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Material Failure (material + failure)
Selected AbstractsFailure Mechanism of Deformed Concrete Tunnels Subject to Diagonally Concentrated LoadsCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 6 2009Wei He Based on the experimental findings, an extended discussion is carried out to select a rational compressive model for concrete that represents the dominant failure modes of deformed concrete tunnels. Three main dominant final failure modes are described: structural failure due to the plastic rotation of softening hinges, tensile failure caused by localized cracks, and material failure due to concrete deterioration. A parametric analysis of the material properties of concrete shows that the compressive strength of concrete has a dominant effect on the load-carrying capacity, although the compressive fracture energy of concrete remarkably influences the post-peak deformation behavior of the tunnel. Moreover, the soil pressure, which is regarded as a distributed external load, plays an important role in controlling the final failure modes and the deformation behavior of concrete tunnels. The size effect on the load-carrying capacities of different-sized concrete tunnels is also discussed based on the numerical simulations. [source] Modelling microseismicity of a producing reservoir from coupled fluid-flow and geomechanical simulationGEOPHYSICAL PROSPECTING, Issue 5 2010D.A. Angus ABSTRACT In this paper, we investigate production induced microseismicity based on modelling material failure from coupled fluid-flow and geomechanical simulation. The model is a graben style reservoir characterized by two normal faults subdividing a sandstone reservoir into three compartments. The results are analysed in terms of spatial and temporal variations in distribution of material failure. We observe that material failure and hence potentially microseismicity is sensitive to not only fault movement but also fluid movement across faults. For sealing faults, failure is confined to the volume in and around the well compartment, with shear failure localized along the boundaries of the compartment and shear-enhanced compaction failure widespread throughout the reservoir compartment. For non-sealing faults, failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden. All shear-enhanced compaction failures are localized within the reservoir compartments. Fault movement leads to an increase in shear-enhanced compaction events within the reservoir as well as shear events located within the side-burden adjacent to the fault. We also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic. The shear-enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centred about 106 Pa, whereas the shear events have pseudo scalar seismic moments that vary over three orders of magnitude. Overall, the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring. [source] Comparison of defects in ProTaper hand-operated and engine-driven instruments after clinical useINTERNATIONAL ENDODONTIC JOURNAL, Issue 3 2007G. S. P. Cheung Abstract Aim, To compare the type of defects and mode of material failure of engine-driven and hand-operated ProTaper instruments after clinical use. Methodology, A total of 401 hand-operated and 325 engine-driven ProTaper instruments were discarded from an endodontic clinic over 17 months. Those that had fractured were examined for plastic deformation in lateral view and remounted for fractographical examination in scanning electron microscope. The mode of fracture was classified as ,fatigue' or ,shear' failure. The lengths of fractured segments in both instruments were recorded. Any distortion in hand instrument was noted. Data were analysed using chi-square, Fisher's exact or Student's t -test, where appropriate. Results, Approximately 14% of all discarded hand-operated instruments and 14% of engine-driven instruments were fractured. About 62% of hand instruments failed because of shear fracture, compared with approximately 66% of engine-driven instruments as a result of fatigue (P < 0.05). Approximately 16% of hand instruments were affected by shear, and either remained intact or was fractured, compared with 5% of engine-driven instruments (P < 0.05). The length of the broken fragment was significantly shorter in hand versus engine-driven group (P < 0.05). Approximately 7% of hand instruments were discarded intact but distorted (rarely for engine-driven instruments); all were in the form of unscrewing of the flutes. The location of defects in hand Finishing instruments was significantly closer to the tip than that for Shaping instruments (P < 0.05). Conclusions, Under the conditions of this study (possibly high usage), the failure mode of ProTaper engine-driven and hand-operated instruments appeared to be different, with shear failure being more prevalent in the latter. [source] Comparison between cohesive zone modelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2004K. Y. Volokh Cohesive zone models (CZMs) are widely used for numerical simulation of the fracture process. Cohesive zones are surfaces of discontinuities where displacements jump. A specific constitutive law relating the displacement jumps and proper tractions defines the cohesive zone model. Within the cohesive zone approach crack nucleation, propagation, and arrest are a natural outcome of the theory. The latter is in contrast to the traditional approach of fracture mechanics where stress analysis is separated from a description of the actual process of material failure. The common wisdom says that only cohesive strength,the maximum stress on the traction,separation curve,and the separation work,the area under the traction,separation curve,are important in setting a CZM while the shape of the traction,separation curve is subsidiary. It is shown in our note that this rule may not be correct and a specific shape of the cohesive zone model can significantly affect results of the fracture analysis. For this purpose four different cohesive zone models,bilinear, parabolic, sinusoidal, and exponential,are compared by using a block-peel test, which allows for simple analytical solutions. Numerical performance of the cohesive zone models is considered. It appears that the convergence properties of nonlinear finite element analyses are similar for all four CZMs in the case of the block-peel test. Copyright © 2004 John Wiley & Sons, Ltd. [source] A lumped mass numerical model for cellular materials deformed by impactINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 11 2001Z. H. Tu Abstract When impacted by a relatively rigid body, cellular materials undergo severe deformation and extensive material failure. However, such behaviour may not be well described using traditional numerical approaches such as the finite element method. This paper presents a lumped mass numerical model which can accommodate high degrees of deformation and material failure. The essence of this model is to discretize a block of material into contiguous element volumes, each represented by a mass point. Interactions between a node and its neighbours are accounted for by defining ,connections' that represent their interfaces which transmit stresses. Strains at a node are calculated from the co-ordinates of the surrounding nodes; these also determine the stresses on the interfaces. The governing equations for the entire solution domain are then converted into a system of equations of motion with nodal positions as unknowns. Failure criteria and possible combinations of ,connection' breakage are incorporated to model the occurrence of damage. A practical contact algorithm is also developed to describe the contact interactions between cellular materials and rigid bodies. Simulations for normal and oblique impacts of rigid rectangular, cylindrical and wedge-tipped impactors on crushable foam blocks are presented to substantiate the validity of the model. The generally good correlation between the numerical and experimental results demonstrates that the proposed numerical approach is able to model the impact response of the crushable foam. However, some limitations in modelling crack propagation in oblique impacts by a rigid impactor on foam blocks are observed. Copyright © 2001 John Wiley & Sons, Ltd. [source] Phase Transition and Failure at High Temperature of Bismuth-Layered Piezoelectric CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2006Liaoying Zheng High-temperature bismuth-layered piezoelectric ceramics (CaxSr1,x)Bi4Ti4O15 have been prepared by the conventional solid reaction method. Our results reveal that there is a "phase transition induced by the composition" taking place in the solid solution at around x=0.4. During transition, the crystal structure changes from orthorhombic to pseudo-tetragonal and then back to orthorhombic. Although all (CaxSr1,x)Bi4Ti4O15 (with any x value) undergo a second-order ferroelectric-to-ferroelectric phase transition at about 200°C below their Curie temperature, only the ceramic with x=0.4 exhibits a failure in piezoelectric properties during the transition. It is suggested that, on the basis of the X-ray photoelectron spectroscopy results, this material failure is induced by the high concentration of oxygen vacancies in the material during the phase transition. [source] Verarbeitung hochfester Aluminiumlegierungen durch umformende VerfahrenMATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 7 2009R. Neugebauer ECAP; rolling; extrusion; incremental forming Abstract Basierend auf experimentell ermittelten Werkstoffkennwerten wird ein Vergleich des Umformverhaltens der beiden Aluminiumlegierungen im Anlieferungszustand (EN AW-7075 T651) sowie im modifizierten Zustand (EN AW-7075 ECAP) bei der Verarbeitung mit herkömmlichen Umformverfahren (Walzen, Fließpressen) angestellt. Zudem werden die Verfahrensgrenzen des Umformgrades durch Variation der Werkzeuggeometrien experimentell lokalisiert. Dabei ist ebenfalls Untersuchungsgegenstand, den Einfluss des Verfahrens auf das Umformvermögen zu ermitteln. Davon ausgehend werden Werkzeugkonzepte entwickelt, die eine Umformung des hochfesten Aluminiums unter Erzeugung von hydrostatischen Druckzuständen in der Umformzone ermöglichen. Sowohl die Simulation als auch die Versuche zeigten Materialversagen der ultrafeinkörnigen Werkstoffe beim Fließpressen. Das Walzen als inkrementelles Umformverfahren ermöglichte hingegen eine schadensfreie Herstellung der Werkstücke. Forming of high-strength aluminium alloys A comparison of the forming behavior of both aluminum alloys in as-received condition (EN AW-7075 T651) as well as modified condition (EN AW-7075 ECAP) when processed with conventional forming processes (rolling, extrusion) is conducted on the base of experimentally determined material characteristics. In the following the process limits of the true strain are located by variegating the tool geometry. The influence of the manufacturing method on the plasticity is also a subject matter to analysis. Based upon the outcome of this analysis special tool conceptions are being developed, which allow the forming of highest-strength aluminum while creating hydrostatical pressure states in the deformed zone. Both simulation and experiments showed material failure of the ultra-fine-grained materials when extruded whereas rolling, being the incremental forming process, allowed damage-free manufacturing of components. [source] Aerated autoclaved concrete: Stochastic structure model and elastic propertiesPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Ilja Kadaschewitsch Aerated autoclaved concrete (AAC) is a modern and important construction material, whose elastic properties are primarily defined by its porosity. The possibility to predict elastic properties of AAC based on the voids distribution is very important. The report describes simulations of the mechanical properties of AAC, based on a stochastic-geometric model of its structure. The model is the well-known "cherry-pit" model, which presents a random system of partially overlapping spheres. In the mechanical analysis the solid phase is approximated by a network model with the help of the so-called radical tessellation with respect to the hard spheres of the "cherry-pit" model. The network edges are modelled in ANSYS as 3D beams. In this approach, the discretized elements (the edges) have in distinction to FE calculations with small polyhedral same dimension as the air voids and so the numerical costs can be drastically reduced. The FE simulations calculate the elastic constants and energy concentrations, which are responsible for the material failures, in large samples. Comparisons with fracture tests showed good matching between simulations and experiments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||