Browsing by Subject "chemical kinetics"

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    Numerical Modeling of Stress Corrosion Cracking in Polymers
    (2015-12) Ge, Hanxiao
    Polymeric materials have been increasingly used for structural purposes in civil infrastructures. However, stress corrosion cracking has been a critical issue that affects the service lifetime of polymer components. My preliminary study showed that polyethylene may be severely corroded in an oxidizing environment and lose its fracture resistance property. Experimental methods have been primarily adopted to investigate stress corrosion cracking in polymers; however, these approaches are expensive to apply, and may fail to account for certain aspects of this chemo-mechanical process. Therefore, a numerical approach is needed to investigate this issue. A unified chemo-mechanical model is developed to predict the stress corrosion cracking (SCC) of a viscoplastic polymer. This model is applied to the specific case of high density polyethylene (HDPE) exposed to a chlorinated environment at a constant stress load. This chemo-mechanical model is comprised of three components, each capturing a critical aspect of SCC. An elastic-viscoplastic constitutive model is adopted to predict the time-dependent creep behavior of HDPE, and the model parameters have been calibrated through tensile testing. This constitutive model has been implemented in finite element analysis by using a user-defined material subroutine. The polymer fracture property is considered to be dependent on the extent of corrosion, and this dependence is implemented with a cohesive zone model. A chemical kinetics and diffusion model is utilized to predict the degradation of fracture properties in the material as a result of reactions and migration of chemical substances. The coupled chemo-mechanical simulation is accomplished by integrating the chemical reaction calculation into finite element analysis via user defined subroutines. Two modes are considered for failure of the polymer: excessive plastic deformation or catastrophic unstable crack growth. At high stresses, the failure is primarily due to excessive plastic deformation. At low stresses, chemical reactions and diffusion are the dominant factors leading to failure. In addition, two distinct patterns of crack growth (reaction-driven or diffusion-driven) are revealed at various disinfectant concentrations at low stress levels. In reaction-driven crack growth, material degradation is localized at the crack tip, and crack growth rate is a constant throughout the simulated lifetime. However, when diffusion dominates, the entire specimen ligament may be severely degraded, and crack growth accelerates at the end of component lifetime. The current simulation framework allows exploring the interaction of various factors in stress corrosion cracking, such as disinfectant concentration, loading, and temperature. The framework is also general enough to be implemented for other polymeric materials and corresponding corrosion mechanisms. In the future, the proposed chemo-mechanical modeling approach may be expanded to analyze the performance of a variety of materials under stress corrosion cracking. In addition, a stochastic methodology may be incorporated to account for the variances in loading, as well as material properties.
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    Pilgrim 2020.1
    (2020-03-19) Ferro-Costas, David; Truhlar, Donald G; Fernandez-Ramos, Antonio; truhlar@umn.edu; Truhlar, Donald G; University of Minnesota Theoretical and Computational Chemistry Group
    Pilgrim is a program written in Python and designed to use direct dynamics in the calculation of thermal rate constants of chemical reactions by the variational transition state theory (VTST), based on electronic structure calculations for the potential energy surface. Pilgrim can also simulate reaction mechanisms using kinetic Monte Carlo (KMC). For reaction processes with many elementary steps, the rate constant of each of these steps can be calculated by means of conventional transition state theory (TST) or of the VTST. In the current version, Pilgrim can evaluate these thermal rates using the canonical version of reaction-path VTST, which requires the calculation of the minimum energy path (MEP) associated with each elementary step or transition structure. Multi-dimensional quantum effects can be incorporated through the small-curvature tunneling (SCT) approximation. These methodologies are available both for reactions involving a single structure of the reactants and the transition state and also for reactions involving flexible molecules with multiple conformations of the reactant and/or of the transition state. For systems with many conformers, the program can evaluate each of the elementary reactions by multi-path canonical VTST or multi-structural VTST. Moreover, the reactant can be unimolecular or bimolecular. Torsional anharmonicity can be incorporated through the MSTor and Q2DTor programs. Dual-level calculations are also available: automatic high-level single point energies can be used to correct the energy of reactants, transition states, products, and MEP points using the interpolated single-point energies (ISPE) algorithm. When the rate constants of all the chemical processes of interest are known, by means of their calculation using Pilgrim or alternatively through analytical fits to the rate constants as functions of temperature, it is possible to simulate the whole process using KMC. This algorithm allows performing a kinetic simulation to monitor the evolution of each chemical species with time and obtain the product yields.