This thesis concerns the production of thermoplastic/carbon fiber composites via aqueous suspension prepregging. Suspension prepregging requires the use of a binder dispersant: i) to disperse the polymer particles so as to get good fiber-matrix distribution in the prepreg, and ii) to improve the handling characteristics of the prepreg, by holding the matrix to the fibers and the fibers to themselves. The effects of the binder on the processability, properties, and performance of composites were assessed. It was found that the addition of the binder can affect both the matrix and the composite performance depending on the binder concentration and chemistry. For example an increase of 250 % on the complex dynamic viscosity of LaRC TPI matrix was measured upon the addition of 21.3 wt% of imidized LaRC TPI polyamic acid binder at a frequency of 1 Hz. The greatest fiber-matrix adhesion was observed for PEEK composites when BisP BTDA polyamic acid was used as a binder. A preliminary, qualitative study on the possibility of using suspension prepregging as means of manufacturing thermoplastic/carbon fabric composites was performed. Good penetration of PEEK polymer particles into the fabric was obtained when the fabric was previously desized by acid digestion. A preliminary study was made on the binding and dispersing capabilities of a novel copolymer poly(pyridine ether-eo-ether ether ketone). The addition of the copolymer to PEEK drastically increased the melt viscosity due to an ionomer effect. This resulted in poorly consolidated panels with high void volumes. / Master of Science

A novel method of impregnation was used to make thermoplastic prepregs with aqueous suspension of polymer powder and AS-4 carbon fibers. The prepregging, consolidation, and laminate evaluation procedures are described in this report. The need and importance of a dispersing and binding agent and its interactions with the composite matrix are assessed. A time-temperature-pressure consolidation cycle was designed for PEEK prepreg. Two thermoplastic polymers were studied : LaRC-TPI, provided by Mitsui Toatsu Chemicals, and Poly(etheretherketone), synthesized in Virginia Tech Chemistry Department. Comparison of LaRC-TPI composites to those made at NASA-Langley Research Center demonstrated good reproducibility of the techniques used in this study. The last PEEK composites fabricated compared well to the ICI standards. / Master of Science

The flash method proposed by Parker et al. in the early sixties is one of the most important experimental procedures to determine the thermal properties of homogeneous materials. Because of the versatility of this method, researchers have attempted to extend its usefulness into the realm of composite material. However, some difficulties arise because of the existence of preferential heat paths in heterogeneous materials, especially in fiber-reinforced composites. In order for experimental flash method results to be meaningful a homogeneous temperature front must exist at the back face of the sample, where the measurements are made. In this work, the parameters that render the radial temperature response of a fiber reinforced composite homogeneous at the back face were investigated. According to the literature three criteria must be met for homogeneity to occur: fiber-to-matrix volume ratio must be high; sample axial dimension must be large compared to radial fiber dimension and the thermal contact between the fibers and the matrix must be high. Since the first two criteria are met by most fiber-reinforced composite samples subject to the flash method, attention was concentrated on the third criterion. An inequality that must be met by the contact conductance term to establish homogeneity is proposed and some sample temperature profiles are presented. / Master of Science

Research effort was directed towards developing a near real-time, acousto-ultrasonic (AU), nondestructive evaluation (NDE) tool to study the failure mechanisms of ceramic composites. Progression of damage is monitored in real-time by observing the changes in the received AU signal during the actual test. During the real-time AU test, the AU signals are generated and received by the AU transducers attached to the specimen while it is being subjected to increasing quasi-static loads or cyclic loads (10 Hz, R = 0.1). The received AU signals for 64 successive pulses were gated in the time domain (T = 40.96 µsec) and then averaged every second over ten load cycles and stored in a computer file during fatigue tests. These averaged gated signals are representative of the damage state of the specimen at that point of its fatigue life. This is also the first major attempt in the development and application of real-time AU for continuously monitoring damage accumulation during fatigue without interrupting the test. The present work has verified the capability of the AU technique to assess the damage state in silicon carbide/calcium aluminosilicate (SiC/CAS) and silicon carbide/magnesium aluminosilicate (SiC/MAS) ceramic composites. Continuous monitoring of damage initiation and progression under quasi-static ramp loading in tension to failure of unidirectional and cross-ply SiC/CAS and quasi-isotropic SiC/MAS ceramic composite specimens at room temperature was accomplished using near real-time AU parameters. The AU technique was shown to be able to detect the stress levels for the onset and saturation of matrix cracks, respectively. The critical cracking stress level is used as a design stress for brittle matrix composites operating at elevated temperatures. The AU technique has found that the critical cracking stress level is 10-15 % below the level presently obtained for design purposes from analytical models. An acousto-ultrasonic stress-strain response (AUSSR) model for unidirectional and cross-ply ceramic composites was formulated. The AUSSR model predicts the strain response to increasing stress levels using real-time AU data and classical laminated plate theory. The Weibull parameters of the AUSSR model are used to calculate the design stress for thermo-structural applications. Real-time AU together with the AUSSR model was used to study the failure mechanisms of SiC/CAS ceramic composites under static and fatigue loading. An S-N curve was generated for a cross-ply SiC/CAS ceramic composite material. The AU results are corroborated and complemented by other NDE techniques, namely, in-situ optical microscope video recordings and edge replication. / Ph. D.

Experimental results of damage development in and mechanical response of heat-treated NicaloniCAS-II laminates subjected to monotonic flexure and axial loading and to cyclic tensile loading are reported. The specimens were subjected to post-processing heat treatments at 900°, 1000°, and l100°C in air for 100 hours. Changes at the fiber/matrix interface/interphase due to post-processing heat treatments were also characterized. The combined effect of fiber debonding and transverse matrix cracking in both 90° and 0° plies plays an important role in damage development in [0/90]₄₅ Nicalon/CAS-II laminates, especially in developing the secondary damage modes such as longitudinal matrix cracking and delamination. Frictional wear effects found in cyclically loaded specimens may be responsible for the observed temperature profiles during the intermediate stage of fatigue life. It is also believed that frictional wear is critical to the failure of notch sensitive fibers. Different damage modes such as "brittle" matrix crack propagation and "quasi-brittle" matrix crack propagation were observed in heat-treated specimens. Results obtained from microanalysis using an analytical scanning transmission electron microscope equipped with an energy dispersive spectrometer, and microindentation indicated that the changes of damage and failure modes were directly related to the changes of characteristics at the fiber/matreix interface/interphase. / Ph. D.

The role of an interfacial barrier at the fiber-matrix interface in the heat conduction behavior of an uniaxial silicon carbide fiber-reinforced reaction-bonded silicon nitride and the effect of fiber orientation on the heat conduction characteristics of carbon fiber-reinforced borosilicate glass was investigated. In the study of the effect of an interfacial thermal barrier, a composite with fibers having a carbon-rich coating of about 3 J.l m was chosen as the reference material. The fiber-matrix interface was then modified by preferential oxidation of the carbon coating on the fibers, using fibers with no carbon coating and using hotisostatic-pressing (HIP) after nitridation. The formation of an interfacial gap at the interface due to thermal expansion mismatch between the fiber and the matrix in reference and HIP'd composites, and removal of carbon coating for oxidized composites, resulted in the dependence of thermal diffusivity/conductivity on the surrounding . atmosphere. This effect was attributed to gaseous heat transfer at the interface. However, no atmospheric effects were observed for composites with fibers without the carbon coating due to very strong bonding between the fiber and the matrix. HIP'ing increased the thermal diffusivity/conductivity of the composites due to densification of the matrix, crystallization of the fibers and increased physical contact at the interface. Removal of the interfacial carbon layer by preferential oxidation lowered the interfacial conductance considerably, due to decrease in the direct thermal contact between the fibers and the matrix. Interfacial contact conductance determined from measurements made in vacuum for reference and HIP'd composites increased rapidly with increasing temperature in accordance with interfacial gap closure. These observations indicate that the heat conduction behavior of all the composites investigated was strongly affected by the existence of an interfacial thermal barrier, for heat transfer transverse to the fiber direction. / Ph. D.

The challenges faced in today’s industry require materials capable of working in chemically aggressive environments at elevated temperature, which has fueled the development of oxidation resistant materials. All-Oxide Ceramic Matrix Composites (OCMC) are a promising material family due to their inherent chemical stability, moderate mechanical properties, and low weight. However, limited information exists regarding their behavior when in contact with other high-temperature materials such as superalloys. In this work three sets of tribological tests were performed: two at room temperature and one at elevated temperature (650 °C). The tests were performed in a pin-on-disk configuration testing Inconel 718 (IN-718) pins against disks made with an aluminosilicate geopolymeric matrix composite reinforced with alumina fibers (N610/GP). Two different loads were tested (85 and 425 kPa) to characterize the damage on both materials. Results showed that the pins experienced ~ 100 % wear increase when high temperature was involved, while their microstructure was not noticeably affected near the contact surface. After high temperature testing the OCMC exhibited mass losses two orders of magnitude higher than the pins and a sintering effect under its wear track, that led to brittle behavior. The debris generated consists of alumina and suggests a possible crystallization of the originally amorphous matrix which may destabilize the system. The data suggests that while the composite’s matrix is stable, wear will not develop uncontrollably. However, as soon as a critical load/temperature combination is attained the matrix is the first component to fail exposing the reinforcement to damage which drastically deteriorates the integrity of the component.