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The two-dimensional stress fields induced by a set of infinitely many parallel edge dislocations are difficult to estimate as those of individual dislocations decay slowly. A simple numerical metho...

The microstructure of an Al-Cu-Li alloy sheet is characterised in the solution treated, underaged, and peak aged tempers. Its mechanical response under uniaxial tension is measured at 90 K , 173 K ...

A linear, periodic, three-dimensional shear-lag model of unidirectionally-reinforced composites that allows for fibre breakage, and matrix failure is proposed. Matrix failure can take the form of matrix splitting or interfacial debonding. A computationally efficient scheme for its solution is developed. This scheme exploits the translation invariance of the elastostatic fields due to failed elements in the periodic cell, and is asymptotically faster than the classical eigensolution-based approach. The new computational scheme is used to illustrate the influence of matrix failure on the elastostatic fields induced by small clusters of fibre breaks in several test problems. Monte Carlo simulations of fracture in model three-dimensional composite specimen with Weibull-distributed fibre segment strengths are also performed. Matrix failure is found to considerably alter fracture development, to weaken the median specimen, and to reduce the variability in composite strength.

Mechanical properties at ambient and cryogenic temperatures of Al-Cu-Li alloy are required for design and fabrication of liquid hydrogen and liquid oxygen tanks of satellite launch vehicles. In the present work, bead-on-sheet, friction stir welding was carried out with three different rotation speeds. The yield and strain hardening behaviors of the welds were evaluated in temperature range of 20 K to 298 K. Both yield stress and strain hardening ability in the specimen increased with decrease in testing temperature. The dependence of yield stress on temperature was modeled on the basis of thermally activated dislocation mobility, while that of strain hardening was modeled on the temperature dependence of dynamic recovery rate parameter. The recovery parameter followed an Arrhenius-type relationship with temperature. The model parameters determined from the experimental data were further used to simulate the stress–strain curves at different sub-zero temperatures for the friction stir welds.

A phenomenological hardening model based on the extended Voce law is proposed to capture the plastic flow of a third generation aluminium-lithium alloy. The model includes a simple precipitation law, which accounts for pre-ageing plastic deformation, and a hardening law that accounts for hardening of the matrix, and for the interaction of matrix glide dislocations with anisotropic and isotropic precipitates. Flow stress evolution in solution treated, underaged, and peak-aged samples is measured through uniaxial tensile tests on specimens cut at 0∘, 45∘, and 90∘ to the rolling direction. The measured flow stress evolution in all the tempers is captured well by the model. Model parameters offer insights into the sub-structural evolution that accompanies plastic deformation.

Monte-Carlo simulations of the fracture of elastic unidirectional model fibre composites are an important tool to understand composite reliability. On account of being computationally intensive, fracture simulations reported in the literature have been limited to simulation patches comprised of a few thousand fibres. While these limited patch sizes suffice to capture the dominant failure event when the fibre strength variability is low (synthetic fibres), they suffer from edge effects when the fibre strength variability is high (natural fibres). On the basis of recent algorithmic developments based on Fourier acceleration, a novel bisection based Monte Carlo failure simulation algorithm is presently proposed. This algorithm is used to obtain empirical strength distributions for model composites comprised of up to $$2^{20} \approx 10^6$$ fibres, and spanning a wide range of fibre strength variabilities. These simulations yield empirical weakest-link strength distributions well into the lower tail. A stochastic model is proposed for the weakest-link event. The strength distribution predicted by this model fits the empirical distributions for any fibre strength variability.

Inverse jet flame (IJF) configuration is a variant of coaxial nonpremixed jet flame which can produce a nonluminous compact blue flame with independent control of air and fuel jet momentum. In the present work, the effect of central air jet velocity on visible topological features such as, visible flame height and base flame height is investigated experimentally. Also the fluid dynamics behind the evolution of base flame in IJF configuration is unraveled in this study. Notably, buoyancy induced oscillations of IJF, which is relatively unexplored in open literature is studied in this work. Moreover, a systematic classification of IJF based on the evolution of its visible flame features with variation in the central air jet and annular fuel jet velocities is reported. In addition, a semi-empirical correlation for visible flame height of IJF with air-fuel momentum ratio is arrived in the present work which can be useful in the design of IJF based burners utilized for impingement heat transfer applications.

The evolution of the yield surface, as predicted by a polycrystal plasticity model of a face-centered cubic material, is studied. Grains in the model polycrystal are endowed with a classical hardening law, which accounts for interaction among the slip system through a hardening matrix. In the literature, the elements of the hardening matrix are assumed non-negative. In the present work, the effect of negative elements in the hardening matrix on the evolution of the yield function, particularly, during monotonic tensile and shear deformation, is systematically studied. In particular, it is shown that certain parametric values simulate a substantial kinematic hardening, similar to experimental observations. The greatest kinematic hardening is obtained when the latent hardening ratio of the reverse slip systems is taken to be −1.2.

The transverse fracture of model unidirectional composite specimen, comprising up to 2^{20} fibers with random strengths, is studied using Monte Carlo simulations. The load sharing from broken to intact fibers is assumed to obey power-law scaling ∼r^{-γ} with distance r from the fiber break. Fiber breaks are assumed to interact in order to remain traction free. The pattern of fiber breaks that propagate catastrophically is interpreted through cluster analysis. The empirical strength distributions obtained from the simulations are interpreted using two probabilistic models of brittle fracture available in the literature. These point to a transition from the brittle to the tough fracture mode as γ↓2. The transitional γ is approximately equal to that reported in the literature for noninteracting fiber breaks.

Tube ballooning of axially confined tubes under increasing thermal loading is studied using a finite element model in an austenitic stainless steel, alloy D9. The model accounts for thermal, plastic and creep deformation of the tube. It is shown that (i) axial restrains can trigger the onset of plasticity, and (ii) activation of plasticity during the thermomechanical loading can significantly reduce the time for the development of large ballooning strains.

A shear-lag model of hybrid materials is developed. The model represents an alternating arrangement of two types of aligned linear elastic fibres, embedded in a linear elastic matrix. Fibre and matrix elements are taken to fail deterministically when the axial and shear stresses in them reach their respective strengths. An efficient solution procedure for determining the stress state for arbitrary configurations of broken fibre and matrix elements is developed. Starting with a single fibre break, this procedure is used to simulate progressive fibre and matrix failure, up to composite fracture. The effect of (1) the ratio of fibre stiffnesses, and (2) the ratio of the fibre tensile strength to matrix shear strength, on the composite failure mechanism, fracture energy, and failure strain is characterised. Experimental observations, reported in the literature, of the fracture behaviour of two hybrid materials, viz., hybrid unidirectional composites, and double network hydrogels, are discussed in the framework of the present model.

The strength of metal matrix composites shows wide scatter on account of variability in the strengths of individual fibres. The relationship between the strength distribution of the fibres, and that of the composite is also affected by the non-linear matrix and fibre/matrix interfacial responses. The present study aims to describe the strength distribution of 2D and 3D commercial Ti/SiC composites. This is accomplished by performing Monte Carlo failure simulations of these composites, comprised of up to 128 fibres. A detailed deformation theory based model, developed and validated against experimental data in previous work, is used to calculate load redistribution in the course of each simulation. The empirical composite strength distribution obtained from the simulations follows weakest-link scaling. A stochastic model for the clustered propagation of fibre breaks, akin to a model proposed for polymer matrix composites in the literature, captures the empirical weakest-link strength distribution. A scaling relationship is derived between the composite strength and composite size for a number of reliability levels.

A detailed study concerning a novel, dynamic control strategy for mitigating thermoacoustic instability in a swirl-stabilized lean premixed, laboratory combustor configuration is presented in this paper. The mitigation strategy is realized by rotating the otherwise static swirler, which is primarily meant for stabilizing the lean premixed flame. The proposed strategy is tested over a range of bulk flow velocities, mixture equivalence ratios, and swirler rotation rates for validating the robustness of this concept. A prominent reduction in the fundamental acoustic mode amplitude by about 25dB is observed with this control technique for the cases that are studied. The physical mechanism responsible for the instability mitigation due to the rotating swirler is investigated by observing the distinct changes associated with the reacting flowfield using particle image velocimetry. An attempt is made to probe into the self-excited flame dynamics using high-speed intensified, chemiluminescence imaging and identifying the instability driving source locations from a spatial Rayleigh indices map. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate strongly positive Rayleigh indices regions (and eventually the acoustic energy source) to render quiet instability mitigated swirling flames.

Monte Carlo simulations of the failure of unidirectional fibre composites typically require numerous evaluations of the stress-state in partially damaged composite patches. In a simulated composite patch comprised of N fibres, of which $$N_b$$ fibres are broken in a common cross-sectional plane transverse to the fibre direction, the stress overloads in the intact fibres are given by the weighted superposition of the unit break solutions associated with each of the breaks. Determining the weights involves solving $$N_b$$ linear equations, and determining overloads in the intact fibres requires matrix-vector multiplication. These operations require $$O(N_b^3)$$ , and $$O(N N_b)$$ floating point operations, respectively. These costs become prohibitive for large N, and $$N_b$$ ; they limit Monte Carlo failure simulations to composite patches of only a few thousand fibres. In the present work, a fast algorithm to determine the overloads in a partially damaged composite, requiring $$O( N_b^{1/3} N \log N)$$ floating point operations, is proposed. This algorithm is based on the discrete Fourier transform. The efficiency of the proposed method derives from the computational simplicity of weighted superposition in Fourier space. Computations of the stress state ahead of large circular clusters of breaks in composite patches comprised of about one million fibres are used to demonstrate the efficiency of the proposed algorithm.

Parameters of five popular continuum damage models are fit to match their creep rate and time to rupture predictions with that of a validated micro-mechanisms based model at a high nominal stress for an austenitic stainless steel. Their predictions are then compared with that of the micro-mechanisms based model at lower stress levels. The creep-strain rate and time to failure predictions of the model due to Wen et al. (Eng Fract Mech 98:169–184, 2013) best agrees with that of the micro-mechanisms based model in the regime of dominance of creep deformation processes. At still lower stress levels, where cavitation-rate is determined by diffusion processes, the Wen et al. model predictions of creep lifetimes become excessively non-conservative. A correction based on a formula due to Cocks and Ashby (Prog Mater Sci 27:189–244, 1982) has been proposed for this regime.

The classical Chin-Hosford-Mendorf (CHM) criterion for deformation twinning is used widely in polycrystal plasticity computer simulations. It assumes that twin nuclei are abundant, and that twin propagation and growth control the realisation of deformation twins. However, recent experimental studies reported in the literature have found that nominally unfavourable twin systems are activated in grains. This has been attributed to twin nucleation in some of the unfavourable twinning systems with low Schmid factors, and its suppression in some of the nominally more favourable ones. Presently, this explanation is quantitatively examined. Full and relaxed constraint versions of the Taylor, ALAMEL, and binary tree based models, all implementing the CHM criterion are used to simulate uniaxial compression of a zirconium billet. Model predictions are compared with experimentally measured twinning statistics reported in the literature for a Zr polycrystal. The ALAMEL and binary tree models, which explicitly represent intergranular interactions, are found to capture the twinning statistics well. These observations suggest that the CHM criterion is adequate to capture twinning in Zr, provided intergranular interactions are represented in the model used to interpret the experiments.

A shear-lag and deformation-theory based model for a metal matrix composite reinforced by continuous unidirectional fibres is proposed. The model accounts for fibre and matrix cracking, matrix plasticity, and fibre-matrix interfacial sliding through seven characteristic non-dimensional parameters, which combine geometric, phase and interface properties. It allows arbitrary tensile loading and unloading history along the fibre direction, and predicts the history-dependent elastoplastic displacement, strain, and stress fields in all the fibre and matrix elements. Broken elements may be present initially, or form during the imposed loading history. Non-linear one-dimensional governing differential and algebraic equations are formulated on the basis of the model. A computationally fast solution methodology based on pseudospectral collocation is implemented. The present model is employed to predict the elastic strain profiles in a Ti/SiC composite tape near pre-existing breaks. These predictions agree well with experimental measurements reported in the literature.

The stress state in a shear-lag model of a unidirectional fiber composite with an arbitrary configuration of fiber breaks is obtained by the weighted superposition of the stress state due to a single broken fiber. In a periodic patch comprised of N fibers located at the points of a regular lattice, a method to determine the stress state due to a single break was proposed by Landis et al. (J Mech Phys Solids 48(3):621–648, 2000). This method entails the determination of the eigenspace of an $$N\times N$$ matrix, at a computational cost of $$O(N^3)$$ . In the present work, an alternative algorithm is proposed. This algorithm exploits the circulant structure of the matrix describing the inter-fiber interactions. The asymptotic computational complexity of the present algorithm equals that of the discrete Fourier transform: $${O}(N \log N)$$ . Run times of the present method with the eigensolution based method are compared, and shown to be very favorable for the present method, even for small N. Power-law scaling of the overloads due to a single break to much larger distances than previously possible has been verified using the present method.

Pilgering is a complex incremental manufacturing process for seamless tubes. In this work, a miniature physical simulator for pilgering was designed and fabricated. This miniature simulator employs a grooved roll-die and a mandrel and can impose controlled reductions in both tube diameter and wall thickness. Pilgering deformation over a range of ratios of reductions in wall thickness and in tube diameter, known as the Q-factor, was imposed on hemi-cylindrical zirconium alloy specimens. The influence of the Q-factor on the microstructure and deformation texture of the deformed specimens was quantified. A polycrystal plasticity calculation based on the binary tree model was used to simulate texture evolution during the simulated pilgering process. The computer model quantitatively captured the variation with Q of the Kearns factors, as measured in the physically simulated specimen. The small differences noticed between the predicted and experimental final textures point to unaccounted transverse components of the flow field. These observations suggest that physical and/or computer simulations can form the basis of a rapid methodology for tool selection to realize prescribed post-pilgering textures. (C) 2016 Elsevier B.V. All rights reserved.

The evolution of mechanical and damage fields, and the time to failure of material points ahead of a stationary crack in a compact tension specimen are computed using finite element simulations for a linear elastic/power law creeping material. These are compared with predictions obtained from fields based on two fracture mechanics based load-parameters: the steady-state $$C^*$$ , and the time-corrected C(t). The finite element calculations predict opening stresses in the crack plane that are non-monotonic in the time interval $$0 \le t \le t_1$$ , where $$t_1$$ denotes the time to transition from small-scale creep to extensive creep. This is in contradiction to the monotonic ‘self-similar’ decay of stress with time given by the C(t) field. Consequently, damage rates and times to failure of material points ahead of a crack are calculated using the finite element stress-field, and the C(t)-based stress-field diverge considerably. These observations suggest that the creep damage rates derived on the basis of self-similarly decaying opening stress fields may be severely inaccurate.

This paper is concerned with the experimental investigation of swirling inverse jet flame (IJF) characteristics such as visible flame appearance, visible flameheight, temperature distribution, post flame emissions and flame noise in a recessed coaxial burner. Furthermore, variation in lean blowout limits of swirling IJF with aerodynamic swirl number is also reported. Based on the observation of visible appearance of swirling IJF with aerodynamic swirl number, two flame regimes are identified. Besides this, semi-empirical correlations for flame height and lean blowout limits are attempted in this work which can be used for the design and development of future burner systems employing swirling inverse jet flame. Apart from this, an attempt has been made to understand the flow features and flame structure of swirling IJF through numerical simulations. The swirling inverse jet flame configuration studied in this work is capable of establishing compact blue flame with better thermal and emission characteristics and hence can be employed in thermal applications.

Although high-temperature material response is known to be history-dependent, many models of creep crack growth assume the history-independent Norton constitutive law. Even so, these models capture the experimentally observed creep crack growth by adjusting only the damage model. This is explained presently by showing that the damage evolution ahead of a stationary crack in a material obeying a history-dependent unified creep-plasticity constitutive law due to Robinson can be 'fit' by simply adjusting the damage parameters in a model implementing Norton's law. The implication of this result to the case of propagating cracks is discussed. � 2015 Elsevier Ltd.

A minimum plastic power principle is proposed for a rigid-viscoplastic crystalline domain subdivided into two sets of lath-shaped regions, called bands. The lattice orientation in each band is assumed uniform and to differ infinitesimally from that in the other band. The proposed minimum principle yields the slip activity in the bands and semi-analytical expressions for the misorientation axis and orientation of band boundaries. These band boundary characteristics are predicted for f.c.c. lattice orientations near the ideal rolling texture components. Surprisingly, it found that the predicted band boundary characteristics closely match those of microstructural features called cell block boundaries reported in the experimental literature, except when the dislocations of activated slip systems are known to interact very strongly. This suggests that except when precluded by strong dislocation interactions, continuum extremum principles may also govern microstructural characteristics. � 2015 Elsevier Ltd. All rights reserved.

The present work focuses on the dynamic blowout sensing of compressed natural gas (CNG) turbulent inverse jet flame (IJF) based on the temporal CH ∗ chemiluminescence signal acquired from the flame zone. The time varying CH ∗ chemiluminescence signal exhibits lower fluctuation level for stable IJF. However, higher level of fluctuation in the temporal CH ∗ intensity is observed as the IJF approaches blowout. In addition, the characterization of CH ∗ chemiluminescence signature from IJF based on histogram and power spectral density are performed for delineating the stable and unstable IJF. Furthermore, a statistical parameter called normalized root mean square (NRMS), computed from the statistical analysis of CH ∗ intensity signal is found to be effective in predicting the onset of blowout in CNG IJF. Apart from this, a methodology for sensing the proximity of blowout in CNG IJF is proposed based on NRMS which can be helpful in averting the unexpected blowout in IJF based combustion systems.

This paper reports the stability limits and near blowout characteristics of unrecessed and recessed compressed natural gas (CNG) inverse jet flames (IJFs) by varying the central air jet velocity for a constant fuel jet velocity. The experimental results indicate that unrecessed IJF has higher flame stability as compared to the recessed case for the same fuel jet velocity. In order to understand the reason for the enhanced flame stability in unrecessed IJF as compared to the recessed IJF, the visible appearance of these two inverse jet flame configurations are examined for understanding the various transient events taking place towards blowout. In the case of unrecessed IJF, initiation of local extinction and reignition phenomena on the flame surface ushers the onset of blowout. In contrast, this phenomenon is not observed in recessed IJF, rather the flame gets partially lifted from the burner rim prior to blowout. The enhanced flame stability of unrecessed IJF is attributed to the presence of base flame near the burner rim that acts as a pilot flame. On the other hand, the absence of base flame in recessed IJF is the reason for its lower flame stability limit as compared to the unrecessed IJF. Based on the nature of blowout events, blowout mechanisms for both unrecessed and recessed IJFs are proposed in the present work.

The development of microstructure and crystallographic texture with effective strain at three through-thickness locations (near rolls, center, and near mandrel) in a partly pilgered Zircaloy-4 tube is described. Pilgering is found to eliminate through-thickness variation in grain size in the starting hot-extruded material and to generate location-dependent asymmetries in crystallographic texture. Deformation texture development during pilgering is modeled with polycrystal plasticity by idealizing the metal flow pattern as axisymmetric flow through a convergent channel. Good qualitative comparison of the predicted and experimental post-pilgering textures is obtained, provided location-dependent transverse shear component is superposed on the gross flow field, and localized deformation at grain boundaries is allowed. Frictional forces between tube and die are deduced from these observations. (C) The Minerals, Metals & Materials Society and ASM International 2015

Components such as bladed rings, and bladed disks fabiricated out of titanium matrix composites were extensively explored in the two decades since about 1990 as light weight replacements for conventional superalloy blades and disks in the intermediate hot stages of gas turbines. One of the challenges, which has hindered their adoption is the relative unreliability of the composite components; nominally identical Ti composite specimen display a much larger variability in strength than their superalloy counterparts. In the present work, we have quantified the reliability of Ti matrix composites by developing a detailed micromechanical-statistical model of their failure. The micromechanical model resolves fibres, matrix, and the interface, and accounts for such failure modes as fibre breakage, matrix cracking, matrix plasticity, interfacial sliding, and debonding. It also accounts for mechanical interaction between these various failure modes. The mechanical model’s predictions are validated against synchotron X-ray measurements reported in the literature, both after loading, and unloading. Using the detailed micromechanical model, Ti matrix composite was simulated following a Monte Carlo framework. These simulations yield the empirical strength distribution of the Ti matrix composite, and insights into the dominant failure mode. The latter allows the construction of a stochastic model of composite failure. The stochastic model can be used to determine safe working loads as a function of composite size for any desired reliability level.

Monte Carlo simulations of the failure of unidirectional fiber composites in a plane transverse to the fiber direction are performed on much larger patches than in previous works, assuming a realistic load redistribution scheme from broken to intact fibers. Computational effort involved in these simulations is substantially reduced using an algorithm based on the quadtree data structure. The empirical strength distribution obtained from the simulations has a weak-link character, regardless of the variability in fiber strengths. The empirical strength distribution is well captured by a probabilistic model based on the growth of a tight cluster of fiber breaks. It is also well captured by regarding composite patch failure as the failure of the weakest equal load-sharing bundle of a certain size, following Curtin [Phys. Rev. Lett. 80, 1445 (1998)PRLTAO0031-900710.1103/PhysRevLett.80.1445]. The approximate coincidence of these two predictions identifies the dominant failure mechanism underlying Curtin's empirical scaling relationship.

Friction stir welds (FSWs) can be considered as an ensemble of elements of material with composite microstructures connected in series. In the present study, bead-on-plate FSW runs were made on an Al–Cu–Li alloy with varying rotation speeds ranging from 400 to 800 rpm. Microstructure of the FSW region was investigated by using optical microscope, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). Thermal stability of various precipitates was evaluated with differential scanning calorimetry (DSC) measurements. Strength variation across FSW cross sections was mapped by microhardness measurements. Average as well as local mechanical properties were evaluated using a digital image correlation (DIC) technique. Irrespective of the process parameters, FSW samples showed similar tensile and strain hardening behaviour along with serrations in stress-strain curves while local strength values showed increasing trend with rotation speed. The FSW alloy produced at intermediate rotation speed exhibited different mechanical behavior and is correlated with the resultant substantial changes in the microstructure. Strain localization occurred at the boundary of nugget zone and thermo-mechanically affected zone which led to failure of the FSW tensile specimens within weld regions. Fractography investigation revealed that the failure is initiation controlled, that is void nucleation at coarse precipitate-matrix interfaces.

Grains in f.c.c. polycrystals deform non-uniformly even under imposed homogeneous deformation and subdivide into domains of different lattice orientations. Intense non-uniformity of grain deformation produces substructural features called deformation bands and shear bands, wherein large deviations from the average lattice orientation and/or slip localization occur. Using a model of grain banding, subdivision of pure copper grains initially oriented along the �

Fully recrystallized commercial Zirconium plates were subjected to uniaxial tension. Tests were conducted at different temperatures (123 K - 623 K) and along two plate directions. Both directions were nominally unfavorable for deformation twinning. The effect of the working temperature on crystallographic texture and in-grain misorientation development was insignificant. However, systematic variation in work hardening and in the area fraction and morphology of deformation twins was observed with temperature. At all temperatures, twinning was associated with significant near boundary mesoscopic shear, suggesting a possible linkage with twin nucleation. A binary tree based model of the polycrystal, which explicitly accounts for grain boundary accommodation and implements the phenomenological extended Voce hardening law, was implemented. This model could capture the measured stress-strain response and twin volume fractions accurately. Interestingly, slip and twin system hardness evolution permitted multiplicative decomposition into temperature-dependent, and accumulated strain dependent parts. Furthermore, under conditions of relatively limited deformation twinning, the work hardening of the slip and twin systems followed two phenomenological laws proposed in the literature for non-twinning single-phase face centered cubic materials. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A model capable of representing the evolution rate of various damage mechanisms pertinent to creep rupture of austenitic stainless steels is utilized to study the role of grain boundary sliding on the damage evolution in the form of cavitational damage at grain boundary facets and wedge cracking at triple lines and on the creep lifetime of a standard creep specimen. Reduced grain boundary sliding reduces the rate of damage evolution and hence prolongs creep lifetime at higher stresses. However, creep lifetime at lower stresses is controlled by diffusional cavitation and remains unaffected by grain boundary sliding.

Effects of the recession of the central air jet on the visible flame height, necking zone, and luminosity of a turbulent compressed natural gas-air inverse diffusion flame in a coaxial burner are investigated in this experimental study. The inner circular tube of the coaxial burner is recessed by 0.25da, 0.5da, and 1.0da, where da is the central tube inner diameter. From the visual observation, the flame height and the necking zone height are observed to decrease exponentially with the air jet Reynolds number with no recession of the central air jet. However, only a marginal reduction in the visible flame height is observed with an increase in the recession height of the air jet as compared to the necking zone height. Interestingly, the necking zone at the flame base disappears beyond the critical recession height of the central jet. Moreover, the recession is found to be effective in eradicating the fuel rich zone and soot ring at the flame base of turbulent compressed natural gas inverse diffusion flame at lower air jet Reynolds numbers.

A rigid-plastic rate-independent crystal plasticity model capable of capturing banding in single crystals subjected to homogeneous macroscopic deformation is proposed. This model treats the single crystal as a ‘stack of domains’. Individual domains deform homogeneously while maintaining velocity and traction continuity with their neighbors. All the domains collectively accommodate the imposed deformation. The model predicts lattice orientation evolution, slip distribution, strain localization and band orientation in copper single crystals with imposed plane strain deformation. In quantitative agreement with experimental observations reported in the literature, macroscopic shear banding and regular deformation banding are predicted in initially copper and rotated cube oriented single crystals, respectively, while banding is not predicted in initially Goss oriented single crystals. The model does not, however, predict the experimentally observed orientation of smaller scale dislocation boundaries such as dense dislocation walls.

The orientation and disorientation of extended cell block boundaries that separate cell blocks in model rate-independent grains accommodating imposed plastic deformation by the mechanism of slip is predicted on the basis of the following three hypotheses: (1) a uniform state of stress prevails throughout the grain; (2) cell blocks are disoriented so as to minimize the power of plastic deformation; and (3) cell block boundaries are oriented so as to minimize plastic incompatibility between neighboring cell blocks. Predicted orientations and disorientations compare favorably with those reported in the experimental literature for copper and aluminum polycrystals deformed plastically in uniaxial tension. This suggests that the assumed hypotheses may represent the physical principles that determine the preferred cell block boundary orientation.

Grains in f.c.c. polycrystals that accommodate imposed deformation purely byslip processes develop a multi-scale dislocation substructure that evolves with deformation.When the polycrystal is subjected to rolling deformation or to channel-die compression, oneof the elements of this substructure, called cell block boundaries, are widely reported to alignparallel to the transverse direction and close to the macroscopic plane of maximum shear. Thisobservation is explained based on standard rate-independent crystal plasticity augmented bythree hypotheses.

A novel ‘stack’ model of a rate-independent polycrystal, which extends the ‘ALAMEL’ model of Van Houtte et al. (2005) is proposed. In the ‘stack’ model, stacks of N neighboring ‘ALAMEL’ domains collectively accommodate the imposed macroscopic deformation while deforming such that velocity and traction continuity with their neighbors is maintained. The flow law and consistency conditions are derived and an efficient solution methodology based on the linear programming technique is given. The present model is applied to study plastic deformation of an idealized two-dimensional polycrystal under macroscopically imposed plane-strain tension and simple shear constraints. Qualitative and quantitative variations in the predicted macroscopic and microscopic response with N are presented. The constraint on individual ‘ALAMEL’ domains diminishes with stack size N but saturates for large N. Computational effort associated with the present model is analyzed and found to be well within one order of magnitude greater than that required to solve the classical Taylor model. Furthermore, implementation of the consistency conditions is found to reduce computation time by at least 50%.

A comparative study of the turbulent Inverse Diffusion Flame (IDF) in recessed coaxial and backstep burners is carried out, based on visible flame appearance, flame length, flame stability, centerline temperature distribution, centerline oxygen concentration, and NO x emissions. The backstep burner is observed to produce a compact flame shape with less luminosity at a higher air-fuel velocity ratio, as compared to the coaxial burner. Moreover, slightly better thermal characteristics and marginal reduction in NO x emissions are provided by the backstep IDF, as compared to the recessed coaxial IDF. Besides this, the centerline oxygen concentration is marginally increased in the backstep IDF due to higher entrainment of ambient air. Interestingly, a lower flame stability limit is seen in the backstep burner than in the coaxial IDF, which can be attributed to its enhanced fuel-air mixing.

The present experimental study characterizes the turbulent LPG Inverse Diffusion Flame (IDF) stabilized in a backstep burner in terms of visible flame length, dual flame structure, centerline temperature distribution, and oxygen concentration. The visible flame length for a fixed fuel jet velocity is found to reduce with increase in air jet velocity. Besides this, the effect of air and fuel jet velocities on visible flame length is interpreted using a new parameter, Global Momentum Ratio (GMR). Interestingly, GMR seems to be correlating well with the visible flame length for the air and fuel velocity ranges considered in the present study. Moreover, the dual flame structure of IDF is identified with the help of CH-chemiluminescence signature. The existence of dual flame structure of IDF is confirmed further with the centerline temperature and oxygen concentration measurements.

A model of a rigid-plastic rate-independent polycrystalline aggregate wherein sub-aggregates are represented as the nodes of a binary tree is proposed. The lowest nodes of the binary tree represent grains. Higher binary tree nodes represent increasingly larger sub-aggregates of grains, culminating with the root of the tree, which represents the entire polycrystalline aggregate. Planar interfaces are assumed to separate the sub-aggregates represented by nodes in the binary tree. Equivalence between the governing equations of the model and a standard linear program is established. The objective function of the linear program is given by the plastic power associated with polycrystal deformation and the linear constraints are given by compatibility requirements between the sub-aggregates represented by sibling nodes in the binary tree. The deviatoric part of the Cauchy stress in each sub-aggregate is deduced as linear combinations of the Lagrange multipliers associated with the constraints. It is shown that the present model allows a generalization of Taylor’s principle to polycrystals. The proposed model is applied to simulate tensile, compressive, torsional, and plane-strain deformation of copper polycrystals. The predicted macroscopic response is in good agreement with published experimental data. The effect of the initial distribution of the planar interfaces separating the sub-aggregates represented by the binary tree on the predicted mechanical response in tension, compression and torsion is studied. Also, the role of constraints relaxation in simulations of plane strain compression is investigated in detail.

A hierarchical model of a polycrystalline aggregate of rigid viscoplastic grains is formulated, and a robust and efficient computational algorithm for its solution is proposed. The polycrystalline aggregate is modeled as a binary tree. The leaves of the binary tree represent grains, and higher tree nodes represent increasingly larger sub-aggregates of grains. The root of the tree represents the entire polycrystalline aggregate. Velocity and traction continuity are enforced across the interface between the children of each non-leaf node in the binary tree. The hierarchical model explicitly models intergranular interactions but is nevertheless comparable in computational effort to the mean field models of polycrystal plasticity. Simulations of tensile, compressive, torsional, and plane strain deformation of copper lead to predictions in good agreement with experiments, and highlight the interconnection between grain deformations and intergranular constraints. It is inferred from the results that a hybrid mean field/hierarchical model represents a computationally efficient methodology to simulate polycrystal deformation while accounting for intergranular interactions.

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, Hp, to visible flame length, Hf, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINOX happens to occur closer to stoichiometric overall equivalence ratio.

The present disclosure relates to a method and apparatus to mitigate thermo-acoustic instabilities in combustor of gas turbine engines using a lean premixed flame; and provides a dynamic control strategy for mitigating thermo-acoustic instability in a swirl stabilized, lean premixed combustor by rotating the otherwise static swirler meant for stabilizing the lean premixed flame. The swirler is subjected to a controlled rotation for imparting increased turbulence intensity and higher tangential momentum to the premixed reactants towards mitigating thermo-acoustic instability. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate periodic interactions found during a particular phase of the instability cycle on account of periodic collision of diverging flame base with the flame segment above the dump plane in the combustor. This prevents flame-flame interactions and emergence of strongly positive Rayleigh indices which contribute as sources of acoustic energy to drive the self-excited instability.

Monte Carlo simulations and probabilistic modeling are employed to understand the strength distribution of a planar bundle of local load-sharing fibers. The fibers are distributed randomly within a unit square according to a Poisson process, and the fiber strengths are Weibull distributed with exponent ?. Monte Carlo failure simulations of bundles comprised of up to 105 fibers suggests that the bundle strength distribution obeys weakest-link scaling for all ?. Also, a probabilistic model of the weakest-link event is proposed. This model introduces a failure event at a size scale between that of the fiber and that of the bundle, whose failure statistics follows that of equal load-sharing bundles. The weakest-link event is modelled as the growth of a tight cluster of these equal load-sharing bundles. The size of the equal load-sharing bundles increases with decreasing ?. The simulated bundle strength distributions and those predicted by the model are compared, and excellent agreement is obtained. � 2015 American Physical Society.

A theory of rigid-plastic single-crystal deformation based on the hypothesis that the crystal follows a mode of deformation that minimizes the plastic power is developed. In addition to the usual homogeneous slip modes, plastic power minimization in the present theory also extends over inhomogeneous deformation modes in the form of deformation bands or shear bands. Bands are assumed to originate from unstable perturbations of the lattice orientation. The evolution of the banded substructure with continued deformation depends on the mobility of the interband dislocation boundaries. The theory predicts the evolution of substructural morphology and lattice orientation with deformation. These predictions agree quantitatively with experimental observations in initially ( 1 1 2 ) / [ 1 1 1 ¯ ] - and (0 0 1)/[1 1 0]-oriented copper single crystals that undergo shear banding and deformation banding, respectively, when subjected to plane strain deformation.

The evolution of in situ elastic strain with cyclic tensile loading in each phase of a single Al 2O3-fiber/aluminum-matrix composite was studied using neutron diffraction (ND). An analytical model appropriate for metal matrix composites (MMCs) was developed to connect the measured axial strain evolution in each phase with the possible micromechanical events that could occur during loading at room temperature: fiber fracture, interfacial slipping, and matrix plastic deformation. Model interpretation showed that the elastic strain evolution in the fiber and matrix was governed by fiber fracture and interface slipping and not by plastic deformation of the matrix, whereas the macroscopic stress–strain response of the composite was influenced by all three. The combined single-fiber composite model and ND experiment introduces a new and quick engineering approach for qualifying the micromechanical response in MMCs due to cyclic loading and fiber fracture. 12 13 14 15 16 17 18 19

In addition to texture, plastic anisotropy of a polycrystalline fcc metal stems from the directional nature of the dislocation substructure within individual grains. This produces the marked work hardening or softening observed immediately following load path changes. Following the framework of Peeters et al., in bcc steel, we develop a dislocation substructure evolution-based stage III hardening model for copper, capable of capturing the constitutive response under load path changes. The present model accounts for the more complicated substructure geometry in fcc metals than in bcc. Using an optimization algorithm, the parameters governing substructure evolution in the model are fit to experimental stress-strain curves obtained during compression along the three orthogonal directions in samples previously rolled to various reductions. These experiments approximate monotonic, reverse, and cross-load paths. With parameters suitably chosen, the substructure model, embedded into a self-consistent polycrystal plasticity model, is able to reproduce the measured flow stress response of copper during load path change experiments. The sensitivity of the parameters to the assumed substructure geometry and their uniqueness are also discussed.