ABSTRACT Carbon fiber/phenolic resin composites have long been used as ablative materials in rocketry. Ablation is a complex multiscale problem where radiative and convective heating leads to the pyrolysis of phenolic resin matrix, resulting in the formation of a porous insulation char as thermal protection. This study investigates the shear properties evolution during the heat treatment of a carbon fiber/phenolic resin nozzle extension entrance (exit cone) which is part of an integrated nozzle of launching and sounding vehicles, developed at the Instituto de Aeronáutica e Espaço (SP), Brazil. Specimens of the material (carbon fiber/phenolic resin composite) were subjected to heat treatment at 500, 1,000, 1,500 and 2,000°C, and measurements of shear strength and shear modulus were performed using the Iosipescu mode. Experimental data were compared with the results obtained theoretically. Also, morphological analysis was accomplished by optical microscopy and the observation of fractured surfaces, by scanning electron microscopy. Significant morphological changes in the microstructure after heat treatments were observed. The lowest value for shear strength obtained experimentally was 4.05 MPa, which is greater than the ultimate value obtained analytically (2.35 MPa), fulfilling its structural function during the propulsion time.

Abstract: Phenolic resin has long been used as matrix for composites mainly because of its flame retardant behavior and high char yield after pyrolysis, which results in a self supporting structure. The addition of ceramic powders, such as SiC and B4C, as fillers to the phenolic resin, results in better thermo-oxidative stability, but as drawbacks, it has poor homogeneity, adhesion and processing difficulties during molding of the composites. The addition of single elements, such as boron, silicon and phosphorus in the main backbone of the thermo-set resin is a new strategy to obtain special high performance resins, which results in higher mechanical properties, avoiding the drawbacks of simply adding fillers, which results in enhanced thermo-oxidative stability compared to conventional phenol-formaldehyde resins. Therefore, the product can have several applications, including the use as ablative thermal protection for thermo-structural composites. This work describes the preparation of a boron-modified phenolic resin (BPR) using salicyl alcohol and boric acid. The reaction was performed in refluxing toluene for a period of four hours, which produced a very high viscosity amber resin in 90% yield.The final structure of the compound, the boric acid double, substituted at the hydroxyl group of the aromatic ring, was determined with the help of the Infrared Spectroscopy, 1H-NMR, TGA-DSC and boron elemental analysis. The absorption band of the group B-O at 1349 cm-1 can be visualized at the FT-IR spectrum. 1H-NMR spectra showed peaks at 4.97-5.04 ppm and 3.60-3.90 ppm assigned to belong to CH2OH groups from the alcohol. The elemental analysis was also performed for boron determination.The product has also been tested in carbon and silicon fibers composite for the use in thermal structure. The results of the tests showed composites with superior mechanical properties when compared with the conventional phenolic resin.

Resumo: O presente trabalho descreve os processos de obtenção de compósitos termoestruturais de carbono reforçado com fibras de carbono. O processamento desses materiais tem início pela definição de uma arquitetura do reforço de fibras de carbono, seja na forma de empilhamento simples do reforço, de tecidos ou na forma de reforço multidirecional. A incorporação de matriz carbonosa no reforço de fibras, pelo preenchimento de vazios e interstícios, promove a densificação do material, e o incremento de massa específica. Duas rotas de processamento são predominantes na obtenção desses materiais, o processo via impregnação líquida e o processo via impregnação em fase gasosa. Em ambos os casos, processos térmicos levam à formação de matriz de carbono com propriedades específicas, que derivam de seus materiais precursores. Os processos diferem entre si, também, pelo rendimento, enquanto os processos executados por impregnação líquida apresentam rendimento de, aproximadamente, 45%, os processos por impregnação em fase gasosa apresentam rendimento em torno 15%.

Abstract: The present work describes the processes used to obtain thermostructural Carbon/Carbon composites. The processing of these materials begins with the definition of the architecture of the carbon fiber reinforcement, in the form of stacked plies or in the form of fabrics or multidirectional reinforcement. Incorporating fiber reinforcement into the carbon matrix, by filling the voids and interstices, leads to the densification of the material and a continuous increase in density. There are two principal processing routes for obtaining these materials: liquid phase processing and gas phase processing. In both cases, thermal processes lead to the formation of a carbon matrix with specific properties related to their precursor. These processes also differ in terms of yield. With liquid phase impregnation the yield is around 45 per cent, while gas phase processing yields around 15 per cent.

Resumo: O principal objetivo do presente trabalho foi a determinação do módulo de cisalhamento G12 e a máxima resistência ao cisalhamento (ô12) utilizando o ensaio de cisalhamento losipescu. Os testes foram conduzidos com dois tipos de compósitos, fibra de carbono/epóxi e fibra de vidro/epóxi utilizados na indústria aeroespacial e também uma matriz de resina moldada. Os resultados indicam a efetiva contribuição das fibras de reforço para a resistência ao cisalhamento (ô12) e para o módulo de cisalhamento (G12) comparado com a matriz polimérica sem reforço.

Abstract: The main aim of the present work was the determination of the shear modulus (GI2) and the maximum shear strength (ôI2) using the losipescu Shear Test. Tests were carried out on two types of composites, carbon fiber/epoxy and glass fiber/epoxy, used in the aerospace industry, and also a molded epoxy resin matrix. The results indicate the effective contribution of fiber reinforcements to the shear strength (ô12) and shear modulus (G12) compared to the no reinforcement polymer matrix.

Abstract. Quartz phenolic composites have been applied to thermal protection systems (TPSs) for reentry vehicles since the late fifties due to their excellent ablative resistance and mechanical performance. TPSs must withstand the aggressive reentry environment, such as atomic oxygen, when submitted to very high temperatures (> 1000° C) and heat flux. The ablative performance of composites is influenced by both base materiais and environmental parameters during the ablation process. For TPS systems phenolic resin is usually used as the base matrix due to its ability to form a stable char during decomposition. This charplays an important role in the absorption of the heat generated during the ablation process. During re-entry, parts of the charred matrix can be abrasively removed by shear force due to high pressure and velocity. In this work the ablative and mechanical properties of quartz phenolic composites were evaluated in order to identify the range of properties suitable for the use of these materiais as thermal protection systems for space vehicles. Quartz fabric having an areai weight of 680 g/m2 and a resole-typephenolic resin were used to prepare the composites. The resin has a viscosity of l65 MPa at 20°C. The prepreg material was cured by heating under pressure of 100 bar in a mold. The resin content of the prepreg obtained was about 50 per cent. The mechanical properties evaluated were, tensile, shear and flexural strength. The results obtained showed that this material has average values of 38.5 MPa, 52 MPa and 85 MPa for tensile, shear andflexural strength, respectively. The ablative tests were carried out in a high-energy air plasma in ambient atmosphere and the mass losses were measured for different exposure time.

Weight reduction and improved damage tolerance characteristics were the prime drivers to develop new family of materials for the aerospace/aeronautical industry. Aiming this objective, a new lightweight Fiber/Metal Laminate (FML) has been developed. The combination of metal and polymer composite laminates can create a synergistic effect on many properties. The mechanical properties of FML shows improvements over the properties of both aluminum alloys and composite materials individually. Due to their excellent properties, FML are being used as fuselage skin structures of the next generation commercial aircrafts. One of the advantages of FML when compared with conventional carbon fiber/epoxy composites is the low moisture absorption. The moisture absorption in FML composites is slower when compared with polymer composites, even under the relatively harsh conditions, due to the barrier of the aluminum outer layers. Due to this favorable atmosphere, recently big companies such as EMBRAER, Aerospatiale, Boing, Airbus, and so one, starting to work with this kind of materials as an alternative to save money and to guarantee the security of their aircrafts.

The main objective of this work was to obtain the damping factor (xi ) as well as the elasticity modulus (E) of two kinds of synthetic graphite (HLM and ATJ), using the modal analysis technique. Prismatic beams of square section (~ 11 x 11 mm) and length over thickness ratio (L/t) of about 22.7 were tested in the free - free boundary condition. The first four modes of vibration were taken into account in the non-destructive evaluation of the materials. In addition, numerical simulations were also carried out in this investigation. The agreement between the theoretical and the experimental results was quite good. The average values of E and xi for the HLM graphite were 20% and 90% higher, respectively, than those presented by the ATJ graphite, indicating that the HLM graphite has, proportionally, more damping mechanisms than the ATJ graphite.