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Aramid fiber reinforced polymer (AFRP) composite materials show promise for prestressed concrete bridge applications. However, there are still some knowledge gaps due to lack of sufficient data to assess the long-term performance and therefore sustainability of beams prestressed with AFRP composite materials. The objective of this research is to effectively characterize the material properties based on the short-term and long-term characteristics of AFRP bars. Tensile, creep-rupture, and relaxation tests are experimentally conducted using AFRP bars to validate testing procedures and expand an existing limited database. Previous results from tensile tests show that the stress-strain behavior of Arapree® AFRP bars is linear until failure with tensile strength of approximately 210 ksi (1448 MPa) and strain of 2.1%. For the creep-rupture tests, three specimens are tested and monitored at four different load levels (50, 60, 75 and 85% of maximum tensile strength) throughout a period of 14 days (short-term evaluation) and 42 days (long-term evaluation). From these tests, it is expected that for a 100-year life span, 55% of the ultimate load, Fu, must be applied as an initial stress to obtain a long-term residual strength of 0.80 Fu. For the relaxation tests, six specimens at four different strain levels (50, 60, 75 and 85% of maximum tensile strain) are tested and monitored throughout a period of 14 days and 42 days. Relaxation loss profiles of the AFRP bars are developed based on the experimental data collected from prestressed AFRP bars, which have been less well understood given lack of sufficient experimental data. Overall, the results of this study provide more insight as to the reliability and potential long-term performance of AFRP bars embedded within prestressed bridge structures.

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Polymer matrix composites are seeing increasing use in structural systems (e.g. ships, bridges) and require a quantitative basis for describing their performance under combined mechanical load and fire. Although much work has been performed to characterize the flammability, fire resistance and toxicity of these composite systems, an understanding of the structural response of sandwich type structures and laminate panels under combined mechanical and thermal loads (simulating fire conditions) is still largely unavailable. Therefore a research effort to develop a model to describe the structural response of these glass/vinyl esters systems under fire loading conditions is relevant to the continuing and future application of polymer matrix composites aboard naval ships. The main goal of the effort presented here is to develop analytical models and finite element analysis methods and tools to predict limit states such as local compression failures due to micro-buckling, residual strength and times to failure for composite laminates at temperatures in the vicinity of the glass transition where failure is controlled by viscoelastic effects. Given the importance of compression loading to a structure subject to fire exposure, the goals of this work are succinctly stated as the:(a)Characterization of the non-linear viscoelastic and viscoplastic response of the E-glass/vinyl ester composite above Tg. (b)Description of the laminate compression mechanics as a function of stress and temperature including viscoelasticity.(c)Viscoelastic stress analysis of a laminated panel ([0/+45/90/-45/0]S) using classical lamination theory (CLT). Three manuscripts constitute this dissertation which is representative of the three steps listed above. First, a detailed characterization of the nonlinear thermoviscoelastic response of Vetrotex 324/Derakane 510A - 40 through Tg was conducted using the Time - Temperature - Stress - Superposition Principle (TTSSP) and Zapas - Crissman model. Second, the modeling approach and viscoelastic relaxation mechanism is validated by substituting the shear relaxation modulus into a compression strength model to predict lifetimes for isothermal and one sided heating of unidirectional laminates. Finally, viscoelastic stress analysis using CLT is performed for a general laminated panel to predict lifetimes under one sided heating. Results indicate that when temperatures remain in the vicinity of Tg, the laminate behavior is controlled by thermoviscoelasticity. / Ph. D.

The aim of this study is to develop a combined experimental and analytical framework for accelerated lifetime estimates of semi-crystalline plastic pipes which is sensitive to changes in structure, orientation, and morphology introduced by processing conditions. To accomplish this task, high-density polyethylene (HDPE) is chosen as the exemplary base material. As a new accelerated test protocol, several characterization tests were planned and conducted on as-manufactured HDPE pipe segments. Custom fixtures are designed and developed to admit uniaxial characterization tests. The yield behavior of the material was modeled using two hydrostatic pressure modified Eyring equations in parallel to describe the characterization test data collected in axial tension and compression. Subsequently, creep rupture failure of the pipes under hydrostatic pressure is predicted using the model. The model predictions are validated using the experimental creep rupture failure data collected from internal pressurization of pipes using a custom-designed, fully automatic test system. The results indicate that the method allows the prediction of pipe service lifetimes in excess of 50 years using experiments conducted over approximately 10 days instead of the traditional 13 months. The analytical model is joined with a commercial finite element package to allow simulations including different thermal-mechanical loading conditions as well as complicated geometries. The numerical model is validated using the characterization test data at different temperatures and deformation rates. The results suggest that the long-term performance of the pipe is dominated by the plastic behavior of the material and its viscoelastic response is found to play an insignificant role in this manner. Because of the potential role of residual stresses on the long-term behavior, the residual stress across the wall thickness is measured for three geometrically different HDPE pipes. As expected, the magnitude of tensile and compressive residual stresses are found to be greater in pipes with thicker walls. The effect of the residual stress on the long-term performance of the pipes is investigated by including the residual stress measurements into the numerical simulations. The residual stress slightly accelerates the failure process; however, for the pipe geometries examined, this acceleration is insignificant. / Doctor of Philosophy / The use of plastic pipes to carry liquids and gases has greatly increased in recent decades, primarily because of their moderate costs, long service lifetimes, and corrosion resistance compared with materials such as corrugated steel and ductile iron. Before these pipes can be effectively used, however, designers need the capability to quickly predict the service lifetime so that they can choose the best plastic material and pipe design for a specific application. This capability also allows manufacturers to modify materials to improve performance. The aim of this study is to develop a combination of experiments and models to quickly predict the service lifetime of plastic pipes. High-density polyethylene (HDPE) was chosen as the plastic material on which the model was developed. Several characterization tests are planned and conducted on as-manufactured HDPE pipe segments. The yielding behavior of the material is modeled and the lifetime predictions are evaluated. The predictions are validated by experimental data captured during pipe burst tests conducted in the lab. The results indicate that the method allows the accurate prediction of pipe service lifetimes in excess of 50 years using experiments conducted over approximately 10 days instead of the traditional 13 months, resulting in significant savings in time (and consequently costs) and making it possible to introduce new materials into production more rapidly.

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A simulation model which incorporates the statistical- and numerical-based Lattice Green Function Local Load Sharing Model and a Fracture Mechanics-based Residual Strength Model has been developed. The model simulates creep rupture by imposing a fixed load of constant stress on the composite over the simulation duration. Simulation of the fatigue of glass fiber-reinforced composites is achieved by replacing the constant stress parameter in the model with a sinusoidal wave function. Results from the creep rupture model using fused silica fiber parameters, compare well with S-2 glass/epoxy systems. Results using Mandell's postulate that fatigue failure in glass fiber-reinforced polymeric composites is a fiber-dominated mechanism, with a characteristic slope of 10 %UTS/decade are consistent with available experimental data. The slopes of fatigue curves for simulated composites for three frequencies namely: 2, 5 and 10 Hz are within 12-14 %UTS/decade compared with that of 10.6-13.0%UTS/decade for unidirectionl glass reinforced composites (epoxy and vinyl ester) obtained from Demers' [40] data. / Master of Science

Current fire induced burnthrough models of aluminum typically rely solely on temperature thresholds and cannot accurately capture either the occurrence or the time to burnthrough. This research experimentally explores the fire induced burnthrough phenomenon of AA6061-T651 plates under multiple sized exposures and introduces a new burnthrough model based on the near melting creep rupture properties of the material. Fire experiments to induce burnthrough on aluminum plates were conducted using localized exposure from a propane jet burner and broader exposure from a propane sand burner. A material melting mechanism was observed for all localized exposures while a material rupture mechanism was observed for horizontally oriented plates exposed to the broader heat flux. Numerical burnthrough models were developed for each of the observed burnthrough mechanisms. Material melting was captured using a temperature threshold model of 633 deg C. Material rupture was captured using a Larson-Miller based creep rupture model. To implement the material rupture model, a characterization of the creep rupture properties was conducted at temperatures between 500 and 590 deg C. The Larson-Miller curve was subsequently developed to capture rupture behavior. Additionally, the secondary and tertiary creep behavior of the material was modeled using a modified Kachanov-Rabotnov creep model. Thermal finite element model accuracy was increased by adapting a methodology for using infrared thermography to measure spatially and temporally varying full-field heat flux maps. Once validated and implemented, thermal models of the aluminum burnthrough experiments were accurate to 20 deg C in the transient and 10 deg C in the steady state regions. Using thermo-mechanical finite element analyses, the burnthrough models were benchmarked against experimental data. Utilizing the melting and rupture mechanism models, burnthrough occurrence was accurately modeled for over 90% of experiments and modeled burnthrough times were within 20% for the melting mechanism and 50% for the rupture mechanism. Simplified burnthrough equations were also developed to facilitate the use of the burnthrough models in a design setting. Equations were benchmarked against models of flat and stiffened plates and the burnthrough experiments. Melting mechanism burnthrough time results were within 25% of benchmark values suggesting accurate capture of the mechanism. Rupture mechanism burnthrough results were within 60% of benchmark values. / Ph. D.

This thesis presents the results of testing and modeling conducted to characterize the performance of 3M™ VHB™ structural glazing tape in both shear and tension. Creep rupture testing results provided the failure time at a given static load and temperature, and ramp-to-fail testing results provided the ultimate load resistance at a given rate of strain and temperature. Parallel testing was conducted on three structural silicone sealants to compare performance. Using the time temperature superposition principle, master curves of VHB tape storage and loss moduli in shear and tension were developed with data from a dynamic mechanical analyzer (DMA). The thermal shift factors obtained from these constitutive tests were successfully applied to the creep rupture and ramp-to-fail data collected at 23°C, 40°C, and 60°C (73°F, 104°F, and 140°F), resulting in master curves of ramp-to-fail strength and creep rupture durability in shear and tension. A simple linear damage accumulation model was then proposed to examine the accumulation of wind damage if VHB tape is used to attach curtain wall glazing panels to building facades. The purpose of the model was to investigate the magnitude of damage resulting from the accumulation of sustained wind speeds that are less than the peak design wind speed. The model used the equation derived from tensile creep rupture testing, extrapolated into the range of stresses that would typically be generated by wind loading. This equation was applied to each individual entry in the data files of several real wind speed histories, and the fractions of life used at each entry were combined into a total percentage of life used. Although the model did not provide evidence that the established design procedure is unsafe, it suggested that the accumulation of damage from wind speeds below the peak wind speed could cause a VHB tape mode of failure that merits examination along with the more traditional peak wind speed design procedure currently recommended by the vendor. / Master of Science

Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular. / <p>QC 20160905</p>