Novel automated method for evaluating the morphological changes of cellulose fibres during extrusion-compounding of plastic–matrix composites

A novel method based on fluorescence optical microscopy has been developed for determining the fibre geometrical changes occurring during the melt processing of cellulose-reinforced composites, which are known to be closely related with composite properties. Determination of these changes is still a tedious and challenging task because existing methods are not well developed yet. The novel method proved its ability for explaining the screw configuration effects on the attrition bore by the fibres during extrusion-compounding of plastic–matrix composites. The percentage of fibres longer than the critical length parameter was revealed to highlight the mechanical degradation of fibres during compounding. The percentage of fines exhibited the clearest correlation with the differences in fibre content of composites. Relationships found between composite tensile properties and fibre characterization parameters revealed the ability of the novel method for explaining the effects of composition and processing on composite properties.     

Non-destructive automatic determination of aspect ratio and cross-sectional properties of fibres

A novel method for computerised estimation of the aspect ratio distribution and various cross-sectional geometrical properties of fibres in short-fibre reinforced composites is proposed. The method, based on X-ray micro-computed tomography, is non-destructive and does not require user intervention. Based on results on specially fabricated model material, the accuracy and precision of the method seems adequate. The method is applied in analysing a manufacturing process of wood fibre reinforced thermoplastic composite. The results indicate a significant decrease of the aspect ratio of fibres during the processing steps. Finally, the feasibility of the method is assessed by estimating parameters of a micromechanical model for flax fibre composites and comparing the results with those from tensile tests.     

SWNT composite coatings as a strain sensor on glass fibres in model epoxy composites

The preparation of a model glass-fibre/epoxy composite with single-walled carbon nanotubes (SWNTs) incorporated as a strain sensor on the fibre surface is described. A micromechanical study of stress transfer at the fibre–matrix interface followed using Raman spectroscopy properties is reported. The SWNTs were distributed along the fibre surface either by dispersing them in an amino-silane coupling agent or coating with an epoxy resin solution containing the SWNTs. The point-by-point mapping of the fibre strain in single fibre fragmentation tests has been undertaken for the first time using SWNTs on the fibres and the interfacial shear stress distribution along the fibre length was determined using the embedded SWNTs. The behaviour was found to be consistent with the classical shear-lag model. The effects of SWNT type and preparation procedure on the sensitivity of the technique were evaluated and optimized from single fibre deformation tests.     

Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling

Recycling carbon fibres from waste composite materials would only be efficient if it were possible to separate the fibres and the matrix and to re-use the recycled fibres as new reinforcements. The challenge is to use non-continuous fibres to produce high-strength materials. The formation of defects in “semi-long” fibre composites has not yet been taken into account. In this paper the influence of fibre length and fibre alignment on the strength and the modulus of composite materials is illustrated. It is shown that the presence of defects may be modelled in order to understand what the quality of a second generation composite material would be.     

The effect of mechanical defects on the strength distribution of elementary flax fibres

Flax fibres are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibres are affected by the natural variability in plant as well as the damage accumulated during processing, and thus have considerable variability that necessitates statistical treatment of fibre characteristics. The strength distribution of elementary flax fibres has been determined at several fibre lengths by standard tensile tests, and the amount of kink bands in the fibres evaluated by optical microscopy. Strength distribution function, based on the assumption that the presence of kink bands limits fibre strength, is derived and found to provide reasonable agreement with test results.     

A combined experimental–numerical approach for generating statistically equivalent fibre distributions for high strength laminated composite materials

A technique is presented where actual experimental distributions, measured from a high strength carbon fibre composite, are considered in the development of a novel method to generate statistically equivalent fibre distributions for high volume fraction composites. The approach uses an adjusted measure of nearest neighbour distribution functions to define inter-fibre distances. The statistical distributions, characterising the resulting fibre arrangements, were found to be equivalent to those in the actual microstructure. Finite element models were generated and used to determine the effective elastic properties of the composite and excellent agreement was obtained. The algorithm developed is simple, robust, highly efficient and capable of reproducing actual fibre distributions for high strength laminated composite materials. It does not require further heuristic steps, such as those seen in fibre stirring/shaking algorithms, in order to achieve high volume fraction microstructures and provides a useful alternative to both microstructure reproduction and random numerical models.     

Stress distribution in discontinuous fibres in a model composite

In order to calculate stress distribution in unidirectional discontinuous fibres embedded in a metal matrix, a method based on the shear-lag analysis was proposed. Using this method, the influence of fibre length, interfacial bonding strength, distance between fibre ends in the longitudinal direction, and applied strain to composite on both stress distribution and average stress of fibres was estimated for a number of examples.     

Gaining knowledge on the processability of PLA-based short-fibre compounds – A comprehensive comparison with their PP counterparts

The present study provides a quantitative overview of bio-based compound processing compared to commonly used composites reinforced with short glass fibres (GF). Three reinforcing fibres were compounded with polylactide and polypropylene: abaca, man-made cellulose and conventional E-GF. The flow behaviour of corresponding melts was determined using melt flow rate (MFR) and online flow spiral test. The composite structures were analysed by means of SEM in order to investigate the fibre fracture during processing and the fibre/matrix bonding affinity. The fibre length distribution was correlated with the results from the melt flow experiments, and the structure–property relationships were determined using SEM images. It was confirmed that the fibre texture, interactions between fibres and fibre–matrix bonding are influenced by subsequent processing steps and have a substantial effect on the further composite melt processing.     

Analysis of fibre orientation distribution in short fibre reinforced polymers: A comparison between optical and tomographic methods

The fibre orientation distribution in a material sample of short fibre reinforced polyamide extracted from an injection moulded notched plate was analysed using two different methods, one based on micro-computed tomography and the Mean Intercept Length concept and the other based on the classical optical section method. The two methods were compared in terms of the preferred fibre orientation at a chosen position, and the agreement was found to be excellent provided the correct section plane was chosen for the optical method. The optical method was applied to different section planes to ascertain the best choice. Comparisons with the optical method, which can provide the full fibre orientation distribution, confirm that the analysis based on the MIL concept is capable of capturing important information about the fibre orientation.     

The influence of fibre properties of the performance of glass-fibre-reinforced polyamide 6,6

We discuss the effect of fibre strength and diameter on the balance of mechanical properties of glass-reinforced polyamide 6,6. The results show that the elastic properties of injection-moulded short-glass-fibre-reinforced polyamide 6,6 are not strongly influenced by fibre diameter in the 10–17 micron range. The ultimate properties of these composites (strength and Izod impact behaviour) showed a clear dependence on fibre diameter and were increased by the presence of high-strength S-2 glass fibres. The relationship between the observed mechanical properties and the length, diameter and orientation of the fibres is explored. We have measured fibre length as a function of diameter in composites containing a single glass-reinforcement product and blends of two glass products. The reduction in glass-fibre length from glass-fibre production to final composite moulding has been followed step by step. The final composite mechanical properties, the fibre length, strength, diameter and orientation are all inter-related.     

Mechanical analysis of bi-component-fibre nonwovens: Finite-element strategy

In thermally bonded bi-component fibre nonwovens, a significant contribution is made by bond points in defining their mechanical behaviour formed as a result of their manufacture. Bond points are composite regions with a sheath material reinforced by a network of fibres’ cores. These composite regions are connected by bi-component fibres — a discontinuous domain of the material. Microstructural and mechanical characterization of this material was carried out with experimental and numerical modelling techniques. Two numerical modelling strategies were implemented: (i) traditional finite element (FE) and (ii) a new parametric discrete phase FE model to elucidate the mechanical behaviour and underlying mechanisms involved in deformation of these materials. In FE models the studied nonwoven material was treated as an assembly of two regions having distinct microstructure and mechanical properties: fibre matrix and bond points. The former is composed of randomly oriented core/sheath fibres acting as load-transfer link between composite bond points. Randomness of material’s microstructure was introduced in terms of orientation distribution function (ODF). The ODF was obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). Bond points were treated as a deformable two-phase composite. An in-house algorithm was used to calculate anisotropic material properties of composite bond points based on properties of constituent fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Individual fibres connecting the composite bond points were modelled in the discrete phase model directly according to their orientation distribution. The developed models were validated by comparing numerical results with experimental tensile test data, demonstrating that the proposed approach is highly suitable for prediction of complex deformation mechanisms, mechanical performance and structure-properties relationships of composites.     

Characterising and modelling variability of tow orientation in engineering fabrics and textile composites

Variability of tow orientation is unavoidable for biaxial engineering fabrics and their composites. Since the mechanical behaviour of these materials is strongly dependent on the fibre direction, variability should be considered and modelled as exactly as possible for more realistic estimation of their forming and infusion behaviour and their final composite mechanical properties. In this study, a numerical code, ‘VariFab’, has been written to model realistic full-field variability of the tow directions across flat sheets of biaxial engineering fabrics and woven textile composites. The algorithm is based on pin-jointed net kinematics and can produce a mesh of arbitrary perimeter shape, suitable for subsequent computational analysis such as finite element forming simulations. While the shear angle in each element is varied, the side-length of all unit cells within the mesh is constant. This simplification ensures that spurious tensile stresses are not generated during deformation of the mesh during forming simulations. Variability is controlled using six parameters that can take on arbitrary values within certain ranges, allowing flexibility in mesh generation. The distribution of tow angles within a pre-consolidated glass–polypropylene composite and self-reinforced polypropylene and glass fabrics has been characterised over various length scales. Reproduction of the same statistical variability of tow orientation as in these experiments is successfully achieved by combining the VariFab code with a simple genetic algorithm.     

Bridging effect on mode I fatigue delamination behavior in composite laminates

This paper discusses the bridging effect of fibres on mode I fatigue delamination growth in unidirectional and multidirectional polymer composite laminates based on a series of double cantilever beam (DCB) tests. From the results, there is sufficient evidence that fibre bridging can decrease the crack growth rate da/dN significantly, and using only one fatigue resistance curve to determine the delamination behavior in composite materials with large-scale fibre bridging may be inadequate. The bridging created in fatigue delamination is different from that of quasi-static delamination at the same crack length. So it is incorrect to use the resistance curve (R-curve) from quasi-static delamination tests to normalize fatigue delamination results.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Design and manufacture of all-PP sandwich panels based on co-extruded polypropylene tapes

This paper describes the creation of polypropylene sandwich panels, based on all-polypropylene (all-PP) composite laminates combined with a polypropylene based honeycomb or foam core. These all-PP composite laminates are based on high modulus polypropylene tape reinforcing a polypropylene matrix. Sandwich panels containing these all-PP composite laminate faces are compared with sandwich panels containing conventional glass fibre reinforced polypropylene laminate faces, and the mechanical properties, failure modes, and design requirements of these different materials are discussed.     

Hygroscopic strain measurement by fibre Bragg gratings sensors in organic matrix composites – Application to monitoring of a composite structure

This study is devoted to the identification of the moisture expansion coefficients of composite materials by means of a novel measurement technique. This method is based on the insertion of Fibre Bragg Grating (FBG) sensors between composite layers. The sensor enables to measure the hygroscopic strain induced by moisture diffusion in the plane of the laminated composite. Experimental results from immersed samples, varying both the direction of measurement and the fibre volume fraction are given according to the water uptake, and leading to the characterisation of moisture expansion coefficient.     

Microbuckling elastic modelling approach of a single carbon fibre embedded in an epoxy matrix

Carbon fibre alignment defect inside of an epoxy matrix is examined by the use of microscopy and video recording. The initiation and the growth of a fibre waviness defect was monitored during the cure of the matrix. The observations established that fibre defect appears during the hot stage of the epoxy matrix thermosetting reaction. The objective is to evaluate the possibility of a fibre microbuckling mechanism to explain the observations. A first elastic microbuckling model is proposed. Analytical expressions of the necessary elastic load for the appearance of a fibre buckling instability and its associated wavelength are established. The compressive load applied onto the fibre has been evaluated by using finite elements calculations based on mechanical characteristics of the materials. Comparison between the critical microbuckling load and the applied load onto the carbon fibre during the cure is in coherence with the observations. The associated wavelength is discussed.     

The use of composite materials in modern orthopaedic medicine and prosthetic devices: A review

The use of fibre reinforced composite materials for biomedical purposes is reviewed. The development of polymer composite materials has, in recent years, led to technological advances across a wide range of applications in modern orthopaedic medicine and prosthetic devices. Composites typically possess a superior strength to weight characteristic compared to monolithic materials and offer excellent biocompatibility. They are, therefore, favourable for both hard- and soft-tissue applications as well as the design of prostheses. In particular, the development of specifically designed carbon fibre sports prostheses now allows lower-limb amputees to actively participate in competitive sports. Sensory feedback systems, porous composite materials for tissue engineering and functional coatings for metallic implants are further developments anticipated to be introduced in next generation orthopaedic medicine.     

Deformation micromechanics of a model cellulose/glass fibre hybrid composite

Interfacial stress transfer in a model hybrid composite has been investigated. An Sm³⁺ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm⁻¹, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres.     

Establishing a new Forming Limit Curve for a flax fibre reinforced polypropylene composite through stretch forming experiments

In developing an understanding of the failure in natural fibre reinforced polymer composites, the failure limits of this class of the material system are required. It is found that the conventional Forming Limit Curve is not suitable to predict the failure initiated in the natural fibre composite as principal strains cannot differentiate the strain on the flax fibres and the polypropylene matrix. This study proposes a new Forming Limit Curve for the composite which expresses limiting fibre strain as a function of forming mode depicted by the ratio of minor strain to major strain. The new Forming Limit Curve, along with the Maximum Strain failure criterion have been successfully implemented in FEA simulations, and numerical simulations suggest that the former is more accurate. The current work provides an innovative method to predict the onset of failure in natural fibre composites, which can be applied in composite forming and structural design.