be the most suited for the optimisation of multi-modal response functions in a discontinuous design space. This paper considers the important characteristics of genetic algorithms and their adaptation for use in parametric design optimisation of internal combustion engines. In order to verify the basic functionality of the proposed optimisation strategy, a genetic algorithm based, optimisation software was developed and tested on a number of analytical functions, selected from optimisation literature, with satisfactory results.
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    A frequency Domain Technique for Structural Parameter Updating
    (SEM SOCIETY FOR EXPERIMENTAL MECHANICS INC, 1993) Nalitolela, Noel Gerald
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    Identification of Unbalanced Forces and Foundation Parameters of Rotating Machines from Vibration Measurements
    (Tanzania Journal of Engineering and Technology, 2011) Nalitolela, Noel Gerald
    A technique is described to use measured vibration data of a rotating machine and its foundation to identify unbalanced forces, stiffness and damping parameters of the mountings, and the parameters of the foundation. It is based on an idealisation treating the rotor, the machine structure and the foundation as rigid masses supported by springs and dampers. Operational vibration data of the machine and its foundation before and after the rotating unbalanced forces are perturbed by adding unbalanced mass to the rotor are used in the identification procedure. Once the parameters are identified, dynamic forces transmitted to the foundation can be estimated. The technique is demonstrated using simulated example for a machine with a two bearings rotor.
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    Localization of Damage in Structures by Analytical Model Improvement and Strain Energy Balance
    (SPIE INTERNATIONAL SOCIETY FOR OPTICAL, 2005-02-13) Nalitolela, Noel Gerald
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    A Mass or Stiffness Addition Technique for Structural Parameter Updating
    (International Journal of Analytical and Experimental Modal Analysis, 1992-07) Nalitolela, Noel Gerald; Penny, J. E. T.; Friswell, I. M.
    Most methods used to update dynamic structural models use either frequency response data or both eigenvalues and mode shape data. This paper presents a technique to adjust the parameters using eigenvalues alone. Eigenvalues may be measured very accurately whereas mode shapes often contain substantial errors. The structure and its theoretical model are perturbed by adding mass of stiffness. The measured eigenvalues before and after each mass or stiffness addition are used to update the parameters by sensitivity analysis. It is shown that with error-free data and a proper choice of the perturbing coordinates, exact parameters can be identified from eigenvalues alone using unconstrained optimization. Due to measurement errors and possible inaccuracies in the structure of the model matrices, the parameters of a real structure are adjusted by incorporating a constraint of minimum changes from the initial estimates using a Bayesian estimator.
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    A New Approach to Update Model Parameters Using the Frequency Response Data
    (SEM SOCIETY FOR EXPERIMENTAL MECHANICS INC, 1992) Nalitolela, Noel Gerald
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    System Identification of Linear Vibrating Structures
    (1991) Nalitolela, Noel Gerald
    Methods of dynamic modelling and analysis of structures, for example the finite element method, are well developed. However, it is generally agreed that accurate modelling of complex structures is difficult and for critical applications it is necessary to validate or update the theoretical models using data measured from actual structures. The techniques of identifying the parameters of linear dynamic models using Vibration test data have attracted considerable interest recently. However, no method has received a general acceptance due to a number of difficulties. These difficulties are mainly due to (i) Incomplete number of Vibration modes that can be excited and measured, (ii) Incomplete number of coordinates that can be measured, (iii) Inaccuracy in the experimental data (iv) Inaccuracy in the model structure. This thesis reports on a new approach to update the parameters of a finite element model as well as a lumped parameter model with a diagonal mass matrix. The structure and its theoretical model are equally perturbed by adding mass or stiffness and the incomplete number of eigen-data is measured. The parameters are then identified by an iterative updating of the initial estimates, by sensitivity analysis, using eigenvalues or both eigenvalues and eigenvectors of the structure before and after perturbation. It is shown that with a suitable choice of the perturbing coordinates exact parameters can be identified if the data and the model structure are exact. The theoretical basis of the technique is presented. To cope with measurement errors and possible inaccuracies in the model structure, a well known Bayesian approach is used to minimize the least squares difference between the updated and the initial parameters. The eigen-data of the structure with added mass or stiffness is also determined using the frequency response data of the unmodified structure by a structural modification technique. Thus, mass or stiffness do not have to be added physically. The mass-stiffness addition technique is demonstrated by simulation examples and Laboratory experiments on beams and an H-frame.
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    Updating Model Parameters by Adding an Imagined Stiffness to the Structure
    (Academic Press, 1993-03-03) Nalitolela, Noel Gerald; Penny, J. E. T.; Friswell, I. M.
    A mass addition technique for structural parameters updating has recently been described by the authors. It uses eigenvalues of the structure, before and after it is perturbed by adding mass, to adjust selected parameters by sensitivity analysis. The technique avoids the use of noisy mode shape data and overcomes the problem of a non-unique set of parameters when eigenvalues alone are used. However, it is not suitable for a large structure because of the difficulty of adding the necessarily large perturbing mass to the existing structure. This paper presents an alternative technique. An "imagined" stiffness is added to the structure and the FRF of this perturbed structure is obtained from the measured FRF of the original structure by a simple structural modification technique. The eigenvalues of the original and perturbed structures are obtained from the measured and constructed FRFs respectively. These eigenvalues, together with the eigenvalues predicted from an analytical model of the structure, are used to adjust the structural parameters by sensitivity analysis. The technique is demonstrated by a simulated example and by an experiment on an H-frame.
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    Updating Structural Parameters of a Finite Element Model by Adding Mass or Stiffness to the System
    (1993) Nalitolela, Noel Gerald; Penny, J. E. T.; Friswell, I. M.