No description available.

No description available.

No description available.

No description available.

Bibliography: pages 92-98. / In this thesis a simplified analysis procedure is developed, in which creep laws are decoupled from damage laws, for the purposb of constructing methods of use in the early stages of high temperature design. The procedure is based on the creep and damage laws proposed by Kachanov and Rabotnov. The creep laws are normalised. with respect to a convenient normalising stress. As a consequence of this normalisation, the dependence of the creep law on the stress constant, the time and temperature functions, and the actual load level is removed. In addition, if the reference stress of the component is chosen as the normalising stress, the creep law becomes insensitive to the stress exponent. The non-dimensional creep laws are then implemented in a standard finite element scheme, from which the results of a stationary state creep analysis are then in non-dimensional form. In order to estimate rupture times, the maximum stationary stresses in a component are used together with the damage laws. Conservative failure criteria are derived from the creep and damage laws to extend the method to residual life assessment and damage monitoring. The procedure is illustrated and tested against simple examples and case studies.

The main source of the time-dependent behavior of fiber-reinforced composites is their polymeric matrix, which causes concerns about their long term durability. Although for composites where organic fibers such as Graphite are used, the fibers are also a contributing factor. A composite material may exhibit an appreciable amount of creep, depending on the state of stress and temperature. Viscoelastic flow in the matrix and internal flaw formation and growth are the main sources of this creep. Thus a study was made on the viscoelastic behavior of GI/BMI fiber reinforced composite. An experimental method for testing a large number of composite materials in compression was developed. The samples were tested according to the test matrix consisting of combinations of static and cyclic loads and temperatures. The fixtures were calibrated to check the validity of measurements and reproducibility of results. Stress gradients were caused by frictional effects between the fixture and samples. The modulus change of samples over a period of time were studied. Bending parameters in samples were measured and analyzed for different stresses, clamping forces, temperatures and time. Mechanical models were used to explain the basic principles behind creep of a viscoelastic material followed by a theoretical explanation and study of creep. The linear and non-linear viscoelastic constants were studied and a methodology to analyze these results was presented. The linear and non-linear constants were used in a prediction model and predictions of a composite creep strain with time were made. Creep data obtained tor [45/0/-45/90]������ for a period of three months were compared to the prediction model. / Graduation date: 1994

The effect of thermoreversible physical aging on the linear viscoelastic creep properties of a thermoplastic composite was investigated. Radel X/IM7, an amorphous composite material considered for use in the next generation high speed transport aircraft, was studied. The operating environment for the aircraft material will be near 188°C (370°F) with a service life in excess of 60,000 hours at temperature. Accurate predictions of the viscoelastic properties of the material are essential to insure that design strength and stiffness requirements are met for the entire service life. The effect of physical aging on the creep response was studied using momentary tensile creep tests conducted at increasing aging times following a rapid quench from above the glass transition temperature (Tg) to a sub-Tg aging temperature. As the aging time increased, the creep response of the material significantly decreased. The tensile creep compliance data for each aging time were fit to the empirical equation for the creep compliance D(t): D(t)-Dₒ<i>e</i> ^{t/tₒ)m} where Dₒ, tₒ,and m are fitting parameters determined using a nonlinear fitting program based on the Levenberg-Marquardt finite difference algorithm. The short-term creep compliance curves, obtained at various aging times, were then shifted to form a momentary master compliance curve. The double logarithmic aging shift rate μ and its dependence on sub-T_g aging temperature were determined. The aging characterization process was conducted on unidirectional specimens with 0, 90, and 45 degree fiber direction orientations. This permitted the calculation of the complete principal compliance matrix for the composite material. The effect of physical aging becomes more apparent during long-term tests when creep and aging occur simultaneously. This results in a gradual stiffening and decrease in the creep response with increased time. Predictions based solely on the Time-Temperature Superposition Principle would significantly over-predict the creep response if physical aging effects were ignored. Theoretical predictions for long-term creep compliance were made using an effective time theory and compared to long-term experimental data for each fiber orientation. Finally, experimental results of a long-term test of a 30 degree fiber angle orientation specimen were compared to theoretical predictions obtained by transforming the principal compliance matrix to the 30 degree orientation. / Ph. D.

This investigation addresses the interaction mechanisms of time dependent material behavior and cyclic damage during fatigue loading of fiber reinforced composite laminates. A new term 'damorheology' has been coined to describe such physical behavior. The lamina has been chosen as the building block and a cross ply laminate configuration was the selected test case. The chosen material system is the Radel X/T65-42 thermoplastic composite by Amoco. The fatigue performance at the lamina level is represented by the dynamic stiffness, residual strength and fatigue life of unidirectional laminates. The time dependent behavior is represented at the lamina level by a Pseudo-Analog Mechanical model. The thermo-rheological characterization procedure combines mechanical (creep) and thermal (dynamic mechanical analysis) techniques. / Ph. D.

A new type of computer-controlled instrument has been developed to measure microviscoelastic properties of thin materials. It can independently control and measure indentation loads and depths in situ revealing information about material creep and relaxation. Sample and indenter positions are measured with a specially designed polarization interferometer. Indenter loadings can be varied between 0.5 and 10 grams and held constant to ±41 mg. The resulting indentation depths can be measured in situ to ±1.2 nm. The load required to maintain constant indentation depths from 0.1 to 5.0 microns can be measured in situ to ±3.3 mg and the depth held constant to ±15 nm.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Ocean Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / Ph.D.