569 research outputs found

    An Object-Oriented Approach for Modeling and Simulation of Crack Growth in Cyclically Loaded Structures

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    We present an object-oriented modeling frame for simulating crack propagation due to cyclic loadings. Central to the approach is that the crack propagates when a user-defined propagation criterion is fulfilled, i.e., the crack propagation rate is not prescribed but predicted. The approach utilizes the commercial finite element software package ABAQUS and its associated Python based scripting interface. The crack propagation is simulated by a generalized node release technique. If the propagation criteria are satisfied in the end of a cycle, the crack is allowed to propagate. The incremental crack growth is inferred from an iterative investigation of the propagation criteria. The propagation criteria are user-defined, and can be based on any parameter or parameter set that can be obtained from the simulations. We illustrate the developed modeling frame by two benchmark problems, where the propagation criterion is based on the dissipated energy in the vicinity of the crack tip

    Assessing Plastically Dissipated Energy as a Condition for Fatigue Crack Growth

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    The suitability of using a proposed condition for simulating cyclic crack propagation in a numerical scheme is qualitatively investigated, employing the finite element method. The propagation criterion is based on a condition that relates the plastically dissipated energy to a critical value. In the finite element simulation scheme, the crack is allowed to propagate when the criterion is satisfied, and the crack propagates until the condition is no longer fulfilled. Experimentally, it is well established that a negative load ratio increases the crack propagation rate, whereas a tensile overload tends to decrease the crack propagation rate. By simulating these load conditions, we show that the proposed propagation criterion closely captures these rate changes

    Numerical Evaluation of Paris-Regime Crack Growth Rate Based on Plastically Dissipated Energy

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    The crack growth rate during cyclic loading is investigated via numerical simulations. The crack advancement is governed by a propagation criterion that relates the increment in plastically dissipated energy ahead of the crack tip to a critical value. Once this critical value is satisfied, crack propagation is modeled via a node release scheme. Thus, the crack growth rate is an output from the numerical simulation. The crack growth rate predicted by the proposed scheme is compared with published experimental crack growth data in the Paris-regime for selected metals. A good match is found between the experimentally observed crack growth rates and the numerically obtained results. The Paris coefficients are subsequently evaluated from the numerically obtained crack growth rates

    Progression in Non-Destructive Spallation Prediction and Elevated Temperature Mechanical Property Evaluation of Thermal Barrier Coating Systems by Use of a Spherical Micro-Indentation Method

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    The accumulation of carbon dioxide in the atmosphere continues to be an important ecological issue associated with global warming. The demand for improved efficiencies in energy conversion in recent years has led to the introduction of land-based gas turbines with significantly increased inlet temperatures. To accomplish this, nickel-based superalloys with protective thermal barrier coatings (TBC) are widely used as systems capable of extending the life and increasing firing temperatures of combustor and stationary turbine components. However, coating durability, thermal-fatigue and erratic spallation failure currently limit the continuous operation of turbine engines. Of the present ceramic coating materials used, yttria stabilized zirconia (YSZ) is most prevalent; its low thermal conductivity, high thermal expansion coefficient and outstanding mechanical strength make it ideal for use in TBC systems. However, residual stresses caused by coefficients of thermal expansion mismatches within the TBC system and unstable thermally-grown oxides (TGO) are considered the primary causes for its premature and erratic spallation failure. The development of new materials, coating technologies and evaluation techniques is required if enhanced efficiency is to be achieved. As a result, several non-destructive evaluation (NDE) techniques have been developed to address this problem yet few comprehensive studies have resulted in the development of true NDE techniques capable of predicting failure location prior to its occurrence.;In this research, a load-based micro-indentation method for NDE of TBCs exposed to thermal loads in air has been developed. Coating surface stiffness responses obtained through use of this technique have been found capable of assessing damage accumulation and macroscopic debonding failure sites following thermal exposure of TBC systems to elevated temperatures. Furthermore, microstructural analyses correlating these surface stiffness response to overall YSZ/TGO interface conditions indicate that high interfacial rumpling and non-uniform oxide growth leads to the development of both in-plane and out-of-plane residual stresses. As a result, areas displaying relative increases in surface stiffness response enable early detection of initial TBC spallation locations. Additionally, with the evolution of nanotechnologies, indentation testing techniques have become more common, yet their ability to evaluate material mechanical properties in harsh environments remains a challenge. Following a classical Hertzian contact mechanics approach, a micro-indentation technique that does not require system compliance calibration or the use of high precision depth sensors has been developed. The removal of these constraints has led to the development of both a portable and high-temperature micro-indentation system for TBC materials mechanical property evaluation up to 1000°C

    Implementation of a Plastically Dissipated Energy Criterion for Three Dimensional Modeling of Fatigue Crack Growth

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    Fatigue crack growth is simulated using three dimensional elastic-plastic finite element analysis. The crack extension per load cycle, da/dN, as well as crack front profile changes (crack tunneling) under cyclic loading is not specified as an input but evaluated based on a condition that relates plastically dissipated energy to a critical value. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone

    Implementation of a Plastically Dissipated Energy Criterion for Three Dimensional Modeling of Fatigue Crack Growth

    Get PDF
    Fatigue crack growth is simulated using three dimensional elastic-plastic finite element analysis. The crack extension per load cycle, da/dN, as well as crack front profile changes (crack tunneling) under cyclic loading is not specified as an input but evaluated based on a condition that relates plastically dissipated energy to a critical value. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone

    Micro-scale fatigue mechanisms in metals: an in situ high frequency experimental approach

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    More than half of all mechanical failures in engineering structures are classified as fatigue failures. Regardless of the constituting material's strength, the limiting factor in service-life is the amount of cyclic damage that the material can accumulate before fracture. Since metals are the material-of-choice for most structural applications, scientists have strived to gain a fundamental understanding of the mechanisms that lead to their failure under cyclic loading, which is necessary for the design of novel and superior alloys. The reason fatigue of metals is such a challenging problem is that different microstructural features are involved and the controlling mechanisms span many length and time scales. In particular, the dislocation plasticity that precedes micro-crack initiation and the nature of micro-crack growth remain poorly understood. Small-scale testing has recently emerged as a valuable methodology to understand micro-scale deformation. Over the past two decades, this method has made many breakthroughs in understanding the physics of dislocation plasticity. However, extending these methods to cyclic loading has been met with many difficulties. In this thesis, fundamental fatigue mechanisms in face-centered cubic metals at the micro-scale are explored. To achieve this, a novel high-frequency micro-fatigue experimental methodology is developed by combining aspects of electron microscopy, micro-mechanical testing, and acoustic emission detection. These experiments are designed to replicate stress states (e.g., bending and uniaxial) and cycle counts (>10^7) common in bulk fatigue experiments, although at a ~10 micron length scale. All experiments are performed in situ in a scanning electron microscope to acquire real-time observations of surface morphology changes that are not obtainable with bulk-scale fatigue experiments. Using this method, a transition from cyclic hardening to softening is found in nickel-base superalloys, which is attributed to the shearing of precipitates. Also, for the first time, persistent slip bands (PSB) were observed in pure nickel in micron-scale single crystals. However, the number of cycles to PSB nucleation was two orders of magnitude higher than those in bulk crystals, which was attributed to the high surface-to-volume ratio in the tested crystals. Additionally, the experiments have shown that PSBs nucleate locally and propagate in a way that current models in literature do not account for. A new mechanism-based model was proposed to capture these new observations. The PSBs were also found to promote micro-crack initiation in nickel and nickel-base superalloys. Finally, intermittent micro-crack propagation events and acoustic emissions associated with micro-plasticity were quantified and characterized statistically

    Hybrid Modeling of Offshore Platforms’ Stress-Deformed and Limit States Taking into Account Probabilistic Parameters

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    Offshore platforms should be referred to critically and strategically important objects of a technosphere due to technological and operational challenges, on the one hand, and the danger potential level, on the other hand. Environmental, social and economic losses occurred over several decades of accidents and disasters in unique Great Britain, Norwegian. The Russian and the USA platforms were evaluated in death of dozens of operators, destruction of platforms, environment contamination and hence in multi-bullion losses. All of these indicate insufficiency of currently taken engineering solutions, providing structure strength, operational life and safety. The scientific, design, expert and supervising organizations in Russia and in the world are developing and improving mathematical and physical methods, implementing the probabilistic formulations for accidents and disasters, risk assessment and risks reduction on offshore platforms. The solutions of the following problems are included: extension of the comprehensive computational and experimental strength, operational life and survivability analysis to the cases of nonroutine events, accidental and catastrophic conditions; numerical justification of modelling of critical elements, zones and points with the maximum tension, deformations and damages occurring under impacts of external extreme seismic, ice, wind, low temperature; implementation of comprehensive diagnostic methods for damage states evaluation within nonlinear and probabilistic fracture mechanics; and use of new structural design and technological systems for reduction of negative extreme impacts as well as emergency protection systems. The solution of the specified problems is illustrated by case studies of the Russian specialists for each life cycle stage of the platforms offshore Caspian and Kara Seas and Sea of Okhotsk

    Stochastic Simulation of Mudcrack Damage Formation in an Environmental Barrier Coating

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    The FEAMAC/CARES program, which integrates finite element analysis (FEA) with the MAC/GMC (Micromechanics Analysis Code with Generalized Method of Cells) and the CARES/Life (Ceramics Analysis and Reliability Evaluation of Structures / Life Prediction) programs, was used to simulate the formation of mudcracks during the cooling of a multilayered environmental barrier coating (EBC) deposited on a silicon carbide substrate. FEAMAC/CARES combines the MAC/GMC multiscale micromechanics analysis capability (primarily developed for composite materials) with the CARES/Life probabilistic multiaxial failure criteria (developed for brittle ceramic materials) and Abaqus (Dassault Systmes) FEA. In this report, elastic modulus reduction of randomly damaged finite elements was used to represent discrete cracking events. The use of many small-sized low-aspect-ratio elements enabled the formation of crack boundaries, leading to development of mudcrack-patterned damage. Finite element models of a disk-shaped three-dimensional specimen and a twodimensional model of a through-the-thickness cross section subjected to progressive cooling from 1,300 C to an ambient temperature of 23 C were made. Mudcrack damage in the coating resulted from the buildup of residual tensile stresses between the individual material constituents because of thermal expansion mismatches between coating layers and the substrate. A two-parameter Weibull distribution characterized the coating layer stochastic strength response and allowed the effect of the Weibull modulus on the formation of damage and crack segmentation lengths to be studied. The spontaneous initiation of cracking and crack coalescence resulted in progressively smaller mudcrack cells as cooling progressed, consistent with a fractal-behaved fracture pattern. Other failure modes such as delamination, and possibly spallation, could also be reproduced. The physical basis assumed and the heuristic approach employed, which involves a simple stochastic cellular automaton methodology to approximate the crack growth process, are described. The results ultimately show that a selforganizing mudcrack formation can derive from a Weibull distribution that is used to describe the stochastic strength response of the bulk brittle ceramic material layers of an EBC
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