Dissertations / Theses on the topic 'Cohesive-frictional'

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 143-148).
In this thesis we develop analytical solutions for the relations between scratch hardness and strength properties of cohesive-frictional materials of the Mohr-Coulomb and Drucker-Prager type. Based on the lower-bound yield design approach, closed form solutions are derived for frictionless scratch devices, and validated against computational upper bound and elastoplastic Finite Element solutions. The influence of friction at the blade{material interface is also investigated, for which a simple computational optimization is proposed. The model is extended to porous cohesive-frictional materials through the use of a homogenized strength criterion based on the Linear Comparison Composite theory. Relations between scratch hardness, porosity and strength properties are proposed in the form of fitted functions. Illustrated for scratch tests on cement paste, we show that the proposed solutions provide a convenient way to determine estimates of cohesion and friction parameters from scratch data, and may serve as a benchmark to identify the relevance of strength models for scratch test analysis.
by Romain Bard.
S.M.

Many previous studies have been focused on the behaviour homogeneous granular soils under the quasi-static loading, however, various soil types exist in the field. Therefore, based on the evaluation of these previous studies, an extensive study has been addressed to expose the dynamic behaviour of cohesive-frictional soils associated with the effects of fines content, the effect of moisture content and the type of impact regime. The proposed study mainly investigates the behaviour of sand – clay mixtures to impact loading, both from a loaded plate dropped from different heights and one dropped repeatedly from a fixed height. The Aberdeen beach sand and the Teuchan clay were used for the study and mixed in different proportions to create soils of varying proportions. The six soil samples used have known volumetric proportions of sand : clay and the tests were carried out under the dry condition and two other moisture contents. The results determine the optimal percentage of fines content and its related moisture condition to obtain more stable performance of the granular soils under dynamic compaction. It can be implemented to enhance the quality of ground improvement techniques for the construction. A Soil Model Tester for 2-Dimension program (SM2D) [Chan (1988)] was used to modify the existing material model before being used for Finite Element simulation. The impact test results were used to verify the numerical model developed using an explicit u-w schemebased finite element program, GLADYS2E [Chan et al. (1992, 1994)]. Such use of explicit schemes requires the use of time step lengths which are smaller than a critical value, in order that stability and accuracy of solution are ensured. A semi-empirical formula has been developed for the critical time step determination using MATLAB.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.
Includes bibliographical references (p. 213-221).
Advanced experimental and theoretical micromechanics such as nanoindentation makes it possible today to break down highly heterogeneous materials to the scale where physical chemistry meets (continuum) mechanics, to extract intrinsic material properties that do not change from one material to another, and to upscale the intrinsic material behavior from the sub-microscale to the macroscale. While well established for elastic properties, the extraction of strength properties of cohesive-frictional materials from nanoindentation tests has not been investigated in the same depth. The focus of this thesis is to investigate in depth the link between nanohardness of cohesive-frictional materials and strength properties. To address our objectives, we develop a rational methodology based on limit analysis theorems and implement this methodology in a finite element, based computational environment. By applying this technique to indentation analysis, we show that it is possible to extract the cohesion and the friction angle from two conical indentation tests having different apex angles. The methodology is validated on a model cohesive-frictional material, bulk metallic glass, and a first application to a highly heterogeneous natural composite material, shale materials, is shown. The results are important in particular for the Oil and Gas industry, for which the reduced strength properties (cohesion and friction angle) are critical for the success of drilling operations.
by Francois P. Ganneau.
S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.
Includes bibliographical references (p. 170-178).
Natural composites in general and sedimentary rocks in particular are highly heterogeneous materials which defy a straightforward implementation of the materials science paradigm of microstructure-properties-performance correlation. The application of nanoindentation to natural composites has provided the geomechanics community with a new versatile tool to test in situ phase properties and structures of geomaterials that cannot be recapitulated ex situ in bulk form. But it requires a rigorous indentation analysis to translate indentation data into meaningful mechanical properties. The development and implementation of such an indentation analysis for the strength properties of cohesive-frictional porous materials is the focus of this thesis. We report the development and implementation of a multi-scale indentation analysis based on limit analysis, which makes it possible to infer from an experimental hardness value and the solid's packing density the strength properties of the cohesive-frictional porous material.
(cont.) Making use of most recent advances in non-linear strength homogenization theory, we implement a homogenized cohesive Cam-Clay type elliptical strength criterion which takes into account the strength properties of the constituents (cohesion and friction), the porosity and the microstructure, into a yield design approach to indentation analysis. Making use of the strong duality of the lower and upper bound theorem, we identify the resulting upper bound problem as a Second-Order Conical optimization problem, for which advanced solvers such as MOSEK became recently available. The originality of our approach lies in the combination of finite element discretization and advanced optimization techniques, which is readily implemented in standard tools of computational mechanics, such as MATLAB. The upper bound yield design solutions are benchmarked against solutions from comprehensive elastoplastic contact mechanics finite element solutions and compared with lower bound solutions, which all show an excellent agreement.
(cont.) Furthermore, from a detailed parameter study based on intensive computational simulations, we show that it is possible to condense the indentation hardness-material properties relation of cohesive-frictional porous materials into a single hardness-packing density scaling relation. On this basis, it is possible to use the hardness-packing density scaling relation for reverse analysis of the strength parameters of cohesive-frictional solids from indentation. The procedure is illustrated for shale materials. From hardness values of six shale materials of different packing density and mineralogy, we deduce that the clay fabric in highly compacted shales is most likely a purely cohesive (friction-less) nano-granular material, having a uniaxial strength of roughly 440 MPa.
by Sophie Cariou.
S.M.

In this thesis, two main topics have been covered: the mechanics of strain localization in plasticity and the performance of several mixed finite elements subjected to plastic strain localization. Throughout the thesis, incompressible and cohesive-frictional, isotropic and orthotropic, elasto- and rigid-plastic solids are analyzed using associated or non-associated flow rules, both in the continuum and the discrete settings. Plastic yielding, strain bifurcation and strain localization are identified in the failure process prior to conducting a detailed analysis of strain localization. The mechanics of strain localization in the continuum and discrete settings, including the constitutive relations, the kinematics for strong and weak discontinuities, and the strain localization conditions are presented. Maxwell’s kinematic condition, the traction rate continuity and the stress rate constraints are explained, thereby distinguishing the correlations and differences between strain bifurcation and strain localization conditions. The analytical prediction of strain localization derived from the stress boundedness condition is proposed and numerically verified through independent simulations. Unlike predicted in classical strain bifurcation analysis, strain localization is independent from the elasticity behavior and is only related to plastic flow. Specifically, the strain localization angle depends on the stress state and plastic potential but not on the yield surface. Uniaxial computational experiments on strips subjected to uniaxial stretching and compressing in plane stress and plane strain to assess the theoretical analysis and Prandtl’s flat punch tests are performed. Numerical results for incompressible and cohesive-frictional, isotropic and orthotropic, associated and non-associated plasticity, with or without inclination angles between the material local axes and the global axes are compelling evidence for the proposed theoretical framework. Various mixed finite elements are used in this thesis. By comparing the numerical outputs, the advantages and disadvantages of the performance of the several mixed finite elements are shown regarding enhanced accuracy, computational efficiency, mesh sensitivity and stress locking.
En esta tesis trata dos temas principales: la mecánica de la localización de deformaciones en plasticidad y el funcionamiento de varios elementos finitos mixtos sometidos a la localización de deformaciones plásticas. A lo largo de la tesis, se estudian sólidos incompresibles y cohesivo-friccionales, isotrópos y ortótropos, elasto- y rígidos-plásticos, utilizando reglas de flujo asociadas o no asociadas, tanto a en formato continuo como discreto. En un análisis detallado del proceso de localización de la deformación, se identifican los puntos de plastificación, bifurcación y localización de la deformación. Se presentan los mecanismos de la localización de deformaciones a nivel continuo y discreto, incluyendo las relaciones constitutivas, la cinemática de las discontinuidades fuertes y débiles y las condiciones de localización de la deformación. Se explican la condición cinemática de Maxwell, la condición de continuidad del incremento de tracción y la condición acotabilidad del incremento de la tensión, su relevancia en la bifurcación de la deformación y las condiciones de localización de la deformación. Se propone y se verifica numéricamente mediante simulaciones independientes la predicción analítica de la localización de la deformación a partir de la condición de acotabilidad de la tensión. A diferencia de lo que predice en el análisis clásico de localización de deformaciones, ésta es independiente del comportamiento elástico y está únicamente relacionada con el flujo plástico. Específicamente, el ángulo de localización de la deformación depende del estado de la tensión y del potencial plástico, pero no de las constantes elásticas ni de la superficie de fluencia. Se realizan experimentos computacionales en placas sometidas a tracción y compresión uniaxial en tensión y deformación plana para evaluar el análisis teórico, así como en tests de punzonamiento de Prandtl. Los resultados numéricos con plasticidad incompresible y cohesivo-friccional, isotrópa y ortotrópica, asociada y no asociada, con o sin ángulos de inclinación entre los ejes locales materiales y los ejes globales proporcionan evidencias convincentes para el marco teórico propuesto. En esta tesis se utilizan varios elementos finitos mixtos. Al comparar los resultados numéricos, se muestran las ventajas y desventajas del funcionamiento de varios elementos finitos mixtos con respecto a su precisión, la eficiencia computacional, la sensibilidad respecto a la alineación de la malla y el bloqueo de tensiones.
Enginyeria civil

L'Institut de Radioprotection et de Sûreté Nucléaire (IRSN) s'intéresse à l'étude des réactions de gonflement interne, dont les Réactions Sulfatiques, et à leur impact sur l'évolution des propriétés du matériau cimentaire. Les Réactions Sulfatiques sont caractérisées par la précipitation de l'ettringite, dans les pores du matériau durci entraînant des gonflements locaux et une fissuration par déformations différentiées. Les fissures créées constituent alors le lieu privilégié de la précipitation d'ettringite et accélèrent le transport des espèces chimiques au sein du milieu poreux. La modification locale des phénomènes de transport induit une accélération de la dégradation du matériau.Ce travail de thèse modélise à l'échelle mésoscopique d'une collection de granulats, le gonflement du béton par les Réactions Sulfatiques et la cinétique de dégradation. Un modèle chimio-mécanique basé sur une description du transport réactif (diffusion d'espèces et réactions chimiques) et mécanique (Modèle de Zones Cohésives) dans un milieu poreux fissuré est proposé et résolu à l'aide d'un couplage étagé générique.Les paramètres chimiques et mécaniques initiaux sont estimés par un calcul d'hydratation et d'homogénéisation analytique.La modélisation chimio-mécanique tridimensionnelle est validée de façon modulaire et appliquée aux Réactions Sulfatiques Externe et Interne. Les effets de la composition du béton et des conditions environnementales chimiques sur la cinétique d'expansion et le faciès de rupture sont étudiés. Les applications mettent en évidence l'influence des granulats et des fissures dans la répartition spatiale inhomogène des zones de précipitation de l'ettringite et les contraintes de gonflement associées
The French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN) conducts researches on the impact of internal swellings reactions on concrete, such as Sulfate Reactions. Such reactions are characterized by the precipitation of ettringite which induces swellings and cracks by differential strain. These cracks are preferential location for ions diffusion and further ettringite precipitations.The aim of the study is to model the degradation of a mature material by ettringite pressure at the aggregate scale.A chemo-mechanical model based on a coupling between reactive transport (species diffusion and chemical reactions) and mechanics in cracked porous medium is developed and is solved with a generic staggered approach.The initial microstructure and poro-mechanical and diffusion parameters are estimated by hydration computing and analytical homogenization.The coupled chemo-mechanical model is validated and then applied to Sulfate External and Internal Attack.The impact of the concrete composition and the chemical environments on the swelling kinetics and crack path is taken into account. Furthermore, our simulations highlight the influences of inclusions and cracks on the inhomogeneous spatial distribution of precipitation areas of ettringite and associated swelling stress

Ce projet de recherche vise à développer un code de calcul adapté à l'étude des structures maçonnées, utilisable en bureau d'études. Le code aux éléments discrets LMGC90 est choisi comme moteur de calcul pour sa capacité à décrire le comportement de milieux divisés comme ceux rencontrés en maçonnerie. Un modèle de zone cohésive frictionnelle (noté FCZM) est proposé permettant de décrire l'endommagement progressif et la dissipation d'énergie associés au comportement quasi-fragile et au frottement de l'interface pierre-mortier. Sous sollicitations combinées de traction et cisaillement, le modèle décrit la rupture en Mode mixte I+II de l'interface, tandis que sous sollicitations combinées de compression et cisaillement, un couplage entre les comportements cohésif et frictionnel, basé sur le niveau d'endommagement, conduit à l'expression d'une contrainte de friction croissante associée au comportement adoucissant de l'interface. Sur cette base, les paramètres cohésifs et frictionnels du modèle FCZM sont déterminés par l'intermédiaire d'essais de caractérisation (traction directe et cisaillement) menées sur des assemblages de pierres calcaires jointoyées au mortier de chaux. Une validation du modèle FCZM est proposée à l'échelle d'un mur. Ce dernier est soumis à un essai de rupture en cisaillement sous charge verticale constante. La confrontation des réponses expérimentales et simulées fournit une large base de validation du modèle FCZM. Si la statique de l'essai est bien décrite à partir des valeurs d'entrée du modèle FCZM issues de la caractérisation, la description de la cinématique nécessite quant à elle de recourir à une calibration des propriétés élastiques des blocs et de dégrader les propriétés cohésives des interfaces par l'intermédiaire d'un champ d'endommagement initial uniforme. Sur cette base, l'utilisation de champs d'endommagements non-uniformes permet de capturer les différents enclenchements de mécanismes de rupture constatés expérimentalement en fonction des murs testés
This work aims to develop a design code suitable for structural masonry design. Discrete element code LMGC90 is chosen as basis of the design code for its capacity to consider masonry discontinuities. A general frictional cohesive zone model (FCZM) is proposed to describe the progressive damage and the release of energy due to the quasi-brittle behavior of materials and to the friction of the interface stone-mortar. Under combined traction and shear loadings, a mixed-mode response based on pure Mode I and Mode II cohesive behaviors is proposed. Under combined compression and shear loadings, a coupling between Mode II cohesive behavior and frictional behavior based on the damage level is proposed and leads to a progressive rising of the frictional stress associated with the softening part of the cohesive behavior of the interface. On this basis, cohesive and frictional parameters of the FCZM are estimated from two characterization tests (direct tensile and direct shear) carried out on samples of limestone blocks assembled by lime mortar joint. A general validation is proposed at the scale of a masonry wall submitted to a shear fracture test under constant vertical load. The confrontation of experimental and simulated responses provides a large validation basis because all the loading modes considered in FCZM are present in the wall during the shear test. If the static of the experimental responses is well described by the model using the parameters resulting from the characterization tests, the kinematic of the responses needs calibrating the elastic properties of the stone blocks and degrading the cohesive properties of the interfaces through a uniform initial damage field. On this basis, the use of a nonuniform initial damage field allows matching the experimental variability observed in the location and chronology of the fracture mechanisms of tested walls

The main purpose of this project is to investigate different tool steels in terms of their ability to withstand material transfer buildup, so-called galling, occurring in SMF (sheet metal forming) operations. The ability to withstand galling is vital to optimize cost-effectiveness and increase the work tool’s effective operational time. This investigation studies four different tool steels, including a TiN-coating, with the intention of evaluating the microstructures, chemical composition and hardness effect on galling resistance in dry conditions using a slider-on-flatsurface (SOFS) tribo-tester which measures the coefficient of friction during sliding.

An OP (optical profilometer) was used to measure the size and geometry of lump growth on the tool and damage on the work sheet. A scanning electron microscope (SEM) was used to identify the interacting tribological mechanisms exhibited at different stages during the slide. The SEM figures confirmed three different types of characteristic patterns exhibited in the tracks after tribo- testing which were categorized as mild adhesive, abrasive and severe adhesive damage.

A SEM figure that illustrates a ragged contact surface and an obvious change in the sheet materials plastic behavior is in this report regarded as a sign of severe adhesive contact, the characteristics could possibly be explained by local high temperature and high pressure followed by a sudden pressure drop and creation of hardened welds or solders between the two surfaces which increase the frictional input needed for further advancement. Friction coefficients observed in the initial 100% mild adhesive stage were, μ=0,22-0,26 succeeded by abrasive SEM characteristics often in association with mild adhesive contact and friction values between μ=0,25-0,4 which where sometimes followed by severe adhesive SEM characteristics in 100% of the contact zone with friction values between μ=0,34- 0,9 respectively. The tool material that performed best according to the friction detection criteria was Sv21 closely followed by Sleipner (TiN coated) and Va40 (HRC 63.3). Unfortunately was the friction criteria, a significant raise in friction for defining a sliding length to galling, not adequate for dry conditions due to immediate material transfer succeeded by cyclic changes between partial or 100% abrasive+mild adhesive and severe adhesive contact. The mechanism that change abrasive wear in association with mild adhesive contact, (moderate friction input), to sever adhesive wear, (higher friction input), is dependent on lump shape (lump geometry) and can appear at comparably low speeds 0,04-0,08 [m/s] and low friction energy input (μ=0,34), the magnitude of the change in friction is therefore not always significant and hardly detectable on the friction graph. This was quite unexpected but could be explained by concentration of friction energy rater than the absolute amount. The problem with using friction graphs for galling evaluation was increased even further when a very small lump size and low corresponding rate of material transfer to the tool surface caused a sustainable high raise in friction (μ≈0,3→0,6) on a TiN-coated tool steel called Sleipner.

A hardly detectable or similar friction raise for Sv21 and Va40 showed much larger corresponding lump size and rate of material transfer. This means that friction graphs demonstrate a clear problem with quantifying lump size [m3] and rate of  material transfer [m3/s]. Another phenomenon called stick slip behavior, material transfer and lump growth followed by a sudden decrease in lump size and transfer of material back to the work sheet, is also not possible to detect on a friction graph. Because a drop in friction can easily be a change in contact temperature and lump attack angle due to a growing lump and not a decreasing lump.

 

The conclusion, a friction graph is not suited for galling evaluation and ranking in dry SOFS conditions. A ranking should primarily be based on dimensional OP measurements of the cross section of formed tracks and scratches or preferably by repeated OP measurements of the tool surface during a single test, the last revel the exact lump growth history and true lump growth even in the sliding direction.

 

civilingenjörsexamen

博士
國立中央大學
土木工程研究所
83
This study carried out direct shear tests and simple shear tests to investigate the frictional properties of the pile-soil interface and to obtain the relationships among the shear stress, the sliding displacement at interface and the shear strain in soil. The theory of nonlinear elasticity was used to analyze the shear deformation of the soil around pile shaft. The proposed the method based on these tests and the theoretical analysis is used to predict the frictional behavior of pile shaft. The results of the analysis show that the relationship between the displacement and the frictional force predicted by the proposed method is consistence with those obtained from the triaxial model pile tests, the tank model pile tests and the pile load test. The results of frictional test show that the total displacement in prefailure stage includes the sliding displacement at interface and the shear deformation in soil. After the failure of the pile-soil interface, the variation of total displacement is come from sliding displacement. Before the failure, the dependency of the shear strain in soil on the friction ratio at the interface is the same as the one on the friction ratio in the soil. It can be find that there is little influence of the diameter of pile on the frictional resistance of shaft in triaxial and tank model tests. Howere, the displacement of pile at failure is increase with the increase of the diameter.

Thss thesis presents the results of an experimental programme on the static mono-tonic response of cohesive-frictional granular materials. The purpose of this experimental programme was to gain insight into the mechanical behaviour of uncemented sands, and sands with small percentages of cementation. With this objective in sight, the research involved understanding and delineating the e ects of four variables: the intermediate principal stress, stress inclination, cohesion (or cementation), and particle morphology. The hollow cylinder torsion (HCT) apparatus, which allows control over both the magnitude and direction of principal stresses, was used in this study to carry out a series of elemental tests on the model materials. The test results were analysed in a plasticity theory based framework of critical state soil mechanics. Drained and undrained HCT tests were conducted on a model angular sand to understand the combined influence of intermediate principal stress ratio (b) and principal stress inclination ( ). Sand specimens were reconstituted to a given density and confining pressure, and were sheared to large strains towards a critical state. The stresses at the critical state with varying `b' were mapped on an octahedral plane to obtain a critical state locus. The shape of this locus closely resembles a curved triangle. Also these specimens showed increased non-coaxiality between the stress and strain increment directions at lower strains. This non-coaxiality decreased significantly, and the response at the critical state was by and large coaxial. The effect of `b' and ` ' on the flow potential, phase transformation, and critical state was also investigated. At phase transformation, ` ' plays a more dominant role in determining the flow potential than `b'. The shape and size of the critical state locus remained the same immaterial of the drainage conditions. Next, small amounts of cohesion (using ordinary Portland cement) was added to this sand ensemble to study the mechanical behaviour of weakly cemented sands. The peak in the stress strain curve was used to signal the breakdown of cohesion further leading to a complete destructuring of the sand at the critical state. The response of the cemented sand changes from brittle to ductile with increase in confining pressure, while reverses with increase in density and `b'. Stress-dilatancy response for the weakly cemented materials shows the non coincidence of peak stress ratio and maximum value of dilation unlike purely frictional materials. This mismatch in peak stress ratio and maximum dilation diminishes with increase in confining pressure. The peak stress (cemented structured sand) locus and the critical state (destructured) locus were constructed on the octahedral plane from these HCT tests. The critical state locus of the cemented sand when it is completely destructured almost coincides with the critical state locus of the clean sand. Using this experimental data set, some important stress-dilatancy relationships (like Zhang and Salgado) and failure criteria (Lade's isotropic single hardening failure criteria and SMP failure criteria) were benchmarked and their prediction capabilities of such models were discussed in detail. The effect of particle morphology was also investigated in this testing programme. Rounded glass ballotini and angular quartzitic sand which occupy two extreme shapes were selected, and a series of HCT tests at different `b' values were con-ducted. A larger sized CS locus was obtained for angular particles and it encompassed the critical state locus of the spherical glass ballotini. Spherical particles exhibit a predominantly dilative behaviour, however present a lower strength at the critical state. The mobilization of strength as a result of rearrangement of angular particles and the consequent interlocking is higher. Even with contractive behaviour which is reflected in the higher values of critical state friction angle and the larger size of the yield locus for sand. Finally, a series of unconfined compression tests were performed to understand if there exists a scale separation in cohesive frictional materials. Specimens were reconstituted to a range of sizes while maintaining a constant aspect ratio and density. As the specimen size increased, the peak strength also increases, counter to an idea of a generalized continuum for all model systems. The observed secondary length scale (in addition to the continuum length scale) is obverse to the one observed in quasi-brittle materials such as concrete, rock. In order to ascertain the reason behind this phenomenon, a series of tomography studies were carried out on these contact-bound ensembles. The presence of cohesion between the grains brings about an \entanglement" between the grains, which contributes to increase in strength, with increase in the size of the sample. This in e ect bringing forth a second length scale that controls the behaviour of these cohesive frictional granular materials. This experimental data set provides quantification of various aspects of the me-chanical response of both cemented and uncemented granular materials under myriad stress conditions. This data set is also extremely useful in developing and bench-marking constitutive models and simulations.

碩士
國立臺灣科技大學
營建工程系
107
In the region there was generally influenced by seasonal weather such as highly rainfall intensity, increasing or decreasing water levels may trigger the changing of failure surface and factor of safety on slopes. Reducing the factor of safety causes the slope failure potential increases. It was indicated by the increase of pore water pressure, therefore the shear strength reduced and shear stress enhanced. The uncertain intensity in every rainfall period causes the water level change needs to be evaluated conscientiously to prevent the slope failures. This study primarily aims to investigate the failure mechanisms and factor of safety that are resulted as an impact of pore-water pressure differences by varying the water level, slope angle, soil strength parameters, and slope height with footing and without footing. In addition, cohesive-frictional soils are concerned in this study because this soil condition is commonly found in practice. Limit analysis as one of the rigorous stability analysis methods is used in this study to predict the slope failure mechanism and factor of safety by using two dimensional (2D) numerical approach. The plastic zone will be observed as well in this study as none studies explained it clearly. Latter, the stability charts are also produced for preliminary assessment by practical engineers

The formation and development of localisation bands and/or cracks have been experimentally identified as the key failure mechanism governing responses of quasi-brittle geomaterials like concrete, sandstones and soft rocks. In fact, key features of the material behaviour, including Lode-angle dependence, size effect and brittle-ductile transition can be considered as consequences stemming from the localised failure mechanism in several loading conditions. For geomaterials with fibre reinforcement (i.e., fibre reinforced concrete), even though adding short fibres into the material significantly changes its mechanical characteristics and performances, the development of cracks and localisation bands remains the central mechanism driving the material responses. In this case, the bridging effect caused by fibres across cracks, together with the material cohesive-frictional resistance, refrains cracks from further developing and forces the material to form more cracks throughout the structure to dissipate the given energy. This prolongs the coalescence of diffuse micro/meso-cracks to form a macro-crack and considerably improve the material strength and ductility. Classical continuum models, in principle, can capture the overall responses of the material with stress-strain relationships formulated from experimental observations at the macroscopic level. However, the material behaviour in these models is homogenous throughout the whole element domain, leading to an incorrect dependence of dissipated energy and specimen responses on the discretisation resolution. This is because they fail to capture the difference of deformation and behaviour between the localisation zone and its surrounding bulk material. As a result, with the presence of crack/localisation band, the definition of averaged quantities such as overall stress and strain is not adequate to describe the volume element and using them for analysing post-localisation behaviour of quasi-brittle geomaterials is inappropriate, if not totally incorrect. Consequently, classical approaches that ignore the strong heterogeneity induced by the localisation of deformation suffer from mesh convergence issues in Boundary Value Problems (BVPs) analysis. In this research, the localised failure mechanism is employed as the basis to develop a continuum-based constitutive model for quasi-brittle geomaterials with and without fibre reinforcement. The cracks/localisation bands are explicitly included as an intrinsic part of the model with its own behaviour in conjunction with the responses of the surrounding bulk material. This allows an additional constitutive relationship, together with its internal variables, to be defined inside the localisation band to describe its microstructural changes under the course of loading. The material inelastic behaviour and important features such as brittle-ductile transition, Lode-angle dependence, size effect and hydrostatic pressure dependence can thus be correctly captured in association with observable failure patterns at constitutive level. The model, constructed in this manner, is also capable of featuring multiple localisation bands/cracks inside the constitutive equations to account for any change of loading path and avoid unphysical stress-locking naturally. In addition, for modelling quasi-brittle geomaterials with fibre reinforcement, the incorporation of cracks within the constitutive model enables the inclusion of separate models describing the fibre bridging effect and material cohesive-frictional resistance. As a result, the interactions between these two components and their contributions to the stress transfer across a crack are naturally captured for different types of fibres and their volume contents. The transition from hardening to softening, corresponding to the change from diffuse cracking phase to localised failure can also be reflected as a consequence. Furthermore, owing to the explicit inclusion of cracks in the model, the resulting constitutive behaviour automatically scales with the discretisation resolution and the results are thus mesh-independent when solving BVPs, without requiring any additional regularisation. Model validations against experimental data show that the proposed model is simple yet effective in capturing the localised failure of quasi-brittle geomaterials with and without fibre reinforcement at both constitutive and structural levels. The model is proven to be reliable and computationally inexpensive, with a few model parameters which can be identified and calibrated in a consistent and physically meaningful manner. The proposed model can thus be applied straightforwardly for the analysis and design of solids/structures made of rocks or concrete, with or without fibre reinforcements.
Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 2019

In this thesis a previously unknown mechanism of failure in multilayered slope profiles is identified. In some conditions this mechanism does not confirm to the known failure models (relating to circular failure) used in slope stability analysis. For this reason, major failures have occurred in the artificial cuts despite the fact that the limit equilibrium methods suggest that these cuts would be stable. The limit equilibrium methods were originally created to apply to earth dam walls. In the open pit mining environment, where we face inhomogeneous and inclined multilayered structures, the assumptions of these limit equilibrium methods appear to be inapplicable (e.g. assumption for the equal shear strength along the failure surface). Analysis starts with a general picture of the stress state in the highwall slope, given extant geological conditions and rock properties. The study then focuses on a comparison of the crack-tip stress changes in the rockmass with and without inclusions at the microscopic level. Basing some assumptions on binocular microscope observations of grain structures, it is possible to measure the size of the different inclusions and show that the microscopic carbon flakes present in the rock fabric make a major contribution to the failure process in a mudstone layer in the slope. The approach adopts the fracture-process zone ahead of a crack tip as the controlling parameter of flaw propagation in rock. Flaw coalescence, which is poorly accounted for in current fracture models, is attributable to two phenomena: the flaw propagation due to high level of applied stress; and the linking of fracture-process zones due to the small distance between neighbouring flaws. A condition of flaw coalescence is given based on these two mechanisms. This development allows defining of two zones along the failure surface (frictional and cohesive). In the slope-stability field the shear strength of the rock along the failure plane is a composite function of cohesive and frictional strength. For instance, the relaxation stress normal to bedding, induced by overburden removal, provides an investigation method for the determination of the weakest minerals, which may act as flaws for fracture propagation in low-porosity rock. A method has been developed to determine the critical stress for tensile fracture propagation due to the rock structure and the stress reduction normal to bedding. A proposed failure mechanism is based on the polygonal failure surfaces theory developed by Kovari and Fritz (1978), Boyd’s field observations (1983), Stead and Scoble’s (1983) analyses, Riedel (1929) Shear Fracture Model, Tchalenko and Ambraseys (1970), Gammond’s (1983) and Ortlepp (1997) observations for natural shear failures, computer modelling by McKinnon and de la Barra (1998), the results of many laboratory experiments reported by Bartlett et al. (1981) and the author’s experience. The proposed failure mechanism evaluates stability of the artificial slope profile due to the embedded weak layer structure, layer thickness, layer inclination and depth of the cut. On the basis of the observations and the above-mentioned modified fracture model, the slope profile is divided into two blocks; passive and active blocks. With this new model, it is possible to calculate slope safety factors for the slope failure cases studied in the industry. It has been found that, whereas the conventional slope stability models predict stable conditions, the new model suggests that the slope is only marginally stable (i.e. that failure can be expected).
Thesis (PhD(Mining Engineering))--University of Pretoria, 2007.
Mining Engineering
unrestricted