Not available until OCRd / Master of Science

A new compact double-notched specimen for in-plane shear testing of isotropic, anisotropic and composite materials was developed. A preliminary test of the specimen was performed on a cross-ply laminate. High sensitivity moire interferometry was used to determine the shear and normal strains on the entire face of the specimen. The results of the test were compared to those of the Iosipescu specimen on the same material. The compact specimen produced a more uniform and more pure shear distribution than the Iosipescu specimen. Stiffness measurements of the composite material were made. A detailed investigation of the specimen was performed and it revealed important attributes and deficiencies of the specimen. Recommendations for future work are cited. The interlaminar shear response of a cross-ply composite was investigated. Shear strains on a ply-by-ply basis were measured using moire interferomety. Qualitative and quantitative information of the interlaminar shear characteristics of the material were obtained. The interlaminar shear modulus of individual plies and the effective modulus for the laminate were determined. Variations of nominally equal plies were observed. / Master of Science

The effect of surface pretreatment on the physical and chemical properties of carbon fiber-polyimide matrix composite surfaces was evaluated. Eight pretreatments were studied: methanol wash, gritblast, sulfuric acid soak, ammonia plasma, argon plasma, argon plasma followed by ammonia plasma, nitrogen plasma, and oxygen plasma. The pretreated surfaces were chemically characterized through the use of XPS (X-ray photoelectron spectroscopy), ISS (ion scattering spectroscopy), and PAS-FTIR (photoacoustic Fourier transform infrared spectroscopy). Surface fluorocarbon contamination was appreciably reduced with gritblasting, argon plasma and oxygen plasma pretreatments. Specific functional groups were incorporated into the composite surfaces through the choice of gases used in the pretreatment. Physical changes were determined through the use of HR-SEM (high resolution scanning electron microscopy). With the exception of the macroroughening produced by gritblasting and the pitting produced by long exposure times in the oxygen plasma, no signlficant differences in the topography of the pretreated composites were observed. The wettability of the pretreated composite surfaces increased, as demonstrated with contact angle measurements, due to a combination of a decrease in the fluorocarbon contamination and an increase in the surface functionality present. / Ph. D.

The simulation program FWCURE [1] models the curing process and layer tension loss of axisymmetric filament wound composite cases during fabrication. For a specified temperature cure cycle, the model predicts the temperature distribution, resin degree of cure, viscosity, layer compaction, and fiber motion throughout the composite case during cure. The scope of the simulation program developed by Tzeng [1] has been extended, and the modifications to the FWCURE program are the goals of this investigation. Major modifications to FWCORE include a more general 2-D layer tension loss model, additions to an element curvature calculation routine, a new cure reaction kinetics model and viscosity model for a Fiberite-974 epoxy resin system, and modifications and additions to Input/Output (I/O) throughout the program. Modifications and additions to FWCORE are implemented in the analysis of an 18 inch diameter test bottle. Results of the simulation are compared with test data obtained during winding and cure of a graphite-epoxy 18 inch test bottle. Excellent agreement was obtained between the results of the model and data. Another major accomplishment involved coupling FWCURE with a thermo-mechanical stress simulation program called WACSAFE. When combined, the coupled program forms an improved comprehensive structural model which characterizes the thermal, chemical, physical, and mechanical processes occurring during winding and cure of filament wound composite cases. The complete simulation program should provide the process engineer with a resource to help select an optimum fabrication cycle, assess the processing characteristics of new matrix reein eystems, and act as a simulator to yield real time, closed loop process control. FWCURE should also provide information on the processing parameters that have the greatest effect on the final filament wound composite structure. / Master of Science

The iso- and non-isothermal crack problems of layered fiber-reinforced composite materials are investigated within the framework of linear anisotropic thermoelasticity and under the state of generalized plane deformation. The crack is assumed to be parallel to the layer bounding surfaces. By employing the Fourier integral transform technique and the flexibility/stiffness matrix formulation, the current mixed boundary value problems are reduced to solving a set of simultaneous singular integral equations with Cauchy-type kernels. The crack·tip stress intensity factors are then defined in terms of the solution of the integral equations. Numerical results are presented addressing the salient and unique features for a class of crack problems involving highly anisotropic fibrous composite materials. Specifically, the cases of a crack embedded i) within a homogeneous and anisotropic slab, ii) between two bonded dissimilar anisotropic half-spaces and iii) within the matrix-rich interlaminar region of a generally laminated anisotropic slab are considered. The effects of relative crack size, crack location and fiber volume fraction on the stress intensity factors are examined as a function of über angle. For the case of layered composites, the matrix-rich interlaminar region is modeled as a separate interlayer. As the interlayer thickness approaches zero, the interlaminar crack model illustrates no smooth transition to the ideal interface crack model of zero interlayer thickness which exhibits oscillatory stress singularities. The mixed-mode crack tip response is shown to involve the simultaneous presence of three fracture modes. It is demonstrated that the corresponding values of stress intensity factors are strongly influenced by the laminate stacking sequence and layer orientation. In addition, the partially insulated crack surface condition is observed to alleviate the severity of thermally-induced stress fields near the crack tip. / Ph. D.