Resumo: A produção de materiais que apresentem alto desempenho em suas aplicações exige avanços tecnológicos e científicos. Os elevados valores de resistência à tração e módulo de elasticidade, aliados à flexibilidade, baixa densidade e alta razão de aspecto, fazem dos nanotubos de carbono excepcionais candidatos para reforçar compósitos poliméricos. Preparamos por laminação sistemas compósitos ternários à base de resina epóxi/tecidos de fibra de carbono (para os binários), acrescendo nanotubos de carbono para os sistemas ternários. Os parâmetros estequiométricos do sistema epoxídico (valor de Phr) e concentração de nanotubos de carbono empregado nos sistemas compósitos tiveram avaliadas suas morfologias (MEV), propriedades térmicas (TG) e mecânicas (ASTM D790). Embora o sistema epoxídico com Phr 10,0 tenha apresentado uma maior estabilidade térmica, os compósitos ternários produzidos a partir do sistema com Phr 26,6 mostraram valores de tensão máxima e módulo de elasticidade até 8 vezes maiores que aqueles produzidos com o sistema Phr 10. A adição de nanotubos de carbono aos compósitos com Phr 26,6 resultou em ganhos adicionais de aproximadamente 38 e 15% na tensão máxima e no módulo de elasticidade, respectivamente. Esses resultados revelaram a limitação nos métodos de integralização de nanoestruturas a sistemas compósitos, onde as dispersões estão limitadas a uma série de fatores inerentes a interação química e/ou física durante a fabricação dos compósitos nanoestruturados. Palavras-chave: Resina epóxi, tecidos de fibra de carbono, nanotubos de carbono, relação estequiométrica, propriedades térmicas, propriedades mecânicas.
Influence of Resin/Hardener Stoichiometric Ratios on the Mechanical Properties of Hierarchical Ternary Composites -Epoxy Resin/Woven Carbon Fiber/Carbon NanotubesAbstract: The production of materials that exhibit high performance in their applications requires scientific and technological advances. The high values of tensile strength and modulus of elasticity, combined with flexibility, low density and high aspect ratio, make carbon nanotubes exceptional candidates for reinforced polymer composites. We prepared by lamination composite systems based on epoxy resin/woven carbon fiber (for binary), plus carbon nanotubes for the ternary systems. The effect of the stoichiometry of the epoxy system (Phr value) and concentration of carbon nanotubes used in the composite systems was evaluated by their morphological (SEM), thermal (TG) and mechanical properties (ASTM D790-10). Although the epoxy system with 10.0 Phr has presented a higher thermal stability, the ternary composite system produced with 26.6 Phr showed values of maximum stress and elastic modulus up to 8 times greater than those produced with the 10 Phr system. The addition of carbon nanotubes to composites with 26.6 Phr resulted in additional increase of approximately 38 and 15% of the maximum stress and elastic modulus, respectively. These results revealed a limitation in the methods of incorporation of the nanostructures in composit...
The new class of bainitic steels can present toughness at room temperature greater than traditional quenched and tempered martensitic steel. This is because the microstructure of steel with high Si content (≈1.5wt%) submitted to bainitic transformation is compose of fine plates of bainitic ferrite separated by retained austenite. The inhibition of cementite precipitation leads to the improvement of toughness. The presence of cementite facilitates the nucleation of cracks. Moreover, the blocks of retained austenite are undesirable. This morphology is rather unstable and tends to transform into hard and brittle untempered martensite under the influence of small stress, contributing to a low toughness. However, it was observed in this work that the greater the volume fraction of retained austenite, the greater is the toughness (10-24 J) for multi-phase steel. The values of toughness were independent whether the retained austenite is present on film or block forms. The decrease of toughness values was observed by the tempered samples after the bainitic transformation (10-14 J). This occurred because the blocks of retained austenite decomposed into carbides, martensite and/or bainite.
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