Woven (also known as textile) composites are one class of polymer matrix composites with increasing market share in aerospace, autmobile, civil infrastructure applications mostly due to their lightweight, their flexibility to form into desired shape, their mechanical properties and toughness. Due to the viscoelasticity of the polymer matrix, time-dependent degradation in modulus (creep) and strength (creep rupture) are two of the major mechanical properties required by engineers to design a structure reliably when using these materials. Unfortunately, creep and creep rupture of woven composites have received little attention by the research community and thus, there is a dire need to generate additional knowledge and prediction models, given the increasing market share of woven composites in load bearing structural applications. In this thesis, an analytical creep model, namely the Modified Equivalent Laminate Model (MELM), was developed to predict tensile creep of plain weave composites for any orientation of the load with respect to the orientation of the fill and warp fibers, using creep of unidirectional composites. The model was validated using an extensive experimental involving the tensile creep of plain weave composites under varying loading orientation and service conditions. Plain weave epoxy (F263)/ carbon fiber (T300) composite, currently used in aerospace applications, was procured as fabrics from Hexcel Corporation. Creep tests were conducted under two loading conditions: on-axis loading (00) and off-axis loading (450). Constant load creep, in the temperature range of 80–2400C and stress range of 1-70% UTS of the composites, was experimentally evaluated for time periods ranging from 1–120 hours under both loading conditions. The composite showed increase in creep with increase in temperature and stress. Creep of composite increased with increase in angle of loading, from 1% under on-axis loading to 31% under off-axis loading, within the tested time window. The experimental creep data for plain weave composites were superposed using TTSP (Time Temperature Superposition Principle) to obtain a master curve of experimental data extending to several years and was compared with model predictions to validate the model. The experimental and model results were found in good agreement within an error range of +1-3% under both loading conditions. A parametric study was also conducted to understand the effect of microstructure of plain weave composites on its on-axis and off-axis creep. Additionally, this thesis generated knowledge on time-dependent damage in woven composites and its effect on creep and tensile properties and their prediction.

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In methanol processing plants, steam-methane reformers consist of hundreds of vertical tubes operating at temperatures up to 1000°C. These reformer tubes fail by creep through the formation of creep voids during service. Preliminary research showed that the occurrence of these voids was not random and may be related to certain microstructural features of the material. In the present research, the technique of serial sectioning was used to generate threedimensional reconstructions of voids in several steam-methane reformer tube samples with creep damage. The serial sectioning method and subsequent 3D reconstruction revealed creep void information such as size, density, location, and shape in three-dimensions, information that cannot be obtained from two-dimensional micrographs alone. Samples were obtained at various locations along the length of an ex-service reformer tube to investigate the effects of service conditions on the characteristics of creep voids. In addition, samples were taken from various positions along the wall thickness where there were differences in temperature, stress, and microstructure. Additionally, the identity and crystallographic orientations of the phases adjacent to creep voids were studied by electron backscatter diffraction (EBSD) to determine the crystallographic trends in creep void locations. Three dimensional observations revealed that creep voids were indeed not uniformly distributed through the volume in terms of their size, shape, and location. All voids appeared next to carbides and these voids came into contact with M₂₃C₆ precipitates somewhere along their perimeter. Most of the voids were found on austenite (ɣ) grain facets (the interface between two ɣ grains) but the larger voids were generally found at grain edges and corners. The grain boundaries where voids were located were generally oriented at 45 degrees with respect to the hoop stress direction. Here, the effective stress due to a combination of loading and temperature were highest. xviii Abstract EBSD results showed that 80% of the M₂₃C₆ precipitates surrounding these voids have an irrational crystallographic orientation relationship (OR) with the austenite matrix. In contrast, grain boundary precipitates in an aged sample always show a rational OR with respect to one adjacent grain. This implied that the preferred sites for creep voids are low registry boundaries between M₂₃C₆ precipitates and austenite. The data obtained from 3D observations were applied to a classic void growth model. Various permutations of the parameters obtained from this work were applied to the model to simulate conditions that may be beneficial to extending the service lives of reformer tubes. It was shown that the void growth model required accurate and representative materials constants for good estimation of life. Furthermore, the model revealed that more work was required in terms of observations of void nucleation in 3D, in order to fully utilize the model. Finally, it was shown that void density measurements are the most critical item for accurate prediction of growth of voids.

Woven (also known as textile) composites are one class of polymer matrix composites with increasing market share in aerospace, autmobile, civil infrastructure applications mostly due to their lightweight, their flexibility to form into desired shape, their mechanical properties and toughness. Due to the viscoelasticity of the polymer matrix, time-dependent degradation in modulus (creep) and strength (creep rupture) are two of the major mechanical properties required by engineers to design a structure reliably when using these materials. Unfortunately, creep and creep rupture of woven composites have received little attention by the research community and thus, there is a dire need to generate additional knowledge and prediction models, given the increasing market share of woven composites in load bearing structural applications. In this thesis, an analytical creep model, namely the Modified Equivalent Laminate Model (MELM), was developed to predict tensile creep of plain weave composites for any orientation of the load with respect to the orientation of the fill and warp fibers, using creep of unidirectional composites. The model was validated using an extensive experimental involving the tensile creep of plain weave composites under varying loading orientation and service conditions. Plain weave epoxy (F263)/ carbon fiber (T300) composite, currently used in aerospace applications, was procured as fabrics from Hexcel Corporation. Creep tests were conducted under two loading conditions: on-axis loading (00) and off-axis loading (450). Constant load creep, in the temperature range of 80–2400C and stress range of 1-70% UTS of the composites, was experimentally evaluated for time periods ranging from 1–120 hours under both loading conditions. The composite showed increase in creep with increase in temperature and stress. Creep of composite increased with increase in angle of loading, from 1% under on-axis loading to 31% under off-axis loading, within the tested time window. The experimental creep data for plain weave composites were superposed using TTSP (Time Temperature Superposition Principle) to obtain a master curve of experimental data extending to several years and was compared with model predictions to validate the model. The experimental and model results were found in good agreement within an error range of +1-3% under both loading conditions. A parametric study was also conducted to understand the effect of microstructure of plain weave composites on its on-axis and off-axis creep. Additionally, this thesis generated knowledge on time-dependent damage in woven composites and its effect on creep and tensile properties and their prediction.

No description available.