An improved knowledge of wood material behavior in hot-pressing environments can provide the benefit of an increased understanding of material properties during the manufacture of wood-based composites as well as insight into the development of new processes and products which manipulate the viscoelastic nature of wood. Two specific areas where additional knowledge is needed are: the high temperature equilibrium moisture content (EMC) behavior and the moisture dependent softening behavior. EMC data was collected and desorption isotherms were generated for mature and juvenile wood of aspen, loblolly pine and yellow-poplar at 50 and 160°C. High temperature EMC behavior was found to be distinct from that at lower temperatures, and considerable differences between the isotherms for juvenile and mature wood were detected. Substantial thermal degradation was observed during desorption at 160 °C and found to be strongly influenced by relative humidity. The thermal softening behavior of wood was evaluated using dielectric thermal analysis (DETA) at moisture levels from 0 to 20 percent. Coincident in situ relaxations of hemicellulose and amorphous cellulose in the range of 20 to 200 °C were observed and found to exhibit the characteristics of a secondary (glass) transition. The moisture dependence of this transition was characterized, and differences in the observed Tg were detected between juvenile and mature wood. Time-temperature superposition was also shown to be applicable to the wood-water system. / Ph. D.

This investigation is concerned with the flexural response of linear viscoelastic plates of constant thickness. Fundamental equations for both quasi-static and dynamic response of such plates are developed and solved for important cases of each. The term quasi-static ls used to indicate that Inertia forces due to deformation are neglected. These are included, of course, in the dynamic analysis. Solutions of the quasi-static equation are compared with experimental results obtained by measuring the deflection of a test plate made of Plexiglas. The basic viscoelastic stress-strain relations used in the derivation of the fundamental plate equations are taken in the form of a differential time operator equation. Use of this equation leads to results that are In a convenient form for reduction to a particular material such as a Kelvin or a Maxwell plate. Using a generalized virtual work principle based upon irreversible thermodynamic considerations the fundamental plate equation, including shear effects, ls established. The procedure involved ls that of determining a stationary value of a certain operational invariant by means of the calculus of variations. A simplified form of this equation, omitting the shear effects, is deduced and solutions for various load conditions obtained. An extended version of this simplified form which includes inertia effects due to deformation is developed by the principle of correspondence. This is used to study free vibrations of rectangular viscoelastic plates simply supported on all edges. Solutions of the simplified form of the fundamental equation for the case of so-called proportional loading, I.e. when the load function is the product of a space function multiplied by a time function, are given in terms of the equivalent elastic solution multiplied by a function of time. For more general types of loading the deflection and the load are expanded Into suitable infinite series and these series representations are inserted directly into the previously mentioned variational expression of the generalized virtual work principle. This leads to a set of ordinary differential situations in time the unknowns of which are the coefficients of the deflection expansion. These equations, as were the similar ones arising in the case of proportional loading, are solved by the Laplace transform method of the operational calculus. As an example of such a general loading the case of a moving line load on a rectangular plate is worked out. As a means of establishing a correlation between the deflection predicted by the analytical solution and actual deflections of Inelastic plates a set of static load tests were carried out on a square plate made or Plexiglas. The results are plotted and a comparison of the theoretical and experimental values given. The problem of determining the dynamic response of viscoelastic plates is treated using the method given above for solving the case of general loading for the quasi-static deflection. Under the assumption of incompressibility of the plate material explicit solutions in terms of the physical parameters involved are presented and discussed. For compressible plate materials methods are developed to give approximate solutions the accuracy of which depends on the degree of approximation used in determining the roots of certain cubics appearing in the transformed form of the governing dynamics equation. Conditions for the dynamic solutions to be oscillatory are indicated. / Ph. D.

Pavement is usually composed of aggregate, asphalt binder, and air voids; rigid pavement is built with hydraulic cement concrete; reinforced pavement contains steel. With these wide ranges of materials, different mechanical behaviors need to be defined in the pavement simulation. But so far, there is no research providing a comprehensive introduction and comparison between various contact models. This paper will give a detail exploration on the contact models that can be potentially used in DEM pavement simulation; in the analysis, it includes both a theoretical part, simulation results and computational time cost, which can reveal the fundamental mechanical behaviors for the models, and that can be a reference for researchers to choose a proper contact model. A new contact model—the power law viscoelastic contact model is implemented into software PFC 3D and is numerically verified. Unlike existing linear viscoelastic contact models, the approach presented in this thesis provides a detailed exploration of the contact model for thin film power-law creeping materials based on C.Y Chueng's work. This model is aimed at simulating the thin film asphalt layer between two aggregates, which is a common structure in asphalt mixtures. Experiments with specimens containing a thin film asphalt between two aggregates are employed to validate the new contact model. / Master of Science

Linear viscoelasticity theory is used to analyze the dynamic stability of composite, viscoelastic flat plates subjected to in-plane, biaxial edge loads. In deriving the associated governing equations, a hereditary constitutive law is assumed. In addition, having in view that composite-type structures exhibit weak rigidity in transverse shear, the associated governing equations account for the transverse shear deformations, as well as the transverse normal stress effect. The integro-differential equations governing the stability are solved for simply-supported boundary conditions by using the Laplace transform technique, thus yielding the characteristic equation of the system. In order to predict the effective time-dependent properties of the orthotropic plate, an elastic behavior is assumed for tile fiber, whereas the matrix is considered as linearly viscoelastic. In order to evaluate the nine independent properties of the orthotropic viscoelastic material in terms of its isotropic constituents, the micromechanical relations developed by Aboudi [24] are considered in conjunction with the correspondence principle for linear viscoelasticity. The stability behavior analyzed here concerns the determination of the critical in-plane normal edge loads yielding asymptotic stability of the plate. The problem is studied as an eigenvalue problem. The general dynamic stability solutions are compared with their quasi-static counterparts. Comparisons of the various solutions obtained in the framework of the Third Order Transverse Shear Deformation Theory (TTSD) are made with its first order counterpart. Several special cases are considered and pertinent numerical results are compared with the very few ones available in the field literature. / Master of Science

Although mechanical alterations to pelvic floor ligaments, such as the cardinal and uterosacral ligaments, are one contributing factor to the development and progression of pelvic floor disorders, very little research has examined their mechanical properties. In this study, the first biaxial elastic and viscoelastic tests were performed on uterosacral and cardinal ligament complexes harvested from adult female swine. Biaxial elastic testing revealed that the ligaments undergo large strains and are anisotropic. The direction normal to the upper vagina was typically stiffer than the transverse direction. Stress relaxation tests showed that the relaxation was the same in both directions, and that more relaxation occurred when the tissue was stretched to lower initial strains. In order to describe the experimental findings, a three-dimensional constitutive model based on the Pipkin-Rogers integral series was formulated and the parameters of such model were determined by fitting the model to the experimental data. In formulating the model, it was assumed that the tissues consist of a ground substance with two embedded families of fibers oriented in two directions and that the ligaments are incompressible. The model accounts for finite strains, anisotropy, and strain-dependent stress relaxation behavior. This study provides information about the mechanical behavior of female pelvic floor ligaments, which should be considered in the development of new treatment methods for pelvic floor disorders. / Master of Science

The increase and change in traffic loading in recent years has resulted in the introduction of a new range of high performance asphalt binders. These new binders known as modified asphalt binders, have a more complex behavior than traditional binders. A review of the current mathematical models shows that most of them suffer from different drawbacks that make them inadequate for their intended application. To describe the behavior of straight and modified binders in the thermorheologically simple linear viscoelastic region, two models are proposed. Models to characterize the absolute value of the complex shear modulus (|G*|) and the phase angle (d) were developed using the matching function approach and validated by an experimental program. The dynamic mechanical properties of two typical paving grade binders and three modified binders were tested at intermediate and high service temperatures. Short-term and long-term aging were simulated by the rolling thin film oven test and the pressure aging vessel test, respectively. A dynamic shear rheometer with parallel plate configuration was used to conduct the dynamic mechanical tests at frequencies between 0.06 to 188.5 rad/sec and temperatures ranging from 5 to 75°C. Prior to the frequency sweeps, strain sweeps were performed to establish the linear viscoelastic region. Results indicated a strong susceptibility to the defined strain at intermediate temperatures; however, strain susceptibility was less pronounced at high temperatures. Frequency sweeps were then conducted at a constant strain corresponding to greater than 95% of the initial complex shear modulus as established by AASHTO TP5 for straight asphalts. The Time-Temperature Superposition Principle was used to construct the master curves. The shift factors were determined based on the complex shear modulus master curves and verified for the phase angle, storage shear modulus and loss shear modulus. After construction of the master curves, non-linear regression was used to fit the proposed models to the experimental data. Comparison between the measured and predicted values indicated a good agreement for frequencies higher than 10⁵ rad/sec. The phase angle model was found to adequately describe unmodified binder with a small percentage of errors (less than 6%). On the other hand, the phase angle model was found unable to simulate the plateau region observed for polymer-modified binders. However, the error in this case was found to be relatively small (from zero to 10%). The ability of the models to estimate other viscoelastic functions, e.g. storage shear modulus (G'), loss shear modulus (G"), and relaxation spectrum (H(t)), was found to be adequate. / Master of Science

Yes / The compression stiffness of a phospholipid microbubble was determined using force-spectroscopy as a function of temperature. The stiffness was found to decrease by approximately a factor of three from 0.08 N m 1, at 10 C, down to 0.03 N m 1 at 37 C. This temperature dependence indicates that the surface tension of lipid coating is the dominant contribution to the microbubble stiffness. The timedependent material properties, e.g. creep, increased non-linearly with temperature, showing a factor of two increase in creep-displacement, from 24 nm, at 10 C, to 50 nm, at 37 C. The standard linear solid model was used to extract the visco-elastic parameters and their determination at different temperatures allowed the first determination of the activation energy for creep, for a microbubble, to be determined. / EPSRC

Yes / Liquid crystal is a new emerging biomaterial. The physical property of liquid crystal plays a role in supporting the adhesion of cells. Nano and microball indentation techniques were applied to determine the elastic modulus or viscoelasticity of the cholesteryl ester liquid crystals in the culture media. Nano-indentation results (108 ± 19.78 kPa, N = 20) agreed well with the microball indentation (110 ± 19.95 kPa, N = 60) for the liquid crystal samples incubated for 24 hours at 37o C, respectively. However, nanoindentation could not measure the modulus of the liquid crystal (LC) incubated more than 24 hours. This is due to the decreased viscosity of the liquid crystal after immersion in the cell culture media for more than 24 hours. Alternatively, microball indentation was used and the elastic modulus of the LC immersed for 48 hours was found to decrease to 55 ± 9.99 kPa (N = 60). The microball indentation indicated that the LC did not creep after 40 seconds of indentation. However, the elastic modulus of the LC was no longer measurable after 72 hours of incubation due to the lost of elasticity. Microball indentation seemed to be a reliable technique in determining the elastic moduli of the cholesteryl ester liquid crystals. / Science Fund Vot. No. S024 or Project No. 02- 01-13-SF0104 and FRGS Vot. No. 1482 awarded by Malaysia Ministry of Education

This study consisted of two main parts, the thermoviscoelastic characterization of Kevlar 49/Fiberite 7714A epoxy composite lamina and the development of a numerical procedure to predict the viscoelastic response of any general laminate constructed from the same material. The four orthotropic material properties, S₁₁, S₁₂, S₂₂, and S₆₆, were characterized by 20 minute static creep tests on unidirectional ([0]₈, [10]₈, and [90]₁₆) lamina specimens. The Time-Temperature-Superposition-Principle (TTSP) was used successfully to accelerate the characterization process. A nonlinear constitutive model was developed to describe the stress dependent viscoelastic response for each of the material properties. A new numerical procedure to predict long term laminate properties from lamina properties (obtained experimentally) was developed. Numerical instabilities and time constraints associated with viscoelastic numerical techniques were discussed and solved. The numerical procedure was incorporated into a user friendly microcomputer program called Viscoelastic Composite Analysis Program (VCAP), which is available for IBM ‘PC’ type computers. The program was designed for ease of use and includes graphics, menus, help messages, etc. The final phase of the study involved testing actual laminates constructed from the characterized material, Kevlar/epoxy, at various temperature and load levels for 4 to 5 weeks. These results were then compared with the VCAP program predictions to verify the testing procedure (i.e., the applicability of TTSP in characterizing composite materials) and to check the numerical procedure used in the program. The actual tests and predictions agreed, within experimental error and scatter, for all test cases which included 1, 2, 3, and 4 fiber direction laminates. The end result of the study was the development and validation of a user friendly microcomputer program that can be used by design engineers in industry to predict thermoviscoelastic properties of orthotropic composite materials. / Ph. D.

The viscoelastic response of polymer-based composite laminates, which may take years to develop in service, must be anticipated and accommodated at the design stage. Accelerated testing is therefore required to allow long-term compliance predictions for composite laminates of arbitrary layup, based solely upon short-term tests. In this study, an accelerated viscoelastic characterization scheme is applied to T300/5208 graphiteepoxy laminates. The viscoelastic response of unidirectional specimens is modeled using the theory developed by Schapery. The transient component of the viscoelastic creep compliance is assumed to follow a power law approximation. A recursive relationship is developed, based upon the Schapery single-integral equation, which allows approximation of a continuous time-varying uniaxial load using discrete steps in stress. The viscoelastic response of T300/5208 graphite-epoxy at 149C to transverse normal and shear stresses is determined using 90-deg and 10-deg off-axis tensile specimens, respectively. parameters In each case the seven viscoelastic material required in the analysis are determined experimentally, using a short-term creep/creep recovery testing cycle. A sensitivity analysis is used to select the appropriate short-term test cycle. It is shown that an accurate measure of the power law exponent is crucial for accurate long-term predictions, and that the calculated value of the power law exponent is very sensitive to slight experimental error in recovery data. Based upon this analysis, a 480/120 minute creep/creep recovery test cycle is selected, and the power law exponent is calculated using creep data. A short-term test cycle selection procedure is proposed, which should provide useful guidelines when other viscoelastic materials are being evaluated. Results from the short-term tests on unidirectional specimens are combined using classical lamination theory to provide long-term predictions for symmetric composite laminates. Experimental measurement of the long-term creep compliance at 149C of two distinct T300/5208 laminates is obtained. A reasonable comparison between theory and experiment is observed at time up to 10 5 minutes. Discrepancies which do exist are believed to be due to an insufficient modeling of biaxial stress interactions, to the accumulation of damage in the form of matrix cracks or voids, and/or to interlaminar shear deformations which may occur due to viscoelastic effects or damage accumulation. / Doctor of Philosophy