abstract: An orthotropic elasto-plastic damage material model (OEPDMM) suitable for impact simulations has been developed through a joint research project funded by the Federal Aviation Administration (FAA) and the National Aeronautics and Space Administration (NASA). Development of the model includes derivation of the theoretical details, implementation of the theory into LS-DYNA®, a commercially available nonlinear transient dynamic finite element code, as material model MAT 213, and verification and validation of the model. The material model is comprised of three major components: deformation, damage, and failure. The deformation sub-model is used to capture both linear and nonlinear deformations through a classical plasticity formulation. The damage sub-model is used to account for the reduction of elastic stiffness of the material as the degree of plastic strain is increased. Finally, the failure sub-model is used to predict the onset of loss of load carrying capacity in the material. OEPDMM is driven completely by tabulated experimental data obtained through physically meaningful material characterization tests, through high fidelity virtual tests, or both. The tabulated data includes stress-strain curves at different temperatures and strain rates to drive the deformation sub-model, damage parameter-total strain curves to drive the damage sub-model, and the failure sub-model can be driven by the data required for different failure theories implemented in the computer code. The work presented herein focuses on the experiments used to obtain the data necessary to drive as well as validate the material model, development and implementation of the damage model, verification of the deformation and damage models through single element (SE) and multi-element (ME) finite element simulations, development and implementation of experimental procedure for modeling delamination, and finally validation of the material model through low speed impact simulations and high speed impact simulations. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2019

Rutting failures are prominent in expedient airfields constructed with AM2 landing mats over soft existing subgrades. There are many issues that must be addressed when approaching this multiaceted problem. The load transfer mechanism occurring at interlocking mat joints and the mat-soil interface bonding condition affect near surface subgrade response. The repeated loading coupled with lateral aircraft wander causes significant principal stress rotation in the subgrade. This kneading action then causes variations in the excess pore-water pressure and a subsequent softening of the soil. The purpose of this study is to investigate the critical factors that lead to subgrade rutting failures in landing mats constructed over soft subgrades. A three dimensional finite element (3D FE) model of a landing mat system over soft subgrade is implemented under both static and pseudo-dynamic loading conditions with aircraft wander. To capture the complex stress histories induced by the simulated moving gear loads over the unique structural features of the AM2 mat system, an elastoplastic kinematic hardening constitutive model, the Multi-Mechanical Model, is developed, calibrated and used to represent the subgrade response. Under both static and pseudo-dynamic loading, the FE model results match very well with the stress and deformation results from full-scale instrumented testing of the AM2 mat over 6 CBR subgrade. Results show that incorporating the load transfer mechanism occurring at the mat joints and varying the mat-soil interface condition affect the near surface subgrade deformation and stress responses that contribute to rutting failures. Furthermore, rotation of the principal stress axes and changes in excess pore-water pressures occur in the subgrade because of the moving tire load. These phenomena contribute to extension of the field of deformation influence around the trafficked area in the subgrade and upheaval at the edges of the test section. Findings of this study show that although layered elastic analysis procedures are the basis of current airfield design methodologies, critical design features and the corresponding deformation responses can be better modeled using the FE approach. Furthermore, the proposed 3D modeling approach implementing aircraft wander can provide a reliable platform for accurately simulating the subgrade response under pseudo-dynamic loading conditions.

This study focuses on molecular dynamics (MD) simulations, coupled with a discrete mathematical framework, and crystal plasticity (CP) simulations to investigate micro void nucleation and the plastic spin. The origin and historical use of the plastic spin are discussed with particular attention to quantifying the plastic spin at the atomistic scale. Two types of MD simulations are employed: (a) aluminum single crystals undergoing simple shear and (b) aluminum triple junctions (TJ) with varying grain orientations and textures undergoing uniaxial tension. The high-angle grain boundary simulations nucleate micro voids at or around the TJ and the determinant of the deformation gradient shows the ability to predict such events. Crystal plasticity simulations are used to explore the stress-state of the aluminum TJ from uniaxial tension at a higher length scale with results indicating a direct correlation between CP stress-states and the location of micro void nucleation in the MD simulations.

Constitutive modeling is a method that is useful in providing precise predictions of material response in components subjected to a variety of operating conditions. A process for optimizing the material constants of the Miller constitutive model for uniaxial modeling was developed and implemented in an automated optimization routine. Generally, up to twenty experiments simulating a range of conditions are needed to identify the material parameters for the model. The research sought to minimize the amount of empirical data that is necessary for optimization, aiming to reduce the costs and time necessary to carry out this procedure for more expensive classes of materials. The ultimate goal was to develop a robust method for determining the material constants of a viscoplastic model using a minimum amount of experimental data. An automated optimization routine was implemented into a program, referred to as uSHARP, developed as part of the research to determine constitutive model parameters. Central to the method was the use of more complex stress, strain, and temperature histories than are traditionally used, allowing for the effects of all material parameters to be captured using as few tests as possible. By carrying out successive finite element simulations and comparing the results to simulated experimental test data, the material constants were determined from 75% fewer experiments. By reducing monetary costs and time required, this procedure will allow for a more widespread application of advanced constitutive models in industry, allowing for better life prediction modeling of critical components in high temperature applications.

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One of the important factors determining the lifetime of polymer electrolyte membrane fuel cells (PEMFCs) is membrane degradation and failure. The lack of effective mitigation methods is largely due to the currently very limited understanding of the underlying mechanisms for mechanical and chemical degradations of fuel cell membranes. In order to understand degradation of membranes in fuel cells, two different experimental approaches were developed; one is fuel cell testing under open circuit voltage (OCV) with bi-layer configuration of the membrane electrode assemblies (MEAs) and the other is a modified gas phase Fenton's test. Accelerated degradation tests for polymer electrolyte membrane (PEM) fuel cells are frequently conducted under open circuit voltage (OCV) conditions at low relative humidity (RH) and high temperature. With the bi-layer MEA technique, it was found that membrane degradation is highly localized across thickness direction of the membrane and qualitatively correlated with location of platinum (Pt) band through mechanical testing, Infrared (IR) spectroscopy, fluoride emission, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) measurement. One of the critical experimental observations is that mechanical behavior of membranes subjected to degradation via Fenton's reaction exhibit completely different behavior with that of membranes from the OCV testing. This result led us to believe that other critical factors such as mechanical stress may affect on membrane degradation and therefore, a modified gas phase Fenton's test setup was developed to test the hypothesis. Interestingly, the results showed that mechanical stress directly accelerates the degradation rate of ionomer membranes, implying that the rate constant for the degradation reaction is a function of mechanical stress in addition to commonly known factors such as temperature and humidity. Membrane degradation induced by mechanical stress necessitates the prediction of the stress distribution in the membrane under various conditions. One of research focuses was on the developing micromechanism-inspired continuum model for ionomer membranes. The model is the basis for stress analysis, and is based on a hyperelastic model with reptation-inspired viscous flow rule and multiplicative decomposition of viscoelastic and plastic deformation gradient. Finally, evaluation of the membrane degradation requires a fuel cell model since the degradation occurs under fuel cell operating conditions. The fuel cell model included structural mechanics models and multiphysics models which represents other phenomena such as gas and water transport, charge conservation, electrochemical reactions, and energy conservation. The combined model was developed to investigate the compression effect on fuel cell performance and membrane stress distribution.

This thesis presents the simulation of chip formation in orthogonal metal cutting to evaluate the predictive capabilities of finite element code DYNA 3D. The Johnson and Cook constitutive model for materials, OFHC Copper, Aluminum 2024 T351, and Aluminum 6061 T6 alloy were incorporated into the simulation to account for the effects of strain hardening, strain rate hardening, and thermal softening effects during machining. Calculated values for the Johnson and Cook constitutive constants for Aluminum 6061 T6 alloy were determined from the literature. The model was compared to experimentally measured shear angles, chip thickness, chip velocity, and forces from the literature to evaluate the accuracy of the finite element code for a range machining strain rates. In an attempt to determine the predictive capabilities of DYNA 3D a strain rate regime of 10+3 s-1 to 10+4 s-1 was defined as the optimal strain rate regime for the orthogonal metal cutting application.

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