Anu Tripathi

University of Minnesota Ph.D. dissertation. December 2020. Major: Civil Engineering. Advisors: Jia-Liang Le, Susan Mantell. 1 computer file (PDF); xii, 122 pages.High density polyethylene (HDPE) is increasingly being used in infrastructure applications with a design service lifetime of several decades. In many cases, the member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mode of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation that deteriorates the load capacity and long-term behavior of HDPE structure exposed to a chlorinated environment. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) of HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model to considers the individual deformation and damage mechanisms of the amorphous and crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with entangled network resistance and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the amorphous phase parameters to the polymer molecular weight which decreases with corrosion level. The proposed model is calibrated using uniaxial tensile tests at different deformation rates, crystallinities, and corrosion levels. The model is used to simulate the double-edge notched (DEN) tension specimens at different corrosion levels. The constitutive model can capture the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment. The decrease in the molecular weight of HDPE due to exposure to bleach environment is captured by a reduced-order corrosion kinetics model. The selective diffusion and chemical reaction of bleach into the amorphous phase lead to polymer chain scission that reduces the molecular weight. The corrosion kinetics model describes this diffusion-chemical reaction of bleach and expresses the extent of chain scission as a function of the bleach concentration. The proposed material constitutive model and the diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves which qualitatively match the measured stress-life data of polymer pipes. The stress-life curve is shown to exhibit different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen. The simulations also provide the fracture kinetics under different environments, which can be used to predict the service life of an HDPE specimen with any geometry and applied load

ABSTRACT Buddhist monasteries in Sikkim Himalayas constitute important religious and architectural heritage. These random rubble (RR) stone masonry structures located in high seismically active regions of the Himalayas have suffered varied degrees of damages in the past earthquakes. The study presents seismic vulnerability assessment of four archetypal monastic temples using finite element (FE) analyses. Linear and nonlinear analyses of these structures were conducted in Abaqus FE environment. These analyses identified the damage prone areas of the structures and provided load-deformation behavior under lateral loads. Fragility analyses indicate a high probability of collapse for the specified design level earthquake of the region. The study shows that performance of the structure can be enhanced by improving the strength and stiffness of the stone masonry walls.

University of Minnesota Ph.D. dissertation. December 2020. Major: Civil Engineering. Advisors: Jia-Liang Le, Susan Mantell. 1 computer file (PDF); xii, 122 pages.High density polyethylene (HDPE) is increasingly being used in infrastructure applications with a design service lifetime of several decades. In many cases, the member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mode of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation that deteriorates the load capacity and long-term behavior of HDPE structure exposed to a chlorinated environment. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) of HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model to considers the individual deformation and damage mechanisms of the amorphous and crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with entangled network resistance and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the amorphous phase parameters to the polymer molecular weight which decreases with corrosion level. The proposed model is calibrated using uniaxial tensile tests at different deformation rates, crystallinities, and corrosion levels. The model is used to simulate the double-edge notched (DEN) tension specimens at different corrosion levels. The constitutive model can capture the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment. The decrease in the molecular weight of HDPE due to exposure to bleach environment is captured by a reduced-order corrosion kinetics model. The selective diffusion and chemical reaction of bleach into the amorphous phase lead to polymer chain scission that reduces the molecular weight. The corrosion kinetics model describes this diffusion-chemical reaction of bleach and expresses the extent of chain scission as a function of the bleach concentration. The proposed material constitutive model and the diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves which qualitatively match the measured stress-life data of polymer pipes. The stress-life curve is shown to exhibit different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen. The simulations also provide the fracture kinetics under different environments, which can be used to predict the service life of an HDPE specimen with any geometry and applied load

The Buddhist monasteries of Sikkim portray the history and culture of the place with their intricate art and architecture. These monasteries, located in the regions of high seismic activity in the Himalayas, have suffered varied degrees of damages in the past earthquakes. Proper restoration programs require knowledge of the seismic performance of these random-rubble stone masonry structures. This paper presents the seismic fragility analysis of four representative study monastic temples, namely Labrang, Enchey, Phodong and Pubyuk. The seismic fragility analyses of these temples were performed based on their pushover curves, using the capacity spectrum method, which accounts for the variability in the seismic loading. The analyses provided the probability of exceedance of a damage state for the earthquake levels expected in seismic zone IV of Indian seismic code. In this study, the damage levels of immediate occupancy (IO), life safety (LS), and collapse prevention (CP) were consider...

Abstract High density polyethylene (HDPE) is increasingly used in infrastructure applications with a design service lifetime of several decades. In many cases, the HDPE member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mechanism of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation, which deteriorates the load capacity of the structure. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) in HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model that separately considers the individual deformation and damage behaviors of the amorphous and the crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with the resistance of the entangled network and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the parameters of amorphous phase to the polymer molecular weight. The diffusion and chemical reaction of chlorine are described by a reduced order kinetics model that links the extent of polymer oxidation to the reduction of the molecular weight. The material constitutive model and diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves and fracture kinetics under different environments. The predicted stress-life curve qualitatively matches the measured stress-life data of polymer pipes. It is shown that the stress-life curve exhibits different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen.

ABSTRACT Buddhist monasteries in Sikkim Himalayas constitute important religious and architectural heritage. These random rubble (RR) stone masonry structures located in high seismically active regions of the Himalayas have suffered varied degrees of damages in the past earthquakes. The study presents seismic vulnerability assessment of four archetypal monastic temples using finite element (FE) analyses. Linear and nonlinear analyses of these structures were conducted in Abaqus FE environment. These analyses identified the damage prone areas of the structures and provided load-deformation behavior under lateral loads. Fragility analyses indicate a high probability of collapse for the specified design level earthquake of the region. The study shows that performance of the structure can be enhanced by improving the strength and stiffness of the stone masonry walls.