Papers by Author: Luiz Claudio Pardini

Abstract: Composite materials have increasingly being used on aerospace industry due to its low density and high mechanical strength as well as high fatigue endurance. Consequently, increase attention is being devoted to study the fatigue behavior of these materials under cyclic loads. This work presents results on fatigue under shear stress by using the Iosipescu method. For fatigue testing a cured neat epoxy resin and a carbon fiber/epoxy composite having orientations of 0/90o and ± 45o in relation to the loading axis were tested by using the Iosipescu coupon. Firstly, the specimens were submitted to static tests in order to obtain the ultimate shear strength (τ) and the in-plane shear modulus (G₁₂). Further batches of specimens were tested under definite levels of stress ratio as a function of number of cycles. So, the S-N curves were obtained. The maximum number of cycles was set at 120,000 cycles, which corresponds approximately to two times the life of a structural element from a civilian airplane. The stress ratio used was R=0.1 and R=0.5. At the limit of 120,000 cycles the epoxy resin exhibited a shear strength of 18 MPa, for a stress ratio of R=0.5, and 10 MPa for a stress ratio of R=0.1. The carbon fiber/epoxy 0/90o composite, at the limit of 120,000 cycles, showed a shear strength of 84 MPa for a stress ratio of R=0.5, and 64 MPa for a stress ratio of R=0.1. For the carbon fiber/epoxy ±45o composite, at the limit of 120,000 cycles, a shear strength of 105 MPa and 90 MPa, where found for a stress ratio of R=0.5 and R=0.1, respectively.

Abstract: This work reports the synthesis and thermal characterization of poly(borosiloxanes) (PBS) derived from methyltrietoxysilane (MTES) and vinyltriethoxysilane (VTES), aiming to use these polymers as precursors of ceramic matrices for the protection of carbon fibers in ceramic matrix composites (CMCs). The resulting materials exhibited better thermal stability than the carbon fiber, especially the Cfiber/SiBCO composite derived of the methyltriethoxysilane (MTES) system prepared with a B/Si ratio of 0.5. This study showed that poly(borosiloxanes) are promising materials for the oxidation protection of carbon fibers, and consequently for thermal protection systems.

Abstract: The densification of carbon fiber substrates by chemical vapor deposition (CVD) is a process quite used since the fifties, for manufacturing carbon reinforced with carbon fiber composites (CRCF). The process is based in thermal decomposition of a gas which contains carbon in its molecules and the resulting pyrolytic carbon is continually deposited onto de carbon fiber substrate. For this experiment the substrate material was made of carbon fiber felt. The deposition was performed by isothermal process at atmospheric pressure and at temperature of 1050 oC. Methane (CH4) gas was used as carbon bearer and nitrogen (N2) as the carrier gas. Different volumetric ratio of N2 and CH4 (N2:CH4) were used in order to get an optimum densification of the substrate. Optical and electron scanning microscopy and density measurements were used to characterize the impregnated material. An increase in ratio of N2 and CH4 increase the overall deposition for the same time interval.