Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1980. / Microfiche copy available in Archives and Science. / Bibliography: leaves 46-50. / by Raymond Joseph Harpin. / M.S.

Short-term creep tests in tension and in compression were conducted on southern pine, Douglas-fir, yellow-poplar, and Parallam™ samples at temperatures ranging between 20 and 80° C and at 6, 9 and 12% moisture content. The principle of time-temperature superposition was applied to form a master curve that extended for a maximum of 2 years. The horizontal shift factors followed an Arrhenius relation with activation energies ranging between 75 and 130 kJ/mole. It was not possible to superpose the compliance curves at 70 and 80° C, this is attributed to the presence of multiple components in wood with different temperature dependence. Long-term creep tests were also conducted in tension and in compression at 20° C and 12% moisture content for over 2 years. The resulting compliance curves were fitted to the power law equation using a nonlinear fitting procedure. The results were compared with those of the short-term creep tests. Finite element analysis was conducted on selected wood structures to determine the effect of creep on serviceability and stability. / Ph. D.

Over the past fifty years, many states have recognized the benefits of making precast, prestressed multi-girder bridges continuous by connecting the girders with a continuity diaphragm. Although there is widespread agreement on the benefits of continuous construction, there has not been as much agreement on either the methods used for design of these systems or the details used for the continuity connections. To aid designers in choosing the most appropriate method, an analytical and experimental study was undertaken at Virginia Tech. Analyses were done to compare the differences in the predicted continuity moments for different design methods and assumptions over a range of commonly used systems of Precast Concrete Bulb Tee (PCBT) girders and cast-in-place slabs. The results of the analyses were used to develop three continuity connection details for testing during the experimental study. Three different continuity connections were tested using full depth PCBT 45 in. deep girders made continuous with a 6 ft wide slab. The bottom of the ends of the girders were made continuous with the continuity connection by extending prestressing strands for one test and extending 180 degree bent bars for the other test. Both connections adequately resisted service, cyclic, and ultimate loads. But, the test with the extended bars remained stiffer during cyclic loading and is recommended for use. A third test was performed on a system using only a slab cast across the top of the girders. Two primary cracks formed above the ends of the girders at the joint during service testing, after which no significant increase in damage took place. Results from the analytical study indicate that the predicted positive thermal restraint moments may be significant, similar in magnitude to the actual positive cracking moment capacities. Results from the experimental study indicate that restraint moments develop early due to thermal expansion of the deck during curing and subsequent differential shrinkage; however, the magnitudes of the early age restraint moments are much less than conventional analyses predict. / Ph. D.

The effect of the interphase/interface region on the static mechanical properties, creep and creep rupture behavior of thermoplastic (J2) composites was investigated. The mechanical properties of the J2 composites were altered by systematic changes in fiber surface chemistry. Four fiber systems were used including the AU4, AS4(1) & (2), and AS4CGP fibers. (AS4(1) and AS4(2) represent different batch numbers.) Surface energies and chemistry of carbon fibers were examined using the Dynamic Contact Angle (DCA) method and X-ray Photoelectron Spectroscopy (XPS), respectively. The meso indentation technique was used to measure the interfacial shear strengths (ISS) of the composites. For the same batch of the composites, the ISS ratios for AS4(2)/J2 to AU4/J2 and AS4CGP/J2 to AU4/J2 were 1.22 and 1.24, respectively. The mechanical properties of these composites in the fiber direction were insensitive to the ISS. The transverse and shear moduli of the J2 composites were also not affected by the ISS. The static strengths, in general, ordered themselves from strong to weak as follows: AS4(2)/J2> AS4CGP/J2> AU4/J2. However, the creep rupture strength revealed a different ordering: AS4CGP/J2> AS4(2)/J2> AU4/J2. This suggests that static mechanical properties may not be a good indicator for long term mechanical performance. Experimental results showed that the interphase/interface region did not affect the degradation rates of the creep rupture strength of the J2 composites. DMA creep tests were performed at elevated temperatures for J2 composites. A master curve of each composite was generated. The shift factors obeyed the Arrhenius type equation. The activation energies of composites were approximately the same. The creep response of the AU4/J2, AS4(2)/J2, and AS4CGP/J2 composites were not dependent upon the ISS. Severe delaminations were observed in the AS4(1)/Jd2 composite laminates. The ([±45/90₂]_s) laminate tensile strength of AS4(1)/J2 composite was less than that of AS4(2)/J2 and AS4CGP/J2 composite. The creep rupture strength of the AS4(1)/J2 composite laminates degraded about two times faster than that of the other three composite systems. / Ph. D.

Concrete undergoes volumetric changes throughout its service life. These changes are a result of applied loads and shrinkage. Applied loads result in an instantaneous recoverable elastic deformation and a slow, time dependent, inelastic deformation called creep. Creep without moisture loss is referred to as basic creep and with moisture loss is referred to as drying creep. Shrinkage is the combination of autogeneous, drying, and carbonation shrinkage. The combination of creep, shrinkage, and elastic deformation is referred to as total strain. The prestressed concrete beams in the Chickahominy River Bridge have been fabricated with a lightweight, high strength concrete mixture (LTHSC). Laboratory test specimens have been cast using the concrete materials and mixture proportions used in the fabrication of the bridge beams. Two standard cure and two match cure batches have been loaded for 329 and 251 days, respectively. Prestress losses are generally calculated with the total strain predicted by the American Concrete Institute Committee 209 recommendations, ACI 209, or the European design code, CEB Model Code 90. Two additional models that have been proposed are the B3 model by Bazant and Baweja, and the GL2000 model proposed by Gardner and Lockman. The four models are analyzed to determine the most precise model for the LTHSC mixture. Only ACI 209 considered lightweight aggregates during model development. GL2000 considers aggregate stiffness in the model. ACI 209 was the best predictor of total strain and individual time dependent deformations for the accelerated cure specimens. CEB Mode Code 90 was the best predictor of total strain for the standard cure specimens. The best overall predictor of time dependent deformations was the GL2000 model for the standard cure specimens. / Master of Science

Clay consolidation has generally been considered from a macro-scale perspective by measuring the macro-scale compression of a clay soil over time. Clay particles in consolidation tests experience shear and normal forces at the inter-particle level due to force applied to the soil at the macro-scale. These shear and normal forces cause the particles to slide at the micro-scale and produce macro-scale changes in soil volume and shape. By considering the inter-particle interactions at the micro-scale, the shear force - normal force - velocity relationship can be described by the Rate Process Theory (RPT). This research investigated the use of the RPT for analyzing sliding at individual clay particle contacts during secondary compression to describe macro-scale clay behavior. The novel micro-scale friction experiments conducted in this research demonstrated that an Atomic Force Microscope (AFM) can be used to obtain coefficient of friction (μ) measurements for montmorillonite. This method allows for the measurements to be performed over spatial scales of a few microns, can be done under dry conditions or a wide range of aqueous solutions, and requires no calibration beyond making a few microscopic measurements of the probe. Control tests of silica on mica (μ = 0.29 ± 0.02) agree with literature values where limits indicate one standard deviation.μ values for wet and dry sodium montmorillonite were determined to be 0.20 ± 0.03 and 0.72 ± 0.03, respectively. The micro-scale AFM and macro-scale triaxial shear, ring shear, and direct shear experimental data ofμ as a function of sliding velocity were found to match well with those calculated using common RPT parameter values. The activation energy for the macro-scale triaxial shear and corresponding micro-scale friction regime experiments fall within the expected range for pure montmorillonite of 84–109 kJ/mol. Additionally, the micro-scale and macro-scale experimental results fall within the expected range for the number of bonds per unit of normal force of 10^7–10^9 bonds/N. A discrete element method (DEM) model was developed to calculate thin, disk-shaped clay particle movement in three dimensions during compression using the RPT as a contact model. The DEM compression results were compared to macro-scale consolidation experiments conducted on the same reference clay as the micro-scale AFM experiments. The influences on the compression of the number of bonds at each clay contact per unit of normal contact force and the activation energy were quantified. Increasing the activation energy decreased the compression, as expected. Similarly, increasing the number of bonds per unit of normal force at the contacts decreased the compression, as expected. Realistic clay fabrics with varying particle sizes, particle size distributions, and aspect ratios led to a compression model with behavior similar to the macro-scale laboratory compression tests. This research provides evidence of the close correspondence between macro-scale and micro-scaleμ measurements and contributes to multi-disciplinary understanding of factors that control friction between clay particles and deformation of clay masses. The results from this work can be applied to a wide range of time-dependent phenomena such as clay secondary compression, shear deformation, and fault dynamics behavior. / Ph. D.

Creep compliance of Hot Mix Asphalt (HMA) is an important property to characterize the material's viscoelatic behavior. It is used to predict HMA thermal cracking at low temperature and permanent deformation at high temperatures. There are several experimental methods to measure the creep compliance. Two of these methods were used in this thesis; uniaxial compressive and indirect tension (IDT) creep compliance. The tests were conducted at five temperatures (-15, 5, 20, 30, and 40°C) with a static loading for 1000-sec to characterize two typical HMA mixes used in Virginia, a base and a surface mix. Creep compliance master curves (CCMC) were developed by shifting the curves to a reference temperature using time-temperature superposition. Three mathematical functions, Prony series, power and sigmoidal, were fitted to the experimental data using regression analysis. Uniaxial CCMC were also predicted based on dynamic modulus measurements using method for interconversion of vicoelastic properties recommended in the literature. Finally, the susceptibility of the mixes to thermal cracking was evaluated based on the creep compliance measurements at low temperature. The regression analysis showed that the three mathematical models considered are appropriate to model the CCMC over a wide ranger of reduced times. The sigmoidal model provided the best fit over the entire range of reduced times investigated. This model also produced the best results when used in the interconversion procedures. However, there were noticeable differences between the CCMC predicted using interconversion and the experimental measurements, probably due to nonlinearity in the material behavior. The m-values for the base mix were higher using the creep results measured with both configurations. / Master of Science

This thesis presents an experimental study of creep and shrinkage of concrete made from local limestone aggregate and type III cement. The cylindrical test specimens were exposed to the atmosphere in order to observe the effects that variations of temperature and relative humidity had upon creep and shrinkage. These effects are shown by graphical presentations in the thesis. Creep strains for the five month period of investigation are shown graphically for each of four conditions of stress, 1000 psi constant stress 1000 psi variable stress 500 psi constant stress, and 500 psi variable stress. Exponential equations for these surveys were derived from the creep data of the five month study. Ultimate creep values were predicted from these equations and are presented together with their respective creep coefficients. These values are given to represent the anticipated creep for limestone aggregate concrete subjected to field conditions. / Master of Science

The creep and creep recovery behavior of a random fiber composite (SMC-R50) at elevated temperature and constant humidity were investigated experimentally and theoretically. The short time creep response for four constant stress levels at each of four selected temperature levels was experimentally determined. It was found that repeatable results can be obtained by applying a mechanical conditioning prior to each creep and creep recovery test. Creep data were modelled using the Findley equation which contains three parameters, ε_O (the instantaneous creep response), m (the amplitude of transient creep), and n (the time exponent). It was found that the time exponent is a function of time but approaches to an asymptotic value when the duration of creep is long. Thus, at a constant temperature level, one long-time creep test and four short-time creep tests were conducted. The long-time creep results were used to determine the proper time exponent n. The short time creep data for constant load were used to determine the Findley parameters ε_O and m. It was found that the Findley equation represented the creep results very accurately. Based on the short-time creep results, the Findley equation was used to predict the long time creep response and the creep response due to multiple step loadings. Five long time creep experiments were conducted. Four of them were 10,000 minutes long and were conducted at the same stress level (6,510 psi) but different temperature levels. The fifth creep experiment was conducted at 5,425 psi and 185°F over a three week period. Three multiple step creep experiments were conducted. These tests were of different load steps and durations. In all cases, it was found that the Findley equation predicted both long time creep response and multiple step creep response very accurately. Since repeatable results were obtained from conditioned specimens, the test results were compared to experimental data obtained from unconditioned specimens. It was found that experimental results of the conditioned specimens fell within the scatter band of the data for the unconditioned specimens. A free energy based failure criterion (proposed by Reiner and Weisenberg) was coupled with the Findley equation to predict the creep rupture time of SMC-R50. It was found that the critical free energy at the time of failure is temperature dependent. For a constant temperature, the critical free energy required for rupture is essentially a constant. It is also concluded that, for limited data, the Reiner-Weisenberg failure criterion provj.dl!S overall good prediction of the time to failure for SMC-R50. / Ph. D.

In-service inspections of high temperature pressureequipment show that weldments are prone to creep and fatiguedamage. It is not uncommon that severely damaged weldments arefound even before the design life of the component has beenreached. In order to improve this situation action has beentaken during the last decades, both from industry, universitiesand research institutes, aiming at an enhanced understanding ofthe weldment response. The work presented in this thesis focuses on numericalsimulation of weldment creep response. For a more profoundunderstanding of the evolution of creep damage in mismatchedlow alloy weldments, simulations are performed using thecontinuum damage mechanics, CDM, concept. Both design and lifeassessment aspects are addressed. The possibility to assessseam welded pipes using results from tests of cross-weldspecimens taken out from the seam is investigated. It is foundthat the larger the cross-weld specimen the better thecorrelation. The advantage to use the CDM concept prior to aregular creep analysis is also pointed out. In order to developthe CDM analysis, a modified Kachanov-Rabotnov constitutivemodel is implemented into ABAQUS. Using this model, a secondredistribution of stresses is revealed as the tertiary creepstage is reached in the mismatched weldment. Creep crack growth, CCG, in cross-weld compact tension, CT,specimens is investigated numerically where a fracturemechanics concept is developed in two steps. In the first one,the C*value and an averaged constraint parameter areused for characterising the fields in the process zone, whilein the second step, the creep deformation rate perpendicular tothe crack plane and a constraint parameter ahead of the cracktip, are used as characterising parameters. The influence oftype and degree of mismatch, location of starter notch as wellas size of CT specimen, is investigated. Results show that notonly the material properties of the weldment constituentcontaining the crack, but also the deformation properties ofthe adjacent constituents, influence the CCG behaviour.Furthermore, the effect of size is influenced by the mismatchof the weldment constituents. A circumferentially cracked girth weld with differentmismatch is assessed numerically by use of the fracturemechanics concept developed. The results show that type anddegree of mismatch have a great influence on the CCG behaviourand that C*alone cannot characterise crack tip fields.Corresponding R5 assessments are also performed. Comparisonwith the numerical investigation shows that the assumption ofplane stress or plane strain conditions in the R5 analysis isessential for the agreement of the results. Assuming the formerresults in a relatively good agreement for the axial stressdominated cases while for the hoop stress dominated cases, R5predicts higher CCG rates by an order of magnitude. <b>Keywords:</b>ABAQUS, constraint effect, continuum damagemechanics, creep, creep crack growth, design, design code,finite element method, fracture mechanics, life assessment,mismatch, numerical simulation, weldment