Abstract: The primary objective of this research study was to evaluate the capabilities for measuring strain of a composite UAV wing with an embedded optical fiber connected to a Rayleigh backscattering distributed sensing system. This research paper summarizes the manufacturing procedure used during the instrumentation of the composite UAV wing. In addition, a Finite Element Model was developed in order to verify the strain distribution of this complex structure under static and dynamic loading conditions. The use of strain gauge data as a means for verification is presented as part of this research. Finally, fatigue tests were carried out to determine the longevity of the embedded fiber during the design life of the structure. The results demonstrate the ability of a distributed sensing system to obtain complex and accurate strain distributions on a single non-grated fiber. In addition, the findings demonstrate current limitations of the system for capturing accurate strain profiles in dynamic loading test cases.
Abstract: An experimental and numerical study on interlaminar thermal fatigue induced delamination in unidirectional carbon fiber reinforced laminate is presented in this work. The studied material consists of a unidirectional prepreg carbon fiber/epoxy (AS4/8552), which has the anti-symmetrical [902/02]T X-ply lay-up. A Teflon® film insert was introduced into the material between the third and fourth outer layers to simulate pre-existing crack. The samples were tested under thermal cycle condition within a temperature range of 130°C and -70°C. The curvature variation with the number of cycles was measured during the tests and presented along with the crack growth rate or delamination rate (da/dN) as a function of energy release rate amplitude (), which is the arithmetic difference between GH and GL that occur at lowest and highest temperatures respectively. The experimental results were compared with numerical results obtained using ABAQUS Finite Element code. A good correlation in terms of crack growth rate (da/dN) as a function of the energy release rate range () was obtained between experimental and numerical results. Furthermore, the results show that the experimental curvature decrease with increasing number of cycles, and its behavior varies with the range of thermal load imposed on the specimen.
Abstract: Composite materials have been increasingly used in the aerospace industry for the manufacturing of structures, because of the associated properties of low weight and high mechanical resistance. On the other hand, they have low delamination resistance. This paper presents the results of an experimental study performed to obtain the values of interlaminar fracture toughness (G) of a laminate under three different temperatures, using 0º carbon-epoxy prepreg fabric plies and manufactured via Hand lay up cured in autoclave (HLUP). Double Cantilever Beam (DCB) tests were performed to evaluate mode I toughness, Four Point Bend End Notched Flexure (4ENF) for mode II and Mixed Mode Bending (MMB) for mixed mode I / mode II at -54°C, 25°C and 80°C. The data were collected and analyzed using a routine developed in Matlab®. Finally, the relation between GI and GII through the failure envelope and the temperature influence on the interlaminar fracture toughness was assessed.
Abstract: The composite prepreg waste is still an environmental challenge. In the last decade, several researchers have been studying the recycling and reuse processes of composite components, mainly to use in secondary (non-structural) components. In this paper, uncured aerospace grade prepreg scraps resulted from the production waste were used to manufacture laminates. The carbon reinforcement of the prepreg presented plain weave style fabric. To manufacture the laminates the hand-lay up process was used, randomly positioning the scraps and without respecting the preferred directions of the warp and weft. Therefore, the resulting laminate is multidirectional (in the plane) and orthotropic. The cure was performed in autoclave at the maximum temperature of (180 ± 1)°C and pressure of 100 psi. This laminate was tested in compression according to standard SACMA SRM 1R-94. The results show an average compression resistance equivalent to that of laminates produced with continuous layers, although with a higher dispersion. In order to understand this variation, the fracture surfaces of the laminates were analyzed by scanning electron microscopy (SEM) in order to identify the failure modes. The specimens present shear and interlaminar failure modes, in addition to a mixed mode of failure (a transition between the shear and interlaminar failures). For the same specimen, the analysis of the internal arrangement of the fibers shows that changes in failure modes occur in regions where the carbon fabric is not continuous, i.e. there are joinings of scraps. It was also observed that none of the compression tests resulted in catastrophic failure of the specimens, possibly due to a greater resistance to fracture growth in the presence of mixed failure mode.
Abstract: An experimental investigation of the resistance welding of PPS/carbon fiber is presented in this manuscript. Currently, one of the main problems of the structural polymer composites consists in its effective integration of the components. The electrical resistance welding has been considered as one of the promising techniques for bonding composites, because it is a quick process with easy surface preparation. To improve the process to welding poly-(phenylene sulfide) (PPS) reinforced with carbon fiber laminates, it was used a full factorial design (23). Considering the factors pressure, electrical current and time, the more appropriate conditions for welding were evaluated based on a criterion of maximum lap shear strength, according to ASTM D1002-10. A comparison between welded and non-welded specimens in terms of analysis dynamic mechanical (DMA), thermomechanical analysis (TMA) and vibration tests was performed. It was demonstrated that large-scale DMA presented a similar results but according to TMA and vibration test were observed that welded specimens presented different results when compared to non-welded laminates, due probably to the presence of metallic heating element.
Abstract: This paper is focused on the processing of thermoplastic composite materials obtained from carbon fibers (CFs) treated by plasma assisted techniques. The treatments employed in this work were the Dielectric Barrier Discharge (DBD), which is done at atmospheric pressure, involving lower energies and the Plasma Immersion Ion Implantation (PIII), which is performed at low pressure, involving higher energies. After the treatments, samples characterizations were performed to determine which treatment is most effective to get better physico-chemical CF surface properties. The techniques employed in this work in order to evaluate the surface treatment were: scanning electron microscopy (SEM); atomic force microscopy (AFM) Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). Treated and untreated CFs/Polyphenylene sulfide (PPS) composites were processed by hot-compression molding technique. These composites were evaluated by interlaminar shear tests (ILSS). After analyzing the results, it was found that the treatments increased the CF roughness and caused slight changes in the CF structure. In addition, there was an increase in the shear strength of the composites obtained from treated fibers by both plasma processes. In conclusion, DBD and PIII treatments are effective tools for improving adhesion between CF and the polymeric matrix.
Abstract: This paper presents a numerical study on the influence of multimodal shunt circuit parameters in the flutter velocity of a typical section under an unsteady airflow. Flutter on typical sections is a kind of self-excited oscillation which can occur due to the interaction with the airflow. In the flutter point, when the critical dynamic pressure is reached, the vibrations of the typical section become unstable and increase fast and significantly in time. As a result, it can lead the structure to failure. Thus, it becomes important to investigate the possibility of reducing the effects of flutter in order to increase the reliability of composite structures during service. In this work, the aero-electromechanical dynamic model formulation is based on the Hamilton principle. The unsteady aerodynamic forces are calculated based on the linearized thin-airfoil theory, proposed by Theodorsen. The passive element responsible for the energy dissipation is a multimodal resonant shunt circuit in series topology, attached to a piezoelectric patch. An iterative solution algorithm is proposed to solve the resultant nonlinear eigenvalue problem. The optimum shunt tuning is firstly performed using Hagood and Flotow’s propositions; then, it is used an heuristic optimization algorithm, based on Differential Evolution. The preliminary results indicate that the flutter speed can be affected by the passive control, both in its mechanical aspect as electrical.
Abstract: The knowledge of how to process composite materials and combine them with radiation absorbing centers, using different components, additives and polymer matrices with suitable electromagnetic properties (dielectric constant and tangent loss), allows the production of multifunctional composites that can function as conductors or microwave absorbing materials. Thus, the purpose of this study was to process and evaluate the electromagnetic properties of multilayered multifunctional composites made with layers of glass fiber cloths or nonwoven glass fiber veils pre-impregnated with formulations based on carbon black. Electromagnetic properties of the multifunctional composites were evaluated by measuring the reflection of microwave radiation using the waveguide technique in the X-band (8.2 to 12.4 GHz). The results show that the multifunctional composites absorbed 90% to 99% of the energy of the incident microwave radiation. The high attenuation of the incident microwave radiation combined with their small thickness indicate that these multifunctional composites could be used in a number of military and civilian applications.
Abstract: Integral structures offer large benefits in terms of manufacturing cost, but suffer from a lower degree of fail safety when compared to built-up structures. In order to achieve an improvement on the fatigue crack propagation (FCP), crack containment features (also known as crenellations) have been used on these structures. The source of the FCP improvement is the stress intensity factor (K) modification due to the geometry change. In the current study, an analysis about means of estimating K from the experimental information, and also to verify the K behavior while the crack propagates was performed. The study tested two AA 7475 panels, one with crenellations and another without. As the crack propagates, the K values were estimated in two forms, based on the crack propagation rate and by using a digital image correlation (DIC) system, coupled with strain gages. Based on DIC system, it was possible to evaluate the K estimation, the singularity dominated zone size and the K increase, as long as the crack propagated, for both test specimens. A comparison between the two methods was also made, and finally the use of a DIC system as a tool for estimating the K parameter was discussed.