Fracture toughness determination of adhesive and co-cured joints in natural fibre composites

Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. On the other hand, natural fibre composites have recently gained interest due to the low cost and density. It is therefore essential to predict the fracture behavior of joints between these materials, to assess the feasibility of joining or repairing with adhesives. In this work, the tensile fracture toughness (Gc n) of adhesive joints between natural fibre composites is studied, by bonding with a ductile adhesive and co-curing. Conventional methods to obtain Gc n are used for the co-cured specimens, while for the adhesive within the bonded joint, the J-integral is considered. For the J-integral calculation, an optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain Gc n than the available methods is proposed (by the J-integral technique), and the fracture behaviour in tension of bonded and co-cured joints in jute-reinforced natural fibre composites is also provided for the subsequent strength prediction. Additionally, for the adhesively- bonded joints, the tensile cohesive law of the adhesive is derived by the direct method.

An approach for reliability-based robust design optimisation of angle-ply composites

Variations of manufacturing process parameters and environmental aspects may affect the quality and performance of composite materials, which consequently affects their structural behaviour. Reliability-based design optimisation (RBDO) and robust design optimisation (RDO) searches for safe structural systems with minimal variability of response when subjected to uncertainties in material design parameters. An approach that simultaneously considers reliability and robustness is proposed in this paper. Depending on a given reliability index imposed on composite structures, a trade-off is established between the performance targets and robustness. Robustness is expressed in terms of the coefficient of variation of the constrained structural response weighted by its nominal value. The Pareto normed front is built and the nearest point to the origin is estimated as the best solution of the bi-objective optimisation problem.

Numerical prediction of delamination onset in carbon/epoxy composites drilling

Despite the fact that their physical properties make them an attractive family of materials, composites machining can cause several damage modes such as delamination, fibre pull-out, thermal degradation, and others. Minimization of axial thrust force during drilling reduces the probability of delamination onset, as it has been demonstrated by analytical models based on linear elastic fracture mechanics (LEFM). A finite element model considering solid elements of the ABAQUS® software library and interface elements including a cohesive damage model was developed in order to simulate thrust forces and delamination onset during drilling. Thrust force results for delamination onset are compared with existing analytical models.

Mathematical modeling and simulation of heat flow through pultrusion die assembly system for thermoset composites

Pultrusion is an industrial process used to produce glass fibers reinforced polymers profiles. These materials are worldwide used when performing characteristics, such as great electrical and magnetic insulation, high strength to weight ratio, corrosion and weather resistance, long service life and minimal maintenance are required. In this study, we present the results of the modelling and simulation of heat flow through a pultrusion die by means of Finite Element Analysis (FEA). The numerical simulation was calibrated based on temperature profiles computed from thermographic measurements carried out during pultrusion manufacturing process. Obtained results have shown a maximum deviation of 7%, which is considered to be acceptable for this type of analysis, and is below to the 10% value, previously specified as maximum deviation. © 2011, Advanced Engineering Solutions.

Hybrid thermoset polymeric composites – an Innovation towards sustainability

In this study, a new waste management solution for thermoset glass fibre reinforced polymer (GFRP) based products was assessed. Mechanical recycling approach, with reduction of GFRP waste to powdered and fibrous materials was applied, and the prospective added-value of obtained recyclates was experimentally investigated as raw material for polyester based mortars. Different GFRP waste admixed mortar formulations were analyzed varying the content, between 4% up to 12% in weight, of GFRP powder and fibre mix waste. The effect of incorporation of a silane coupling agent was also assessed. Design of experiments and data treatment was accomplished through implementation of full factorial design and analysis of variance ANOVA. Added value of potential recycling solution was assessed by means of flexural and compressive loading capacity of GFRP waste admixed mortars with regard to unmodified polymer mortars. The key findings of this study showed a viable technological option for improving the quality of polyester based mortars and highlight a potential cost-effective waste management solution for thermoset composite materials in the production of sustainable concrete-polymer based products.

Damage assessment of fibre reinforced laminates

The use of solid wastes and natural fibres to the production of innovative composites is now a matter of environmental need. A set of hybrid composites constituted of waste rubber particles and sugarcane bagasse fibres into a thermoset composite material are drilled to determine the effect of rubber particle addition and size, sugarcane fibre addition and length and fibre chemical treatment on damage extension and related mechanical properties. Damage extension is determined by enhanced radiography for further damage measurement – diameter and areas – and correlated with mechanical test results — bearing test. The results demonstrated significant effect of the rubber and fibre additions on mechanical properties of the composites.

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.