Mode III interlaminar fracture of carbon/epoxy laminates using the edge crack torsion (ECT) test

The mode III interlaminar fracture of carbon/epoxy laminates was evaluated with the edge crack torsion (ECT) test. Three-dimensional finite element analyses were performed in order to select two specimen geometries and an experimental data reduction scheme. Test results showed considerable non-linearity before the maximum load point and a significant R-curve effect. These features prevented an accurate definition of the initiation point. Nevertheless, analyses of non-linearity zones showed two likely initiation points corresponding to GIIIc values between 850 and 1100 J/m2 for both specimen geometries. Although any of these values is realistic, the range is too broad, thus showing the limitations of the ECT test and the need for further research.

Estudo da Delaminação em Compósitos de Epóxido Reforçado com Borracha e Fibras Naturais

Nos últimos anos, a utilização dos materiais compósitos tem vindo a tornar-se cada vez mais comum em várias indústrias, onde se verifica uma ascensão na procura pelos mesmos. Características como o baixo peso aliado à sua alta resistência e rigidez permitem que estes materiais possuam diversas aplicações em variadas áreas, desde a medicina, aeronáutica, indústria automóvel e aeroespacial, até à indústria eletrónica. Hoje em dia, o uso de desperdícios sólidos de borracha e fibras naturais na produção de materiais compósitos é, mais que uma opção, uma necessidade ambiental. De forma a reduzir as enormes quantidades de desperdícios, foi criado um material compósito constituído por uma resina termoendurecível reforçada com esses dois tipos de desperdícios. Parâmetros de fabrico como a percentagem de borracha, o tamanho das partículas de borracha, a percentagem de fibras de cana-de-açúcar e o comprimento dessas fibras foram variados, com o objetivo de estudar a influência destes dois materiais nas propriedades mecânicas do compósito. Apesar da maior parte dos compósitos serem fabricados na forma de uma peça funcional quase pronta a ser utilizada, por vezes é necessário recorrer à maquinação de furos. Apesar das muitas técnicas de furação existentes, os defeitos resultantes deste processo aplicado aos materiais compósitos são ainda muito comuns. Desses defeitos o que mais se destaca é sem dúvida a delaminação. Trinta e seis provetes de epóxido reforçado com borracha e fibra de cana-de-açúcar foram fabricados e furados, de modo a possibilitar o estudo das propriedades mecânicas do material compósito, assim como a análise da zona danificada durante a furação. Diferentes condições de furação, como tipos de broca e velocidades de avanço diferentes, foram impostas aos provetes de forma a variar o mais possível a zona de dano de uns furos para os outros. Parâmetros como a área de dano ou ainda o fator de delaminação provam ser muito úteis na caracterização e quantificação do dano na zona periférica de um furo. Recorrendo a técnicas de processamento de imagem foi possível obter esses parâmetros. O processamento e análise de imagem pode ser feito através de vários métodos. O método utilizado neste trabalho foi o software MATLAB® associado a ferramentas de processamento de imagem. Depois de feita a análise dos furos foram realizados ensaios de esmagamento a todos os provetes. Este passo permitiu assim avaliar de que forma os parâmetros de furação influenciam a resistência mecânica do material, e se a avaliação realizada aos furos é um método viável para a avaliação da extensão de dano nesses furos.

Plasma polymerization of C[subscript 4]F[subscript 8] thin film on high aspect ratio silicon molds

High aspect ratio polymeric micro-patterns are ubiquitous in many fields ranging from sensors, actuators, optics, fluidics and medical. Second generation PDMS molds are replicated against first generation silicon molds created by deep reactive ion etching. In order to ensure successful demolding, the silicon molds are coated with a thin layer of C[subscript 4]F[subscript 8] plasma polymer to reduce the adhesion force. Peel force and demolding status are used to determine if delamination is successful. Response surface method is employed to provide insights on how changes in coil power, passivating time and gas flow conditions affect plasma polymerization of C[subscript 4]F[subscript 8].

Modelización constitutiva y computacional del daño y la fractura de materiales compuestos

En el trabajo se definen modelos constitutivos que permiten reproducir el proceso de fallo de estructuras de materiales compuestos en distintas escalas bajo cargas estáticas. Se define un modelo constitutivo para determinar la respuesta de estructuras de materiales compuestos mediante la teoría de laminados. El modelo es validado mediante un programa de ensayos experimentales con probetas con un agujero central geométricamente similares. Se muestra la capacidad del modelo de detectar el efecto tamaño. Se define un modelo constitutivo para materiales transversalmente isótropos bajo estados tridimensionales de tensión. El modelo se valida analizando numéricamente el proceso de agrietamiento de la matriz. Finalmente se desarrolla un modelo analítico para determinar el agrietamiento de la matriz y la delaminación entre las capas.

Anthraquinone catalysis in the glucose-driven reduction of indigo to leuco-indigo

Anthraquinone immobilised onto the surface of indigo microcrystals enhances the reductive dissolution of indigo to leuco-indigo. Indigo reduction is driven by glucose in aqueous NaOH and a vibrating gold disc electrode is employed to monitor the increasing leuco-indigo concentration with time. Anthraquinone introduces a strong catalytic effect which is explained by invoking a molecular "wedge effect'' during co-intercalation of Na+ and anthraquinone into the layered indigo crystal structure. The glucose-driven indigo reduction, which is in effective in 0.1 M NaOH at 65 degrees C, becomes facile and goes to completion in the presence of anthraquinone catalyst. Electron microscopy of indigo crystals before and after reductive dissolution confirms a delamination mechanism initiated at the edges of the plate-like indigo crystals. Catalysis occurs when the anthraquinone-indigo mixture reaches a molar ratio of 1:400 (at 65 degrees C; corresponding to 3 mu M anthraquinone) with excess of anthraquinone having virtually no effect. A strong temperature effect ( with a composite E-A approximate to 120 kJ mol(-1)) is observed for the reductive dissolution in the presence of anthraquinone. The molar ratio and temperature effects are both consistent with the heterogeneous nature of the anthraquinone catalysis in the aqueous reaction mixture.

Evolution of the eye-end design of a composite leaf spring for heavy axle loads

This paper presents the design evolution process of a composite leaf spring for freight rail applications. Three designs of eye-end attachment for composite leaf springs are described. The material used is glass fibre reinforced polyester. Static testing and finite element analysis have been carried out to obtain the characteristics of the spring. Load-deflection curves and strain measurement as a function of load for the three designs tested have been plotted for comparison with FEA predicted values. The main concern associated with the first design is the delamination failure at the interface of the fibres that have passed around the eye and the spring body, even though the design can withstand 150 kN static proof load and one million cycles fatigue load. FEA results confirmed that there is a high interlaminar shear stress concentration in that region. The second design feature is an additional transverse bandage around the region prone to delamination. Delamination was contained but not completely prevented. The third design overcomes the problem by ending the fibres at the end of the eye section.

Finite element aided design evolution of composite leaf spring

This paper shows the process of the virtual production development of the mechanical connection between the top leaf of a dual composite leaf spring system to a shackle using finite element methods. The commercial FEA package MSC/MARC has been used for the analysis. In the original design the joint was based on a closed eye-end. Full scale testing results showed that this configuration achieved the vertical proof load of 150 kN and 1 million cycles of fatigue load. However, a problem with delamination occurred at the interface between the fibres going around the eye and the main leaf body. To overcome this problem, a second design was tried using transverse bandages of woven glass fibre reinforced tape to wrap the section that is prone to delaminate. In this case, the maximum interlaminar shear stress was reduced by a certain amount but it was still higher than the material’s shear strength. Based on the fact that, even with delamination, the top leaf spring still sustained the maximum static and fatigue loads required, the third design was proposed with an open eye-end, eliminating altogether the interface where the maximum shear stress occurs. The maximum shear stress predicted by FEA is reduced significantly and a safety factor of around 2 has been obtained. Thus, a successful and safe design has been achieved.

ASSOCIATED A-TYPE SUBALKALINE AND HIGH-K CALC-ALKALINE GRANITES IN THE ITU GRANITE PROVINCE, SOUTHEASTERN BRAZIL: PETROLOGICAL AND TECTONIC SIGNIFICANCE

The 590-580 Ma Itu Granite Province (IGP) is a roughly linear belt of post-orogenic granite plutons similar to 60 km wide extending for some 350 km along the southern edge of the Apia-Guaxupe Terrane in southeastern Brazil. Typical components are subalkaline A-type granites (some with rapakivi texture) that crystallized at varied, but mostly strongly oxidizing conditions, and contrast with a coeval association of also oxidized high-K calc-alkaline granites in terms of major (e. g., lower Ca/Fe) and trace elements (higher Nb, Y, Zr). Mantle-derived magmas (such as those forming the LILE-rich Piracaia Monzodiorite, with epsilon(Nd(t)) = -7 to -10, (87)Sr/(86)Sr((t)) = 0.7045-0.7055) are inferred to derive from enriched subcontinental lithosphere modified during previous subduction, and may have played a role in the generation of the A-type granites, adding melts or fluids or both to the lower crust from which the latter were generated. The IGP is interpreted as a reflection of crust uplift and increased heat flux during ascent of hot, less dense asthenosphere after continental collision, probably reflecting breakoff of an oceanic slab coeval to the right-lateral accretion of a terrane related to the Mantiqueira Orogenic System.

Effect of sonication on the particle size of montmorillonite clays

This paper reports on the effect of sonication on SAz-1 and SWy-1 montmorillonite suspensions. Changes in the size of the particles of these materials and modifications of their properties have been investigated. The variation of the particle size has been analyzed by DLS (dynamic light scattering). In all cases the clay particles show a bimodal distribution. Sonication resulted in a decrease of the larger modal diameter, as well as a reduction of its volume percentage. Simultaneously, the proportion of the smallest particles increases. After 60 min of sonication, SAz-1 presented a very broad particle size distribution with a modal diameter of 283 nm. On the other hand, the SWy-1 sonicated for 60 min presents a bimodal distribution of particles at 140 and 454 nm. Changes in the properties of the clay suspensions due to sonication were evaluated spectroscopically from dye-clay interactions, using Methylene Blue. The acidic sites present in the interlamellar region, which are responsible for dye protonation, disappeared after sonication of the clay. The changes in the size of the scattering particles and the lack of acidic sites after sonication suggest that sonication induces delamination of the clay particles. (c) 2008 Elsevier Inc. All rights reserved.

Influence of z-pinning on the buckling of composite laminates under edge-wise compression

Z-pinning is a newly developed technique to enhance the strength of composite laminates in the thickness direction. Recent experimental and theoretical studies have shown that z-pins significantly improve mode I and mode II fracture toughness. In practice, buckling accompanying delamination is a typical failure mode in laminated composite structures. For a complete understanding of the z-pinning technique towards improvements of the overall mechanical properties of laminated composites, a numerical model is developed in this paper to investigate the influence of z-pins on the buckling composite laminates with initial delaminations under edge-wise compression. The numerical results indicate that z-pinning can indeed effectively increase the compressive strength of the composite laminates provided that the initial imperfection is within a certain range. The magnitude of the improvement is consistent with available experimental data.

Investigation of failure mechanisms in aged aerospace composites

The effect of isothermal ageing on two high temperature, bismaleimide composite materials, a novel CSIRO CBR 320/328 composite and a commercial CIBA GEIGY Matrimid® 5292 composite, was examined at 204 and 250 °C. Delamination is a major cause of failure in composite materials, therefore, the Mode I interlaminar fracture toughness (GIC) of both materials was measured using the double cantilever beam (DCB) test. Chemical degradation of the matrix was monitored concurrently using Fourier transform infrared (FTIR) and Raman spectroscopy. Chemical changes at the core of both of these materials were found to occur concomitantly with the observed changes in interlaminar fracture toughness. FTIR analysis of both matrix materials revealed the predominant degradation mechanism to be the oxidation of the methylene group bridging two aromatic rings common to the structure of both resins, and was substantiated by the ingrowth of a broad peak centred at 1600 cm⁻¹ . In addition to this, the pyromellitic anhydride unit present only in the CBR 320/328 composites was found to be highly resistant to the effects of ageing, whereas the saturated imide, common to the cured structures of both materials, was observed to degrade. Raman spectroscopy indicated that the predominant degradation mechanism of the composites differed at the two ageing temperatures.

Sol-gel derived HA/TiO2 double coatings on Ti scaffolds for orthopaedic applications

Hydroxyapatite/titania (HA/TiO₂) double layers were coated onto Ti scaffolds throughout for orthopaedic applications by sol-gel method. Differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and X-ray diffractometry (XRD) were used for the characterisation of the phase transformations of the dried gels and coated surface structures. Scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS) was used for the observation and evaluation of the morphology and phases of the surface layers and for the assessment of the in vitro tests. The in vitro assessments were performed by soaking the HA/TiO₂ double coated samples into the simulated body fluid (SBF) for various periods. The TiO₂ layer was coated by a dipping-coating method at a speed of 12 cm/min, followed by a heat treatment at 600 °C for 20 min. The HA layer was subsequently dipping-coated on the outer surface at the same speed and then heat-treated at difference temperatures. The results indicat that the HA phase begins to crystallize after a heat treatment at 560 °C. The crystallinity increases obviously at 760 °C. SEM observations find no delamination or crack at the interfaces of HA/TiO₂ and TiO₂/Ti. The HA/TiO₂ coated Ti scaffolds displays excellent bone-like apatite forming ability when it is soaked into SBF. Ti scaffolds after HA/TiO₂ double coatings can be anticipated as promising implant materials for orthopaedic applications

Sol-gel derived hydroxyapatite/titania biocoatings on titanium substrate

A simple sol–gel method was successfully developed for a hydroxyapatite (HA)/TiO₂ double layer deposition on a pure titanium substrate. Phase formation, surface morphology, and interfacial microstructure were investigated by differential scanning calorimetry analysis (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The TiO₂ layer was coated by a spin coating method at a speed of 1500 rpm for 15 s, followed by a heat treatment at 560 °C for 20 min. The HA film was subsequently spin coated on the outer surface at the same speed and then heat-treated at difference temperatures. Results indicated that the HA phase began to crystallize after a heat treatment at 580 °C; and the crystallinity increased obviously at a temperature of 780 °C. The HA film showed a porous structure and a thickness of 5–7 μm after the heat treatment at 780 °C. SEM observations revealed no delamination and crack at the interfaces of HA/TiO₂ and TiO₂/Ti. The HA film with a porous structure is expected to be more susceptible to the natural remodeling processes when it is implanted in a living body.

Z-pin bridging in composite laminates and some related problems

This paper reviews our recent studies on z-pinning of composite laminates. The contents include theoretical, numerical and experimental studies on the Mode I and Mode II z-pinned delamination growth and the corresponding bridging laws. Test methods to evaluate the z-pin bridging law will be discussed. Comparisons of experimental results and theoretical predictions for the z-pinned double-cantilever-beam (DCB) subjected to mode I delamination with a pre-determined bridging law are provided to confirm the reliability of the methods. A parametric study by finite element method (FEM) is presented for both Mode I and Mode II z-pinned delaminations. In addition, the effect of loading rate on z-pinned DCB delamination and the bridging effect of z-pinning on the buckling of composite laminates are also given.