Optimisation of Fabric Reinforced Polymer Composites Using a Variant of Genetic Algorithm

Fabric reinforced polymeric composites are high performance materials with a rather complex fabric geometry. Therefore, modelling this type of material is a cumbersome task, especially when an efficient use is targeted. One of the most important issue of its design process is the optimisation of the individual laminae and of the laminated structure as a whole. In order to do that, a parametric model of the material has been defined, emphasising the many geometric variables needed to be correlated in the complex process of optimisation. The input parameters involved in this work, include: widths or heights of the tows and the laminate stacking sequence, which are discrete variables, while the gaps between adjacent tows and the height of the neat matrix are continuous variables. This work is one of the first attempts of using a Genetic Algorithm (GA) to optimise the geometrical parameters of satin reinforced multi-layer composites. Given the mixed type of the input parameters involved, an original software called SOMGA (Satin Optimisation with a Modified Genetic Algorithm) has been conceived and utilised in this work. The main goal is to find the best possible solution to the problem of designing a composite material which is able to withstand to a given set of external, in-plane, loads. The optimisation process has been performed using a fitness function which can analyse and compare mechanical behaviour of different fabric reinforced composites, the results being correlated with the ultimate strains, which demonstrate the efficiency of the composite structure.     

Potentiality of utilising natural textile materials for engineering composites applications

This paper gives an overview of utilising natural textile materials as reinforcements for engineering composites applications. The definition and types of textile materials are addressed to provide readers a thoughtful view on the role of these materials in a structural composite system. Available material properties of natural textile and their composites are critically reviewed here. In general, these materials are categorised into fibre, yarn and fabric forms. The load bearing capacity of natural textile fibre reinforced polymer composites is governed by the quantity, alignment and dispersion properties of fibres. It has been found that the natural fibre reinforced composites are limited to use in low to medium load bearing applications. However, a limited research work has been performed to date and there is a significant gap between the high performance textile fabric and their use as reinforcement in fibre reinforced composite materials.     

Strengthening in and fracture behaviour of CNT and carbon-fibre-reinforced epoxy–matrix hybrid composite

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in strength and fracture resistance properties as compared with their bulk, monolithic counterparts. In the present work, mode-I (tensile) fracture behaviour of the neat epoxy (without nano- or hybrid reinforcements), nanocomposite (with amino-functionalized multi-walled carbon nanotube (MWCNT) reinforcement to neat epoxy) and hybrid composite (with amino MWCNT and carbon fibre reinforcements to neat epoxy) along with their flexural strength and interlaminar shear strength has been reported and discussed. Limited topological studies have also been conducted to understand the nature of material fracture and its dependence on the notch orientation. The results thus obtained are analysed and discussed in detail to elucidate: (i) alignment of fibre and its influence on the anisotropy in strength and fracture resistance, (ii) dependence of notch root radii on the apparent fracture toughness and concurrence to strain-controlled fracture and (iii) finally, the nature of J–R curves. The results thus obtained have revealed that the resistance to fracture is significantly increased with the addition of amino-functionalized MWCNTs and carbon fibres. In the hybrid composite, fracture resistance is greater in the longitudinal orientation of fibres than in the transverse orientation and it exhibits a significantly higher strength–fracture toughness combination.     

Post-Impact Mechanical Characterisation of Glass and Basalt Woven Fabric Laminates

Two woven fabric laminates, one based on basalt fibres, the other on E-glass fibres, as a reinforcement for vinylester matrix, were compared in terms of their post-impact performance. With this aim, first the non-impacted specimens were subjected to interlaminar shear stress and flexural tests, then flexural tests were repeated on laminates impacted using a falling weight tower at three impact energies (7.5, 15 and 22.5J). Tests were monitored using acoustic emission analysis of signal distribution with load and with distance from the impact point. The results show that the materials have a similar damage tolerance to impact and also their post-impact residual properties after impact do not differ much, with a slight superiority for basalt fibre reinforced laminates. The principal difference is represented by the presence of a more extended delamination area on E-glass fibre reinforced laminates than on basalt fibre reinforced ones.     

On the design,manufacture and testing of trajectorial fibre steering for carbon fibre composite laminates

Trajectorial fibre steering of composite tows within composite laminates has been investigated as a means of producing composite structures with increased strength. Strategies to design the fibre trajectories in laminates have included aligning the fibres with principal stress vectors and a newly developed concept of load paths. In the laminate, the steered plies were inter-mingled with unidirectional or fabric plies. A manufacturing technique for the steered plies has been proposed. Applications include a specimen containing an open hole and a specimen with a pin-loaded hole. Specific strength increases of 62 and 85%, respectively, were achieved.     

Strength improvement by fibre steering around a pin loaded hole

A fibre steering technique has been applied around boltholes in carbon fibre reinforced epoxy composite laminates to locally enhance the bearing strength of bolted joints. The procedure can precisely place dry tows of fibre on a prepreg fabric following both the tensile and compressive principal stress trajectories around the hole. The bearing test results indicate that fibre steering improved the peak load of the composite bolted joints approximately in linear proportion to fibre addition by weight. The best result achieved an increase for the peak load by a factor of 2.69. The best improvement of bearing strength was by a factor of 1.36 for a specimen reinforced by 3 k fibre tows in tensile principal stress patterns and 6 k fibre tows in compressive principal stress patterns. The bearing strength improved due to significant increase in peak load and moderate change in thickness.     

A fast algorithm for the elastic fields due to interacting fibre breaks in a periodic fibre composite

Monte Carlo simulations of the failure of unidirectional fibre composites typically require numerous evaluations of the stress-state in partially damaged composite patches. In a simulated composite patch comprised of N fibres, of which \(N_b\) fibres are broken in a common cross-sectional plane transverse to the fibre direction, the stress overloads in the intact fibres are given by the weighted superposition of the unit break solutions associated with each of the breaks. Determining the weights involves solving \(N_b\) linear equations, and determining overloads in the intact fibres requires matrix-vector multiplication. These operations require \(O(N_b^3)\), and \(O(N N_b)\) floating point operations, respectively. These costs become prohibitive for large N, and \(N_b\); they limit Monte Carlo failure simulations to composite patches of only a few thousand fibres. In the present work, a fast algorithm to determine the overloads in a partially damaged composite, requiring \(O( N_b^{1/3} N \log N)\) floating point operations, is proposed. This algorithm is based on the discrete Fourier transform. The efficiency of the proposed method derives from the computational simplicity of weighted superposition in Fourier space. Computations of the stress state ahead of large circular clusters of breaks in composite patches comprised of about one million fibres are used to demonstrate the efficiency of the proposed algorithm.     

Crack resistance curve in glass matrix composite reinforced by long Nicalon<Superscript>®</Superscript> fibres

Theoretical micromechanical analysis of bridged crack development at chevron-notch tip of three-point bend specimens has been applied to determine the crack resistance curve for a composite made of a glass matrix reinforced by continuous Nicalon^® fibres. Fracture toughness (K _IC) values were determined using the chevron-notch technique at room temperature. The theoretical predictions were based on micromechanical analysis exploiting weight functions. Detailed FEM analysis using the ANSYS package was applied to determine numerically the weight functions for orthotropic materials. Appropriate bridging models for the theoretical prediction of the R-curve behaviour typical of the investigated composite were applied together with the weight functions. Experimental observations confirmed the theoretical calculations.     

Influence of the Hybrid Combination of Multiwalled Carbon Nanotubes and Graphene Oxide on Interlaminar Mechanical Properties of Carbon Fiber/Epoxy Laminates

An effective strategy to improve the mode I and mode II interlaminar fracture toughness (G _IC and G _IIC ) of unidirectional carbon fiber/epoxy (CF/E) laminates using a hybrid combination of multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO) is reported. Double cantilever beam (DCB) and end notched flexure (ENF) tests were conducted to evaluate the G _IC and G _IIC of the CF/E laminates fabricated with sprayed MWCNTs, GO and MWCNTs/GO hybrid. Scanning electron microscopy was employed to observe the fracture surfaces of tested DCB and ENF specimens. Experimental results showed the positive effect on the G _IC and G _IIC by 17% and 14% improvements on CF/E laminates with 0.25 wt.% MWCNTs/GO hybrid content compared to the neat CF/E. Also, the interlaminar shear strength value was increased for MWCNTs/GO-CF/E laminates. A synergetic effect between MWCNTs and GO resulted in improved interlaminar mechanical properties of CF/E laminates made by prepregs.     

All-cellulose composite laminates

A new processing method for fibre-reinforced all-cellulose composite (ACC) laminates has been developed in this work on the basis of a conventional hand lay-up method. Four layers of a man-made cellulosic textile (Cordenka rayon) and four layers of a natural fibre textile (linen), respectively, were impregnated with the ionic liquid (IL) 1-butyl-3-methylimidazolium acetate (BMIMAc). The impregnated layers were heated under pressure for the fibre surfaces to partially dissolve in the IL and to achieve compaction of the single laminae to form a composite. A matrix phase is formed in situ by the regeneration of dissolved fraction of the fibre via solvent exchange.Both textiles could be processed into thick (>1 mm) cellulose laminates. Analysis of the composite microstructure and phase transformations revealed that the dissolution behaviour of the man-made cellulose fibres was better than of the linen textile resulting in a more homogenous fibre–matrix-interphase and therefore better tensile properties.     

Modelluntersuchungen zur Faserverstärkung keramischer Werkstoffe

Model Investigation of the Fibre Reinforcement of Ceramic Materials A mechanism for fibre reinforcement of high-modulus brittle materials, in particular ceramics, is investigated both experimentally and theoretically. A simple mathematical formalism representing crack extension in a brittle matrix reinforced by ductile fibres is derived, and characteristic property values, adapted to composite structures, are introduced. Alumina with unidirectional reinforcement by thin 80Ni20Cr wires is used as a model material. The mechanism presented allows to reduce the high sensivity of ceramic materials to cracks. It is necessary for the fibres to bridge matrix cracks under load. For this purpose, the fibres must not be bound too tightly to the matrix, and while a high ductility is certainly of value, it is not a necessity. The efficiency of the mechanism depends on

• – interface energy and friction at the matrix-fibre interface,
• – deformation and fracture behavior of the fibre material,
• – radius of the fibres,
• – volume fraction of the fibres,
• – critical stress intensity factor of the matrix,
• – size of the largest crack.

Among these, the fibre radius appears to be the most suitable one for variation with the goal of optimizing material quality.     

Analyses of the mechanical properties and microstructure of bamboo-epoxy composites

Bamboo reinforced epoxy possesses reasonably good properties to waarrant its use as a structural material, and is fabricated by utilizing bamboo, an abundant material resource, in the technology of fibre composites. Literature on bamboo-plastics composites is rare. This work is an experimental study of unidirectional bamboo-epoxy laminates of varying laminae number, in which tensile, compressive, flexural and interlaminar shear properties are evaluated. Further, the disposition of bamboo fibre, the parenchymatous tissue, and the resin matrix under different loading conditions are examined. Our results show that the specific strength and specific modulus of bamboo-epoxy laminates are adequate, the former being 3 to 4 times that of mild steel. Its mechanical properties are generally comparable to those of ordinary glass-fibre composites. The fracture behaviour of bamboo-epoxy under different loading conditions were observed using both acoustic emission techniques and scanning electron microscopy. The fracture mode varied with load, the fracture mechanism being similar to glass and carbon reinforced composites. Microstructural analyses revealed that natural bamboo is eligibly a fibre composite in itself; its inclusion in a plastic matrix will help solve the problems of cracking due to desiccation and bioerosion caused by insect pests. Furthermore, the thickness and shape of the composite can be tailored during fabrication to meet specific requirements, thereby enabling a wide spectrum of applications.     

Evaluation of the bearing capacity of fiber reinforced concrete sections under fire exposure

Aim of the paper is the evaluation of the bearing capacity of fiber reinforced concrete (FRC) sections, without any traditional steel reinforcement, subjected to different values of the fire duration. At this purpose, bending moment (M)–axial force (N) interaction envelopes are defined, through an analytical model based on the direct integration of the hot or residual mechanical properties of the material, throughout the member cross section. Finally, a parametric survey, specifically related to tunnel segments, with different geometries and FRC materials allows highlighting the worst (or better) scenarios.     

Advantages of regenerated cellulose fibres as compared to flax fibres in the processability and mechanical performance of thermoset composites

Man-made cellulosic fibres (MMCFs) have attracted widespread interest as the next generation of fibre reinforced composite. However, most studies focused entirely on their performance on single fibre level and little attention has been paid to their behaviour on a larger application scale. In this study, MMCFs were utilized as reinforcement in unidirectionally (UD) manufactured thermoset composites and compared to several commercial UD flax fibre products. Specimens were prepared using a vacuum bag based resin infusion technique and the respective laminates characterized in terms of void fraction and mechanical properties. MMCF laminates had comparable or better mechanical performance when compared to flax fibre laminates. Failure mechanisms of MMCF laminates were noted to differ from those of flax-reinforced laminates. The results demonstrate the potential of MMCFs as a viable alternative to glass fibre for reinforcement on a larger scale of UD laminates. These results were utilized in the Biofore biomaterial demonstration vehicle.     

Effect of fibre/matrix adhesion on residual strength of notched composite laminates

Effects of fibre/matrix adhesion and residual strength of notched polymer matrix composite laminates (PMCLs) and fibre reinforced metal laminates (FRMLs) were investigated. Two different levels of adhesion between fibre and matrix were achieved by using the same carbon fibres with or without surface treatments. After conducting short-beam shear and transverse tension tests for fibre/matrix interface characterisation, residual strength tests were performed for PMCLs and FRMLs containing a circular hole/sharp notch for the two composite systems. It was found that laminates with poor interfacial adhesion between fibre and matrix exhibit higher residual strength than those with strong fibre/matrix adhesion. Major failure mechanisms and modes in two composite systems were studied using SEM fractography. The effective crack growth model (ECGM) was also applied to simulate the residual strength and damage growth of notched composite laminates with different fibre/matrix adhesion. Predictions from the ECGM were well correlated with experimental data.     

Improving the mechanical properties of glass-fibre-reinforced polyester composites by modification of fibre surface

Polyalkenyl-poly-maleic-anhydride-ester/amide type new experimental additives have been developed and used to achieve the better properties of glass-fibre-reinforced polyester composites. Two different commercial reinforcements have been investigated: a chopped glass fibre and a glass woven [0/90°] fabric materials. Based on their chemical structures, both were E-type. The surfaces of reinforcements have been treated with the dissolved form of polyalkenyl-poly-maleic-anhydride-ester, polyalkenyl-poly-maleic-anhydride-amide and polyalkenyl-poly-maleic-anhydride-ester-amide type experimental coupling additives then they have been used in fibre-reinforced thermoset. The coupling additives have had differences not only in their chemical structure and physical properties, but also in the fibre–matrix interaction that can also be affected by them. That is the reason why additives have resulted in numerous differences in the mechanical properties of the reinforced specimens. The most favourable effects have been found in the case when the glass-fibre surface was modified by polyalkenyl-poly-maleic-anhydride-ester-amide type additives. Moreover, results have referred to more favourable effects in case of chopped glass-fibre mat than in glass woven fabric composites. Tensile properties could be improved by 38.9% with that additive and flexure properties with 21.9% in those laminates. Tensile and flexure properties of glass woven [0/90°] fabric reinforced composites could be improved by 18.0%, and 40.1% comparing to the untreated glass fibres containing polyester composites with the same reinforcement. The polyalkenyl-poly-maleic-anhydride-ester type surface modifying additive has deteriorated the tensile and flexural properties of the laminates, but the dynamic properties have been more favourable than those of specimens with untreated glass fibres. Fibre–matrix interaction responsible for increased or decreased mechanical properties has been studied on SEM micrographs of the fractured face of composites. It has been found that the unfavourable results had been caused by the fibre slipping out the polyester matrix. Nevertheless, it has been supported by visual observation that the polyalkenyl-poly-maleic-anhydride-ester-amide additive managed to improve the adhesion between the fibres and the matrix.     

Fatigue behaviour of aligned short carbon-fibre reinforced polyimide and polyethersulphone composites

Fibre reinforced plastics, with 50 to 55 vol % of aligned short carbon fibres of approximately 3 mm in length, have good mechanical properties and advantages in deformability during the manufacturing process of structural components. The mechanical properties and damage mechanisms of this kind of composite have not been investigated deeply in the past. In the present paper results of an examination programme on laminates with various stacking sequences and two thermoplastic matrix systems (polyimide and polyethersulphone) are given. It will be shown that composites reinforced with aligned discontinuous carbon fibres can be an alternative material to continuous-fibre reinforced composites when considering their static and fatigue properties.     

Characterization of graphite-aluminium composites using analytical electron microscopy

Exposure of graphite fibre/aluminium composites to elevated temperatures, such as those used for processing, can lead to degradation of the mechanical properties. Analytical electron microscopy has been used to examine the phases formed during the heat treatment of the following materials: (a) wire tows of AA6061 aluminium, reinforced with TiB _x -coated fibres, and (b) bulk composites produced by hot pressing the tows within a cladding of AA6061. In the tow material, relatively little interaction between the fibres and matrix was observed. Precipitation at the fibre/matrix interface was considerably more advanced in the as-processed bulk composite. Heat-treating both materials at temperatures in the range 580–700 °C resulted in increasingly severe interfacial reactions. Generally, the microstructures at the fibre/matrix interfaces were considerably more complex than reported in previous work. The following phases have been identified following processing or heat-treatment: Al₄C₃, FeSiAl, magnesium spinel, elemental silicon, TiAl₃, MgO, FeSiAl and a quarternary phase, MgSiAl(NiCu), together with amorphous oxides and porosity.     

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.     

Impact and post-impact damage characterisation of hybrid composite laminates based on basalt fibres in combination with flax,hemp and glass fibres manufactured by vacuum infusion

The impact and flexural post-impact behaviour of ternary hybrid composites based on epoxy resin reinforced with different types of fibres, basalt (B), flax (F), hemp (H) and glass (G) in textile form, namely FHB, GHB and GFB, has been investigated. The reinforcement volume employed was in the order of 21–23% throughout. Laminates based exclusively on basalt, hemp and flax fibres were also fabricated for comparison. Hybrid laminates showed an intermediate performance between basalt fibre reinforced laminates on the high side, and flax and hemp fibre reinforced laminates on the low side. As for impact performance, GHB appears to be the worst performing hybrid laminate and FHB slightly overperforms GFB. In general, an increased rigidity can be attributed to all hybrids with respect to flax and hemp fibre composites. The morphological study of fracture by SEM indicated the variability of mode of fracture of flax and hemp fibre laminates and of the hybrid configuration (FHB) containing both of them. Acoustic emission monitoring during post-impact flexural tests confirmed the proneness to delamination of FHB hybrids, whilst they were able to better withstand impact damage than the other hybrids.