• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • No language data
  • Tagged with
  • 333
  • 38
  • 18
  • 16
  • 14
  • 14
  • 8
  • 7
  • 7
  • 7
  • 7
  • 6
  • 6
  • 6
  • 4
  • 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.
41

The application of smooth particle hydrodynamics to the modelling of solid materials

Fuller, Mark Daniel January 2010 (has links)
This thesis explores the mesh-free computer modelling technique of Smooth Particle Hydrodynamics (SPH), and explores its usage in solid mechanics applications. A review of the context in which SPH can be used, the theory behind the governing equations and the adaptations carried out to enable elastic body problems to be simulated is presented. An algorithm is proposed to improve the ability for SPH to model contact between deformable surfaces. Non-linear behaviour is simulated via the introduction of plasticity, a statistical damage model and the introduction of friction between surfaces. It is shown how SPH can successfully model the buckling in slender rods and match predictions even under extreme deformations. The culmination of these techniques is used to simulate the mechanical properties of thermal barrier coatings (TBC). The simulated effect of increasing the size of gaps in the columnar structure of TBC is shown to reduce hardness. Higher coefficients of inter-column friction are suggested to be able to enhance the load bearing properties of the TBC system. The future research and improvements to the SPH technique are explored along with a discussion about the appropriate methodology of adapting SPH to meet a range of modelling requirements.
42

The use of a CFBG sensor for detecting damage in composite laminates and adhesively bonded joints

Palaniappan, Jayanthi January 2008 (has links)
Reliable in-situ damage detection techniques which can detennine the existence and location of damage in composite materials and structures are critical for the effective use of these materials. In this work, embedded chirped fibre Bragg grating (CFBG) sensors have been shown to be successful for both detection and location of matrix cracks in composite laminates and disbond detection in bonded composite joints. In all the cases, the CFBG reflection spectra were predicted using commercial software and agreed well with the experimental results. In the matrix cracking work, single matrix cracks in cross-ply GFRP (glass' fibre reinforced plastic) laminates were detected and located using a CFBG sensor embedded within the 0° plies, near the 0/90 interface. The CFBG sensor showed an approximately sinusoidal variation of the intensity of the reflected spectrum at the position of the crack, enabling both crack development and crack position to be identified. It was shown that the precise position of the cracks does not correspond with the bottom of a dip in the reflected spectrum, as has previously been thought. Disbond initiation and progression from either end of a composite bonded joint was monitored by embedding the CFBG sensor in one of the GFRP adherends, with the low wavelength end ofthe sensor positioned at the cut end ofthe adherend. A shift in the low wavelength end of the spectrum to lower wavelengths indicated disbond initiation and movement of a perturbation in the reflected spectrum towards higher wavelengths indicated disbond propagation. In a related fashion, disbond initiation and propagation was detected from the high-wavelength end ofthe spectrum (adjacent to the other cut end of the adherend). With the aid of a parametric study based on a closed-form solution for the strain field in the bonded joint (available in the literature), it has been shown that the sensitivity ofthe CFBG sensor in detecting the disbond depends mainly on the position of the sensor within the adherend and the strain distribution in the adherend. Finally, artificial manufacturing defects were introduced into GFRP-GFRP bonded joints using Teflon inserts and it has been demonstrated that the location of the defects is possible using the CFBG technique.
43

Probabilistic fracture mechanics by boundary element method

Huang, Xiyong January 2010 (has links)
In this work, a new boundary element method is presented for the Probabilistic Fracture Mechanics analysis. The method developed allows the probabilistic analysis of cracked structure accomplished by the dual boundary element method (DBEM), in which the traction integral equation is used on one of the crack faces as opposed to the usual displacement integral equation. The stress intensity factors and their first order derivatives are evaluated for mode-I and mixed-mode fracture problems. A new boundary element formulation is derived and implemented to evaluate the design variables sensitivities. This method involves the solution of matrix systems formed by the direct differentiation of the discretised dual boundary element equations with respect to the each random parameter. The derivatives of fracture parameters with respect to design variables are calculated using implicit differentiation method (IDM) in DBEM for mode-I and mixed-mode fracture problems. The gradient of performance function is determined analytically and the total derivative method (TDM) is used in probabilistic fatigue crack growth problems. The randomness in the geometry, material property and the applied stress are considered in 2-D fracture problems; while initial crack size, final crack size, material property and applied stress are considered in fatigue crack growth. Uncertainties in other aspects of the problem can be included. First-Order Reliability Method (FORM) is used for predicting the reliability of cracked structures. The Hasofer Lind Rackwitz Fiessler algorithm is used to find the most probable point, referred as reliability index. Finally, the validation and applications of the stochastic boundary element coupled with FORM are presented. Numerical calculations are shown to be in good agreement either with the analytical solution or Monte Carlo Simulation.
44

Fatigue crack characterization by image correlation

Lopez-Crespo, Pablo January 2007 (has links)
The structural integrity of many materials from aircraft structures, pressure vessels and offshore structures is closely related to the cracks existing in the component. The stress intensity factor is widely recognized as a major parameter for assessing the severity of a crack, and is extensively used in fatigue lifetime predictions. This work presents a new methodology for experimental determination of the effective stress intensity factor from image correlation data. In addition, the methodology allows the reconstruction of the full stress field. Based on a multi-point over-deterministic method, the full-field displacement information collected from the neighbourhood of the crack tip is combined with an elastic model based on Muskhelishvili's complex variable formulation to determine the opening and the sliding mode stress intensity factors. Utilization was made of edge-finding routines for locating the crack tip coordinates from the displacement fields. The technique was applied successfully to study real fatigue cracks in different aluminium alloy specimens, subjected to static and cyclic loads. Good agreement was also obtained for a crack emanating from a hole, where complex passing stress fields exist. Different factors such as crack face contact and rubbing effects, local mixed-mode arising from wedge opening or tilting of the crack front are identified as being responsible of the differences between the nominal and measured values. Finally the non-contacting tool was extended to the analysis of crack face interaction and crack closure problems, and it was found to be able to estimate the opening load.
45

Modelling the impact of tennis balls on court surfaces

Dignall, Richard John January 2005 (has links)
A model of tennis balls impacting obliquely on tennis courts was developed in this study. Balls were impacted normally on a force plate to read impact force data, and filmed at high speed during oblique impacts. A normal model was created and then extended to cover oblique impacts. The experimental data was used to verify the model in each case. A study of surface testing methods found that tennis courts are significantly stiffer than tennis balls; so much so that they can be considered rigid. A coefficient of friction between ball and surface was all that was necessary to define a surface. Normal impacts were performed on a force plate for four different ball constructions at speeds between 3 and 20 ms-I . Impact speed had a significant effect on coefficient of restitution (ratio of rebound speed to inbound speed) - for example for a pressurised ball, from about 0.8 at an impact speed of 3 msI to about 0.6 at 20 msI. Pressureless balls bounce at a similar speed to pressurised balls at low impact speeds, but slower at high impact speeds. Punctured balls bounce slower throughout the range of impact speeds. All balls showed a rapid increase in force during the initial part of the impact. An iterative model was created to simulate normal impact. A numerical method was used to find the effect of deformation shape on the relationship between centre of mass movement and ball deformation. A total force during impact was created by combining structural stiffness, material damping and impulsive reaction forces. This model worked well for all ball types and used quasi-static compression data and a low speed drop test to find the parameters. The impulsive force simulated the initial increase in force well. A thorough experimental study of oblique impacts was performed by isolating in turn each of the key incoming properties of impact. The incoming speed, spin and angle, together with the ball and surface construction were individually varied in turn and the effect on outgoing characteristics measured using high speed video footage. In most cases there was a distinct change in rebound properties when rolling happened. Footage at up to 7000 frames per second was used to qualitatively explain the effect of deformation shapes on energy losses. It was found that impacts with backspin caused more deformation and an increased energy loss compared to normal impacts with the same vertical velocity. Impacts with topspin had a reduced vertical energy loss. The normal model was extended to include the horizontal and rotational forces necessary to simulate an oblique impact. A damping compensation factor was included to adjust the vertical energy losses at different spin rates. The oblique test data was used to verify the model, and there was a very good correlation.
46

Predicting malfunction in quasi steady state rotating machines

Kitsos, Christos January 2007 (has links)
Dry vacuum pumps appeared in the mid-1980s in order to address problems caused by conventional fluid-sealed pumps. However, their working environment is often harsh, sometimes resulting in catastrophic faults. Continuously monitoring the state of the system and scheduling maintenance as appropriate is thus desirable.;Sliding mode techniques have been widely used in condition monitoring and fault detection schemes in recent years. Their main advantage is a fundamental robustness against certain kinds of parameter variations. They also enable faults and/or values of un-measurable system parameters to be reconstructed.;The principal aim of this thesis is to apply sliding mode techniques in order to reduce the occurrence of unplanned pump stoppages, by monitoring appropriate subsystems and parameters, for the onset of cooling system failure, bearing failure and exhaust blockage. This is achieved using the concept of the equivalent injection signal that is necessary to maintain a sliding motion. Experimental test results acquired from the dry vacuum pump test-bed illustrate the usefulness of the approach for condition monitoring. Further, the method is cost effective since it requires only low cost temperature transducers and an exhaust pressure sensor that is already part of the typical sensor package for some pump processes.;The thesis concludes with ideas and recommendations regarding possible future work, including the application of fault classification techniques and the development of processes for generating an efficient and implementable code, suitable for the vacuum pumps' embedded control systems.
47

Microstructural and residual stress characterisation of laser deposited nickel-base superalloy test structures

Moat, Richard James January 2009 (has links)
No description available.
48

Mode III Fracture in Advanced Engineering Materials

Pennas, Dimitrios Christou January 2007 (has links)
Delamination is a fundamental failure mode of laminated composites. When laminates are subjected to out-of-plane loading, the resin matrices between the plies often fail, resulting in ply separation or delamination failure. Such loading conditions occur in various ways and depend heavily on the load transfer mo~es in the composite structures. Delamination cracks can be visualised in the fracture mechanics sense to grow, and thus can be characterised in Mode I (opening), Mode II (shear) and Mode III (scissoring or anti-plane shear) and their combinations. __The test techniques for delamination in Mode I and Mode II, as well as mixed Mode IIII, have been well established. However, few workers have investigated the Mode III interlaminar fracture properties of composites, partly because of the experimental difficulties associated with the introduction of pure Mode III loading to the delamination crack. The aim of this study was to investigate the Mode III interlaminar fracture properties of composite materials based on both thermoplastic and thermosetting matrices, adhesive joints and bi-material samples. For this reason, both numerical and experimental methods were used. The Edge Crack Torsion (ECT) test geometry was employed in order to evaluate the Mode III response of the samples studied, as it is commonly accepted that this is the most effective technique for cllaracterising this mode of delamination. The Virtual Crack Closure Integral has also been extensively used to characterise the loading conditions at the crack tip in these samples. Once the quasi-static response of the composites were analysed, dynamic tests were undertaken to investigate the rate-sensitive behaviour of these high-performance materials. Finally, the Mode III geometry was modified to look at possible mixed-mode failure involving Mode II contributions.
49

Flux response technology : a universal tool for material characterisation

Palmer, Candice Alemay January 2011 (has links)
No description available.
50

Bifurcation of spherical and cylindrical shells at finite deformation

Haughton, D. M. January 1979 (has links)
In this dissertation we investigate the possibility of bifurcations in the solution paths of symmetrically loaded spherical and cylindrical shells, both thick-walled and membrane shells of incompressible, isotropic materials capable of withstanding finite deformations elastically. At the points of interest it becomes possible for non-symmetric solutions to exist, for example, bulges in pressurized cylindrical shells. Several authors have considered various aspects of the problems considered here, usually from the viewpoint of stability, using either the equations of small deformations superposed on large, the incremental equations, as formulated by Green, Rivlin and Shield (1952), using the Cauchy stress tensor on current axes, or by using an energy method. We have found, however, that the incremental equations are more appropriately formulated using the conjugate stress-deformation pair, nominal stress and deformation gradient, referred to an arbitrary reference configuration. The main advantage of this formulation is that changes in geometry due to the incremental displacements do not have to be calculated explicitly. Using this formulation of the equations we then investigate spherical and cylindrical shells subjected to internal and external pressures and also rotating cylindrical membranes. In all cases it is possible to conclude the analysis while making no assumptions on the form of the strain-energy function, other than those relating to incompressibility and isotropy. This then allows an investigation of the relationship between the form of strain-energy function, which we express as a function of the principal stretches directly, rather than through invariant functions of the stretches, and the occurrence of the possible bifurcation modes.

Page generated in 0.0424 seconds