In SiC/Ti composites, the physical-chemical behaviour at the interface at fabrication temperatures, is important. Ideally, only a thin bonding film is desired after fabrication with zero growth during the actual service conditions. In this study, two mechanisms which lower the growth rate of silicides about the SiC/Ti interface were investigated. Planar composites that incorporated Mo₅Si₃ or Al films, were compared with the uncoated SiC/Ti standards to investigate the "barrier" and "rejection" mechanisms respectively. Also, SiC/Ti₃Al/Ti composites were used to study the effectiveness of the two mechanisms combined, with the Ti₃Al acting as a barrier and with Ti(Al) retarding the reaction by rejection. Samples were vacuum-annealed at 875°C for various times and examined by X-ray diffraction techniques. The results from the SiC/Al/Ti and SiC/Ti₃Al/Ti composites indicated that silicide formation would be retarded in the presence of an Al saturated Ti matrix. Also, the formation of a Ti₃Al layer near the SiC/Ti interface serves as an effective barrier to diffusion of Si atoms. In the SiC/Mo₅Si₃/Ti composites however, the dissociation of the Mo₅Si₃ phase formed during sample preparation results in additional silicide formation at the interfaces. If stoichiometric Mo₅Si₃ films are sputter-deposited, they could be used as effective barrier. The inter-diffusion of Al films, sputter deposited on Ti substrates, at elevated temperatures have been analyzed. Samples were vacuum annealed at 635°C and 900°C for various times and then examined using X-ray diffraction. In each case, intensity bands from alpha-Ti and diffraction lines from an intermetallic compound have been observed. For subsequent annealing times, the integrated intensities from the compound decreased slightly, indicating that the compound partially decomposed and released Al into the alpha-Ti lattice. This resulted in an alpha-Ti diffracted intensity band that results from a range of compositions. Intensity bands from Ti(101) were used to obtain composition profiles for Al in alpha-Ti, by using X-ray diffraction techniques and computer simulations. In each case, mass conservation of Al atoms was used to determine the interface motion. It was assumed that the Al profiles extend continuously into the_disordered Ti₃Al and Ti₂Al composition regions. This assumption was verified by preparing a Ti₂Al (disordered) powder standard and obtaining accurate lattice parameters. An iterative solution was used to determine composition dependent diffusion coefficients of one-dimensional zones, for two-phase systems. Diffusion zones extending from a few tenths to several microns have been examined. Al diffusion coefficients in alpha-Ti and the Ti₃Al phase have been determined. Activation energies were calculated from the diffusion coefficients at 635°C and 900°C and compared with those obtained from a melting point correlation. Lower activation energies from the present investigation indicated that there was some grain boundary diffusion at these intermediate temperatures. / Ph. D.

Composite materials are considered here as structural materials of highway bridge superstructures. Bridge deck designs can be done according to <i>AASHTO</i>¹ specification and elastic design concepts. In order to evaluate the feasibility of composites as structural materials of highway bridge superstructures, composite materials are compared not only to composite materials themselves but also to the most popular bridge structural materials, which are reinforced concrete and structural steel. The <i>AASHTO</i>¹ HS2O-44 truck load is selected as the standard loading condition of all designs. Loads other than dead load and live load are not considered. Configurations of the bridges are different. Appropriate cross-section of girders are selected according to the material types. For fiber reinforced composite materials, box girder is used, for reinforced concrete, T-beam is selected; in addition, steel concrete composite section is another case. Design methods are different from material to material. Reinforced concrete T-beam design is based on the 'Ultimate Strength Design' method. Steel concrete composite sections are designed according to the 'Load & Resistance Factor Design'. For composite materials, 'Elastic Design' is selected. The results derived are as expected. Substantial weight saving is achieved by simply replacing concrete or steel with composite materials. This also results in many other advantages such as construction time, cost, foundation settlement and support requirements. / Master of Science

The wave propagation mechanism of mode transitions was studied in unidirectional composite materials. Theoretical calculations were compared with experimental measurements. Mode transitions occur when the wave vector orientation is varied in unidirectional samples of T300/5208 graphite/epoxy composite with a 0.6 fiber volume fraction. The mode transition occurs at 51.85° wave vector orientation with respect to the fiber direction. Energy flux deviation and particle displacement directions and amplitudes were also compared with theory. To show this mode transition, an attenuation study was performed. The attenuation coefficient, measured in units of reciprocal time, does not appear to depend on the wave vector orientation and the wave type (quasi-transverse and quasi-longitudinal waves) at 5 MHz frequency. But the attenuation coefficient, expressed in units of reciprocal length, does depend on the wave type and the wave vector orientation because the wave velocity is included in the calculation of this coefficient. / Master of Science

An investigation of the behavior of shear deformable plate finite elements is conducted to determine why and under what conditions these elements lock, or become overly stiff. For this purpose, a new analytical technique is developed to derive the exact form of the shear constraints which are imposed on an element when its side-tothickness ratio is large. The constraints are expressed in terms of the nodal degrees of freedom, and they are easily interpreted as being either the proper Kirchhoff constraints or spurious locking constraints. Moreover, the technique is applicable to any displacement-based shear deformable beam, plate or shell element regardless of the shear deformation theory or the order of the Gauss-Legendre integration rule which is used to numerically evaluate the stiffness coefficients. To gain a better understanding of locking phenomena, the constraints which arise under full and reduced integration are derived for various Mindlin and Reddy-type beam and plate elements. These analytical findings are then compared with numerical results of isotropic and laminated composite plates, verifying the role that shear constraints play in determining the behavior of thin shear deformable elements. The results of the present study lead to definitive conclusions regarding the origin of locking phenomena and the effect of reduced integration. / Master of Science

The Illinois Institute of Technology Research Institute (IITRI) composite compression test method is probably the most widely accepted as producing accurate results. Difficulties associated with the application of this method, however, have fueled continued research into alternate methods. In this study, the Wyoming End-Loaded Side-Supported (ELSS) test scheme was investigated in a combined shear/end loaded mode. This system was examined experimentally and analytically to establish the method as a reasonable alternative to IITRI testing. Combined shear/end loading was accomplished by applying a controlled amount of clamping force to the stabilizing blocks, allowing them to transmit shear force to the coupon faces through friction. The fabrication of test specimens from a BASF graphite IBMI material system is described in detail, including the application of a novel "press-clave" processing system developed at Virginia Tech. This system allows for cost-effective manufacture of flat panels under conditions similar to autoclave processing. For the material system employed in this study, this process proved superior to conventional hot press processing. Experimental results include strengths and failure modes for both tabbed and untabbed coupons tested by the combined loading ELSS method, and these data are compared to the results by the IITRI method. Strength was found to increase with increasing clamping, but reached a maximum with both untabbed and tabbed specimens. The maximum strength achieved with untabbed specimens was still significantly below the IITRI value, but the tabbed specimens reached strengths comparable to the IITRI data. The failure mode also changed with increased clamping, from end crushing to brooming with untabbed specimens, and from end crushing to shear failure with tabbed specimens. As the strength and failure mode achieved with the combined loading ELSS method using tabbed specimens are both similar to those seen in the IITRI tests, this modified technique can be considered equivalent to the IITRI method. Preliminary data from tests using novel nonbonded tabs were also similar to those given by the IITRI method. The finite element method was used to analyze the effect of increasing the clamping force. Competing effects of locally complex and severe stresses at the coupon end and a stress concentration which develops at the end of the gage section were shown to limit the maximum strength. Tabs appear to reduce the stress concentration while further decreasing the stresses at the end, allowing results comparable to those given by the IITRI method to be achieved. / Master of Science

Accurate mechanical properties are critical to the design and use of composite material structures. Due to the available processing methods, the properties of ceramic matrix materials are especially sensitive to the geometry of the component and how it is made. A method is presented by which the ply-level elastic properties of a composite material can be obtained for a common structure; a thick, laminated tube. The mechanical and thermal response of the tubes is modeled by a planar cylindrical elasticity solution. Properties are determined from surface strain measurements of a thick tube subject to axial, torsional, pressure, and thermal loads. All elastic properties (including thermal expansion coefficients) can be obtained except the out-of-plane shear moduli (G13, G23) which are not involved in the planar elasticity solution employed. The ply-level properties are estimated by inversion of the elasticity solution in terms of the global strain measurements. A Least Squares optimization approach is used for the inversion of the elasticity solution. Application of the method for a filament wound aluminum oxide-aluminum oxide tube is presented. Advantages and limitations of the method are identified. / Master of Science

The influence of layer waviness in thick cross-ply composite cylinders subjected to hydrostatic pressure is investigated. The cylinders considered are graphite-epoxy with a 2: 1 ratio of circumferential to axial layers. All cylinders considered contain 104 total layers with a layup of [90/(90/0/90h71s, where a '0° 1 layer is taken to be in the axial direction. The influence of a single isolated group of wavy layers in an otherwise perfect cylinder is evaluated. Layer waviness in only the circumferential direction is considered, and the analysis is assumed to be valid only away from the cylinder ends. A parametric investigation is performed to determine the combined influence of wave location, wave amplitude, and cylinder geometry on hydrostatic response of the cylinder, particularly the stresses generated in and around the wave. The wave is assumed to be located either at the inner or the outer radius of the cylinder. Three wave amplitudes, 0, are considered: 1/2, 1, and 2 layer thicknesses. Only waves with a half wave length of 10 layer thicknesses are considered. Three cylinder geometries are considered, specifically ones with radius to thickness ratios of 5, 10, and 20. Finite element analysis is used to determine the stress state within the imperfect, i.e., wave included, cylinders. Based on a maximum stress failure criterion, failure pressures are determined for each of the various wave and cylinder geometries. Failure pressures for the imperfect cylinders are compared with those for a perfect cylinder to determine the failure pressure reduction ratios due to fiber waviness. It is shown that pressure capacity reductions of approximately 50% are possible for the range of parameters studied. Failure is primarily due to fiber compression, though interlaminar shear and interlaminar tension are a factor. Finite element analysis is also used to deter ine the failure pressure of the perfect cylinder due to buckling. This is done to determine whether failure due to buckling may overshadow material failure due to fiber waviness. It is shown that buckling is a factor in only one of the cylinder geometries considered, and only in the cases of mild layer waviness. In addition to results, details about the finite element model are presented. These details include geometry of the wave, changes in material properties due to local fiber rotation and local volume fraction changes, boundary conditions, and justifications for modeling simplifications that were made in an effort to reduce computational costs and analysis times. / Master of Science

High-performance polymeric composites are currently being considered as state-of-the-art material systems for future supersonic aircraft and space structures. However, the long-term durability and environmental stability of these materials continue to be under question. Continued application of these composites in aerospace structures is contingent upon the long-term durability of these material systems. Polymeric materials have been known to undergo both physical as well as chemical aging. The aging time, temperature, and environment play a significant role in affecting the physical and chemical aging behavior in the polymers. Currently, there is a dearth of information on the combined effects of physical and chemical aging in polymer-based composites. This study describes the effect of sub-T_g environmental aging on the mechanical properties of two high-performance polymeric composite systems. The effect of chemical degradation on the durability of the material systems is discussed. Further, the effect of environmental stress cracking (ESC) behavior of high-performance composite materials in the presence of organic solvents is investigated and the implications of ESC on durability are studied. Also included in this thesis is a study of the physical aging characteristics of the composites, via measurement of the viscoelastic (creep) properties. Accelerated characterization techniques were employed to predict long-term physical aging behavior. Fiberite 954-2 (a thermoplastic toughened cyanate ester resin) and its graphite-reinforced composites, and Fiberite ITX (a semicrystalline thermoplastic resin) and its graphite fiber-reinforced composites (IM8/ITX) were used in the study. These material systems were under consideration for usage in high-speed civil transport (HSCT) aircraft. This aircraft is expected to have an operating temperature of around 150°C (based on a 2.4 Mach number), an operating pressure at service altitude of 2 psi (0.136 atm), and a flight life in the excess of 60,000 hours at the above-mentioned conditions. The aging of the specimens was carried out for periods of up to 9 months at temperatures between 140°C to 200°C in three different environments; an inert nitrogen environment, an environment with a reduced air pressure of 2 psi (0.136 atm), and ambient atmospheric air. The results from stress-strain, flexure, and micro-indentation tests indicated a substantial reduction in material properties with aging in the different environments. The bending strength, strain to failure, and hardness values of the two composite systems decreased by as much as 20-50%. Tensile modulus on the other hand showed an increase of 20% after 6 months of aging in air, indicating apparent embrittlement with aging. Chemical degradation/damage was also monitored by penetrant enhanced x- radiography, scanning electron microscopy (SEM), and scanning acoustic microscopy (SAM). The chemical aging/degradation was seen to be sensitive to the oxygen partial pressure in the aging environment. The greater the amount of oxygen in the aging environment, the more the loss in the material properties. The glass transition temperatures (T​​_g) of the two material systems were sensitive to both the aging environment and aging time. The T_g of both systems increased over long aging times as seen from dynamic mechanical analysis (DMA) measurements. However, increased oxygen concentrations appear to reduce the T_g. Changes in the T_g of both material systems were a complex behavior attributable to the varying oxygen concentrations in the aging environments, and the combined occurrence of physical aging, degradation, etc. in the materials. The chemical degradation in the composites appears to be via an oxidation mechanism and the micro-indentation results further indicate diffusion-controlled oxidation. Weight changes of samples (neat resin and composites) were also monitored over the entire period of the study and these showed a sensitivity to the oxygen concentration in the aging environment. The greater the oxygen in the environment, the greater the weight loss in the specimens, indicating an oxidation phenomenon. DMA and tensile creep were performed to study the interaction of creep and physical aging in these material systems. Long-term creep predictions of the composites were made using Time-Temperature Superposition (TTSP) and Effective Time Theory (ETT) techniques. The IM8/954-2 composites behaved in an anomalous fashion at times. This may be attributable to the blended nature of the 954-2 resin system, possible post-curing and phase separation of the resin, and thermal decomposition at elevated temperatures. The solvent testing of composites based on thermoplastic polymers revealed susceptibility to ESC. Bending strength losses up to 30% were seen from flexure tests on unidirectional composites. It was also seen that residual stresses in cross-ply laminates were sufficient to trigger ESC after exposure to common organic solvents for an hour. The damage/failure modes were captured by SEM micrographs. / Master of Science

The feasibility study of subscale notched specimens to determine the shear response of composites is presented. The investigation consists of finite element analyses, conventional strain-gaged testing, and photomechanics experiments. Several notch geometries of the subscale specimens are studied, the standard 900 V -notch, U-notch, and circular notch. The investigation is performed on two different material systems, a standard high performance graphite/epoxy (AS4/3501-6) material and an SMC R-28 material reinforced with 28% volume fraction strand glass fiber. The moduli obtained from the subscale specimens are compared with those obtained from the standard specimens. Different degrees of twisting observed in testing the sub scale specimens are discussed. Numerical and experimental results of the SMC R-28 materials are presented. The dependence of the measured shear modulus on the relative orientation of the specimen in the panel is identified. The application of the subscale, circular notched specimens to obtain the shear modulus of the SMC material is discussed. / Master of Science

The behavior of ARALL (ARamid ALuminum Laminates) subjected to tension-tension cyclic loading was experimentally investigated as a first step towards the understanding of the fatigue damage mechanisms in Arall laminates, and the relationships between damage and stiffness change, remaining strength and life. / Master of Science