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.
This research consisted in comparing the fatigue crack growth (FCG) performance of four HSLA/ AHSS steels used in automotive applications and with different microstructures, and the application of some prediction models for the da/dN versus ΔK traditional sigmoidal curve as a function of the R load ratio. FCG tests were carried out on C(T) test specimens with R-ratios varying between 0.03 and 0.7. Using the original and empirical methodology proposed by Paris and Erdogan to describe the da/dN-ΔK relationship, the results showed significant differences in function of microstructure, and a deleterious effect of R-ratio increase on the crack growth rate. In order to check existing methodologies based on physical considerations for predicting the fatigue behavior of materials and the effect of the R-ratio mainly in the fatigue threshold ΔK th region, the well-known crack closure model proposed by Elber, an approach using two parameters as a driving force for the crack growth proposed by Vasudevan and co-authors and a combination of these two models recently proposed by Zhu and co-authors were compared. The manifestation of crack closure and its qualitatively expected dependence on the R-ratio were verified for the studied steels, but the Elber model was not able to provide a master curve that accurately summarized the effect of the R-ratio on the sigmoidal fatigue curve of steels. The combined use of two critical thresholds, ΔK th * and K max *, for predicting fatigue crack growth according to the Vasudevan model also did not provide accurate results in evaluating the effect of the R-ratio. Regardless of the verified dispersions, there is a connection between the two-parameter methodology and crack closure, hence the model by Zhu and co-authors could be a promising alternative. However, this model also showed significant dispersions and was unable to create a master curve to adequately predict the effect of R-ratio on crack growth. Thus, it can be concluded that this research topic is still open, requiring a more in-depth phenomenological knowledge to predict the effect of the R-ratio on FCG.
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