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Articles 31 - 60 of 256
Full-Text Articles in Mechanical Engineering
Turbine Passage Vortex Response To Upstream Periodic Disturbances, Mitchell Lee Scott
Turbine Passage Vortex Response To Upstream Periodic Disturbances, Mitchell Lee Scott
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Flow through the turbine section of gas turbine engines is inherently unsteady due to a variety of factors, such as the relative motion of rotors and stators. In low pressure turbines, periodic wake passing has been shown to impact boundary layer separation, blade surface pressure distribution, and loss generation. The effect of periodic disturbances on the endwall flow is less understood. Endwall flow in a low-pressure turbine occurs in the boundary layer region of the flow through the blade passage where the blade attaches to the hub in the turbine. The response of an endwall vortical structure, the passage vortex, …
Accelerated Relaxation In Disordered Solids Under Cyclic Loading With Alternating Shear Orientation, Nikolai V. Priezjev
Accelerated Relaxation In Disordered Solids Under Cyclic Loading With Alternating Shear Orientation, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The effect of alternating shear orientation during cyclic loading on the relaxation dynamics in disordered solids is examined using molecular dynamics simulations. The model glass was initially prepared by rapid cooling from the liquid state and then subjected to cyclic shear along a single plane or periodically alternated in two or three dimensions. We showed that with increasing strain amplitude in the elastic range, the system is relocated to deeper energy minima. Remarkably, it was found that each additional alternation of the shear orientation in the deformation protocol brings the glass to lower energy states. The results of mechanical tests …
Aging And Rejuvenation During Elastostatic Loading Of Amorphous Alloys, Nikolai V. Priezjev
Aging And Rejuvenation During Elastostatic Loading Of Amorphous Alloys, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
Using molecular dynamics simulations, we investigate the effect of uniaxial elastostatic compression on the potential energy, structural relaxation, and mechanical properties of binary glasses. We consider the three-dimensional Kob-Andersen binary mixture, which was initially cooled from the liquid state to the glass phase with a slow rate at zero pressure. The glass was then loaded with a static stress at the annealing temperature during extended time intervals. It is found that the static stress below the yielding point induces large-scale plastic deformation and significant rejuvenation when the annealing temperature is smaller than a fraction of the glass transition temperature. By …
Atomistic Modeling Of Heat Treatment Processes For Tuning The Mechanical Properties Of Disordered Solids, Nikolai V. Priezjev
Atomistic Modeling Of Heat Treatment Processes For Tuning The Mechanical Properties Of Disordered Solids, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
We investigate the effect of a single heat treatment cycle on the potential energy states and mechanical properties of metallic glasses using molecular dynamics simulations. We consider the three-dimensional binary mixture, which was initially cooled with a computationally slow rate from the liquid state to the solid phase at a temperature well below the glass transition. It was found that a cycle of heating and cooling can relocate the glass to either rejuvenated or relaxed states, depending on the maximum temperature and the loading period. Thus, the lowest potential energy is attained after a cycle with the maximum temperature slightly …
The Potential Energy States And Mechanical Properties Of Thermally Cycled Binary Glasses, Nikolai V. Priezjev
The Potential Energy States And Mechanical Properties Of Thermally Cycled Binary Glasses, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The influence of repeated thermal cycling on mechanical properties, structural relaxation, and evolution of the potential energy in binary glasses is investigated using molecular dynamics simulations. We consider a binary mixture with strongly non-additive cross interactions, which is annealed across the glass transition with different cooling rates and then exposed to one thousand thermal cycles at constant pressure. We found that during the first few hundred transient cycles, the potential energy minima after each cycle gradually decrease and the structural relaxation proceeds via collective, irreversible displacements of atoms. With increasing cycle number, the amplitudes of the volume and potential energy …
The Influence Of Complex Thermal Treatment On Mechanical Properties Of Amorphous Materials, Nikolai V. Priezjev, Qing-Long Liu
The Influence Of Complex Thermal Treatment On Mechanical Properties Of Amorphous Materials, Nikolai V. Priezjev, Qing-Long Liu
Mechanical and Materials Engineering Faculty Publications
We study the effect of periodic, spatially uniform temperature variation on mechanical properties and structural relaxation of amorphous alloys using molecular dynamics simulations. The disordered material is modeled via a non-additive binary mixture, which is annealed from the liquid to the glassy state with various cooling rates and then either aged at constant temperature or subjected to thermal treatment. We found that in comparison to aged samples, thermal cycling with respect to a reference temperature of approximately half the glass transition temperature leads to more relaxed states with lower levels of potential energy. The largest energy decrease was observed for …
The Influence Of Periodic Shear On Structural Relaxation And Pore Redistribution In Binary Glasses, Nikolai V. Priezjev, Maxim A. Makeev
The Influence Of Periodic Shear On Structural Relaxation And Pore Redistribution In Binary Glasses, Nikolai V. Priezjev, Maxim A. Makeev
Mechanical and Materials Engineering Faculty Publications
The evolution of porous structure, potential energy and local density in binary glasses under oscillatory shear deformation is investigated using molecular dynamics simulations. The porous glasses were initially prepared via a rapid thermal quench from the liquid state across the glass transition and allowed to phase separate and solidify at constant volume, thus producing an extended porous network in an amorphous solid. We find that under periodic shear, the potential energy decreases over consecutive cycles due to gradual rearrangement of the glassy material, and the minimum of the potential energy after thousands of shear cycles is lower at larger strain …
The Effect Of Cryogenic Thermal Cycling On Aging, Rejuvenation, And Mechanical Properties Of Metallic Glasses, Nikolai V. Priezjev
The Effect Of Cryogenic Thermal Cycling On Aging, Rejuvenation, And Mechanical Properties Of Metallic Glasses, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The structural relaxation, potential energy states, and mechanical properties of a model glass subjected to thermal cycling are investigated using molecular dynamics simulations. We study a non-additive binary mixture which is annealed with different cooling rates from the liquid phase to a low temperature well below the glass transition. The thermal treatment is applied by repeatedly heating and cooling the system at constant pressure, thus temporarily inducing internal stresses upon thermal expansion. We find that poorly annealed glasses are relocated to progressively lower levels of potential energy over consecutive cycles, whereas well annealed glasses can be rejuvenated at sufficiently large …
Structural Transformations During Periodic Deformation Of Low-Porosity Amorphous Materials, Nikolai V. Priezjev, Maxim A. Makeev
Structural Transformations During Periodic Deformation Of Low-Porosity Amorphous Materials, Nikolai V. Priezjev, Maxim A. Makeev
Mechanical and Materials Engineering Faculty Publications
Atomistic simulations are employed to study structural evolution of pore ensembles in binary glasses under periodic shear deformation with varied amplitude. The consideration is given to porous systems in the limit of low porosity. The initial ensembles of pores are comprised of multiple pores with small sizes, which are approximately normally distributed. As periodic loading proceeds, the ensembles evolve into configurations with a few large-scale pores and significantly reduced number of small pores. These structural changes are reflected in the skewed shapes of the pore-size distribution functions and the appearance of a distinct peak at large length scales after hundreds …
Structural Relaxation Of Porous Glasses Due To Internal Stresses And Deformation Under Tensile Loading At Constant Pressure, Nikolai V. Priezjev, Maxim A. Makeev
Structural Relaxation Of Porous Glasses Due To Internal Stresses And Deformation Under Tensile Loading At Constant Pressure, Nikolai V. Priezjev, Maxim A. Makeev
Mechanical and Materials Engineering Faculty Publications
The time evolution of the pore size distributions and mechanical properties of amorphous solids at constant pressure is studied using molecular dynamics simulations. The porous glasses were initially prepared at constant volume conditions via a rapid thermal quench from the liquid state to the glassy region and allowing for simultaneous phase separation and material solidification. We found that at constant pressure and low temperature, the porous network becomes more compact and the glassy systems relocate to progressively lower levels of the potential energy. Although the elastic modulus and the average glass density both increase with the waiting time, their dependence …
Spatiotemporal Analysis Of Nonaffine Displacements In Disorderedsolids Sheared Across The Yielding Point, Nikolai V. Priezjev
Spatiotemporal Analysis Of Nonaffine Displacements In Disorderedsolids Sheared Across The Yielding Point, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The time evolution and spatial correlations of nonaffine displacements in deformed amorphous solids are investigated using molecular dynamics simulations. The three-dimensional model glass is represented via the binary mixture, which is slowly annealed well below the glass transition temperature and then sheared at a constant strain rate. It is shown that with increasing strain,the typical size of clusters of atoms with large nonaffine displacements increases, and these clusters remain spatially homogeneously distributed, until the yielding point when mobile atoms become localized within a system-spanning shear band. Furthermore, the yielding transition is associated with an abrupt change in the spatial correlation …
Shear Band Formation In Amorphous Materials Under Oscillatoryshear Deformation, Nikolai V. Priezjev
Shear Band Formation In Amorphous Materials Under Oscillatoryshear Deformation, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The effect of periodic shear on strain localization in disordered solids is investigated using molecular dynamics simulations. We consider a binary mixture of one million atoms annealed to a low temperature with different cooling rates and then subjected to oscillatory shear deformation with a strain amplitude slightly above the critical value. It is found that the yielding transition occurs during one cycle but the accumulation of irreversible displacements and initiation of the shear band proceed over larger number of cycles for more slowly annealed glasses. The spatial distribution and correlation function of nonaffine displacements reveal that their collective dynamics changes …
Design And Manipulation Of A Power-Generating System With High-Temperature Fuel Cells For Hypersonic Applications, Jack Randolph Chalker
Design And Manipulation Of A Power-Generating System With High-Temperature Fuel Cells For Hypersonic Applications, Jack Randolph Chalker
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Current hypersonic vehicles tend to be incapable of producing onboard power with traditional generators due to their use of supersonic combusting ramjets (scramjets). Because of this, they seek additional energy sources for supporting advanced electronics or other auxiliary power-dependent devices while requiring elaborate thermal management systems to combat temperatures exceeding 700ºC. The incorporation of Solid Oxide Fuel Cell (SOFCs) stacks is an efficient solution, capable of generating large quantities of power through the use of natural fuel sources at high temperatures. Developments in this thesis include the design, construction, and support of a system operating at hypersonic-environment conditions with a …
Design And Implementation Of Periodic Unsteadiness Generator For Turbine Secondary Flow Studies, Nathan James Fletcher
Design And Implementation Of Periodic Unsteadiness Generator For Turbine Secondary Flow Studies, Nathan James Fletcher
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A primary source of periodic unsteadiness in low-pressure turbines is the wakes shed from upstream blade rows due to the relative motion between adjacent stators and rotors. These periodic perturbations can affect boundary layer transition, secondary flow, and loss generation. In particular, for high-lift front-loaded blades, the secondary flowfield is characterized by strong three-dimensional vortical structures. It is important to understand how these flow features respond to periodic disturbances. A novel approach was taken to generate periodic unsteadiness which captures some of the physics of turbomachinery wakes. Using stationary pneumatic devices, pulsed jets were used to generate disturbances characterized by …
Effects Of Lubrication Starvation On Flash Temperature For Thermal Mixed Elastohydrodynamic Gear Contacts, Danielle D. Massé
Effects Of Lubrication Starvation On Flash Temperature For Thermal Mixed Elastohydrodynamic Gear Contacts, Danielle D. Massé
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Lubrication is provided to the gear trains in automotive and aerospace transmission systems to prevent mechanical contact through the formation of a full lubricant film, which in turn removes heat generated at the gear contact surfaces. When debris blocks the inlet nozzle, the flow of lubricant is restricted and mechanical components experience lubrication starvation. Under starved lubrication the temperatures of the contact surfaces become elevated which can lead to the formation of a weld between them, a catastrophic failure mode called scuffing. For spur gears, the occurrence of scuffing is due to high sliding in the vicinity of the root …
An Investigation On Spur Gear Rolling Contact Fatigue Crack Initiation And Crack Propagation Under Ehl Condition, Vignesh Dharmajan
An Investigation On Spur Gear Rolling Contact Fatigue Crack Initiation And Crack Propagation Under Ehl Condition, Vignesh Dharmajan
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Pitting is a rolling contact fatigue phenomenon commonly observed in mechanical rolling elements, such as gears and bearings. In case of gear contacts, pitting usually takes place in the dedendum region, where both sliding and contact load are high. In this study, a model is developed to predict surface breaking crack formation fatigue lives, including both nucleation and propagation stages, for spur gear contacts operating under mixed elastohydrodynamic lubrication (EHL) condition. The model utilizes a gear load distribution model for tooth contact Analysis. A mixed EHL formulation is implemented to evaluate the surface normal pressure and tangential shear, incorporating the …
Simulation, Experimentation, Control And Management Of A Novel Fuel Thermal System, Austin L. Tipton
Simulation, Experimentation, Control And Management Of A Novel Fuel Thermal System, Austin L. Tipton
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Modern aircraft experience increasing thermal loads from electronics, electromechanical actuators, and directed energy weapons. These aircraft also have a reduced ability to transfer thermal energy to the atmosphere due to the use of composite skins and a limited number of air intake ports. For aircraft that use fuel as a heatsink, these factors can cause the fuel at points of the system to exceed temperature limits, which can result in fuel coking, damage to subsystems, and even complete system failure. This thesis investigates the fuel thermal management shortcomings of contemporary aircraft systems and suggests a new methodology to extend performance. …
Aerodynamics Of Fan Blade Blending, Clint J. Knape
Aerodynamics Of Fan Blade Blending, Clint J. Knape
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Blending is a method of fan and compressor blade repair. The goal of the blending process is to remove stress concentration points such as cracks and nicks along the leading, trailing, or tip edges of the blade. The stressed areas are typically removed by grinding or cropping away the surrounding material. For integrally bladed rotor (IBR) disks, repairing a damaged blade is much more economical than replacing the entire disk. However, the change in shape of the blade will change the local aerodynamics and result in mistuning, both structurally and aerodynamically. In a worst case scenario, the change in the …
Analytical And Experimental Investigation Of Time-Variant Acceleration Fields, Justin A. Williams
Analytical And Experimental Investigation Of Time-Variant Acceleration Fields, Justin A. Williams
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Devices expected to operate in elevated or non-standard acceleration fields are often tested in similar conditions prior to deployment. Typically these tests only simulate steady-state acceleration fields in one direction. However, real acceleration fields often vary both directionally and temporally. Designing experiments to produce these conditions requires careful forethought and analysis in order to understand the emergent acceleration components that result from the methodology. An experiment was designed and executed on a horizontal centrifuge in which the radial acceleration varied sinusoidally between -10 < a_r < 10 g. Negative acceleration was achieved by rotating the test article relative to the radial acceleration vector using a servo motor. A model was developed that predicted the acceleration field at every point along the test article. The model provided important information such as the acceleration magnitude and direction anywhere on the test device at any point in time. This model was then used to optimize the velocity profile of the servo motor to minimize experimental artifacts.
Rasters Vs Contours For Thin Wall Ultem 9085 Fdm Applications, Vausman Kota
Rasters Vs Contours For Thin Wall Ultem 9085 Fdm Applications, Vausman Kota
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Currently many components are additively manufactured via fused deposition modeling (FDM). However, FDM results in gaps between passes which produces a poor surface finish and porous material that is difficult to hold pressure. Commercial scale air systems require a pressure to be maintained within thin walled components with minimal post processing and clean up after fabrication. A design of experiments (DOE) was created to identify the optimal raster vs contour ratio for UTLEM 9085 CG fabricated using FDM at different build angles and wall thicknesses. A custom-built pressurized test system was developed, the leak rates were calculated and the surfaces …
Adaptive Identification Of Classification Decision Boundary Of Turbine Blade Mode Shape Under Geometric Uncertainty, Ian M. Boyd
Adaptive Identification Of Classification Decision Boundary Of Turbine Blade Mode Shape Under Geometric Uncertainty, Ian M. Boyd
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Integrally Bladed Rotors (IBR) of aircraft turbine engines suffer from fluctuations in the dynamic response that occurs due to blade to blade geometric deviations. The Stochastic Approach for Blade and Rotor Emulation (SABRE) framework has been used to enable a probabilistic study of mistuned blades in which a reduced order modeling technique is applied in conjunction with sets of surrogate models, called emulators, to make predictions of mistuned mode shapes. SABRE has proven useful for non-switching mode shapes. However, switching mode shapes have non-stationary or discontinuous response surfaces which reduce the accuracy of the surrogate models used in SABRE. To …
Using Cfd To Improve Off-Design Throughflow Analysis, Troy J. Lanchman
Using Cfd To Improve Off-Design Throughflow Analysis, Troy J. Lanchman
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In turbomachinery design, complex internal flows give rise to significant losses and blockage whose effects are difficult to properly analyze without detailed computational fluid dynamics (CFD) methods or experiments. In a typical design method, CFD is used in conjunction with simpler throughflow or cascade codes to hasten the process. However, the lesser physical accuracy of the design codes demands the inclusion of models to improve the accuracy of the throughflow codes. This thesis aims to use CFD data to generate improved loss and blockage models for a 2D compressor throughflow code by matching throughflow data to CFD data using optimizations. …
Unsteady Effects Of A Pulsed Blowing System On An Endwall Vortex, Molly Hope Donovan
Unsteady Effects Of A Pulsed Blowing System On An Endwall Vortex, Molly Hope Donovan
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The low-pressure turbine is an important component of a gas turbine engine, powering the low-pressure spool which provides the bulk of the thrust in medium- and high-bypass engines. It is also a significant fraction of the engine weight and complexity as it can comprise up to a third of the total engine weight. One way to drastically reduce the weight of the low-pressure turbine is to utilize high lift blades. To advance high-lift technology, the Air Force Research Laboratory (AFRL) designed the L2F blade profile, which was implemented in the linear cascade at AFRL/RQT’s low speed wind tunnel facility. The …
Development Of A Computer Program For Transient Heat Transfer Coefficient Studies, Sri Prithvi Samrat Samayamantula
Development Of A Computer Program For Transient Heat Transfer Coefficient Studies, Sri Prithvi Samrat Samayamantula
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At the present time, the magnitude of transient convective heat transfer is approximated using heat transfer coefficient correlations developed for steady state conditions. This is done by necessity, as transient heat transfer correlations are not readily available. There is a rare transient heat transfer correlation found in the literature, but the number of correlations available can be counted on one hand. In addition, the literature does provide some plots of Nusselt numbers for specific cases of transient convective heat transfer, but these are limited to the specific case for which they were developed. The work presented in this thesis is …
Power/Thermal Interaction Within An Adaptive Turbine Engine, Andrew K. Desomma
Power/Thermal Interaction Within An Adaptive Turbine Engine, Andrew K. Desomma
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Usually power take off (PTO) with a two-spool turbofan engine has been accomplished via the high pressure (HP) shaft and bleed air from the high-pressure compressor (HPC). The PTO is used to run various aircraft components such as generators and hydraulic pumps, which also produce waste heat. To better understand the coupled transient nature of balancing engine thrust, power take off and thermal management, a transient variable cycle three stream turbofan engine model has been developed to investigate the integrated behavior. The model incorporates many dynamic features including a third-stream heat exchanger as a heat sink for thermal management and …
Structural Transformations In Porous Glasses Under Mechanical Loading. Ii. Compression, Nikolai V. Priezjev, Maxim A. Makeev
Structural Transformations In Porous Glasses Under Mechanical Loading. Ii. Compression, Nikolai V. Priezjev, Maxim A. Makeev
Mechanical and Materials Engineering Faculty Publications
The role of porous structure and glass density in response to compressive deformation of amorphous materials is investigated via molecular dynamics simulations. The disordered, porous structures were prepared by quenching a high-temperature binary mixture below the glass transition into the phase coexistence region. With decreasing average glass density, the pore morphology in quiescent samples varies from a random distribution of compact voids to a porous network embedded in a continuous glass phase. We find that during compressive loading at constant volume, the porous structure is linearly transformed in the elastic regime and the elastic modulus follows a power-law increase as …
Slow Relaxation Dynamics In Binary Glasses During Stress-Controlled, Tension-Compression Cyclic Loading, Nikolai V. Priezjev
Slow Relaxation Dynamics In Binary Glasses During Stress-Controlled, Tension-Compression Cyclic Loading, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
The effect of cyclic loading on relaxation dynamics and mechanical properties of metallic glasses is studied using molecular dynamics simulations. We consider the Kob-Andersen three-dimensional binary mixture rapidly cooled across the glass transition and subjected to thousands of tension-compression cycles in the elastic range. It was found that during cyclic loading at constant pressure, the system is relocated to progressively lower levels of the potential energy, thus promoting greater densification and higher strength. Furthermore, with increasing stress amplitude, the average glass density increases and the minimum of the potential energy becomes deeper, while the elastic modulus is reduced. The typical …
The Critical Pressure For Microfiltration Of Oil-In-Water Emulsions Using Slotted-Pore Membranes, Nikolai V. Priezjev, Tohid Darvishzadeh, Bishal Bhattarai
The Critical Pressure For Microfiltration Of Oil-In-Water Emulsions Using Slotted-Pore Membranes, Nikolai V. Priezjev, Tohid Darvishzadeh, Bishal Bhattarai
Mechanical and Materials Engineering Faculty Publications
The influence of geometrical parameters and fluid properties on the critical pressure of permeation of an oil micro-droplet into a slotted pore is studied numerically by solving the Navier-Stokes equations. We consider a long slotted pore, which is partially blocked by the oil droplet but allows a finite permeate flux. An analytical estimate of the critical permeation pressure is obtained from a force balance model that involves the drag force from the flow around the droplet and surface tension forces as well as the pressure variation inside the pore. It was found that numerical results for the critical pressure as …
Structural Transformations In Porous Glasses Under Mechanical Loading. I. Tension, Nikolai V. Priezjev, Maxim A. Makeev
Structural Transformations In Porous Glasses Under Mechanical Loading. I. Tension, Nikolai V. Priezjev, Maxim A. Makeev
Mechanical and Materials Engineering Faculty Publications
The evolution of porous structure and mechanical properties of binary glasses under tensile loading were examined using molecular dynamics simulations. We consider vitreous systems obtained in the process of phase separation after a rapid isochoric quench of a glass-forming liquid to a temperature below the glass transition. The porous structure in undeformed samples varies from a connected porous network to a random distribution of isolated pores upon increasing average glass density. We find that at small strain, the elastic modulus follows a power-law dependence on the average glass density and the pore size distribution remains nearly the same as in …
The Yielding Transition In Periodically Sheared Binary Glasses At Finite Temperature, Nikolai V. Priezjev
The Yielding Transition In Periodically Sheared Binary Glasses At Finite Temperature, Nikolai V. Priezjev
Mechanical and Materials Engineering Faculty Publications
Non-equilibrium molecular dynamics simulations are performed to investigate the dynamic behavior of three-dimensional binary glasses prepared via an instantaneous quench across the glass transition. We found that with increasing strain amplitude up to a critical value, the potential energy approaches lower minima in steady state, whereas the amplitude of shear stress oscillations becomes larger. Below the yielding transition, the storage modulus dominates the mechanical response, and the gradual decay of the potential energy over consecutive cycles is accompanied by reduction in size of transient clusters of atoms with large nonaffine displacements. In contrast, above the yield strain, the loss modulus …