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The thesis presents novel formulations of hereditary cohesive zone models able to capture rate-dependent crack propagation along a defined interface. The formulations rely on the assumption that the measured fracture energy is the sum of an intrinsic fracture energy, related to the rupture of primary bonds at the atomic or molecular level, and an additional dissipation caused by any irreversible mechanisms present in the material and occurring simultaneously to fracture. The first contribution can be accounted for by introducing damage-type internal variables, which are to be driven by a rateindependent evolution law in order to be coherent with the definition as intrinsic energy. It is then proposed that the additional dissipation can be satisfactorily characterised by the same continuum-type material constitutive law obeyed by the interface material considered as a continuum: it is postulated that the dimensional reduction whereby a three-dimensional thin layer is idealized as a surface does not qualitatively alter the functional description of the free energy. The specific application considered is mode-I crack propagation along a rubber interface. After focusing on viscoelasticity as a suitable candidate to reproduce rubber’s behaviour, firstly the most common relaxation function, namely a single exponential term, is considerd after which the attention is turned to the use of fractional calculus and the related fractional integral kernel. A comparison with experimental results is presented. A shortcoming of the proposed approach is then noted, in that certain features of experimentally measured responses (i.e.the non-monotonicity of the critical energy-release rate with respect to crack speed) will be shown to be out of reach for the described modelling paradigm. A novel micromechanical formulation is then sketched in an attempt to qualitatively understand the phenomenon. An additional interface damaging mode is introduced, physically inspired by the desire to reproduce the formation of fibrils in a neighbourhood of the crack tip. Fibril formation is then driven by a variational argument applied to the whole of the interface, yielding its non-local character. Upon the introduction of an anisotropic fracture energy, motivated by experimental considerations, it is noted how the model can predict a non-monotonic energy-release rate vs crack speed behaviour, at least for a simple loading mode.

The use of classical creep continuum damage mechanics, constitutive and damage equations is restricted, to model certain types of creep deformation and fracture mechanisms, under isothermal conditions; and, to extend their predictive capabilities for a wider range of problems they have to be modified. The constitutive and damage equations are modified to represent the bi-linear, log. stress vs. log. rupture, and the log, stress vs. minimum strain rate, characteristics of materials; so that the change in material behaviour, as a mechanism change occurs, is represented in the constitutive model, by a change in the slope of these characteristic lines. Uni-axial creep tests of as-cast (OFHC) Copper have been performed at 150°C, 250°C and 500°C; and, an anisothermal constitutive model has been developed for the temperature range 150°C to 500°C, which highlights how the constitutive equations may be modified, to model creep behaviour under varying temperature conditions. The model predictions are in good agreement with the test results. A compact tension specimen has been studied, which has shown the importance of modelling the effects on rupture, of the high tri-axial stress-state present at the crack-tip, which accelerates void growth. Modified constitutive equations, have been used to model the mechanism of constrained cavity growth, and has enabled improved damage distribution and. lifetime predictions to be obtained for the compact tension specimen, similar to those expected from experimental tests. Non-local damage techniques are developed to model the effects of grain size characteristic dimension, on the failure of large and small cracked tension specimens. Non-local damage techniques are shown to be necessary to give accurate, physically related, finite element solutions. Suitably modified constitutive and damage rate equations are used to model the high temperature failure of a circumferential weld, in a thick steam-pipe, operating at a constant temperature and pressure. The models developed predict, the growth of damage in certain microstructural regions of the weld, and the lifetime of the component; which are observed to be in close agreement with the results from a fullsize pressure vessel tests. It will be shown that it is essential to use creep constitutive and damage equations in computer models, which accurately represent the underlying physics of IX the predominant creep mechanisms present. The implications of the research work on future computer modelling and on design are discussed.

Short crack growth behaviour was studied using a method of plastic replication on hour-glass shaped specimens of a medium carbon steel which were subjected to push-pull fatigue testing at ambient temperature. Crack lengths were measured from replicas using an optical microscope from which the growth rate could be calculated. A theory for short crack growth is presented which may be expressed mathematically by the equation:- = (for a < d) da dN where (a) is crack length, (d) is a characteristic dimension between adjacent microstructural obstacles to crack propagation, C1 is a function of stress or strain range and a is a constant. For the medium carbon steel used in this study (d) was equated to the ferrite band length which contained the growing crack. This theory was used to model short crack growth in an Aluminium alloy T6-7075 Al and for the medium carbon steel used in this project. iii By using a second equation to describe "long" crack growth of the form:- da dN = where C2 is a function of the applied strain range and 0 is a constant, it was then possible to describe the complete history of crack growth. By obtaining short crack growth data for different stress levels, quantitative expressions of these two equations were calculated from which fatigue lifetime predictions could be made by integration of the equations for any stress level. Using this method an estimate could also be made of the percentage of fatigue lifetime spent in the initiation and growth of short cracks.

The object of this study was to help clarify some of the fundamental problems occurring in plain and reinforced concrete structural elements suffering from the degradation phenomenon of alkali silica reaction. A laboratory based testing programme was used, so that extreme conditions for generating alkali silica reaction could be used which are not readily achieved in actual structures. Special concrete mixes were also used in which the alkali silica reaction occurs within a period of months rather than years so that observation of the properties of the reacted concrete could be taken throughout the reacti ve process. The laboratory tests undertaken are as follows: fresh and hardened behaviour of alkali silica reactive (ASR) model mixes; mechanical properties and expansion behaviour of ASR concrete; structural behaviour of reinforced ASR concrete beams and columns; bond between reinforcement and ASR concrete in prisms and beams; restraint effect of ASR upon plain and reinforced concrete structural elements. This work shows that, ASR causes an unacceptable level of progressive damage to plain and reinforced concrete. The degradation and expansion in plain ASR concrete is higher than that in reinforced concrete. ASR in concrete reduces the strength and elastic modulus; the stiffness of reinforced beams; the axial load and ultimate bending moment capacity of columns; the ultimate bond between concrete and reinforcement. As a result of expansion in non-symmetrically reinforced structural elements, ASR causes either sagging or hogging displacement. The restraint imposed by reinforcement in ASR concrete elements is however beneficial in terms of inhibiting expansion and crack development, and exerting a small degree of post tensioning which generally improves the strength of members. Preloading of ASR concrete also results in an improvement in the mechanical properties. PhD

This doctoral thesis concerns fatigue of welded structures. Welding is one of the world’s most common joining methods and it is frequently used in several structural applications in many fields. Some examples are construction vehicles, loader cranes, trucks, busses, forestry and agricultural machines, bridges and ships. Since these structures are subjected to repeated loading, fatigue is the most common cause of failure. A novel numerical algorithm has been developed which assesses the welded surface and calculates and quantifies weld quality parameters and the presence of defects which are critical in fatigue applications. The algorithm is designed for implementation in serial production. It will provide robust and reliable feedback on the quality being produced, which is essential if high strength steels are utilized. Two welding procedures which can increase the weld quality in as welded conditions have been assessed. These procedures utilize welding in different positions and pendling techniques, which can be accomplished using the existing welding equipment. It was found that by using these methods, the fatigue strength can be increased compared to normal weld quality. Furthermore, two fatigue assessment methods ability to account for increased weld quality in low cycle and high cycle fatigue applications has been studied. One of these methods showed sufficient accuracy in predicting the fatigue strength with small scatter and also account for increased weld quality. When implementing thinner high strength steels, the overall stress level in the structure increase. Therefore, other locations such as the steel cut edges may become critical for fatigue failure unless they are not designed and manufactured with the same quality as the welded joint. The influence of surface quality on cut edges was studied and the fatigue strength was estimated using international standards and a fatigue strength model for cut edges. / <p>QC 20160613</p> / WIQ / LIGHTSTRUCT / ONWELD

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

A fracture and permeability analysis was performed on the Santana Tuff because of its similarity to the Topopah Springs unit at the Yucca Mountain site. The Topopah Springs unit is the proposed horizon for the spent nuclear fuel repository. Because of the impossibility of completely characterizing the flow properties of the unit without destroying the characteristics that make it desirable as a repository, other ash flow tuffs must be studied. The Santana Tuff and the Topopah Springs tuff both are rhyolitic in composition, nonwelded to densely welded and fractured. Fractures were examined at six outcrop locations spanning a five mile area. Stereonets and rose diagrams were constructed from over 312 fracture orientations. Although the composite data showed two major orientations of nearly vertical fractures, fracture trends at individual outcrops showed a variety of preferred orientations. Over 900 surface permeability measurements were taken using a mini-permeameter. The samples were categorized by three observed types of surface weathering: fresh, weathered, or varnished. Fracture surfaces were generally classified as weathered. The average permeabilities for the samples are 55.33 millidarcies, 5.03 millidarcies, and 3.31 millidarcies, respectively. The one-way statistical analysis performed on the data indicated that the permeability of fresh tuff surfaces is significantly different than both the permeabilities of the weathered and varnished tuffs, using both a least significant difference and greatest significant difference test. However, no difference was shown to exist between the weathered and varnished tuff permeabilities. Samples of fresh, weathered, and varnished tuffs were examined by X-Ray Defraction, the Scanning Electron Microscope, and in thin section. The SEM analysis showed surface differences between the three weathering classifications. The weathered and varnished samples were similar, exhibiting a platy, lamellate texture. The fresh surfaces were irregular and jagged. In thin section, a thin rind of dark minerals (FE-oxides) is observed on the edges of the varnished samples and in microcracks. This fills surface pores and causes the reduction in permeability. Two other zones of weathering have been identified in some of the samples, which may also cause changes in permeability. Tuff permeabilities were also analyzed for directional dependence. After an ash flow tuff is deposited and cooled, it may undergo flattening of pumice fragments and glass shards. These flattened fragments can be identified in handsamples, and are indicative of the direction of flow emplacement. The analysis showed that permeability is enhanced parallel to the emplacement direction, which is generally horizontal. Cut surfaces showed a 30% decrease in permeability perpendicular to flow direction. On varnished surfaces, this trend is still evident, although decreased in magnitude. This is expected because of the clay particles which make up the desert varnish. This study indicates that the formation of low permeability weathering rinds in association with vertical fractures may inhibit infiltration at the surface. It may accelerate infiltration at depth and allow more fluid to penetrate vertically into the tuff. In the event that fluid is absorbed into the matrix, it will travel horizontally, along the enhanced permeability parallel to the emplacement direction. / text