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A Simple Numerical Method Of Cycle Jumps For Cyclically Loaded Structures, D. Cojocaru, Anette M. Karlsson
A Simple Numerical Method Of Cycle Jumps For Cyclically Loaded Structures, D. Cojocaru, Anette M. Karlsson
Mechanical Engineering Faculty Publications
A method for accelerated numerical simulations of structures subjected to cyclic loading is investigated. Of particular interest is a class of structures where the structural properties evolve with time. The proposed method is based on conducting detailed finite element analysis for a set of cycles to establish a trend line, extrapolating the trend line spanning many cycles, and use the extrapolated state as initial state for additional FEA simulations. This includes a control function that automatically monitors the length of the cycle jump to ensure a realistic solution. We compare the proposed method to a reference calculation, where all incremental …
Mechanical Response Of Fuel Cell Membranes Subjected To A Hygro-Thermal Cycle, Ahmet Kusoglu, Anette M. Karlsson, Michael H. Santare, Simon Cleghorn, William B. Johnson
Mechanical Response Of Fuel Cell Membranes Subjected To A Hygro-Thermal Cycle, Ahmet Kusoglu, Anette M. Karlsson, Michael H. Santare, Simon Cleghorn, William B. Johnson
Mechanical Engineering Faculty Publications
The mechanical response of fuel cell proton exchange membranes subjected to a single hygro-thermal duty cycle in a fuel cell assembly is investigated through numerical means. To this end, the behavior of the membrane with temperature and humidity dependent material properties is simulated under temperature and humidity loading and unloading conditions. The stress-evolution during a simplified operating cycle is determined using finite element analysis for two clamping methods and two alignments of the bipolar plates. It is shown that compressive, plastic deformation occurs during the hygro-thermal loading, resulting in tensile residual stresses after unloading. These residual in-plane stresses in the …
Evolution Of Surface Morphology Of Thermo-Mechanically Cycled Nicocraly Bond Coats, J. Shi, Anette M. Karlsson, B. Baufeld, M. Bartsch
Evolution Of Surface Morphology Of Thermo-Mechanically Cycled Nicocraly Bond Coats, J. Shi, Anette M. Karlsson, B. Baufeld, M. Bartsch
Mechanical Engineering Faculty Publications
We investigate morphological surface instabilities on bond coat surfaces of thermal barrier coatings, induced due to thermo-mechanical loading. Experimental results of hollow circular cylindrical specimens, consisting of a directionally solidified superalloy (IN 100 DS) coated with a NiCoCrAlY bond coat, show that the morphological instabilities are strongly dependent on the load conditions. In particular, the morphological instabilities develop during thermal cycling with a thermal gradient over the cylinder wall, whereas the surface remains smooth for thermal cyclic conditions without a gradient. Furthermore, if a cyclic, axial tensile force is applied (synchronized with the thermal cycling), the morphological instabilities become aligned …
Task-Driven Multi-Formation Control For Coordinated Uav/Ugv Isr Missions, Herbert G. Tanner
Task-Driven Multi-Formation Control For Coordinated Uav/Ugv Isr Missions, Herbert G. Tanner
Mechanical Engineering Faculty Publications
The report describes the development of a theoretical framework for coordination and control of combined teams of UAVs and UGVs for coordinated ISR missions. We consider the mission as a composition of an ordered sequence of subtasks, each to be performed by a different team. We design continuous cooperative controllers that enable each team to perform a given subtask and we develop a discrete strategy for interleaving the action of teams on different subtasks. The overall multi-agent coordination architecture is captured by a hybrid automaton, stability is studied using Lyapunov tools, and performance is evaluated through numerical simulations.
Transient Response Of Rotor On Rolling-Element Bearings With Clearance, David P. Fleming, Brian T. Murphy, Jerzy T. Sawicki, J. V. Poplawski
Transient Response Of Rotor On Rolling-Element Bearings With Clearance, David P. Fleming, Brian T. Murphy, Jerzy T. Sawicki, J. V. Poplawski
Mechanical Engineering Faculty Publications
Internal clearance in rolling element bearings is usually present to allow for radial and axial growth of the rotor-bearing system and to accommodate bearing fit-up. The presence of this clearance also introduces a “dead band” into the load-deflection behavior of the bearing. Previous studies demonstrated that the presence of dead band clearance might have a significant effect on synchronous rotor response. In this work, the authors investigate transient response of a rotor supported on rolling element bearings with internal clearance. In addition, the stiffness of the bearings varies nonlinearly with bearing deflection and with speed. Bearing properties were accurately calculated …
An Initial Non-Equilibrium Porous-Media Model For Cfd Simulation Of Stirling Regenerators, Roy C. Tew, Terry Simon, David Gedeon, Mounir B. Ibrahim, Wei Rong
An Initial Non-Equilibrium Porous-Media Model For Cfd Simulation Of Stirling Regenerators, Roy C. Tew, Terry Simon, David Gedeon, Mounir B. Ibrahim, Wei Rong
Mechanical Engineering Faculty Publications
The objective of this paper is to define empirical parameters for an initial thermal non-equilibrium porous-media model for use in Computational Fluid Dynamics (CFD) codes for simulation of Stirling regenerators. The two codes currently used at Glenn Research Center for Stirling modeling are Fluent and CFD-ACE. The codes’ porous-media models are equilibrium models, which assume solid matrix and fluid are in thermal equilibrium. This is believed to be a poor assumption for Stirling regenerators; Stirling 1–D regenerator models, used in Stirling design, use non-equilibrium regenerator models and suggest regenerator matrix and gas average temperatures can differ by several degrees at …
Obtaining Mode Mixity For A Bimaterial Interface Crack Using The Virtual Crack Closure Technique, A. Agrawal, Anette M. Karlsson
Obtaining Mode Mixity For A Bimaterial Interface Crack Using The Virtual Crack Closure Technique, A. Agrawal, Anette M. Karlsson
Mechanical Engineering Faculty Publications
We review, unify and extend work pertaining to evaluating mode mixity of interfacial fracture utilizing the virtual crack closure technique (VCCT). From the VCCT, components of the strain energy release rate (SERR) are obtained using the forces and displacements near the crack tip corresponding to the opening and sliding contributions. Unfortunately, these components depend on the crack extension size, Δ, used in the VCCT. It follows that a mode mixity based upon these components also will depend on the crack extension size. However, the components of the strain energy release rate can be used for determining the complex stress intensity …
Modeling Dynamics And Exploring Control Of A Single-Wheeled Dynamically Stable Mobile Robot With Arms, Eric M. Schearer
Modeling Dynamics And Exploring Control Of A Single-Wheeled Dynamically Stable Mobile Robot With Arms, Eric M. Schearer
Mechanical Engineering Faculty Publications
This paper focuses on simulations of a dynamically stable mobile robot (Ballbot) with arms. The simulations are of Ballbot lifting its arms in various directions. A PD arm controller works independently of an LQR-designed balancing/station keeping controller. The PD controller drives the arms to follow desired trajectories. When the arms are raised, Ballbot assumes a leaning equilibrium (the physical equilibrium) as opposed to the standing equilibrium (body stands totally upright - a predefined desired equilibrium) that the LQR drives toward. The conflict between these two equilibria causes the robot to lose its balance when lifting heavy (10 kg) loads. A …
An Initial Non-Equilibrium Porous-Media Model For Cfd Simulation Of Stirling Regenerators, Roy Tew, Terry Simon, David Gedeon, Mounir B. Ibrahim, Wei Rong
An Initial Non-Equilibrium Porous-Media Model For Cfd Simulation Of Stirling Regenerators, Roy Tew, Terry Simon, David Gedeon, Mounir B. Ibrahim, Wei Rong
Mechanical Engineering Faculty Publications
The objective of this paper is to define empirical parameters (or closwre models) for an initial thermai non-equilibrium porous-media model for use in Computational Fluid Dynamics (CFD) codes for simulation of Stirling regenerators. The two CFD codes currently being used at Glenn Research Center (GRC) for Stirling engine modeling are Fluent and CFD-ACE. The porous-media models available in each of these codes are equilibrium models, which assmne that the solid matrix and the fluid are in thermal equilibrium at each spatial location within the porous medium. This is believed to be a poor assumption for the oscillating-flow environment within Stirling …
An Experimental Investigation Of Humidity And Temperature Effects On The Mechanical Properties Of Persfluorosulfonic Acid Membrane, Y. L. Tang, Anette M. Karlsson, Michael H. Santare, Michael Gilbert, Simon Cleghorn, William B. Johnson
An Experimental Investigation Of Humidity And Temperature Effects On The Mechanical Properties Of Persfluorosulfonic Acid Membrane, Y. L. Tang, Anette M. Karlsson, Michael H. Santare, Michael Gilbert, Simon Cleghorn, William B. Johnson
Mechanical Engineering Faculty Publications
The mechanical properties of a perfluorosulfonic acid (PFSA) membrane have been investigated at different humidities and temperatures in a custom-designed environmental chamber. Tensile tests were conducted to determine Young’s modulus, the proportional limit stress (“yield strength”), break stress, and break strain. In-plane dimensional changes of the membrane at different temperature and humidities were also determined. The results indicate that Young’s modulus and the proportional limit stress of the PFSA membrane decrease as humidity and temperature increase. Higher temperature leads to lower break stress and higher break strain. However, humidity has little effect on the break stress and break strain. A …
Determination Of Uniaxial Residual Stress And Mechanical Properties By Instrumented Indentation, Manhong Zhao, Xi Chen, Jin Yan, Anette M. Karlsson
Determination Of Uniaxial Residual Stress And Mechanical Properties By Instrumented Indentation, Manhong Zhao, Xi Chen, Jin Yan, Anette M. Karlsson
Mechanical Engineering Faculty Publications
We propose an improved technique to determine the uniaxial residual stress, elastic modulus, and yield stress of a linear elastic, perfectly plastic bulk material from the force–displacement curve of one conical indentation test. Explicit relationships between the indentation loading–unloading parameters, material properties, and residual stress are established through extensive finite element analyses. Good agreement is found between the input material parameters used in numerical indentation tests and the properties identified from the reverse analysis, with an error of less than 10% in most cases. The technique is applied to a nanoindentation experiment on the crosssection of a thermal barrier system, …
Stresses In Proton Exchange Membranes Due To Hygro-Thermal Loading, Yaliang Tang, Michael H. Santare, Anette M. Karlsson, Simon Cleghorn, William B. Johnson
Stresses In Proton Exchange Membranes Due To Hygro-Thermal Loading, Yaliang Tang, Michael H. Santare, Anette M. Karlsson, Simon Cleghorn, William B. Johnson
Mechanical Engineering Faculty Publications
Durability of the proton exchange membrane (PEM) is a major technical barrier to the commercial viability of polymer electrolyte membrane fuel cells (PEMFC) for stationary and transportation applications. In order to reach Department of Energy objectives for automotive PEMFCs, an operating design lifetime of at least 5000 h over a broad temperature range is required. Reaching these lifetimes is an extremely difficult technical challenge. Though good progress has been made in recent years, there are still issues that need to be addressed to assure successful, economically viable, long-term operation of PEM fuel cells. Fuel cell lifetime is currently limited by …
Thermodynamically Consistent Relations Involving Plasticity, Internal Energy And Thermal Effects, Howard Schreyer, Paul Maudlin
Thermodynamically Consistent Relations Involving Plasticity, Internal Energy And Thermal Effects, Howard Schreyer, Paul Maudlin
Mechanical Engineering Faculty Publications
Experimental data associated with plastic deformations indicate that the temperature is less than that predicted from dissipation based on plastic work. To obtain reasonable correlation between theoretical and experimental results, the plastic work is often multiplied by a constant β. This paper provides an alternative thermodynamic framework in which it is proposed that there is an additional internal energy associated with dislocation pile-up or increase in dislocation density. The form of this internal energy follows from experimental data that relates flow stress to dislocation density and to equivalent plastic strain. The result is that β is not a constant but …
Increasing The Accuracy Of Cooperative Localization By Controlling The Sensor Graph, Deepti Kumar, Herbert G. Tanner
Increasing The Accuracy Of Cooperative Localization By Controlling The Sensor Graph, Deepti Kumar, Herbert G. Tanner
Mechanical Engineering Faculty Publications
We characterize the accuracy of a cooperative localization algorithm based on Kalman Filtering, as expressed by the trace of the covariance matrix, in terms of the algebraic graph theoretic properties of the sensing graph. In particular, we discover a weighted Laplacian in the expression that yields the constant, steady state value of the covariance matrix. We show how one can reduce the localization uncertainty by manipulating the eigenvalues of the weighted Laplacian. We thus provide insight to recent optimization results which indicate that increased connectivity implies higher accuracy and we offer an analysis method that could lead to more efficient …
On The Determination Of Residual Stress And Mechanical Properties By Indentation, Xi Chen, Jin Yan Yan, Anette M. Karlsson
On The Determination Of Residual Stress And Mechanical Properties By Indentation, Xi Chen, Jin Yan Yan, Anette M. Karlsson
Mechanical Engineering Faculty Publications
Residual stresses are of practical importance in bulk materials and coatings, which critically affects their mechanical integrity and reliability. Comparing with traditional techniques, the depth-sensing indentation technique provides a quick and effective method of measuring the residual stress field. In this study, we have used the finite element method to investigate the effect of in-plane residual stress on hardness and stiffness measurements of a bulk material/thick coating. It is found that the contact hardness, stiffness, and indentation work are sensitive to the residual stress, in particular for materials with a relatively high yield strain. Based on the reverse analysis, a …