Mirabel Cerqueira Rezende

The influence of porosity on the interlaminar shear strength of carbon/epoxy and carbon/bismaleimide fabric laminates

The resin system affects the shape and size of voids in composite laminates and the associated fracture parameters. Most work on the effects of voids in the literature deals with carbon/epoxy laminates; little experimental data are available on other resin systems of practical importance. This work aims at comparing the effect of voids on carbon/epoxy and carbon/bismaleimide fabric laminates. Specimens with void contents in the range 0 to 6% were produced from both materials. The void content is correlated to ultrasonic attenuation and microscopic inspection was carried out to analyze the sizes and shapes of the voids. The interlaminar shear strength was measured by the short-beam method described in ASTM D2344 and the adequacy of a fracture criterion to represent the experimental data for both materials is assessed. The influence of voids on the interlaminar shear strength of both materials is compared in terms of the corresponding fracture parameters and the shape and the location of the voids.

Mechanical behavior of carbon fiber reinforced polyamide composites

The purpose of this work is to compare tensile, compressive and interlaminar shear properties of different carbon reinforcement/polyamide composites obtained by interfacial polymerization and hot compression molding techniques. Two types of composite matrices were studied: polyamide 6 and polyamide 6/6, both reinforced by fabric and unidirectional carbon fibers. The effects of the fiber volume fraction and the matrix on mechanical properties were analyzed through tensile, interlaminar shear and compressive tests. In general, the results have shown a slight increase of the composite elastic modulus, tensile and compressive strength with the increase of carbon fiber content. The microscopic damage development within selected composites during the loading has been observed through optical and scanning electron microscope techniques and has shown that shear failure at the fiber/matrix interface has been mostly responsible for damage development, initiated at relatively low stress.

Dielectric microwave absorbing material processed by impregnation of carbon fiber fabric with polyaniline

It is a known fact that the adequate combination of components and experimental conditions may produce materials with specific requirements. This study presents the effect of carbon fiber fabric impregnation with polyaniline conducting polymer aiming at the radar absorbing material processing. The experiments consider the sample preparation with one and two impregnations. The prepared samples were evaluated by reflectivity measurements, in the frequency range of 8-12 GHz and scanning electron microscopy analyses. The correlation of the results shows that the quantity of impregnated material influences the performance of the processed microwave absorber. This study shows that the proposed experimental route provides flexible absorbers with absorption values of the incident radiation close to 87%.

Dependence of microwave absorption properties on ferrite volume fraction in MnZn ferrite/rubber radar absorbing materials

Abstract We report the analysis of measurements of the complex magnetic permeability (μr) and dielectric permittivity (er) spectra of a rubber radar absorbing material (RAM) with various MnZn ferrite volume fractions. The transmission/reflection measurements were carried out in a vector network analyzer. Optimum conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the impedance matching equation. Both the MnZn ferrite content and the RAM thickness effects on the microwave absorption properties, in the frequency range of 2–18xa0GHz, were evaluated. The results show that the complex permeability and permittivity spectra of the RAM increase directly with the ferrite volume fraction. Reflection loss calculations by the impedance matching degree (reflection coefficient) show the dependence of this parameter on both thickness and composition of RAM.

Microwave properties of EPDM/PAni-DBSA blends

Conductive elastomer blends of ethylene-propylene-diene rubber, EPDM, and polyaniline, PAni, doped with dodecylbenzene sulfonic acid, DBSA, were prepared and evaluated as a radar absorbing material, RAM. Reflectivity measurements in the 8-12 GHz frequency range, using the NRL arch method, were done to evaluate the shielding effectivity. The results showed that the dopant and PAni concentrations and also the sample thickness affect these measurements. Good shielding values were obtained (-15 dB) with the 50/50, 40/60 and 20/80 (w/w %) EPDM/PAni blends.

Evaluation of carbon fiber surface treated by chemical and cold plasma processes

Sized PAN-based carbon fibers were treated with hydrochloric and nitric acids, as well as argon and oxygen cold plasmas, and the changes on their surfaces evaluated. The physicochemical properties and morphological changes were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), tensile strength tests and Raman spectroscopy. The nitric acid treatment was found to cause the most significant chemical changes on the carbon fiber surface, introducing the largest number of chemical groups and augmenting the roughness. The oxygen plasma treatments caused ablation of the carbon fiber surface, removing carbon atoms such as CO and CO2 molecules. In addition, the argon plasma treatment eliminated defects on the fiber surface, reducing the size of critical flaws and thus increasing the fibers tensile strength.

Hygrothermal effects on dynamic mechanical snalysis and fracture behavior of polymeric composites

Polymer composites used above their glass transition temperatures Tg present a substantial degradation of physical properties; therefore a materials glass transition temperature and its change with moisture absorption are of practical importance. Little attention has been paid to the role of the adhesive bonding between the reinforcing fiber and matrix, particularly for BMI matrix. In this work the effect of moisture on the dynamic mechanical behavior and the fiber/matrix interface was investigated. Two systems were evaluated: carbon fabric/epoxy and carbon fabric/bismaleimide laminates. The results demonstrated that the moisture absorbed by the laminates causes either reversible or irreversible plasticization of the matrix. The humidity combined with the temperature effects may cause significant changes in the Tg matrix and toughness affecting the laminate strength. Moisture absorption was correlated to the fracture mode of the laminate demonstrating the deleterious effect of moisture on the interface. This leads to debonding between fiber and matrix. This behavior was investigated by scanning electron microscopy and dynamic mechanical analysis.

Strength of Hygrothermally Conditioned Polymer Composites with Voids

It is well known that voids and environmental conditions may affect the strength of composite laminates. Despite the fact that voids facilitate moisture absorption, the effect of voids on the composite laminates in the presence of moisture has not been addressed in the literature. Moreover, most of the attention has been focused on composites made of unidirectional carbon-epoxy prepreg tape. This article presents experimental results on the combined effect of voids and moisture in polymer composites. The experiments consider two types of reinforcement (eight harness satin fabric and unidirectional tape), two types of resin systems (epoxy and bismaleimide), and two types of loading (interlaminar shear and compression). The effect of these factors on the fracture parameters of laminates with and without environmental conditioning is discussed.

Effect of Void Content on the Moisture Absorption in Polymeric Composites

This study assesses the effect of porosity in the moisture absorption of polymeric composites. The preparation of composites samples with porosity, control, and measurement of void content, and moisture absorption behavior of carbon/epoxy laminates are discussed. Rheological analyses are used to characterize the viscosity behavior of prepregs either in the presence or absence of moisture. Polymeric composites with different levels of voids were produced and submitted to hygrothermal conditioning to study the moisture absorption behavior. It was found that the rate of water uptake and the maximum level of moisture absorption in carbon/epoxy composites depends on the void content and specimen geometry. The dependence on the void content may be quite significant. The nature of the matrix system was also verified to affect the moisture absorption properties of the composite.

Characterization of cure of carbon/epoxy prepreg used in aerospace field

Carbon/epoxy 8552 prepreg is a thermoplastic toughened high-performance epoxy being used in the manufacture of advanced army material. Understanding the cure behavior of a thermosetting system is essential in the development and optimization of composite fabrication processes. The cure kinetics and rheological behavior were evaluated using a differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and a rheometer. Values of the kinetic parameters were obtained from dynamic DSC scans using an nth order reaction model. Rheological measurements as a function of temperature and time were made for the prepreg system. The manufacturers recommended cure cycle was evaluated and considered adequate to consolidated the studied system.