Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study

One disadvantage of multi-layer forming of unidirectional (UD) prepreg tape is the risk of out-of-plane wrinkling. This study aims to show how mixed ply material properties affect global wrinkling behaviour.An experimental study was performed using pre-stacked UD prepreg on a forming tool with varying cross sections. Parameters studied include local interply friction, effects of co-stacking and fibre stresses in critical fibre directions. Experimental evaluation was performed on out-of-plane defect height, type and location. The study shows that fibre stresses in some fibre directions were crucial for the shearing required to avoid wrinkling. The same fibre stresses may cause wrinkling if the lamina is stacked in a non-beneficial order. Changing the friction locally, or reducing the number of difficult combinations of fibre angles, improves the forming outcome slightly. However, in order to make a significant improvement, co-stacking or different fibre stacking is required.     

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.     

Ductile unidirectional continuous rayon fibre-reinforced hierarchical composites

Endless rayon fibres (Cordenka®) were used to reinforce polyhydroxybutyrate (PHB) nanocomposites containing 2.5 wt.% nanofibrillated cellulose (NFC) to create truly green hierarchical composites. Unidirectional (UD) composites with 50–55% fibre volume fraction were produced using a solvent-free continuous wet powder impregnation method. The composites exhibit ductile failure behaviour with a strain-to-failure of more than 10% albeit using a very brittle matrix. Improvements at a model composite level were translated into higher mechanical properties of UD hierarchical composites. The Young’s moduli of rayon fibre-reinforced (NFC-reinforced) PHB composites were about 15 GPa. The tensile and flexural strength of hierarchical PHB composites increased by 15% and 33% as compared to the rayon fibre-reinforced neat PHB composites. This suggests that incorporation of NFC into the PHB matrix binds the rayon fibres, which does affect the load transfer between the constituents resulting in composites with better mechanical properties.     

Consistent Finite Element mesh generation for meso-scale modeling of textile composites with preformed and compacted reinforcements

A new automated method to generate smooth Finite Element meshes for the meso-scale representation of textile composites unit cells with preformed and compacted reinforcements is presented. The reinforcement geometry is obtained from textile geometry preprocessors or by modeling of the preforming step. It is then transformed into a geometric model of the entire composite unit cell, taking into account the real yarn shape after preforming and compaction, the contact interactions between the yarns, as well as the resulting local variations of fiber volume fraction. A consistent mesh of the complete cell, conformal between the parts, is ensured, without the need for inserting artificial matrix layers between the yarns. The methodology is illustrated by mechanical modeling of the representative unit cell of a multi-layer composite with a compacted and nested woven reinforcement.     

Constitutive modelling of fibre-reinforced composites with unidirectional plies using a plasticity-based approach

This paper presents the development of a constitutive model able to accurately represent the full non-linear mechanical response of polymer-matrix fibre-reinforced composites with unidirectional (UD) plies under quasi-static loading. This is achieved by utilising an elasto-plastic modelling framework. The model captures key features that are often neglected in constitutive modelling of UD composites, such as the effect of hydrostatic pressure on both the elastic and non-elastic material response, the effect of multiaxial loading and dependence of the yield stress on the applied pressure.The constitutive model includes a novel yield function which accurately represents the yielding of the matrix within a unidirectional fibre-reinforced composite by removing the dependence on the stress in the fibre direction. A non-associative flow rule is used to capture the pressure sensitivity of the material. The experimentally observed translation of subsequent yield surfaces is modelled using a non-linear kinematic hardening rule. Furthermore, evolution laws are proposed for the non-linear hardening that relate to the applied hydrostatic pressure.Multiaxial test data is used to show that the model is able to predict the non-linear response under complex loading combinations, given only the experimental response from two uniaxial tests.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Development of a new generation of filament wound composite pressure cylinders

A new generation of composite pressure vessels for large scale market applications has been studied in this work. The vessels consist on a thermoplastic liner wrapped with a filament winding glass fibre reinforced polymer matrix structure. A high density polyethylene (HDPE) was selected as liner and a thermosetting resin used as matrix in the glass reinforced filament wound laminate.     

复合材料热膨胀性能预报的随机扰动模型与实验验证

为了探究连续纤维增强的复合材料的热膨胀性能,综合考虑了单向复合材料横截面上纤维的分布情况以及随机模型的真实周期性边界条件等,发展了一种随机扰动模型;并针对高纤维体积分数的随机模型,提出了随机扰动法（RDM）,此方法可以处理的最大纤维体积分数不小于65%。利用本随机模型对M40J/TDE-85的热膨胀性能进行了预测,同时对该复合材料的热膨胀系数进行了高精度测试。结果表明,预测结果与试验结果吻合良好,同时也证明本随机模型能较好地预测复合材料的热膨胀系数。利用本随机扰动模型可迅速准确地预测出复合材料的热膨胀性能,便于材料研究和工程应用。     

The structure of the interface in carbon fibre composites by scanning Auger microscopy

Scanning Auger microscopy (SAM) has been used to study the fibre/matrix interface of composite materials. This novel application of Auger spectroscopy enables further understanding of the mechanism of failure in composites when applied to carbon fibre-reinforced epoxy material. Initial work was carried out on carbon fibres prior to their incorporation into the resin matrix. Auger spectroscopy can be used to detect the presence of thin polymeric layers on the carbon fibres if a suitable, matrix-specific element is chosen to form the scanning Auger image. Two composite materials have been investigated. They differ from each other by the fracture mechanisms. In the case of unidirectional continuous fibre composites, a high volume fraction of conducting carbon fibres makes Auger analysis possible although the failure surface is predominantly polymer residues. For short-fibre composites the technique is more difficult considering the low volume fraction of fibres, but Auger spectroscopy enables the identification of microfailure mechanism and of the effect of fibre surface treatment on the failure mode.     

Interstrand void content evolution in compression moulding of randomly oriented strands (ROS) of thermoplastic composites

Compression moulding of randomly oriented strands (ROS) of thermoplastic composite is a process that enables the forming of complex shapes with keeping final properties close to that of continuous fibre composites. During forming several deformation mechanisms occur. In this paper we focus on the interstrand void content (ISVC) reduction: the squeezing of each single strand during compression enables filling of the gaps between strands. A modelling of this deformation mechanism was developed. The compaction is ruled by an ordinary differential equation that was solved numerically. The model was validated experimentally using an instrumented hot press with Carbon-PEEK prepreg strands. The model accurately predicted ISVC in four characteristic cases. Using the proposed model, the influence of several process and material parameters were investigated. Finally, a design chart giving the final ISVC for a wide range of pressure, strand geometry and part thickness, was constructed.     

Compressibility of carbon woven fabrics with carbon nanotubes/nanofibres grown on the fibres

Growth of carbon nanotubes (CNT) or carbon nano-fibres (CNF) on carbon fibrous substrates is a way to increase the fracture toughness of fibre reinforced composites (FRC), with encouraging results reported in the recent years. If these nano-engineered FRC (nFRC) are destined to leave laboratories and enter industrial-scale production, a question of adapting the existing composite manufacturing methods will arise. The paper studies compressibility of woven carbon fibre performs (two types of fabrics) with CNT/CNF grown on the fibres using the CVD method. The results include pressure vs thickness and pressure vs fibre volume fraction diagrams for one and four layers of the fabric. Morphology of the nFRC is studied with SEM. It is shown that the pressure needed to achieve the target fibre volume fraction of the preform increases drastically (for example, from 0.05 MPa to more than 0.5 MPa for a fibre volume fraction of 52%) when CNT/CNF are grown on it. No change in nesting of the fabric plies is noticed. The poor compressibility can lower the achievable fibre volume fraction in composite for economical vacuum assisted light-RTM techniques and increase the pressure requirements in autoclave processing.     

Mechanical performance optimization through interface strength gradation in PP/glass fibre reinforced composites

A new design for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS) has been developed with the aim of coupling high impact resistance with high static properties. IGIS laminates have been prepared by properly alternating layers of woven fabric with layers of compatibilized or not compatibilized polymeric films. To prove the new concept, polypropylene (PP) and glass fibres woven fabrics have been used to prepare composites by using the film stacking technique. Maleated PP, able to compatibilize polypropylene with glass fibres, has been used to manage the interface strength layer by layer.The flexural and low-velocity impact characterizations have shown that the presence of the coupling agent in conventional composite structures (prepared with fully compatibilized polymeric layers) improves the static flexural properties through the strengthening of the matrix/fibre interface but considerably lowers the low velocity impact resistance of the composite, in terms of maximum load before fibre breakage and recovered energy after impact. The use of the IGIS design, that grade the interface strength through the laminate thickness, allows to prepare composites with both high flexural properties and high impact resistance, without affecting the balance and type of the reinforcement configuration.     

On the recyclability of a cyclic thermoplastic composite material

The recyclability of a novel cyclic thermoplastic composite material has been investigated. The virgin composite was prepared by liquid molding the cyclic thermoplastic resin and a knitted glass fabric. The resultant high fiber content composite (58.7% fiber weight fraction) was recycled by grinding/compounding/injection molding process. A variety of physical and mechanical tests were then conducted on the blended cyclic composite and a baseline, commercially available short fiber reinforced thermoplastic composite. In general, the recycled cyclic composite demonstrated comparable properties to the baseline material. The only exception being the ultimate tensile elongation of the recyclate, which was almost 25% lower than that of the baseline.     

Preparation of carbon fibre reinforced aluminium via ultrasonic liquid infiltration technique

Abstract

An ultrasonic liquid infiltration technique has been developed for the fabrication of carbon fibre reinforced aluminium (CF/Al) precursor wires. The principal effect of ultrasound on aluminium infiltration into carbon fibres is considered to be caused by cavitation. The acoustic power required to produce cavitation in the present experimental system has been approximately calculated to be about 150 W, which is much greater than the requirement, several tens of watts, for overcoming the capillary pressure among carbon fibres. The observations on the morphology of the CF/Al precursor wires show that there are generally four states of infiltration: totally non-infiltrated, non-infiltrated in the centre and in some local regions of the wires, and completely infiltrated. It is found that carbon fibres can be sufficiently impregnated by molten aluminium given the appropriate application of ultrasound. Furthermore, a single fibre tensile test shows that there is no strength degradation of carbon fibres after aluminium infiltration. The CF/Al precursor wires obtained have an average fibre volume fraction of 26%. The maximum longitudinal tensile strength of the CF/Al wires is 605 MN m^?2, which implies a fibre strength transfer efficiency of 0·76.

MST/1715     

Microstructure and Properties of M40 Carbon Fibre Reinforced Mg-Re-Zr Alloy Composites

M40 carbon fibre reinforced rare earth magnesium alloy ZM6 composites with fibre volume fraction about 60%were fabricated by pressure infiltration method.The microstructure, interfacial morphology, and precipitation were studied by scanning electron microscopy, transmission electron microscopy, and energy dispersive Xray spectrometer.It was shown that the interfaces between Mg alloy and fibres were well bonded and free from cracks.The Mg12Nd phase was preferentially precipitated at the fibre/matrix interfaces, leading to the segregation of Nd at the interfaces and the dramatic decrease of Mg12Nd precipitation in the matrix far from interfaces.Crystal defects such as high-density dislocations and twins were observed in the matrix near the fibre/matrix interface.A high bending strength (1393 MPa) and elastic modulus (190 GPa) were achieved in M40/ZM6 composite.     

High performance hybrid PP and PLA biocomposites reinforced with short man-made cellulose fibres and softwood flour

This paper presents the research on hybrid thermoplastic biocomposites reinforced with a combination of short man-made cellulose fibres and softwood flour. The introduced short fibre composites are meant to be processed with injection moulding and may be an alternative to glass-fibre reinforced thermoplastics on account of their comparable specific strengths. The occurring positive hybrid effect enables to substitute up to half the weight of short fibre cellulose reinforcement with softwood flour without a significant reduction of material flexural strength. The flexural modulus of investigated hybrid biocomposites remained approximately at the same level, while impact strength was reduced with increasing softwood flour content. The proposed hybridisation leads to establishing biocomposites of suitable performance with competitive density, price and recycling possibilities in comparison to standard glass fibre reinforced counterparts. Moreover, the application of biobased polymers like polylactide as biocomposite matrix, contributes to the development of so called “green” high performance materials.     

A novel processing technique for thermoplastic manufacturing of unidirectional composites reinforced with jute yarns

This paper primarily investigates the fabrication process of long-fibre reinforced unidirectional thermoplastic composites made using jute yarns (both untreated and treated). Tubular braiding technique was used to produce an intermediate material called “microbraid yarn” (MBY) with jute yarn as the straightly inserted axial reinforcement fibre and polymer matrix fibre being braided around the reinforcing jute yarns. Microbraid yarns were then wound in a parallel configuration onto a metallic frame and compression molded to fabricate unidirectional composite specimens. In this study, two types of polymeric materials (biodegradable poly(lactic) acid and non-biodegradable homo-polypropylene) were used as matrix fibres. Basic static mechanical properties were evaluated from tensile and 3 point bending tests. Test results were analyzed to investigate the effects of molding temperature and pressure on the mechanical and interfacial behaviour. For the unidirectional jute fibre/poly(lactic) acid (PLA) composites, the results indicated that the molding condition at 175 °C and 2.7 MPa pressure was more suitable to obtain optimized properties. Improved wettability due to proper matrix fusion facilitated thorough impregnation, which contributed positively to the fibre/matrix interfacial interactions leading to effective stress transfer from matrix to fibre and improved reinforcing effects of jute yarns. For the jute/PP unidirectional composites, specimens with only 20% of jute fibre content have shown remarkable improvement in tensile and bending properties when compared to those of the virgin PP specimens. The improvements in the mechanical properties are broadly related to various factors, such as the wettability of resin melts into fibre bundles, interfacial adhesion, orientation and uniform distribution of matrix-fibres and the lack of fibre attrition and attenuation during tubular braiding process.     

Experimental studies and analysis of the draping of woven fabrics

This study investigates and compares the draping and forming of four types of woven fabrics, namely a loose plain weave (basket weave), a tight plain weave, a satin and a twill weave. The fabrics were draped over a hat mould consisting of a hemispherical dome surrounded by a flat base. The draping of each fabric was examined in terms of wrinkle formation, boundary profile of the draped fabric, distribution of fibre orientation and local shear angles. A theoretical analysis of the experimental results involved the calculation of the distributions of the fibre volume fraction and mechanical properties, in terms of components of the reduced stiffness matrix, from the experimental data of local shear angles.     

Thermal de-consolidation of thermoplastic matrix composites—I. Growth of voids

Thermoplastic matrix composites have one unique advantage, being reprocessable or recycleable through post-thermoforming and fusion bonding subject to a re-heating/cooling cycle. However, this advantage is not unconditionally available, and thermoplastic matrix composites may suffer greatly from unexpected deterioration in meso-structures during the thermal processing, leading to de-consolidation. Based on experimental observations, the mechanisms of thermal de-consolidation are discussed in detail, and a void growth model is established to evaluate the degree of de-consolidation in a post-thermal operation. Using this model the dependence of void growth on material properties and processing parameters, such as the compressibility of fibre reinforcement, the effects of elasticity/viscosity of matrix, the applied external pressure and processing temperature is evaluated. It is shown that the results obtained are in a good agreement with the experimental data.     

Rigidity of diamond reinforced metals featuring high particle contents

Diamond/metal composites with diamond contents between 57 and 72 vol% have been produced by gas pressure assisted liquid metal infiltration using Ag–3 wt% Si and Al–2 wt% Cu as matrix. The experimental data cover a range of Young’s moduli from 300 to 425 GPa and 245 to 370 GPa for the Ag–3Si and the Al–2Cu-based composites, respectively. Experimental Young’s moduli are compared to the Mori–Tanaka mean field scheme (MTM), the generalized self-consistent scheme (GSCS), the bimodal hard sphere model (TBHS), and the differential effective medium scheme (DEM). At the lower end of volume fractions investigated, the predictions by the GSCS, the TBHS, and the DEM are very close to each other and to the experimental results while the MTM is clearly lower. With increasing volume fraction the differences between the models accentuate and the data up to 72 vol% of diamond are best described by the DEM.