Atomistic simulations of chemomechanical processes in nanomaterials under extreme environments

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 142-146).The complex chemomechanical behavior of nanomaterials under extreme thermal and mechanical environments is of interest for a range of basic science and defense applications. By the limitation of experimental approaches for objects of nanometer, novel computational methods have been developed to investigate such phenomena in nanomaterials under extreme environments. In this thesis, novel continuum and atomistic mechanical modeling and simulations are implemented and constructed for the analysis of the chemomechanical behavior of the dissimilar nano-scale metals, Nickel and Aluminum under a variety of thermal and mechanical stimuli. These studies form the basis of preliminary research on the predictive design principles for reactive polymer nanocomposites.by Hansohl Cho.S.M

Data-Driven Statistical Reduced-Order Modeling and Quantification of Polycrystal Mechanics Leading to Porosity-Based Ductile Damage

Predicting the process of porosity-based ductile damage in polycrystalline metallic materials is an essential practical topic. Ductile damage and its precursors are represented by extreme values in stress and material state quantities, the spatial PDF of which are highly non-Gaussian with strong fat tails. Traditional deterministic forecasts using physical models often fail to capture the statistics of structural evolution during material deformation. This study proposes a data-driven statistical reduced-order modeling framework to provide a probabilistic forecast of the deformation process leading to porosity-based ductile damage, with uncertainty quantification. The framework starts with computing the time evolution of the leading moments of specific state variables from full-field polycrystal simulations. Then a sparse model identification algorithm based on causation entropy, including essential physical constraints, is used to discover the governing equations of these moments. An approximate solution of the time evolution of the PDF is obtained from the predicted moments exploiting the maximum entropy principle. Numerical experiments based on polycrystal realizations show that the model can characterize the time evolution of the non-Gaussian PDF of the von Mises stress and quantify the probability of extreme events. The learning process also reveals that the mean stress interacts with higher-order moments and extreme events in a strongly nonlinear and multiplicative fashion. In addition, the calibrated moment equations provide a reasonably accurate forecast when applied to the realizations outside the training data set, indicating the robustness of the model and the skill for extrapolation. Finally, an information-based measurement shows that the leading four moments are sufficient to characterize the crucial non-Gaussian features throughout the entire deformation history

Mechanics of elastomeric copolymers

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references.Elastomeric copolymers have been versatile materials for a broad variety of engineering applications of critical importance ranging from ballistic protective coatings to self-healing microstructures, possessing a backbone structure composed of alternate hard and soft segments, where the hard/soft nomenclature corresponds to the thermodynamic glassy/rubbery state at ambient temperature. The thermodynamic incompatibility of microstructures often lead to a phase-separated morphology of the hard and soft domains which can be tailored depending on the chemical composition, molecular dispersion, processing and synthesis to give a variety of physical properties. The mechanical behavior of elastomeric copolymers is hence governed by the chemical composition as well as the morphology providing a hybrid performance by virtue of simultaneous contributions from constituent homopolymers often offering new and unique properties. In this research, the mechanics and physics of large deformation behavior of elastomeric copolymers are addressed in terms of their resilience and dissipation involving elastomeric "segmented" copolymers and elastomeric "ionic" copolymers. The presence of hard and soft domains yields to multiple molecular relaxations and hence multiple viscous dissipation mechanisms in elastomeric copolymers. In addition to the viscous dissipation, stretch-induced softening due to microstructural evolution revealed via x-ray scattering observation during deformation provides another major dissipation pathway. Furthermore the segmented copolymers exhibit a substantial shape recovery upon unloading in tandem with a remarkable amount of hysteresis. A microstructurally-informed constitutive model is proposed to address the main features of mechanical behavior of exemplar copolymers under a variety of loading conditions, employing multiple micro-rheological mechanisms representing hard and soft domains. The proposed model was found to be capable of capturing the salient features of resilient yet dissipative stress-strain behavior of materials at a wide range of strains and strain rates. The model was then furthered to examine the effect of weight fraction, morphology and segmental dynamics of hard and soft microstructures. Next, the resilience and dissipation in elastomeric segmented copolymers are examined in their connections to shape recovery under microindentation testing in experiments and numerical simulations. Numerical simulations imparting the proposed constitutive model were found to be capable of capturing the microindentation behavior of materials including force-displacement capable of capturing the microindentation behavior of materials including force-displacement responses under complicated loading scenarios. Additionally, the microindentation behavior revealed a substantial shape recovery of indented surfaces which was due to inelastic flow beyond elastic resilience. The elastically- and inelastically-driven shape recovery provides critical insight into a better understanding of shape memory, recovery and self-healing mechanisms in this class of segmented elastomers. The extreme nature of elastomeric copolymers under harsh mechanical environments is then addressed via Taylor impact testing, where an ultrafast deformation event is incurred. Numerical simulations of Taylor impact behavior of elastomeric copolymers are compared to experimental results in terms of overall and localized deformation profiles, revealing a three-dimensional capability of our framework under dynamic, inhomogeneous deformation field. Furthermore, energy dissipation under such an extreme event is examined by comparison to that found in "model" glassy and rubbery polymers revealing that copolymeric materials enable a highly recoverable, protective composite architecture for shock and ballistic mitigation by taking advantages of hybrid performance of glassy and rubbery polymers. Lastly, the mechanics of elastomeric "ionic" copolymers is addressed for a broad understanding of their resilience, dissipation and shape recovery under a wide range of mechanical loading conditions. Our viscoelastic-viscoplastic constitutive framework is further developed to address the large deformation behavior of ionic elastomers including ethylene methacrylic acid (EMAA) copolymer and its chemically-modified counterparts which are recently finding new avenues towards multi-functional soft materials involving their self-healing ability under severe deformation events. This study provides a simple yet intuitive framework to rationalize physically-sound deformation mechanisms of diverse elastomeric copolymers by employing a combination of novel modeling, experimentation and computation. Finally, potential topics for further research, to which the present work can directly contribute, are discussed in a wide variety of engineering contexts.by Hansohl Cho.Ph. D

Deformation mechanisms of thermoplastic elastomers: Stress-strain behavior and constitutive modeling

This work addresses the large strain behaviors of thermoplastic polyurethanes (TPUs) spanning a range of fractions of hard and soft contents in both experiment and theoretical modeling. The key mechanical features involve a combination of elasticity and inelasticity, and are quantified experimentally under a broad variety of loading scenarios. A finite deformation constitutive model is then presented to capture the main features of the stress-strain data, which are strongly dependent on fractions of hard and soft contents. The stress-strain behavior of these TPUs is characterized by highly nonlinear rate-dependent hyperelastic-viscoplasticity, in which substantial energy dissipation is accompanied by shape recovery as well as softening. Agreement between the model and the experimental data for the representative TPUs provides physical insight into the underlying deformation mechanisms in this important class of soft materials that exhibit both elastomeric and plastomeric characteristics