Rheological characterisation of discrete long glass fibre (LGF) reinforced thermoplastics

Three experimental techniques have been employed to assess the rheological behaviour of discrete long glass fibre reinforced polypropylene and propylene/ethylene copolymers. A Carri Med cone and plate rheogoniometer has been used to determine shear viscosity as a function of strain rate and time at temperatures relevant to the extrusion and injection moulding processes. A bubble inflation test (BIT) has been designed and used to characterise the behaviour of these composites under the extensional flow fields typical of blow moulding and thermoforming. Finally a squeeze load test (SLT), similar to those developed for sheet moulding compounds (SMC) and glass mat thermoplastics (GMT), has been used to explore the rheological behaviour of the long glass fibre (LGF) materials under compression moulding conditions, in particular to assess the relative importance of shear and extensional flow.     

Characterisation of the draping behaviour of unidirectional non-crimp fabrics (UD-NCF)

Thin composites shell structures manufactured from stitched unidirectional non-crimp fabrics (UD-NCF) in a liquid composite moulding process provides high lightweight design capabilities. However, draping behaviour of UD-NCF has been investigated only sparsely, in contrast to research on woven fabrics or biaxial non-crimp fabrics. Hence, this contribution focuses on fundamental investigations of the draping behaviour of UD-NCF. Within this investigation picture frame tests and uniaxial bias extension tests are performed to examine the in-plane shear behaviour of UD-NCF. Furthermore, a new method is presented to examine ambivalent tensile behaviour of UD-NCF transverse to the carbon fibre roving orientation. In particular, the influence of thin glass fibres on transverse tensile behaviour of UF-NCFs is investigated using a new clamping mechanism in tensile testing. Finally, hemisphere tests are performed to observe the forming behaviour of UD-NCF in a realistic forming process and to evaluate the proposed material characterisation methods regarding its suitability for UD-NCFs.     

Analysis of the thermomechanical shear behaviour of woven-reinforced thermoplastic-matrix composites during forming

Shear behaviour of a glass fibre/polypropylene composite is characterized over a wide range of strain rates and forming temperatures using the bias extension test. A temperature- and rate-dependent material model is here introduced to describe the observed behaviour. The model is based on a continuous approach and formulated considering a stress objective derivative based on the warp and weft yarns rotation. The effects of temperature and strain rate on the shear behaviour are analysed via bias extension test simulations. Temperature change in the sheet during forming was measured. This data is used to model cooling during forming. Isothermal and transient forming simulations were performed in order to show the effects of temperature and forming speed on the obtained shear angle distribution. It was found that at low forming speeds the assumption of isothermal forming is not valid anymore since the cooling of the sheet affects the shear behaviour.     

Characterising the shear–tension coupling and wrinkling behaviour of woven engineering fabrics

Modelling the forming process for engineering fabrics and textile composites using a mechanical approach, such as the finite element method, requires characterisation of the material’s behaviour under large shear deformation. For woven engineering fabrics, a coupling between in-plane tension and both shear compliance and the onset of wrinkling is to be expected. This paper focuses on a novel testing technique, the biaxial bias extension test, as a means to investigate this shear–tension coupling and fabric wrinkling. Novel methods of determining the wrinkling behaviour are demonstrated. The main difficulty with the technique lies in extracting the material contribution to the recorded signal. To do this, an experimental method is proposed and demonstrated using a plain weave glass fabric. Biaxial bias extension test results are compared against picture frame and uniaxial bias extension results.     

Investigation of the squeeze flow behaviour of Sheet Moulding Compounds (SMC)

The compression moulding flow of SMC has been investigated by squeeze flow testing between parallel circular plates at room temperature and at elevated temperatures of 30 and 70°C. A model based on a variational approach was adopted to characterise the squeeze flow of SMC. The present results indicate that dissipation in the squeeze flow of SMC at 30 and 70°C is predominantly extensional with shear dissipation only becoming significant at very small plate separations. Measurements of the pressure distribution across the plate also confirm that the flow is predominantly extensional. The presence of edge effects due to fibre length is also evident. The strong dependence of the compression moulding flow characteristics of SMC with temperature is demonstrated by changes in the flow parameters.     

The effect of fibre content on the mechanical properties of glass fibre mat/polypropylene composites

Glass fibre mat was prepared by the fibre mat-manufacturing machine developed in our laboratory. Glass fibre mat reinforced polypropylene (PP) composites were fabricated with the variation of glass fibre content. Tensile, flexural and high rate impact test was conducted to investigate the effect of glass fibre content on the mechanical properties of the glass fibre mat/PP composite. Deformation and fracture behaviour of the glass fibre mat/PP composites was investigated to study the relationship with the mechanical property data. The tensile and flexural modulus increased with the increment of glass fibre content. However, the tensile and flexural strengths exhibited maximum values and showed a decrease at the higher glass fibre content than this point. The impact absorption energy also exhibited a similar result with the tensile and flexural property data.     

Through-thickness strength measurements using Arcan’s method

This paper presents results obtained from the experimental study on the behaviour of glass/epoxy butterfly-shaped specimens tested under pure shear and biaxial loadings using an Arcan device. Notch-to-notch longitudinally and perpendicularly oriented fibre specimens, respectively referred to as Mat31 and Mat32 ones have been tested with different loading angles ranging from 0° to 90°. The fracture mode was more or less repetitive for Mat31 specimens whereas it depended on the loading direction for Mat32 type, for which the number of cracks and the obliqueness relative to the notch-to-notch line, decreased as the test angle increased. The comparison of failure envelopes has demonstrated that Mat31 specimens are much more resistant whatever the loading angle. However, strengths at failure of Mat32 type are in good agreement with Hashin’s failure criterion, for pure shear tests up to 30° loading angles. Mat31 specimens’ failure stresses were overestimated by the criterion, although similarities in trends were noticed.     

Normalisation of biaxial bias extension test results considering shear tension coupling

A theoretical background is proposed for the normalisation of biaxial bias extension results for rate-independent fabrics, whose shear compliance depends on both the shear angle and the fibre tension within the fabric. The theory is used to predict the form of biaxial bias extension results from known shear force–shear angle–fibre tension behaviours. Hypothetical data sets are used to perform a parametric study of the likely influence of the nature of the shear–tension coupling on the form of the biaxial bias extension test results. The theory is then used in implementing an iterative numerical code designed to retrieve the underlying material response from biaxial bias extension test results and examples predictions are given. A discussion of the information required in order to perform the normalisation, and the methods by which this information can be obtained, is presented. Finally, assumptions behind the theory are outlined and critically assessed.     

Fabrication of Woven Metal Fibre Reinforced Glass Matrix Composites

A processing route has been developed for the fabrication of metallic fibre mat reinforced glass matrix composites. For the model experiments reported here, a commercially available satin woven stainless steel 316L fibre mat was used as the reinforcement and soda-lime glass as the matrix. The process involves two steps: (1) the infiltration of the intra- and inter-tows regions of the fibre mats with silica sol using electrophoretic deposition and (2) the fabrication of a composite by cold uniaxial pressing and pressureless sintering of impregnated fibre mats sandwiched between layers of the matrix glass powder. The sintering took place in air at 670°C, and composite materials of sufficient integrity could be obtained without damaging the fibres. The deposited silica remained amorphous at the processing temperature providing a porous interface between the glass matrix and the metallic reinforcement. Obervation of fracture surfaces revealed that both fibre pull-out and fibre deformation occur, which should lead to a significant toughness enhancement. The presence of the interfacial silica layer, deposited using electrophoresis, is thought to be responsible for this behaviour.     

The shear ramp: A new test method for the investigation of coated fabric shear behaviour – Part II: Experimental validation

The shear ramp is a new test method for the investigation of coated fabric shear behaviour. The implementation of the method with a biaxial test rig is given and compared to the off-axis biaxial extension of a specimen with 45° oriented fibres. Based on experimental observations and finite element analyses, it is shown that both test methods give similar results for a wide range of coated fabrics if the proper stress correction factors are used. An estimation of these factors is given for both test methods for PVC–polyester as well as PTFE–glass fabrics.     

Hyperelastic modelling for mesoscopic analyses of composite reinforcements

A hyperelastic constitutive law is proposed to describe the mechanical behaviour of fibre bundles of woven composite reinforcements. The objective of this model is to compute the 3D geometry of the deformed woven unit cell. This geometry is important for permeability calculations and for the mechanical behaviour of the composite into service. The finite element models of a woven unit cell can also be used as virtual mechanical tests. The highlight of four deformation modes of the fibre bundle leads to definition of a strain energy potential from four specific invariants. The parameters of the hyperelastic constitutive law are identified in the case of a glass plain weave reinforcement thanks to uniaxial and equibiaxial tensile tests on the fibre bundle and on the whole reinforcement. This constitutive law is then validated in comparison to biaxial tension and in-plane shear tests.     

Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy

Raman spectroscopy has been used to monitor deformation micromechanics in a model discontinuous fibre composite comprising a single glass fibre in an epoxy resin. The glass fibre was coated with a diacetylene-containing urethane copolymer that was subsequently cross-polymerised thermally. During composite deformation, the stress-induced Raman band shifts of the polydiacetylene sequences in the cross-polymerised coating were used to map the distributions of strain along glass fibres inside the epoxy resin matrix. The fragmentation of the fibre has been followed in detail and the behaviour analysed using classical shear-lag analysis. Values of the interfacial shear stress along fibre fragments were determined from the measured fibre strain distributions and were shown to be limited by the shear yield stress of the matrix. The effect of adhesion between the coating and the fibre upon the strain distributions has been investigated in detail. The fibre strain distributions can only be determined accurately when the adhesion is good. However, in the case of poor adhesion, although the strain distribution in the coating follows that of the matrix, the fragmentation process can still be monitored.     

A new method to normalize the effect of matrix properties on the value of interfacial shear strength obtained from the fragmentation test

A new method, based on tensile yield strength and strain, has been developed to normalize the effect of matrix properties on the critical fibre length and the interfacial shear strength obtained from the fragmentation test. It is argued that the conventional data normalization technique which employs elastic properties of the matrix, is fundamentally flawed because the model employed to calculate interfacial shear strength assumes perfect plasticity. Single embedded fibre fragmentation in a range of epoxy resins with differing mechanical properties has been used to validate the new method. Stoichiometric quantities of the current agent were used to keep the same interfacial chemistry. The proposed method provides more consistent interfacial shear strength data than existing theories. Furthermore, this normalization technique can also be used to predict the interfacial shear strength of glass fibres embedded in a range of support resins, such as vinyl ester or epoxy resins. For these cases, a thin layer of the phenolic resin was used on the glass fibre to keep the interface chemistry the same. This revised version was published online in November 2006 with corrections to the Cover Date.     

Low stress abrasion of laser irradiated GFRP composites: an experimental and microstructural study

The present paper reports, the mechanism of material removal during low stress abrasive wear of high weight percent glass fibre reinforced polymer (GFRP) composites. Two different geometries of glass fibre reinforcement namely woven roving (WR) and chopped strand mat (CSM) were used. Unsaturated isophthalic polyester and bisphenol based epoxy resins were used as matrix for the reinforcement. Rubber Wheel Abrasion Tester (RWAT) was used for evaluating the abrasive wear behaviour of the composites. The composite samples were irradiated using a low power He-Ne laser for different time periods, having intensity of 5 mW. The abrasive wear performance of the composites has been determined as a function of applied load, sliding distance and laser irradiation time. The microstructural features of the abraded surfaces of both the laser irradiated and unirradiated composites have been observed by using a scanning electron microscope. Unsaturated polyester based glass fibre woven roving (WR) composite had a higher wear volume as compared to the epoxy based composite. The trend reversed in the case of chopped strand mat (CSM) composites, in which epoxy-based composite showed higher wear volume. The abrasive wear volume of all the composites decreased on irradiating it with laser. These results have been discussed, based on experimental wear data and observed microstructural features of the abraded surfaces.     

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.     

Biaxial experimental determination of in-plane matrix fracture envelope of unidirectional composite

A test procedure for the determination of the in-plane fracture envelope of unidirectional fibre reinforced polymers (FRP) is presented. In particular, the determined fracture envelope covers combined in-plane shear and transverse (perpendicular to the fibre direction) matrix strength. The proposed test procedure allows the manufacture of specimens for material fracture characterisation in the same way that real composite structures are usually produced for the automotive industry. The biaxial testing is performed using a custom-made dual actuator test machine and keeping the ratio of transverse and shear load constant until fracture. The experimentally obtained transverse–shear strength relation can be well represented by the matrix fracture model by Puck. It is shown that the stress concentrations in the gauge section of the flat biaxial specimens can be avoided by the introduction of a thickness reduction, whereas the stress concentrations within biaxial specimens without such a thickness reduction lead to significantly lower strength.     

A study of the effect of injection speed on fibre orientation in simple mouldings of short glass fibre-filled polypropylene

The pattern of fibre orientation in injection moulded strips of glass fibre reinforced polypropylene has been studied using the technique of contact micro-radiography. It has been found that the fibre orientation in the core of the mouldings is very dependent on injection speed. High injection speed gives alignment of fibres transverse to the flow direction, while for very low speeds the fibres align parallel to the flow. The associated changes in topography of the mouldings have been studied using scanning electron microscopy. The rheological properties of both glass fibre-filled and unfilled polypropylene have been studied in a capillary rheometer. At low shear rates, the fibres cause a significant increase in viscosity, but at the shear rates likely to be encountered in injection moulding, the filled and unfilled melts have very similar viscosities. The rheological data can be used to interpret the pattern of fibre orientation in the mouldings.     

Characterisation of material properties for draping of dry woven composite material

Three test methods, uniaxial bias extension, biaxial and picture frame tests are used to characterise the shear behaviour of dry woven fabric during draping. The deformation of the bias extension and biaxial specimens is measured from images of a central gauge section. The forces applied to the material are resolved into forces along and parallel to the tow directions. The deformation of the material in the bias extension and biaxial tests is found to behave in a manner which is reasonably well described by a pin-jointed net analysis. There is negligible change in the shear resistance of the material during biaxial loading, while a slight increase in shear resistance is observed in the picture frame tests. Microscopic examination of the tow architecture [Compos Sci Technol 63 (2003) 99], which shows a significantly smaller crimp amplitude for picture frame tests than for the bias extension and biaxial tests, supports the suggestion raised by Harrison et al. [Proceedings of the 10th European Conference on Composite Materials, 2002], that the increase in resistance in the picture frame tests is associated with an increase in tow cross-over force generated by large loads along the tows.     

The thermal behaviour of glass fibre investigated by thermomechanical analysis

A thermomechanical analysis (TMA) procedure has been developed with the capability of probing the thermal behaviour of glass fibre. A single glass fibre was successfully mounted into TMA fibre configuration and several thermomechanical programmes were carried out over a wide temperature range from 20 to 900 °C. It was found that measured coefficient of linear thermal expansion of boron-free E-glass fibre remained constant below 300 °C and the values had an excellent agreement with that found in the literature. At higher temperatures an abrupt length change in glass transition region allowed for the determination of glass transition temperature. The results from isothermal measurement showed significant fibre length shrinkage, which was a function of both temperature and time. It follows that there exist two mechanisms, thermal expansion and structural relaxation, which together account for overall thermomechanical responses of glass fibre. The former is related to the decrease of Young’s modulus at elevated temperatures and the latter is considered responsible for the observed increase of room-temperature Young’s modulus after thermally conditioning glass fibre at various temperatures.     

Damage tolerance assessment by bend and shear tests of two multilayer composites: Glass fibre reinforced metal laminate and aluminium roll-bonded laminate

The damage tolerance of an aluminium roll-bonded laminate (ALH19) and a glass fibre reinforced laminate (GLARE) (both based on Al 2024-T3) has been studied. The composite laminates have been tested under 3-point bend and shear tests on the interfaces to analyze their fracture behaviour. During the bend tests different fracture mechanisms were activated for both laminates, which depend on the constituent materials and their interfaces. The high intrinsic toughness of the pure Al 1050 layers present in the aluminium roll-bonded laminate (ALH19), together with extrinsic toughening mechanisms such as crack bridging and interface delamination were responsible for the enhanced toughness of this composite laminate. On the other hand, crack deflection by debonding between the glass fibres and the plastic resin in GLARE was the main extrinsic toughening mechanism present in this composite laminate.