Geraldo Maurício Cândido

Effect of the free edge finishing on the tensile strength of carbon/epoxy laminates

Abstract High productivity can be achieved in the fabrication of composite laminates if molded edges are used. In this work, the fabrication process of laminates with this type of edge finishing is described. The stacking sequence effect for laminates with molded edges is experimentally studied and compared to that of laminates with machined edges. The tensile strengths of five different laminates fabricated with carbon/epoxy unidirectional tape and woven fabrics were measured. It is shown that the strength of laminates with molded edges is about 10% lower than that of laminates with machined edges but less sensitive to stacking sequence. The presence of small pockets of pure resin near the free edge of laminates with molded edges causes a reduction in the tensile strength. The stacking sequence effect may be very pronounced for laminates with woven fabric layers and machined edges.

Nanostructured composites based on carbon nanotubes and epoxy resin for use as radar absorbing materials

Nanostructured polymer composites have opened up new perspectives for multifunctional materials. In particular, carbon nanotubes (CNTs) present potential applications in order to improve mechanical and electrical performance in composites with aerospace application. The combination of epoxy resin with multiwalled carbon nanotubes results in a new functional material with enhanced electromagnetic properties. The objective of this work was the processing of radar absorbing materials based on formulations containing different quantities of carbon nanotubes in an epoxy resin matrix. To reach this objective the adequate concentration of CNTs in the resin matrix was determined. The processed structures were characterized by scanning electron microscopy, rheology, thermal and reflectivity in the frequency range of 8.2 to 12.4 GHz analyses. The microwave attenuation was up to 99.7%, using only 0.5% (w/w) of CNT, showing that these materials present advantages in performance associated with low additive concentrations.

Fractografia de compósito estrutural aeronáutico submetido à caracterização de tenacidade à fratura interlaminar em modo I

Many components of modern aircrafts are now manufactured from polymer composites. Reinforced laminates with continuous carbon fibers and modified epoxy resin are employed in primary and secondary structures to reduce weight and improve the aircraft performance. However, if a circumstantial failure happens, the complex fracture process of the laminates may involve interlaminar damage mechanisms. The delamination is the interlaminar discontinuity which may propagate catastrophically with the application of mechanical loads. The Double Cantilever Beam (DCB) is the most used method to determine the Mode I fracture toughness of structural composites. In this work samples prepared from a plain weave fabric laminate were submitted to Mode I delamination under static load at room temperature. The analysis of the delaminated surfaces was performed with scanning electron microscopy (SEM). The results show that the fracture process initiates at the resin pockets after a Teflon® insert and propagates along the resin rich areas at the crossing of weft and warp tows. The main fractographical aspects revealed are identified, reported and discussed.

Avaliação da temperatura de transição vítrea de compósitos poliméricos reparados de uso aeronáutico

This work shows the evaluation of the glass transition temperature (T g ) by DMTA of three different families of repaired polymeric composites, manufactured with carbon fiber fabric/epoxy F584, glass fabric/epoxy F161 and aramide fabric/epoxy F161, respectively. The composites were laminated by conventional hand lay-up process used in aeronautical industry. Afterwards, simulated damages, scarf type, were made and overlapping layers of F155 epoxy resin prepregs repaired them. The DMTA curves show the effects of combination of different systems resins on the T g values and, consequently, on the service temperature of the repaired laminate. The used repairing material (based on epoxy resin F155 and on epoxy adhesive film FM73) decreased the T g of the repaired laminates in approximately 30 °C for the carbon and glass laminates. The conditioning of aramide laminate under elevated humidity and temperature provoked a more decrease of the T g (reduction of approximately 40 °C). The T g reduction for the repaired laminates is attributed to the adhesive film migration into the F155 resin system during the cure process and also to the resin plasticization by water diffusion during the hygrothermal conditioning. The T g reduction leads to a decrease of the service temperature of the repaired polymeric composite.

Hygrothermal and Stacking Sequence Effects on Carbon Epoxy Composites with Molded Edges

Moisture and high temperature may cause a variety of effects on the mechanical and thermophysical properties of polymeric composites. The diffusion of moisture in the laminate occurs primarily by capillarity until a state of equilibrium is reached. Moisture affects the matrix and the fiber/matrix interface. The stacking sequence and the free edge finishing may affect the laminate tensile strength causing a free edge delamination; they can also affect the moisture absorption process. Laminates produced with molded edges do not require machining operations for the trimming and finishing of the free edges causing a gain in productivity. However, the mechanical behavior of laminates with molded edges saturated with moisture has not been characterized. This work presents a comparative study of the tensile strength of cross ply laminates with molded and machined edges under dry condition and saturated with moisture.

Accelerated aging effects on carbon fiber PEKK composites manufactured by hot compression molding

The accelerated aging effects on mechanical properties of carbon fiber-reinforced polyetherketoneketone (PEKK) thermoplastic composites are reported. Ultraviolet (UV) radiation and moisture absorption may induce property changes that weaken the polymeric matrix and/or deteriorate the matrix fiber interface by debonding or microcracking. The aim of the present work is to correlate the influence of the accelerated aging effects on compression and shear properties of carbon fiber-reinforced PEKK thermoplastic composite manufacturing by hot compression molding. The hot compression molding process was shown to be a good alternative for producing the thermoplastic composite, with an appropriate impregnation. The UV radiation/condensation conditioning effect did not show significant influence on shear strength properties. However, the most significant change was observed in the compression and shear strength after the specimens were submitted to the hygrothermal conditioning, which presented a marked increase. This behavior suggests an antiplasticization effect.

Processing of Carbon Fiber/PEI Composites Based on Aqueous Polymeric Suspension of Polyimide

Thermoplastic can be classified by their molecular structure as either amorphous or semicrystalline polymer. Semicrystalline polymers crystallize to form structures with excellent chemical resistance, mechanical properties, and high service temperature, whereas the amorphous are unable to form ordered structures, showing up either in glassy or rubbery state. The glass transition it is not just determined by the chemical structure but, also, by the available free volume between the chain, allowing its rotational movement (Cogswell, 1992; Nohara, 2005).

FATIGUE BEHAVIOUR STUDY ON REPAIRED ARAMID FIBER/EPOXY COMPOSITES. doi: 10.5028/jatm.2009.0102217221

Aramid fiber reinforced polymer composites have been used in a wide variety of applications, such as aerospace, marine, sporting equipment and in the defense sector, due to their outstanding properties at low density. The most widely adopted procedure to investigate the repair of composites has been by repairing damages simulated in composite specimens. This work presents the structural repair influence on tensile and fatigue properties of a typical aramid fiber/epoxy composite used in the aerospace industry. According to this work, the aramid/epoxy composites with and without repair present tensile strength values of 618 and 680MPa, respectively, and tensile modulus of 26.5 and 30.1 GPa, respectively. Therefore, the fatigue results show that in loads higher than 170 MPa, both composites present a low life cycle (lower than 200,000 cycles) and the repaired aramid/epoxy composite presented low fatigue resistance in low and high cycle when compared with non-repaired composite. With these results, it is possible to observe a decrease of the measured mechanical properties of the repaired composites.