The effect of the cylindrically orthotropic morphology known to exist in graphite fibers on the effective properties of a composite material was studied using the composite cylinder assemblage model. The cylindrical orthotropy of the fibers was found to have no effect on the properties of a composite with purely orthotropic fibers. For fibers with a transversely isotropic core both the size of the core and the morphology of the sheath were found to have an effect on the composite properties. The stress states resulting in the composite cylinder for axial, radial, axial shear, and thermal loads were examined. Singular stresses were observed to occur at r=0 in some fibers in some load conditions. The presence of a transversely isotropic core, which must exist in a real fiber, removed this singularity. The strength of the composite cylinder was found to depend on uÌ ber morphology. The size of the transversely isotropic core within the uÌ ber also affected the strength. The strength of the uÌ ber increased with increasing transversely isotropic core size in some instances. In general, for axial loading failure is expected to be caused by fiber breakage. For radial, axial shear, and thermal loading the failure mode is uÌ ber splitting. / Master of Science

A time-dependent characterization study was performed on T300/934 graphite/epoxy in a simulated space environment. Creep tests on irradiated and non-irradiated graphite/epoxy and bulk resin specimens were carried out at temperatures of 72 °F and 250 °F. Irradiated specimens were exposed to dosages of penetrating electron radiation equal to 30 years 'exposure at GEO-synchronous orbit. Radiation was shown to have little effect on the creep response of both the composite and bulk resin specimens at 72 °F while radiation had a significant effect at 250 °F. A healing process was shown to be present in the irradiated specimens where broken bonds in the epoxy due to radiation recombined over time to form cross-links in the 934 resin structure. An analytical, micromechanical model was also developed to predict the viscoelastic response of fiber reinforced composite materials. The model was shown to correlate well with experimental results for linearly viscoelastic materials with relatively small creep strains. / Master of Science

The need for better impact resistant composites has resulted in the development of many toughened resin systems. A combination of a tougher resin system along with higher strength fibers increases the impact resistance of the composite. The use of an adhesive layer between two plies of the improved prepreg system has been found to considerably increase the impact resistance. This concept is known as "Interleafing." This investigation studies the response of the interleaf materials to instrumented drop weight impact as compared with the response of non-interleaved materials. Two non-destructive quality evaluation techniques, namely, ultrasonics and eddy currents, are used to qualitatively evaluate the damage developed in the specimens. Several different energy levels of damage are studied. The interleaved laminate had significantly better impact response than the non-interleaved laminate for the same impact energy. The onset of delamination was delayed by the use of the interleaf. Whereas damage could be detected at an impact energy as low as 1.75 ft-lb in the baseline laminate; the interleaved laminate did not show any ultrasonic C-scan indication up to an impact of 2.45 ft-lb. The increase of delamination with increasing impact energy was slower in the interleaved specimen. The eddy current method is not effective in detecting damage in the interleaved laminate because of the shielding effect of the interleaf. Compression Strength After Impact (CSAI) could not be used for the test laminates in this project, but the Tensile Strength After Impact test provided useful results. The tensile strength after impact of the interleaved specimen was between 20%-80% more than the baseline laminate up to impact energy of 10 ft-lb. The advantage of the interleaved specimen reduced at higher energy levels of impact. / Master of Science

Shape memory alloy (SMA) hybrid adaptive composites are a class of materials which combine the strain recovery and elastic properties transformation capabilities of SMA fibers with the structural characteristics of advanced composite materials. This study utilizes the Rayleigh-Ritz method and finite panel acoustic radiation theory to investigate the use of SMA hybrid composite materials for adaptive structural acoustic control by active structural tuning. Analytical models are formulated considering classical laminated plate theory (CLPT) and first-order shear deformation theory (FSDT), to predict modal and structural acoustic response to incident low frequency plane wave acoustic excitation. The analysis is further developed to consider simply supported adaptive panels that are tuned by local fiber activation, such that a panel composed of elastically uniform sections can be evaluated in a piece-wise fashion. / Master of Science

An analytical/experimental investigation was performed to study the effect of material nonlinearities on the response of composite tubes subjected to combined axial and torsional loading. An elasticity based analytical model was developed to predict stresses and deformations in composite tubes subjected to combined thermomechanical loading. Material nonlinearities were modeled using the Endochronic Theory. The effect of residual stresses on subsequent mechanical response was included in the investigation. Subsequently, experiments were performed on P75/934 graphite/epoxy tubes with a stacking sequence of [15/1/ ± 10/0/-15], using pure torsion and combined axial/torsional loading. The in-plane material properties needed for incorporation into the analytical model were determined using tests on flat coupons made from P75/934. In the presence of residual stresses. the analytical model predicted a reduction in the Initial shear modulus of a tube subjected to torsional loading. Experimentally. a difference in the nonlinearity of the stress-strain response was found between pure torsion loading and combined proportional loading. This difference is due to coupling between axial loading and shear strain. These phenomena were predicted by the nonlinear analytical model where a linear model did not. The experimentally observed linear limit of the global shear response was found to correspond to the analytically predicted first ply failure. The observed nonlinear response thus appears to be due to a combination of material response at the ply level and gradual damage accumulation. Further, based on cyclic torsion tests, the failure of the tubes was found to be path dependent above a certain critical load level. / Master of Science

Analysis of the effects of matrix cracking on composite laminates is a well-known problem which has attracted considerable attention for the past decade. An approximate analytical solution is introduced in this thesis to study this type of problem. The subjects of primary concern are the degradation of effective laminate properties, such as axial stiffness, Poisson's ratio, shear modulus, and coefficient of thermal expansion, as a function of crack density and the axial stress redistribution due to the existence of matrix cracks. Both transverse cracks (2-D problem) and cross (transverse and longitudinal) cracks (3-D problem) are studied. Results for graphite/epoxy cross-ply laminates are presented and compared to those of other approaches. Some other materials, for instance, glass/epoxy, are also studied. The results and comparisons will appear where appropriate. In general, the agreement between the results of the present analysis and those of other approaches, in particular, the finite element method, is good for the lower crack density. The present study shows that Poisson's ratio may be a good indicator of the degree of damage for a cracked laminate. / Master of Science

Intelligent materials are a class of material systems usually consisting of a composite or hybrid material system with fibrous or distributed actuators, various sensors and a control system. One type of actuator being developed for intelligent material systems is made of nitinol or shape memory alloy wire. In order for nitinol and other actuators to be a reliable part of the system, the effect of composite manufacturing on the actuators’ performance and behavior must be determined. The results of a study investigating the effects of a "high temperature" thermoplastic composite processing cycle on the nitinol actuator’s performance is presented. A study of the interfacial strength between the actuators and APC-2 thermoplastic composite is also reported. The nitinol actuators were exposed to high temperature (400°C) composite processing cycles. Critical parameters of the processing cycles were varied to determine their effect on the actuators’ performance. After the processing cycles, the nitinol actuators demonstrated useable recovery stresses (σ_r^u) of 173-265 MPa. The σ_r^u of a nitinol actuator in the virgin state, subjected to a thermoset processing cycle, and embedded in a specimen of APC-2 thermoplastic composite was also tested to develop a basis for comparison. The quality of the actuator-composite interface bond was tested by pull-out testing and fatigue loading to determine if the actuator is adequately bonded with the host composite. Pull-out forces of 30-50 N could fracture the actuator-composite interface, but 1000 activation cycles of the actuator produced no damage in the bond between actuator and host composite. / Master of Science

The response of a flake mat to the mechanical stress applied during consolidation is a function of mat structure, raw material properties and the environmental conditions created during pressing. This research project was aimed at improving the understanding of raw material behavior during the hot-pressing of wood based composites by examining the response of a wood flake mat to the compression encountered during press closure in the flakeboard manufacturing process. The structure of a flakeboard mat can be considered to be that of a cellular material, the properties of which are governed by the cellular geometry, or arrangement of cells, and the properties of the solid cell wall material. A method for quantifying the cellular structure of a wood flake mat was developed and implemented. The structure of thin mat sections and small flake mats was quantified using image analysis techniques. The applicability of theories developed for modelling the compressive behavior of cellular materials to the consolidation of wood flake mats was investigated using thin mat sections. Narrow mat sections and small laboratory mats has similar void sizes but significantly different void shapes. Void size was not significantly affected by flake orientation, but void shape was. / Master of Science

Delamination at the free edge is analyzed as a mode of failure for uniaxial compression of postbuckled structural components. Analyses are performed for I-section stiffeners and for a dropped-ply laminate, all of which failed due to free edge delamination in earlier experimental studies. These specimens were made of AS4/3502 graphite-epoxy unidirectional tape. The nonlinear response of the specimens is modeled by a geometrically nonlinear finite element analysis. A variable-complexity modeling scheme is developed to economize computer resources for the nonlinear analysis. The three-dimensional stress field near the free edge is investigated by employing 20-node solid elements in that region. A dimensionless delamination index is calculated to investigate the severity of the interlaminar stress state. Numerical results correlated reasonably well with the experiments on load-end shortening plots. The nodallines and/or inflection points of the out-of-plane deflection along the free edge in the postbuckled configuration are found to be delamination critical sites. Both interlaminar shear stress tangent to the free edge and tensile interlaminar normal stress are significant at these critical locations and are likely to initiate delamination. / Master of Science

Reliable estimation of thermal properties is extremely important in the utilization of new advanced materials, such as composite materials. The accuracy of these estimates can be increased if the experiments are designed carefully. The objectives of this study are to design optimal experiments to be used in the prediction of these thermal properties and to then utilize these designs in the development of an estimation procedure to determine the effective thermal properties (thermal conductivity and volumetric heat capacity). The experiments were optimized by choosing experimental parameters that maximize the temperature derivatives with respect to all of the unknown thermal properties. This procedure has the effect of minimizing the confidence intervals of the resulting thermal property estimates. Both one-dimensional and two-dimensional experimental designs were optimized. A heat flux boundary condition is required in both analyses for the simultaneous estimation of the thermal properties. For the one-dimensional experiment, the parameters optimized were the heating time of the applied heat flux, the temperature sensor location, and the experimental time. In addition to these parameters, the optimal location of the heat flux was also determined for the two- dimensional experiments. Utilizing the optimal one-dimensional experiment, the effective thermal conductivity perpendicular to the fibers and the effective volumetric heat capacity were then estimated for an IM7-Bismaleimide composite material. The estimation procedure used is based on the minimization of a least squares function which incorporates both calculated and measured temperatures and allows for the parameters to be estimated simultaneously. / Master of Science