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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Fracture mechanics analysis of multiple edge cracks

Abdul Manan, Muhamad Saifuildin January 2008 (has links)
This thesis is concerned with the evaluation of crack propagation in the presence of other cracks within the same two-dimensional body. The parameter known as the Stress Intensity Factor (SIF) is used within Linear Elastic Fracture Mechanics (LEFM) to predict crack propagation rate knowing the appropriate material properties. Unfortunately the number of SIF solutions for multiple cracks is very small especially for real engineering cases. This thesis investigates the use of the SIF weight function for the efficient evaluation of SIFs for two one-dimensional cracks in close proximity to each other. The SIF weight function has proved to be a powerful tool for the evaluation of single cracks however, has not before been used to solve multiple crack problems. The main objective of the thesis is to investigate the mechanics of multiple cracks through an experimental and numerical analysis programme and to develop engineering solutions for prediction of multiple crack behaviour in steels and materials that exhibit LEFM characteristics. A Finite Element (FE) approach was employed to model multiple crack interaction. In addition, the FE model was used to study the non-uniform stress distribution caused by the interaction effect between cracks. Over one hundred FE models were analysed for this study. A major experimental programme was conducted to study the interaction effect between two edge cracks. A total of seven specimens with different crack geometries were completed under fatigue loading in tension. The purpose of this experimental work was to better understand the mechanisms of crack interaction and to provide information for validation of the numerical anlayses. The experimental results show that cracks in close proximity to each other interact to varying degrees depending on relative crack lengths, crack separation and plate width. A novel weight function method was developed in order to predict SIFs of two edge cracks under uniform tension. The crack interaction effect was established using the idea of non-uniform stress distributions along the potential crack plane due to the presence of an additional edge crack. Generally the novel weight function approach shows good results compared to finite element analysis. Finally, further work to explore the wide range of SIFs for multiple cracks using the weight function method is identified and proposed.
2

Non-destructive testing of masonry arch bridges

Colla, Camilla January 1997 (has links)
Stone masonry arch bridges form a critical part of the transportation system. Present methods of assessment are sometimes too conservative and a number of bridges fail the assessment even though they appear in good condition. Non-Destructive Testing can play a key role and three Non-Destructive techniques - radar, sonics and conductivity measurements- are proposed for bridge testing with the aim of obtaining structural dimensions, material characteristics and integrity information which would lead to a more accurate assessment of the structural conditions being made. After discussing problems and limitations with current analytical and load testing methods of assessment, a review of archetypal forms of bridge construction methods employed along the centuries ismade, showing that a greater variety of bridges than commonly believed, exists. The review also enables an Engineer to have some indication of construction type relating to the area, era and designer. The work then includes site work on two masonry bridges and laboratory experiments. On site, the three Non-Destructive Techniques mentioned were used for testing two Scottish stone masonry bridges (one with a brick arch ring) with the aim of obtaining information about the condition and nature of the materials in the fill, the internal configuration of the structure and the geometrical dimensions of the elements. Data from each technique were plotted in the form of cross-sectional tomographic maps and the results interpreted and compared. Limitations are also discussed. In the laboratory, experiments with radar were undertaken to calibrate the technique in controlled conditions and also, and more importantly, to obtain information about phenomena of signal behaviour and material properties as would be found in a masonry arch bridge. The findings served the purpose of aiding a better planning of radar surveys to be made and an improved understanding and interpretation of the radar data to be obtained.
3

The influence of internal pressure on the bending stability of thin cylindrical shells

Coutie, Martin George January 1960 (has links)
No description available.
4

Impact dynamics of rods, beams and panels

Feroz, Kottamal Thanduparakal January 1997 (has links)
A methodology to study the impact dynamics of rods, beams and panels using modern numerical solution technique and instrumentation is developed. The numerical study is carried out using the commercial finite element package ABAQUS. The general approach of this tool is illustrated and verified for the static analysis of the stress distribution in a castellated beam and for the dynamic analysis of a cantilevered beam subjected to multiple impact loading. The finite element analysis technique is used to assess the optimal performance of steel plate panels of different cross-sections and thicknesses under the same blast loading conditions. Also, the structural response of folded plate panels of different cross-sections and thicknesses subjected to various blast pressures is studied numerically and compared with the existing experimental measurements. It is shown that the strength to weight ratio of the folded plate panels is higher than those of the single and double plate panels and that the folded plate panel is the more blast-resistant design. Stress wave propagation in circular mild steel rods is studied both numerically and experimentally. The rods are impacted longitudinally using spherical balls. The Hertzian law of impact and the associated non-linear ordinary differential equation of motion are used to determine the force-time history of impact. This force-time history is used in a finite element analysis of the rods to predict the propagation of pulses in the rods. The use of finite element simulation in predicting the wave propagation phenomena and its application to non-destructive testing (NDT) of rods and bars is demonstrated. For the experimental measurements, the stress waves propagated in rods and bars are monitored using PZT patches of size 5x3 mm which are calibrated by means of a finite element approach and by the use of a standard wire-resistance strain gauge. A time domain, frequency domain, regression analysis and autocorrelation procedures are developed to detect defects in rods and bars using wave propagation data. The defects are introduced in the form of slots. By analysing the stress wave data for the defect-free rods and bars and for the rods and bars with defects, it is possible to pinpoint the location of the defects. The results show that defects can be identified using any of the procedures and that their location can be estimated using the time domain technique. It is also shown that a high degree of correlation is obtained between measured and predicted characteristics.
5

Echinodome response to dynamic loading

El-Deeb, Khaled Mohamed Mahmoud January 1990 (has links)
The response of an Echinodome to static and dynamic point loads, and explosive type loadings was examined both theoretically and experimentally. The finite element method of analysis was employed in the theoretical investigation. Semi-loof thin shell elements were used to model a GRP prototype on which the experiments were performed. The stress distribution of the Echinodome under a static symmetric point load were investigated both experimentally and theoretically. Then the Southwell technique was employed in estimating the critical buckling load from deflection measurements. Experimental estimates were then compared with the numerical predictions in the form of non-linear collapse and non-linearbifurcation buckling loads. A free vibration was performed to determine the structural natural frequencies and typify the mode shapes. The shock response spectra of several pulse shapes were determined using the finite element method. The most severe loading function was established to be a step loading with infinite duration and zero ramping time and was then employed as the load-time history in an axisymmetric and symmetric non-linear dynamic buckling analysis. The dynamic collapse buckling loads were found to be smaller in magnitude than their static correspondents. A modal testing was then carried out on the Echinodome prototype to determine the experimental modal parameters (natural frequencies, damping values and mode shapes). Newly developed correlation techniques were adopted in the comparison of the experimentally derived parameters with those predicted and poorly modelled regions were identified. Great improvement was achieved by correcting the experimental data and updating the finite element model's boundary conditions. A set of underwater free field experiments was performed to determine the pulse characteristics for a specific explosive charge, followed by another set while the prototype was in a floating submerged state and acting as the target for the same explosive charge. A theoretical simulation was accomplished by employing a finite element-boundary element approximation for the modelling of the structure and infinite fluid media respectively. Measured responses were compared with the numerical predictions and means of acquiring better theoretical approximations are mentioned. The loading conditions to be experienced by an underwater LNG Echinodome vessel are reviewed with emphasis on accidental dynamic loads (impact and explosion). A state of the art storing configuration is proposed for the Echinodome in order to limit the extent of damage and hence minimise risk during upset conditions. Finally, appropriate design, construction and prestressing procedures were recommended.
6

Parametric instabilities in structures

McWhannell, D. C. January 1972 (has links)
No description available.
7

Non-destructive testing of metal ducted post-tensioned bridge beams using sonic impact-echo techniques

Martin, Julia January 1997 (has links)
On 25 September 1992 the Department of Transport (DoT) issued a press notice stating that it would not be commissioning any new grouted duct post-tensioned bridges in England. The decision was taken due to fears that existing grouted duct post-tensioned concrete bridges were badly corroded and could be in a state of imminent collapse. The press notice also announced that existing grouted duct post-tensioned bridges were to undergo detailed inspection. Non-destructive techniques needed to be developed to allow detailed investigation of these structures. The results of these investigations had to be accurate to a high level of confidence as decisions on repair, renovation or destruction would be made on the findings of the investigation. This thesis will give the reasons for the DoT's decision followed by an overview of possible non-destructive techniques available at the time of issue. The main body of work carried out investigates the use of the Sonic Impact-Echo method of non-destructive testing. This involves the development of suitable testing equipment and preliminary laboratory and field investigation. Detailed numerical simulations were carried out using the Finite Element Method in order to quantify the probable limits of the Sonic Impact-Echo method. Final laboratory investigations were carried out on a model with known defects. Detailed field testing was carried out on test beams manufactured by the Transport Research Laboratory in Crowthorne and by TBV Stanger at Elstree.
8

Strain measurements in silicon using X-ray interferometry

Walmsley, C. F. January 1979 (has links)
The aim of this work was to investigate by X-ray interferometer techniques the strain induced in silicon after thermal oxidation and boron diffusion processing. An elasticity model was developed by which the resulting strain and interferometer moire fringe pattern could be predicted when the oxidation or diffusion took the form of a circular disc. By this means, the stress in oxide thermally grown at 950 °C was measured to be 1.65 x 10' dynes/cm2. Double crystal techniques were used to measure the wafer bowing after oxidation, giving good agreement with the interferometer measurements. The solute lattice contraction coefficient for boron was measured by interferometry and found to be 6.0 x 10-2'tm'/atom. Combining this result with four point probe measurements, the mobility of holes in degenerate silicon was found to be 122 cm2/volt sec. By keeping the total dose of boron below the critical value of 3 x 10 5 atoms/cm2 in order to avoid lattice dislocation, it was found by double crystal measurements that this was insufficient to produce a significant diffraction peak from the diffused region under conditions of total Bragg reflection. The possibility of using oxidation or diffusion processing to alter the lattice parameter of silicon or germanium suitably in order to construct an X-ray monochromator or resonator capable of performing at room temperature was shown to be impracticable. Finally, the Burger's vectors of dislocations appearing in an interferometer after oxidation were successfully determined.
9

Buckling strength of steel thin cylindrical shells under elevated local axial compression

Cai, Minjie January 2003 (has links)
First, the buckling analyses of perfect cylinder under various uniform/non-uniform axial compression were performed using the finite element method in order to obtain convergence mesh for further study, to find the reference elastic critical load, and to explore the accuracy of the loading and boundary conditions applied. Linear elastic analysis of perfect cylinder under local elevated axial compression was presented second. The meridional membrane stress distribution in the midplane, which subjected to highly elevated stresses of the whole cylinder, was examined in detail and a linear buckling analysis was performed for the buckling strength of the cylinder limited in the small deflections, geometrically and materially linear range. Two different buckling phenomena were identified, with corresponding and distinct buckling mode forms, by using the deflected shapes and axial membrane stress distribution of various geometry and loading configurations. The introduction of this notion is a key feature of the whole study. Third, a modified RIKS analysis was performed to get the load-displacement curve for some representative cases in order to deal with possible instabilities in the extent of geometric non-linearity as well as material non-linearity with and without imperfections. A new self-design approach was set up to obtain the critical buckling strength in the geometrically non-linear domain more efficiently. Geometrically nonlinear elastic analyses were performed afterwards, using large deflection theory. Both perfect and imperfect cylinders were studied under various geometric and loading configurations. This provided the database for the following sets of simply formulae aiming for design purpose. The influence of material non-linearity was studied via some representative cases. Fourth, the stress pattern in the pre-buckling state before buckling under local elevated axial compression was studied to get the relationship between the high local axial stresses and the buckling strength.
10

Approximate theories for the flexural vibration of uniform beams and their derivation from the general elastic equations

Barr, A. D. S. January 1956 (has links)
No description available.

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