Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 162-165). Also available via the Internet.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 75-76). Also available via the Internet.

Thesis (Ph. D.)--University of Wisconsin, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 119-127).

Long-term creep and recovery models (master curves) were developed from short-term data using the time temperature superposition principle (TTSP) for kiln-dried southern pine loaded in compression parallel-to-grain and exposed to constant environmental conditions (~70°F, ~9%EMC). Short-term accelerated creep (17 hour) and recovery (35 hour) data were collected for each specimen at a range of temperature (70°F-150°F) and constant moisture condition of 9%. The compressive strain was measured using bonded electrical resistance strain gages. For each specimen, the compliance curves for all the temperature levels were plotted against log-time on the same plot. The curve segments at successively higher temperature levels were shifted along the log-time axis with respect to the curve section at 70°F to construct a master curve for each specimen. The extrapolation of the developed master curves ranged from 0.23 to 6.4 years. The requirement that the shift factors below glass transition temperature follow Arrhenius formulation was satisfied by the empirical shift factors. The activation energy for creep and recovery of kiln-dried southern pine derived from the slope of the plot of horizontal shift factor and the inverse of the absolute temperature was 28 KCal/mole. Creep and recovery master curves were represented by power functions and the nonlinear regression analysis was used to estimate the model parameters. Linear regression models were developed to predict one parameter in creep and recovery models from Young's modulus. The other model parameter showed weak correlations with material properties; therefore, an average value was recommended. The validity of the master curves for predicting creep of wood exposed to normal interior environmental conditions in buildings was tested by conducting long-term (10 month) creep tests in a heated/cooled laboratory environment. The fluctuating test environmental conditions caused geometry changes in the surface of the wood specimens in addition to mechanosorptive creep leading to fluctuating long-term data. Therefore, a good agreement between the master curves and long-term data was not found. Creep behavior of shallow southern pine arches was studied to demonstrate the application of the finite element method, incorporating the long-term curves based on TISP, to predict creep in wood structures. Creep tests were conducted at various load levels applied at ambient environmental conditions for two months. One arch failed (i.e., snapped-through) nine days after the tests began indicating that creep can indeed cause instability failure in shallow structures. It was found that the supports in the arch test fixture deflected elastically; therefore, the arches were modeled as three pin structures with base pin joints supported by zero-length linear elastic springs. However, the elastic analysis results revealed the presence of other factors affecting the experimental response which complicated the modeling procedure. The creep analysis was performed using a finite element model incorporating the developed creep master curves; however, due to the complexity in the creep experimental apparatus, the numerical predictions were not validated experimentally. / Ph. D.

To date there are no long-term creep models or practical methods to investigate the effect of creep on the safety and serviceability of modem wood structures and structural wood composites. Long-term creep models were developed for wood in tension and compression using the Time-Temperature Superposition Principle (TTSP). The principle states that the long-term response of a polymer at lower temperature is equivalent to the short-term response at a higher temperature. Accelerated creep tests were conducted in tension and compression using small clear specimens of Douglas-fir, southern pine and yellow-poplar. The specimens were tested at moisture contents of 6 %, 9 %, and 12 %, and at temperatures between 20°C and 80°C. The strain was measured using bonded strain gages. The individual creep compliance for each temperature was shifted along the log-time axis to obtain a "master" curve that describes the creep response of the specimens. All compliance curves also required vertical shifting. The experimental horizontal shift factors followed the Arrhenius formulation that describes the shift factor relation for polymers in the glassy region. / Master of Science

Ultrasonic velocity and attenuation were both measured on samples containing various degrees of damage due to high temperature creep. These results were compared with parameters associated with creep damage such as strain and cavity formation, in order to better understand the mechanisms of creep in Si/SiC and to determine if ultrasonics can be used in evaluating the severity of damage. The data indicated that both ultrasonic velocity and attenuation are directly related to creep strain and can be used in evaluating creep damage. Ultrasonic velocity was found to be exponentially related to creep strain. Cavity formation was found not to significantly affect either of the measured ultrasonic properties. The results indicated that Si/SiC behaves as a two phase material in that high frequency ultrasound propagates primarily through the silicon carbide phase and not by the silicon phase. / Master of Science