Thesis (M.Eng.)--Memorial University of Newfoundland, 2005. / Bibliography: leaves 116-121.

This thesis work describes a new experimental method for the determination of Mode II (shear) fracture toughness, KIIC of rock and compares the outcome to results from Mode I (tensile) fracture toughness, KIC, testing using the International Society of Rock Mechanics Chevron-Bend method.<br><br>Critical Mode I fracture growth at ambient conditions was studied by carrying out a series of experiments on a sandstone at different loading rates. The mechanical and microstructural data show that time- and loading rate dependent crack growth occurs in the test material at constant energy requirement.<br><br>The newly developed set-up for determination of the Mode II fracture toughness is called the Punch-Through Shear test. Notches were drilled to the end surfaces of core samples. An axial load punches down the central cylinder introducing a shear load in the remaining rock bridge. To the mantle of the cores a confining pressure may be applied. The application of confining pressure favours the growth of Mode II fractures as large pressures suppress the growth of tensile cracks.<br><br>Variation of geometrical parameters leads to an optimisation of the PTS- geometry. Increase of normal load on the shear zone increases KIIC bi-linear. High slope is observed at low confining pressures; at pressures above 30 MPa low slope increase is evident. The maximum confining pressure applied is 70 MPa. The evolution of fracturing and its change with confining pressure is described.<br><br>The existence of Mode II fracture in rock is a matter of debate in the literature. Comparison of the results from Mode I and Mode II testing, mainly regarding the resulting fracture pattern, and correlation analysis of KIC and KIIC to physico-mechanical parameters emphasised the differences between the response of rock to Mode I and Mode II loading. On the microscale, neither the fractures resulting from Mode I the Mode II loading are pure mode fractures. On macroscopic scale, Mode I and Mode II do exist. / Diese Arbeit beschreibt eine neue experimentelle Methode zur Bestimmung der Modus II (Schub) Bruchzähigkeit, KIIC, von Gestein und vergleicht die Ergebnisse mit Resultaten aus Versuchen zur Bestimmung der Modus I (Zug) Bruchzähigkeit, KIC.<br><br>An einer Serie von Versuchen mit verschiedenen Belastungsraten wurde das kritische Modus I Rißwachstum eines Sandsteines untersucht. Die mechanischen Daten zeigen, daß zeit- und belastungsratenabhängiges Rißwachstum in dem Material bei konstantem Energieverbrauch stattfindet. <br><br>Der neu entwickelte Versuchsaufbau zur Ermittlung der Modus II Bruchzähigkeit wurde Punch- Through Shear Test genannt. Die Proben werden aus Bohrkernen hergestellt in deren Endflächen Nuten eingebracht werden. Eine Last auf den Innenzylinder induziert eine Schubspannung. Auf die Mantelfläche der Proben kann ein Umlagerungsdruck aufgebracht werden. Da durch Normalspannungen das Modus I Rißwachstum unterdrückt wird, wird das Modus II Rißwachstum gefördert.<br><br>Die PTS- Probengeometrie wurde bezüglich Nutentiefe, -durchmessers, -breite und des Probendurchmessers optimiert. KIIC steigt bi-linear mit Zunahme des Umlagerungsdruckes an. Ein starker Anstieg ist bis zu Umlagerungsdrücken von etwa 30 MPa zu beobachten, oberhalb ist die Steigung geringer. Bisher wurden Umlagerungsdrücke bis maximal 70 MPa aufgebracht. Die Entwicklung der entstehenden Risse und deren Variation mit Umlagerungsdruck wird beschrieben.<br><br>Ob die Entstehung eines Modus II Risses in Gestein möglich ist, wurde vielfach in der Literatur diskutiert. Der Vergleich der Ergebnisse der Modus I und II Experimente, insbesondere bezüglich der Rißmuster und der Korrelationsanalysen von KIC und KIIC zu physiko-mechanischen Parametern, zeigt die Unterschiede der Reaktion auf Modus I und Modus II Belastung auf. Mikroskopisch gesehen wachsen die Risse weder unter Modus I noch unter Modus II Belastung in einem reinen Modus. Allerdings existieren Modus I und Modus II Risse auf der makroskopischen Betrachtungsebene.

This thesis is on the fatigue assessment of cast components with special attention to defects. The primary material in view is nodular cast iron, but also cast steel is considered. However, the fatigue behaviour is in principle valid for general use on other cast metals.The first two papers is about general cast material behaviour in fatigue loading. The materials considered are a high strength alloyed cast steel and a medium strength nodular cast iron. It is concluded that cast defects is the main fatigue initiation cause and it is only in exception that the fatigue life is not ruled by fracture mechanics. The third paper is a fracture mechanics evaluation of a nodular iron cast sleeve. The analysis of the component is based on crack initiation from cast defects and low-cycle fatigue. Fracture mechanics material parameters for Paris law, c and m, are extracted for the materials considered. In paper D design quality rules for nodular cast iron based on the Swedish standard SS 11 40 60 is presented. The quality rules regard cast defects in fatigue assessments and facilitate defect-based component design. In paper E, a finite element tool that is capable to predict and calculate 3D crack propagation for embedded cracks and defects is presented. The tool is an add-on for ANSYS finite element program. In paper F, closure equations for nodular cast iron are proposed in parallel to refined fracture mechanics material data. The paper includes crack propagation at different load ratios and in different microstructures.Summarized, the thesis composes a further development of the fatigue assessment of cast components. The central role of defects in fatigue is clarified and tools are provided for fracture mechanics evaluations of defects as well as for defect based design. The quality rules are also fit for application in manufacturing and for acceptance tests, hence covering the span from design to complete product. / QC 20100712 / Gjutdesign

En esta Tesis Doctoral se analiza el fenómeno de la pérdida de confinamiento en el plano de aplicación de la carga de componentes industriales y estructuras. El análisis ordinario propuesto por la mecánica de la fractura no tiene en cuenta este fenómeno, dando lugar a resultados que en muchos casos son excesivamente conservadores.Los orígenes de la pérdida de confinamiento son diversos, destacando la superficialidad del defecto, el predominio de cargas de tracción y el efecto entalla. Las distintas teorías existentes analizan estos fenómenos de forma independiente.Este trabajo propone un modelo global justificado teóricamente que analiza de forma conjunta las diferentes fuentes de pérdida de confinamiento. Con objeto de validarlo, se presentan los resultados del programa experimental realizado al efecto y, posteriormente, se comparan dichos resultados con las predicciones del modelo. / This Doctorate Thesis analyses the phenomenon of the loss of constraint on the load application plane on industrial components and structures. The ordinary assessments proposed by the Fracture Mechanics Theory do not take into account this phenomenon, giving many times overconservative results.Two clearly distinct sources of low constraint are distinguished: that caused by the predominance of tensile loads and shallow defects and that produced by the notch effect. Both types of loss of constraint have been analysed by several authors, but in a rather disjointed way and without a global focus that might allow cases in which both situations arise simultaneously to be analysed.This work proposes an overall constraint procedure, theoretically justified, that allows analysing situations where the different sources of constraint coexist. Also, the results of the validation experimental programme are presented and compared to the predictions of the model.

The continual deterioration of infrastructure has motivated researchers to look for new ways of repairing and monitoring existing structures. A particularly challenging problem confronting engineers in the revival of the infrastructure is the rehabilitation of reinforced concrete (RC) structures. Traditionally, the repair of RC beams has been achieved by bonding steel plates to the structure. Although this technique has proven to be reasonably effective, it has several distinct disadvantages such as susceptibility of the steel plates to corrode and the excessive weight of steel plates when used in long-span beams. Recently, there has been an emergence of structural engineering applications employing fibre reinforced polymer (FRP) composites as an alternative to steel plates. FRP composites are well known for their high strength- and stiffness-to-weight ratios, corrosion resistance, durability, and ease of application. Numerous studies have been conducted to prove the efficiency of bonding FRP on structural elements. In spite of this, industrial practitioners are still concerned about premature debonding of the plates before reaching the desired strength or ductility. Premature debonding initiates from the ends of the plate or from intermediate cracks (IC) in the concrete. While end initiated debonding and peeling mechanisms have been researched extensively, researchers have unanimously recognized the lack of data for the FRP-RC structural members subjected to IC debonding. The scarcity of data compiled exemplifies the need to develop more refined numerical analysis tools to reduce the high cost and significant time required to conduct full-scale physical testing. In this study, the results of a comprehensive numerical investigation are presented to assess the failure mechanisms caused by different types of flexural and shear crack distributions in RC beams strengthened with FRP composites. The model is based on damage mechanics modeling of concrete and a bilinear bond-slip relationship with softening behaviour to represent the FRP-concrete interfacial properties. A discrete crack approach was adopted to simulate crack propagation through a nonlinear fracture mechanics based finite element analysis to investigate the effects of crack spacing and interfacial parameters such as stiffness, local bond strength, and fracture energy on the initiation and propagation of the debonding and structural performance. Results from the analysis reveal that the debonding behaviour and load-carrying capacity are significantly influenced by interfacial fracture energy and crack spacing. The debonding propagation is mainly governed by mode II fracture mechanisms. The results provide an insight on the long-term behaviour of a repair system that is gaining widespread use and will be of interest to researchers and design engineers looking to successfully apply FRP products in civil engineering applications.

This thesis is investigating how the effective notch method can be used for fatigue assessment of welded joints.  The effective notch method is based on a finite element analysis where the joint is modeled with all notches fictitiously rounded with a radius of 1 mm. Analyses are performed on a cruciform fillet welded joint where parameters such as, load case, steel plate thickness and weld size, are varied. The achieved lifetime estimations are then compared to calculations with other fatigue assessment methods, linear elastic fracture mechanics and the nominal method. The goal is to draw conclusions about pros and cons of the effective notch method. The results are also compared to experimental fatigue tests performed on the same geometry. The results indicates that the effective notch method tends overestimating the lifetime, especially when the steel plate thickness is small. This leads to a non conservative method that is dangerous to use as guidance when designing. The estimations are though better when considering a toe crack then when considering a root crack. Due to a large scatter in experimental test results, it is hard to validate a fatigue assessment method in an absolute sense. That is also the case for the effective notch method, and more results from experimental fatigue tests are needed before the effective notch method can be fully used. For relative analysis, when variations of the same design needs to be compared, the effective notch can be a very powerful tool. This is because of the flexibility for different geometries that this method grants.

The continual deterioration of infrastructure has motivated researchers to look for new ways of repairing and monitoring existing structures. A particularly challenging problem confronting engineers in the revival of the infrastructure is the rehabilitation of reinforced concrete (RC) structures. Traditionally, the repair of RC beams has been achieved by bonding steel plates to the structure. Although this technique has proven to be reasonably effective, it has several distinct disadvantages such as susceptibility of the steel plates to corrode and the excessive weight of steel plates when used in long-span beams. Recently, there has been an emergence of structural engineering applications employing fibre reinforced polymer (FRP) composites as an alternative to steel plates. FRP composites are well known for their high strength- and stiffness-to-weight ratios, corrosion resistance, durability, and ease of application. Numerous studies have been conducted to prove the efficiency of bonding FRP on structural elements. In spite of this, industrial practitioners are still concerned about premature debonding of the plates before reaching the desired strength or ductility. Premature debonding initiates from the ends of the plate or from intermediate cracks (IC) in the concrete. While end initiated debonding and peeling mechanisms have been researched extensively, researchers have unanimously recognized the lack of data for the FRP-RC structural members subjected to IC debonding. The scarcity of data compiled exemplifies the need to develop more refined numerical analysis tools to reduce the high cost and significant time required to conduct full-scale physical testing. In this study, the results of a comprehensive numerical investigation are presented to assess the failure mechanisms caused by different types of flexural and shear crack distributions in RC beams strengthened with FRP composites. The model is based on damage mechanics modeling of concrete and a bilinear bond-slip relationship with softening behaviour to represent the FRP-concrete interfacial properties. A discrete crack approach was adopted to simulate crack propagation through a nonlinear fracture mechanics based finite element analysis to investigate the effects of crack spacing and interfacial parameters such as stiffness, local bond strength, and fracture energy on the initiation and propagation of the debonding and structural performance. Results from the analysis reveal that the debonding behaviour and load-carrying capacity are significantly influenced by interfacial fracture energy and crack spacing. The debonding propagation is mainly governed by mode II fracture mechanisms. The results provide an insight on the long-term behaviour of a repair system that is gaining widespread use and will be of interest to researchers and design engineers looking to successfully apply FRP products in civil engineering applications.

Ferroelectric materials continue to find increasing use in actuator, sensor and transducer design. Questions regarding lifetime and reliability remain a concern due to the inherent low fracture toughness and complex material behavior. The poling procedure required for use in actuator and sensing devices introduces anisotropy in elastic and dielectric coefficients as well as piezoelectric coupling between the mechanical and electrical fields. This introduces complex fracture behavior which necessitates advanced analytical techniques and fracture characterization. In this dissertation, fracture mechanics of ferroelectric materials is evaluated by employing different analytical techniques and experimental methodology. The theoretical work has focused on linear piezoelectric coupling that accounts for the influence of anisotropy and heterogeneity on fracture. A new orthotropic rescaling technique is presented that explicitly solves the anisotropic linear elastic piezoelectric crack problem in terms of material coefficients. The effects of heterogeneities on electric field induced microfracture are analyzed by implementing a crack at the edge of a heterogeneous piezoelectric inclusion. A positive, flaw-localized driving force is realized when permeable crack face boundary conditions are considered. The experimental portion of the work evaluates fracture behavior in the ferroelectric ceramic, lead zirconate titanate (PZT), and the ferroelectric relaxor single crystal PZN-4.5%PT. Relative humidity and electric boundary conditions are shown to have significant effects on crack kinetics in PZT. Fracture anisotropy in single crystal PZN-4.5%PT is characterized using the Single-Edge V-notch Beam (SEVNB) method and Vickers indentations. Scanning electron micrographs are used to determine the crack profile which leads to a prediction of crack tip toughness and local energy release rate. A weak cleavage plane is identified in the single crystal relaxor which contains a significantly lower toughness in comparison to the ferroelectric ceramic PZT.

A fracture mechanics protocol appropriate for small fibers (35 micron diameter) is presented, which allows for the determination of the strength limitations of high performance nylon 6,6 fibers. Specifically, linear elastic fracture mechanics (LEFM) techniques are employed in addition to elastic-plastic fracture mechanics (EPFM) theories to achieve this. We assume that a minute semi-elliptical flaw of an unknown size exists in the specimen, as a result of the detrimental effects of the manufacturing process (melt spinning). Next, we seek to propagate this flaw in a stable manner through an ancillary process such as high cycle or low cycle fatigue (load-unload). After propagation, uniaxial tensile experiments are performed on the fatigued samples, by which the crack growth eventually becomes catastrophic during the process. After performing scanning electron microscopy (SEM) techniques and reviewing fractography, we are able to determine the critical flaw size and ligament length that leads to unstable crack propagation. These results are substituted into the appropriate LEFM equations and are in close agreement with material properties for nylon 6,6. A discussion is provided that draws parallel to the topics discussed in the literature investigation and the experimental results of this study.

The scaling of specific cutting energy is studied when micro-cutting ductile metals. A unified framework for understanding the scaling in specific cutting energy is first presented by viewing the cutting force as a combination of constant, increasing, and decreasing force components, the independent variable being the uncut chip thickness. Then, an attempt is made to isolate the constant force component by performing high rake angle orthogonal cutting experiments on OFHC Copper. The data shows a trend towards a constant cutting force component as the rake angle is increased. In order to understand the source of this constant force component the chip-root is investigated. By quickly stopping the spindle at low cutting speeds, the chip is frozen and the chip-workpiece interface is examined in a scanning electron microscope. Evidence of ductile tearing ahead of the cutting tool is seen at low and high rake angles. At higher cutting speeds a quick-stop device is used to obtain chip-roots. These experiments also clearly indicate evidence of ductile fracture ahead of the cutting tool in both OFHC Copper and Al-2024 T3. To model the cutting process with ductile fracture leading to material separation the finite element method is used. The model is implemented in a commercial finite element software using the explicit formulation. Material separation is modeled via element failure. The model is then validated using the measured cutting and thrust forces and used to study the energy consumed in cutting. As the thickness of layer removed is reduced the energy consumed in material separation becomes important. Simulations also show that the stress state ahead of the tool is favorable for ductile fracture to occur. Ductile fracture in three locations in an interface zone at the chip root is seen while cutting with edge radius tool. A hypothesis is advanced wherein an element gets wrapped around the tool edge and is stretched in two directions leading to fracture. The numerical model is then used to study the difference in stress state and energy consumption between a sharp tool and a tool with a non-zero edge radius.