Dwell time low cycle fatigue (DLCF), low cycle fatigue (LCF), and creep tests were performed at ambient temperature on Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242Si). Test specimens were solution annealed at various temperatures below the beta transus to control the volume fraction of primary alpha. The influence of the changes in primary alpha phase on low cycle dwell time fatigue life were determined and compared to the conventional low cycle fatigue properties of the alloy. A dwell period significantly decreased the number of cycles to failure, but by a decreasing factor with decreasing stress. The increased primary alpha phase present associated with lower solution anneal temperatures significantly increased susceptibility to low cycle dwell time fatigue. It is believed that dwell fatigue sensitivity may be associated with cyclic, ambient temperature, time-dependent plasticity (creep). / Graduation date: 1999

An curvature measurement technique was used to characterize the stress evolution during reaction of a Ni film and a silicon substrate to form nickel silicide. Stress changes were measured at each stage of the silicide growth. When the nickel films were subjected to long-time isothermal annealing, stresses that developed during silicide formation gradually relaxed. Fitting the experimental results with a kinetic model provides insight into the volumetric strain and relaxation behavior of the reacting film and the reaction product. / Singapore-MIT Alliance (SMA)

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes the useful life of coke drums to be shortened. In order to better understand the failure mechanisms, a experimental study of elastic/plastic mechanical properties and deformation behaviors of typical coke drum materials was performed. A new biaxial thermal-mechanical material testing system has been successfully developed. Basic characterization of mechanical properties of coke drum materials is achieved through uniaxial monotonic and cyclic loading tests. In addition, strain-rate dependence and creep of coke drum materials were further experimentally investigated. Complex thermal-mechanical cyclic tests were conducted. The experimental findings help us to understand the damage mechanisms of coke drums such as bulging. In addition, experimental data serve as benchmark data to verify the predictions of the temperature dependent elastoplastic constitutive model.

This study is a continuation of a project to characterise ASTM A437 Grade B4B martensitic stainless steel for use In Hitachi Canadian Industries Ltds (HCI) steam turbine casing bolts. ASTM A437 Grade B4B steel is commercially available and was chosen for the study due to its chemical similarity to a proprietary steel currently used by HCI.<p>High creep resistance is essential for any candidate so creep-rupture and creep-strain tests were performed at and above the intended service temperature of 538°C. Hardness measurements and transmission electron microscopy were performed on the steel in the as-received condition as well as on crept samples to determine the effect of elevated temperature on the development of the steels microstructure.<p>During testing, it was found that ASTM A437 Grade B4B steel has a well defined second stage leading to an abrupt transition into the third stage. The second stage begins in the first 10% of its creep life, while the third stage begins at 90% of its creep life. This equates to 5% and 30% of the final strain, respectively, with an average final strain of 20%.<p>Time-to-Rupture data show good similarity to the creep life as predicted using the Larson-Miller method. When plotted, the steady-state creep rate shows a definite correlation between the creep stress and temperature. From this an empirical relationship was developed to predict the steady-state creep rate.<p> Transmission electron microscopy (TEM) results showed a significant change in the icrostructure between crept and as-received steel. Coarsening of carbides along grain boundaries most likely led to a recovery of the microstructure in the crept samples. Literature suggests that the composition of the carbides is most likely tungsten and molybdenum intermetalics and carbides that coarsened from the depletion of chromium from solution. This was supported by energy dispersive spectroscopy (EDS) analysis.<p>The coarsening of carbides correlates with the decrease in creep resistance of the material and it is likely that the growth of precipitates and recovery of the microstructure causes the entry of the steel into third stage creep.

Polyethylene has been studied from many different perspectives; a final application property perspective, in which the response of the material to loads is the topic; a micromechanical point of view, in which the macroscopic state of the material is related to its microstructure, e.g., Alvarado (2007), and a chemical point of view in which the molecular structure and the processes that create polyethylene are investigated. This thesis focuses on the mechanical behavior of polyethylene observed from testing and relates the mechanical behavior to the molecular structure of the material. High density polyethylene is a material used in civil engineering applications such as pipes and containers. There are two general modes of failure for polyethylene: ductile failure that happens at relatively large stresses (up to 200MPa) and in short amount of time, and brittle failure that occurs when a much lower stress is sustained over a long period of time (Cheng 2008). Other than these two modes of failure, excessive deformation of the material that is usually caused by creep is also to be avoided. This thesis studies the relationship between short-term and long-term creep of polyethylene and its molecular structure. In this work three types of mechanical tests were performed on six samples of polyethylene. The existing models that prescribe the constitutive behavior of the material were then critically evaluated against the observed data. Furthermore the molecular properties of the samples that had been obtained from previous research by Cheng (2008) were compared against the mechanical behavior observed from testing in order to assess what molecular properties are important in determining the mechanical behavior of polyethylene. This information can also help polyethylene designers to produce longer lasting material, or a material that has high stiffness, by knowing what molecular properties to control and optimize.

Polyethylene has been studied from many different perspectives; a final application property perspective, in which the response of the material to loads is the topic; a micromechanical point of view, in which the macroscopic state of the material is related to its microstructure, e.g., Alvarado (2007), and a chemical point of view in which the molecular structure and the processes that create polyethylene are investigated. This thesis focuses on the mechanical behavior of polyethylene observed from testing and relates the mechanical behavior to the molecular structure of the material. High density polyethylene is a material used in civil engineering applications such as pipes and containers. There are two general modes of failure for polyethylene: ductile failure that happens at relatively large stresses (up to 200MPa) and in short amount of time, and brittle failure that occurs when a much lower stress is sustained over a long period of time (Cheng 2008). Other than these two modes of failure, excessive deformation of the material that is usually caused by creep is also to be avoided. This thesis studies the relationship between short-term and long-term creep of polyethylene and its molecular structure. In this work three types of mechanical tests were performed on six samples of polyethylene. The existing models that prescribe the constitutive behavior of the material were then critically evaluated against the observed data. Furthermore the molecular properties of the samples that had been obtained from previous research by Cheng (2008) were compared against the mechanical behavior observed from testing in order to assess what molecular properties are important in determining the mechanical behavior of polyethylene. This information can also help polyethylene designers to produce longer lasting material, or a material that has high stiffness, by knowing what molecular properties to control and optimize.

The viscoelastic and creep properties of salt create challenges in the design of salt mines. Salt undergoes steady state creep for a long period of time, and the time of failure is not easily predicted. Developing functions for creep behavior is important in predicting the deformation of salt pillars. Through literature reviews, it was found that there are many relationships to determine the deformation rate of salt specimens through constitutive models. Mine panels have also been modeled to understand the stress and deformational behavior of the pillars. The purpose of this was project was to develop a relationship that determines the convergence rate from knowing the pillar width to pillar height ratio and thickness of the salt strata immediately above and below the mine. The third power law was adopted in the modeling of salt pillars, which is applicable to low stresses of less than 10 MPa that is typical of salt mine conditions. The finite difference software, FLAC3D was used for the simulations of salt pillar models. A square pillar was modeled using four pillar width to pillar height ratios from 1.5 to 4.6. In mining practices, the pillar width to pillar height ratios are designed to be 1.0 to 5.0. Three sets of pillar dimensions were used for each pillar width to pillar height ratio, this was done to determine whether different room and pillar dimensions for each pillar width to pillar height ratio resulted in different convergence rates. Eight salt thicknesses of 0 m to 26 m were modeled for each set of pillar dimensions, which was sufficient to determine the effect of salt thickness on convergence rate. From the modeled results, general trends among the various pillar width to pillar height ratios were observed. The convergence rate increased as the pillar width to pillar height ratio decreased. In addition, an exponential relationship was found between the convergence rate and the pillar width to pillar height ratio. There was a strong correlation between convergence values calculated from the developed function and the modeled values for the power law exponent of three. The developed expression can be used to estimate the convergence rate due to pillar compression and room convergence.

The purpose of this thesis is to provide an overview of ETFE foil, as it applies to pneumatic cushion cladding, with a focus on creep behavior of the material. Characteristics of ETFE, including weight, optics, insulation, flexibility, environmental properties, fire performance, cushion span and other features are discussed, and, where possible, are compared to the characteristics of glass panels used in similar applications. Relevant chemical and mechanical properties of ETFE are given. Load carrying concepts of tension structures and inflated cushions are discussed, as well as structural design methods for ETFE cushions. Several prominent structures constructed using ETFE foil are introduced and benefits and design issues associated with these structures are reviewed. When used in cushion applications, ETFE films are placed in constant tension, and are therefore subject to creep. Quantifying this creep is desirable so that it can be predicted during the design phase. Therefore, this thesis summarizes the findings of other researchers in the area of creep of ETFE as well as the general mechanical behavior of the material, and presents the results of uniaxial creep tests done for the purpose of this study. These tests included 24 hour uniaxial creep tests done at four stress levels on both the transverse and longitudinal directions of three different brands of film. Two thicknesses of the third film were acquired and both were tested. The stress levels were chosen to coincide with typical design tensile stresses for ETFE film, and to be similar to the levels tested by other researchers. The effects of the different stresses, brands, directions and thicknesses are evaluated and discussed. Three seven day creep tests were also done on one of the films, each at a different stress level. Constitutive viscoelastic and viscoplastic models were developed to represent the 24-hour creep data. The viscoelastic models were based on a four-element Kelvin model and the viscoplastic models were based on a power-law model. The model parameters were determined from the data using linear least squares fitting. Models were also developed for the seven day creep data. Several of these models were based only upon the first 24 hours of data, and were used to determine the applicability of the 24-hour creep models to long-term behavior. It was found that while a viscoelastic model appears to fit long-term creep most closely, the 24-hour models are inadequate for modeling longer time frames. Another method is required for predicting long-term creep. Nonlinear fitting of the parameters is recommended as a possible alternative for creating more accurate models. Longer-term creep tests are also recommended. Tensile tests were also done on the films to confirm mechanical properties supplied by the film manufacturers. Good agreement to the given values was found in the test data.

This study is a continuation of a project to characterise ASTM A437 Grade B4B martensitic stainless steel for use In Hitachi Canadian Industries Ltds (HCI) steam turbine casing bolts. ASTM A437 Grade B4B steel is commercially available and was chosen for the study due to its chemical similarity to a proprietary steel currently used by HCI.<p>High creep resistance is essential for any candidate so creep-rupture and creep-strain tests were performed at and above the intended service temperature of 538°C. Hardness measurements and transmission electron microscopy were performed on the steel in the as-received condition as well as on crept samples to determine the effect of elevated temperature on the development of the steels microstructure.<p>During testing, it was found that ASTM A437 Grade B4B steel has a well defined second stage leading to an abrupt transition into the third stage. The second stage begins in the first 10% of its creep life, while the third stage begins at 90% of its creep life. This equates to 5% and 30% of the final strain, respectively, with an average final strain of 20%.<p>Time-to-Rupture data show good similarity to the creep life as predicted using the Larson-Miller method. When plotted, the steady-state creep rate shows a definite correlation between the creep stress and temperature. From this an empirical relationship was developed to predict the steady-state creep rate.<p> Transmission electron microscopy (TEM) results showed a significant change in the icrostructure between crept and as-received steel. Coarsening of carbides along grain boundaries most likely led to a recovery of the microstructure in the crept samples. Literature suggests that the composition of the carbides is most likely tungsten and molybdenum intermetalics and carbides that coarsened from the depletion of chromium from solution. This was supported by energy dispersive spectroscopy (EDS) analysis.<p>The coarsening of carbides correlates with the decrease in creep resistance of the material and it is likely that the growth of precipitates and recovery of the microstructure causes the entry of the steel into third stage creep.

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