Browsing by Subject "Elevated temperature"

Now showing 1 - 4 of 4

Behavior of beam shear connections in steel buildings subject to fire
(2011-08) Hu, Guanyu; Engelhardt, Michael D.; Frank, Karl H.; Helwig, Todd A.; Kallivokas, Loukas F.; Ezekoye, Ofodike A.
This dissertation presents the results of experimental and computational investigations on the behavior of steel simple beam end framing connections subjected to fire. While significant progress has been made in understanding the overall structural response of steel buildings subject to fire, the behavior of connections under fire conditions is not well understood. Connections are critical elements for maintaining the integrity of a structure during a fire. Fire can cause large force and deformation demands on connections during both the heating and cooling stages, while reducing connection strength and stiffness. Of particular importance are simple beam end framing connections. These are the most common type of connection found in steel buildings and are used at beam-to-girder and girder-to-column connections in the gravity load resisting system of a building. This dissertation focuses on one particular type of beam end connection: the single plate connection, also known as a shear tab vii connection. This connection is very commonly used in U.S. building construction practice. In this study, material properties of ASTM A992 structural steel at elevated temperatures up to 900°C were investigated by steady state tension coupon tests. Experimental studies on the connection subassemblies at elevated temperatures were conducted to understand and characterize the connection strength and deformation capacities, and to validate predictions of connection capacity developed by computational and design models. In the computational studies, a three-dimensional finite element connection model was developed incorporating contact, geometric and material nonlinearity temperature dependent material properties. The accuracy and limitations of this model were evaluated by comparison with experimental data developed in this research as well as data available in the literature. The computational studies investigated the typical behavior of the connection during heating and cooling phases of fires as well as the connection force and deformation demands. The finite element model was further used to study and understand the effects of several key building design parameters and connection details. Based on the test and analysis results, some important finding and conclusions are drawn, and future work for simple shear connection performance in fire are discussed.
Creep and dynamic abnormal grain growth of commercial-purity molybdenum
(2005-08) Ciulik, James R.; Taleff, Eric M.
In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed.
Effects of elevated temperature on the physical aging and gas transport of sub-micron polybenzimidazole gas separation membranes
(2020-07-16) Merrick, Melanie Mae; Freeman, B. D. (Benny D.); Paul, Donald R; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel A
The promising potential of polybenzimidazole (PBI) membranes for high temperature (~200 °C), hydrogen-selective gas separations has been reported for many membrane geometries (e.g., bulk, composite, and hollow fiber), but never for sub-micron, spin-cast membranes. Numerous studies have shown that the performance of spin-cast membranes, which simulate commercially relevant thicknesses, declines more quickly with time than that of thick membranes due to accelerated physical aging. However, because most existing membranes are used near ambient temperature, the physical aging of sub-micron, spin-cast membranes has never been studied at temperatures above 55 °C. Because physical aging is dependent on both thickness and temperature, emerging high temperature membrane applications make it both intellectually and practically imperative to characterize spin-cast membranes in this new temperature regime. For the first time, physical aging studies of spin-cast, sub-micron membranes have been extended to elevated temperatures. PBI membranes, cast from commercial-grade Celazole®, were aged in a high-temperature permeation system while the gas permeabilities were periodically measured over more than 1500 hours. When aging at 190 °C, membrane gas permeabilities decreased rapidly then plateaued after 300 hours of aging. The observation of a plateau (i.e., equilibration) had never before been seen for a membrane, nor, to our knowledge, for any polymer ~250 °C below its glass transition temperature. Decreases in membrane permeability were accompanied by increases in selectivity for H₂, which are traditionally represented by Robeson upper bound plots. These shifts were consistent with previous membrane physical aging studies and indicate membrane size-sieving ability improves with aging. Celazole®’s permeability reductions at lower aging temperatures (e.g., 175 °C) were qualitatively similar to those at 190 °C, but occurred over a longer time period. When graphed vs. the logarithm of aging time, the permeability reductions at various temperatures could be superimposed via time-temperature superposition, which is a hallmark of physical aging. A thorough review of physical aging studies in the polymer physics literature is presented to give context for this unexpectedly short equilibration time far below the glass transition. Comparisons are then made between the current study and previous aging studies in the polymer physics field. Overall, the observation of a plateau at short aging times for a polymer deep in the glassy state casts doubt on our understanding of physical aging’s temperature-dependence and our ability to predict membranes’ long-term stability in elevated temperature applications.
Steel fracture modeling at elevated temperature for structural-fire engineering analysis
(2015-12) Cai, Wenyu; Engelhardt, Michael D.; Helwig, Todd A.; Tassoulas, John; Ghannoum, Wassim; Ezekoye, Ofodike
One of the key elements of performance-based structural-fire safety design is the ability to accurately predict thermal and structural response to fire. For steel structures, significant advances have been made in using finite element models for predicting the response of members, connections and entire structural systems exposed to fire. However, predicting the initiation and propagation of fracture of structural steel at elevated temperatures is still very difficult and uncertain using even the most advanced finite elements software. Fracture plays a critical role in the response of steel structures to fire, and is particularly important in connection response, where fracture often controls both strength and deformation capacity. While advances have been made in computational prediction of the initiation and propagation of fracture in steel at room temperature, much less is known at elevated temperature. The objective of the research described in this dissertation was to evaluate the ability of existing ductile fracture models for metals to predict initiation and propagation of fracture in structural steel at elevated temperatures. The general finite element program Abaqus was used in this research to explore and evaluate various approaches for simulation of fracture. In the first part of this study, true stress-strain curves were developed for structural steel at ambient and elevated temperatures that extend to very large, post-necking strains. Then two different fracture criteria were studied for modeling steel fracture at ambient and elevated temperatures in Abaqus. These two fracture criteria are referred to as the ductile fracture criterion and the shear fracture criterion. Both predict the equivalent plastic strain at fracture as a function of the state of stress, most notably the stress triaxiality, but have different formulations and model parameters. Model parameters for each fracture criterion were estimated by a calibration process that involved developing finite element models of various tests reported in the literature of structural steel materials, members, and connections at ambient and elevated temperatures. To evaluate the capabilities and limitations of each model, a number of comparisons were made between tests of steel components that failed by fracture, and simulations of those tests. These evaluations were conducted for tests conducted at temperatures ranging from ambient up to 1000C. Results of this work showed that the calibrated ductile fracture model was able to reproduce the experimentally observed behavior of tension coupons at elevated temperatures, all the way up through complete fracture. However, this same calibrated ductile fracture model was significantly less accurate in predicting the experimentally observed elevated temperature behavior of bolted steel connections. The model significantly overestimated the measured deformation capacity of the connections. This implies that the model overestimated the equivalent plastic strain at fracture for the states of stress developed in the regions of the bolted connections that experienced fracture. The calibrated shear fracture model, on the other hand, was capable of predicting the observed behavior of a wide range of bolted connection tests with reasonable accuracy. At any given temperature, the same shear fracture model parameters were able to reasonably predict the fracture of a variety of steel grades as well as high strength bolts. This suggests that the fracture model parameters may not be highly sensitive to changes in steel strength. Based on information in the literature and observations from this research, neither the ductile fracture model nor the shear fracture model is applicable across a full range of stress triaxiality values. The ductile fracture model appears to be most appropriate for predicting fracture under high levels of stress triaxiality, whereas the shear fracture model appears most appropriate for states of stress characterized by lower levels of stress triaxiality. The attempts at fracture simulations in this dissertation are based on limited experimental data and should be considered preliminary in nature. Far more work is needed to further develop these capabilities. Nonetheless, the numerous comparisons between simulations and experiments provided in this dissertation offer the hope that fracture behavior of steel connections and members at elevated temperatures can ultimately be simulated with confidence.