Rita de Cássia Mendonça Sales

The temperature effects on the fracture toughness of carbon fiber/RTM-6 laminates processed by VARTM:

The increasing use of composite in the aircraft industry has raised the interest for a better understanding of the failure process in these materials, which can be also influenced by the manufacturing process of the laminate. Some materials used in vacuum assisted resin transfer molding process have been studied in the open literature but very few data have been published for resin transfer molding-6 epoxy based laminates, in particular studies showing the influence of the temperature on the interlaminar fracture behavior of this type of laminates. The aim of this article is to investigate the interlaminar fracture behavior of resin transfer molding-6 based carbon composite laminates manufactured by vacuum assisted resin transfer molding subjected to Modes I and II at 25℃ and 80℃. The results show the influence of the temperature on the interlaminar fracture toughness of composites and provide a database to design composite aerostructures subjected to temperatures commonly experienced in civil aviation. The fracture aspects of the tested laminates were also investigated and directly related to the trend in results found for the fracture toughness values.

Durability study of stiffened composite panels subject to compressive loads in the plan and post buckling regime

The aim of this work was the study of the durability of stiffened composite panels submitted to the post-buckling regime under compressive loads. The study was based on fatigue test using compression loads in specimens made with initial failures in the bonded area. To monitor the delamination process, the ultrasonography technique was used as a non-destructible test. In the end, it was possible to observe the relationship between delamination propagation and the quantity of cyclic loads that the material supports. It was concluded that the stiffened panels supported a series of cycles with a low rate of delamination propagation. This shows that the applications of these materials as aeronautical structures are safe.

Modes I and II fatigue induced delamination in co-cured composite structural joints

An experimental procedure was developed to characterize fatigue delamination growth in co-cured composite joints. No-growth criterion was used to guarantee that crack growth does not reach an unsafe size during the lifetime of the component. The tests were performed for Modes I and II loading configurations, using the double cantilever beam (DCB) and the three-point bend end notched flexure (3-ENF) specimens respectively. The DCB tests were carried out following the ASTM D6115 procedure. There is no standard test procedure for conducting Mode II fatigue delamination tests. The ENF test was chosen based in previous works available in the literature and the ASTM D 7905. Information from previous quasi-static tests (using specimens with the same material batch and geometry) was used to define the load and displacement values for various G-levels, leading to a relationship between the maximum strain energy release rate (SERR-Gmax) and the number of cycles associated with the onset of delamination growth (G– N curve). Experimental results show that the co-cured joints exhibit an overall better fatigue performance in pure mode II loading. Additionally, SEM fractography studies were performed to both loading modes in order to analyze their morphology surfaces. The testing procedures used in this study, and the associated test results will be useful to develop robust criteria based on safe life design in composite aerostructures.

Understanding the water uptake in F-161 glass-epoxy composites using the techniques of luminescence spectroscopy and FT-NIR

This paper investigates the application of the luminescence spectroscopy technique in steady–state to study the moisture influence in glass fiber/epoxy prepreg and their laminates. The studies were monitored by intrinsic luminescence comparing the results with gravimetric analysis and near infrared with Fourier transform. Samples are cured and submitted to humidity controlled at 60 and 80 °C until 90 days. It is verified that the decrease in the maximum emission of the samples is directly related to the material moisture content. However, for very short periods, there is an increase in the relative intensity and blue shift of the emission band for all samples treated at 60 °C, which is related to an increase of the rigidity of the polymeric matrix. The results in this paper have a great significance because it brings a wide discussion on the interaction of water in epoxy composites materials.