Thesis (M.S.)--University of Wisconsin--Madison, 1968. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Welding is an essential process in many industries for both the production and repair of engineering plant, notably pressure vessels and piping. However, welding processes cause large magnitudes of residual stress to be induced within the structure. Residual stress can be defined as a stress that exists in a material when it is under no primary loading. Whilst residual stresses can be reduced by post weld heat treatment, such treatments are not always possible, and so high residual stresses can remain in serviceThe current methodology for evaluating fracture toughness from specimens, particularly if these contain weld residual stresses is presented in BS7448-1997. This method relies on the assumption that the effect of residual stress on fracture toughness measurements can be negated by the application of a local compression, to the ligament ahead of the pre-crack in the test specimen. Recent research has investigated the validity of this assumption. The results suggest that, far from being removed, the residual stresses are modified or even enhanced via local compression. This can reduce the value of measured fracture toughness below its true value. In order to ensure the validity of fracture toughness measurements in materials that contain residual stress, a more robust method for its quantification is developed.The aim of this project was to extend current understanding regarding the magnitude and distribution of residual stresses retained in standard fracture mechanics specimens removed from welds and the consequent effects of these stresses on measured fracture toughness, both in terms of the crack driving force and crack-tip constraint. Furthermore, the project aimed to derive improved methods for the quantification of valid values of fracture toughness from laboratory specimens containing residual stresses. This was achieved via a combination of analytical and experimental work.The effect of specimen extraction on the level of retained residual stress in specimens extracted from non stress-relieved welds was investigated using parametric finite element analyses. Simplified methods to quantify the levels of residual stresses in fracture mechanics specimens removed from welds and their significance, in terms of contribution to crack driving force, are proposed.The influence of residual stresses on the measured fracture toughness properties of ferritic pressure vessel steel, tested in the cleavage fracture regime, has also been studied. A refined method of out-of-plane compression was devised and used to generate significant residual stresses in three-point bend specimens. This method was then used experimentally, alongside supporting elastic-plastic analyses, to quantify the effects of the residual stresses on fracture toughness in terms of both crack driving force and crack-tip constraint in geometrically high and low constraint specimens. A method whereby fracture toughness data, obtained from specimens containing residual stresses, can be corrected to provide valid fracture toughness properties using constraint based fracture mechanics alongside a simple fracture model has been proposed. The main conclusions from the work are as follows. Significant weld residual stresses have been shown to be retained in certain laboratory specimens post extraction from non stress-relieved welds. The magnitude and distribution of retained residual stress has been shown to be dependant on: • Material yield and flow properties • Specimen size; where larger specimens are more likely to retain significant levels of residual stress than smaller specimens • Specimen type; either compact tension (CT) or single edge notched bend (SENB), where there is a tendency for specimens to retain higher relative levels of residual stress in the directions of their largest dimensions; i.e. bend specimens retain more residual stress along their length than CT specimens and CT specimens retain more residual stress across their width than bend specimens • Extraction location, e.g. full thickness, near surface, mid-thickness etc. The stress partitioning method has been shown to provide a useful estimating approach for assessing the levels of residual stress retained in fracture mechanics specimens extracted from non stress-relieved welds in certain orientations.Retained residual stresses have been shown to affect both crack driving force and crack-tip constraint in both low and high geometrically constrained 50mm bend specimens manufactured from A533B ferritic steel. The residual stress has been shown to dominate the level of crack-tip constraint condition over and above the geometric and loading factors during the early stages of loading. The effects of residual stress on crack driving force and crack-tip constraint have been shown to result in fracture loads and, therefore, measured fracture toughness values that vary widely from those to be expected in the material under small-scale yielding conditions; i.e. if a standard specimen were to be tested containing no residual stress. Two-parameter (J-Q) fracture mechanics has been shown to provide a valid approach for quantifying fracture toughness properties from high and low constraint specimens, with and without residual stresses, with all data being shown to be consistent with a J-Q failure locus for a given level of cleavage probability.

A large number of bone fractures are treated with stabilisation devices that utilise metal wires or screws, which traverse the bone and are connected to an external frame or internal plate. Clinically, fixation devices are required to be able to: sustain loads; minimise patient discomfort and possible implant loosening; and promote healing. In the recent years locking plates have become increasingly popular for osteoporotic or complex fractures, which can be difficult to manage. It, however, remains unclear as to how these devices need to be configured for optimum clinical performance. This thesis investigates the mechanics of locking plates, factors that influence their performance and provides guidance to optimise the placement of screws. Finite element simulation and analytical models were developed and validated using lab-based experimental models. The local behaviour around the screw-bone interface is considered and the implications of different modelling assumptions assessed. A novel method of simulating the effect of radial interference due to pilot-hole size is proposed. Different screw types are evaluated: osteoporotic bone is found to be particularly susceptible to the screw tightening preload used in compression screws; far-cortical locking screws are found to slightly reduce device stiffness but substantially increase strain levels around screw holes. Finite element simulations show that many of the local effects, such as preloads and contact modelling, can profoundly influence the prediction of strains around screws but do not generally influence the global load-displacement behaviour; the screw-plate connection and bone/plate material and geometric properties are found to have an influence on global stiffness predictions. The key determinants of load-displacement behaviour evaluated through models are the loading and restraint conditions, which explain the huge range of stiffness predictions in the literature (three orders of magnitude). An analytical model based on 7 bone-plate construct parameters is developed. Despite its simplicity, the model is found to be able to predict the axial stiffness for experimental tests conducted and for 16 other cases from five previous studies with an average error of 20%. The manner of load application, not considered in the literature, is shown to dramatically alter predictions of plate stress, strains within the bone and conclusions regarding screw placement. Even with the inclusion of muscles forces, the choice of restraint condition dominates the mechanical behaviour. Using the models, the influence of screw position is systematically evaluated in varying bone qualities under axial loading and torsion and guidance for optimising fixation is developed.

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Thesis--Universität Paderborn. / Includes bibliographical references.

Semi-circular specimens (SCB) under three point-bending which are commonly used for fracture testing of rocks were used here for fracture mechanics tests. A total of 65 specimens were tested by using Ankara andesite rock. Investigations including the effects of initial notch thickness, different loading span ratios (S/R), flattened loading end, and little dimensional variations when preparing the specimens were carried out. Stress intensity factors for specimens with different geometries were computed individually by using a 3D finite element program ABAQUS. Specimens with a preliminary notch thickness varying from 0.84 to 3.66 mm were tested under three point bending. For a second group of specimens loading span was changed and fracture toughness variation was studied. Another change in the specimen geometry was made by machining a flat loading end at the upper load application point. Fracture toughness values were computed using the stress intensity values computed from numerical modeling and failure loads from the experiments. It was found that up to 2 mm fracture toughness was not affected by variations in the thickness of preliminary notches. Fracture toughness was not affected by changing the loading span. For specimens with flat loading ends, fracture toughness was about 16% lower than the value found from regular SCB type specimens loaded at a point at the top by a steel roller. As a result of about 46 experiments average fracture toughness of Ankara G&ouml / lbasi andesite was found as 1.36 MPa .

Fracture toughness is an important rock property for rock fracturing and fragmentation applications. Theory and practice of opening mode (mode I) and shearing mode (mode II) fracture toughness tests are still in a developing stage for the cylindrical rock cores. A new circular plate type test specimen is used for mode II fracture toughness testing on rock cores. This involves a straight edge notched circular plate type core disc geometry under three-point bending load / new method and its associated specimen geometry is referred as straight edge notched disc bend (SNDB) specimen under three-point bending. Mode II fracture toughness results of the tests with this new geometry were compared to the results of the tests commonly employed for mode II fracture toughness testing. Specimen geometries were modeled and mode II stress intensity factors were computed by finite element modeling using ABAQUS program. For comparison purposes, mode II or shearing mode fracture toughness KIIc of two different rock types were determined by different testing methods commonly employed in recent practice. Core specimens of Ankara andesite and Afyon marble rock types were tested with cracked chevron notched Brazilian disc and cracked straight through Brazilian disc specimens under Brazilian type loading, semi-circular bend specimen and straight edge notched disc bending specimen geometries under three-point bending.For all testing groups, cylindrical cores with diameters varying from 7.5 cm to 12.5 cm were prepared with notch lengths changing from 1.5 cm to 2.6 cm. Effect of specimen thickness on mode II fracture toughness was investigated for three different testing methods. Fracture toughness values remained constant when thickness of the specimens was increased for cracked straight through Brazilian disc, semi-circular bend and straight notched disc bend methods. For cracked straight through Brazilian disc method KIIc values of Ankara andesite and Afyon marble were 0.99 MPa&radic / m and 0.86 MPa&radic / m, respectively. Mode II fracture toughness with semi-circular bend specimen was 0.43 MPa&radic / m for andesite and 0.46 MPa&radic / m for marble. When the results of the two three-point bending type tests were compared straight notched disc under three-point bending resulted in higher KIIc values (0.61 MPa&radic / m for andesite and 0.62 MPa&radic / m for marble) than the results found by semi-circular bend tests.

This thesis aims to measure the mechanical properties of ceramics on the microscale using microcantilever beams. Focussed Ion Beam milled triangular cross-sectional beams (approximately 3 x 5 x 20µm) were fractured using a nanoindenter to measure the Young’s modulus, fracture strength, and fracture toughness. By developing the technique with a sapphire bicrystal, it was found that the mechanical properties could be successfully ascertained if correction factors were used. Experiments and theoretical work showed that sapphire and polycrystalline alumina beams undergo moisture assisted sub-critical crack growth when tested in air. Whilst corrections for the Young’s modulus have been previously reported, this is the first reported attempt to correct for the notch tip residual stress and the first to consider sub-critical crack growth. Once these factors were characterised using the sapphire bicrystal, the technique was applied to a range of different ceramics, such as polycrystalline α-alumina and silicon nitride. These are the first reported direct measurements the grain boundary toughness of these ceramics using microcantilever beams. The grain boundary toughness was correlated with the macroscopic fracture properties and the characteristics of the ceramic (grain boundary composition, impurities, and fracture mode). Two grades of α-alumina were used and the macro- and micro-scale properties extensively compared. The damage evolution during uniaxial compression of alumina was investigated in depth, and compared to a previous reported microcrack evolution model using the measured grain boundary toughness. Investigation of whether deformation twins formed during loading was undertaken and the phenomenon was shown to not occur.

This diploma thesis deals with computation modeling of spur gearing using FEM. The methodology of checking calculation was suggested for gears which are not possible to check only by using standard ČSN 01 4686. The analysis of influence of vicinity of stress concentration on stress distribution in tooth root was performed. It is possibility to use the gears with thinner rim than the standard ČSN 01 4686 recommends. The main part of this work is devoted to fatigue crack propagation study in gears with thin rim. Aiding T-stress the influence of constraint and stress multiaxiality on crack propagation was taking into account. The predicted fatigue crack propagation paths were compared with certain experimental data. It is demonstrated that the considering of influence of constraint improves estimation of consequent crack propagation direction and in some cases the influence of constraint can significantly change the estimated crack propagation path.