Tensile strength of discontinuous fibre-reinforced composites

A stochastic Monte-Carlo approach, based on Eyring's chemical activation rate theory, is used to study the factors controlling the tensile strength of discontinuous fibre-reinforced composites. The model explicitly takes into account the local distribution of stress near fibre ends. Both the fibre and the matrix are allowed to break during fracture of the composite. The stress-strain curves and the modes of failure of the composite are found to be strongly dependent on the volume fraction and aspect ratio of the fibres. The importance of adhesion at the fibre/matrix interface is also studied. The results are compared with available experimental data.     

Storage and loss moduli in discontinuous composites

The paper deals with viscoelastic, rubber-like material unidirectionally reinforced with discontinuous fibres. The longitudinal storage modulus is calculated not only from an equation based on an existing force balance treatment but also from the elastic strain energy stored in matrix and fibres, using two different models to derive the stress and strain distributions from which the stored energy is calculated. There is very good agreement between all the calculations. The energy calculations reveal that loss modulus is also greatly increased by discontinuous reinforcement and enable its value to be estimated. Experiments on storage and loss modulus are reported and show that the calculations underestimate storage modulus and overestimate loss modulus. In both cases the factor of error ～ 2, and arises because the amplified matrix strain is underestimated and is partly hydrostatic; the hydrostatic strain is non-dissipative and therefore does not contribute to the loss modulus. Discontinuous reinforcement can increase loss modulus as well as storage modulus by more than 100 times, and this should help sound and vibration deadening. An estimate is made of the wide ratio of compliance ÷ breaking strength available with discontinuous but not with continuous reinforcement, which opens up new design latitude for components hitherto reinforced with continuous fibres.     

Thermo-visco-plastic behaviour of fibre-reinforced polymer composites

The effect of temperature and strain rate on the tensile behaviour on a series of polymeric matrix-unidirectional glass–fibre composites was studied. Dynamic mechanical analysis (DMA) experiments, as well as tensile tests at three different strain rates and three different temperatures below T_g were performed on off-axis specimens of three different orientations. The strong temperature and strain rate dependence, exhibited by the materials examined, was further described theoretically by applying a formulation of finite elastoplasticity. Constitutive laws based on the material anisotropy, were applied, in combination with constitutive equations of hypoelasticity, written in their objective form. Moreover, empirical equations for the hardening coefficients, arising from the thermal activation theory, were proposed to formulate the temperature and strain rate effect.     

Modeling of progressive damage in aligned and randomly oriented discontinuous fiber polymer matrix composites

Damage constitutive models based on micromechanical formulation and a combination of micromechanical and macromechanical damage criterions are presented to predict progressive damage in aligned and random fiber-reinforced composites. Progressive interfacial fiber debonding models are considered in accordance with a statistical function to describe the varying probability of fiber debonding. Based on an effective elastoplastic constitutive damage model for aligned fiber-reinforced composites, micromechanical damage constitutive models for two- and three-dimensional (2D and 3D) random fiber-reinforced composites are developed. The constitutive relations and overall yield function for aligned fiber orientations are averaged over all orientations to obtain the constitutive relations and overall yield function of 2D and 3D, random fiber-reinforced composites. Finally, the present damage models are implemented numerically and compared with experimental data to show the progressive damage behavior of random fiber-reinforced composites. Furthermore, the damage models will be implemented into a finite element program to illustrate the dynamic inelastic behavior and progressive crushing in composite structures under impact loading.     

Structure and properties of aligned short fibre-reinforced intermetallic matrix composites

Powder injection moulding techniques were utilized to align short fibres (Al₂O₃ and SiC) in a variety of intermetallic matrices (NiAl, MoSi₂ and TaTiAl₂). The alignment was accomplished by extruding a mixture of powders and short fibres with a polymer-based binder through a constricting nozzle. The binder was removed and the powder and fibres were consolidated, producing an aligned short fibrous composite. The effects of powder morphology, fibre volume fraction and fibre diameter on the alignment were demonstrated. Small diameter powders were required to ensure alignment of an appreciable loading of fibres in a powder matrix. Tensile and hardness tests were used to evaluate the effectiveness of the aligned short fibres to strengthen and toughen the matrices. The mechanical behaviour of these aligned short fibrous composites were found to be comparable to similar aligned continuous fibrous composites produced by conventional techniques.     

Time-dependent damage in carbon fibre-reinforced polymer composites

Time-dependent damage (matrix cracks) evolution in AS4/3501-6 cross-ply laminates was studied using constant strain rate and constant stress tests. First ply failure stress and strain as well as the matrix crack density at a given stress level were found to be strongly dependent on strain rate. Matrix crack density increased with creep time at a constant stress level. The compliance and creep rate of the laminate increased in the presence of these cracks. These results emphasize the importance of the knowledge of time-dependent damage evolution in a lamina/laminate of a polymer composite for reliable prediction of creep and creep rupture.     

纤维增强热固性聚合物基复合材料层间增韧研究进展

综述了纤维增强热固性聚合物基复合材料（PMC）层间增韧的最新研究进展。热固性复合材料由于基体树脂高的交联密度而呈脆性,表现出低的冲击损伤阻抗和损伤容限特征。柔性聚合物层间增韧是改善聚合物基复合材料层间断裂韧性和抗冲击性的有效手段,且不会降低热固性树脂的热性能和高模量。目前有3种层间增韧方法：颗粒增韧、聚合物纤维增韧和薄膜增韧。讨论了3种方法的概念、实施方案、增韧机理及研究成果。最后重点阐述了创新性的复合材料“离位”增韧思想,介绍了具有全部自主知识产权的“离位”复合材料高性能化技术体系,包括预浸料和液态成型两大复合材料产品系列。     

Strategic reinforcement of hybrid carbon fibre-reinforced polymer composites

A secondary fibre has been used to improve the impact properties of carbon fibre-reinforced composites. Steel wires possessing similar elastic properties to Type III carbon fibres have been added strategically to the composite cross-section. This has resulted in a 100% improvement in the fracture energy provided that the wires were placed in close proximity to the compressive or impacted face. Such a result is achieved with small increases in longitudinal and interlaminar shear strength. Only minor changes in specific properties occurred through the introduction of the high-density wires. The increase in fracture energy occurs because of the elimination of a compressive failure mode, believed to be brought about by the steel wires increasing the resistance to buckling at the impacted face. Hence, more energy-intensive processes, such as multiple delamination, fibre and wire pull out, are permitted to take place over larger areas of the fracture face.     

Flexed plate impact testing of carbon fibre-reinforced polymer composites

Some initial results on the impact properties of a number of carbon fibre-reinforced resin composites are presented. Composites manufactured with either polyether etherketone or epoxy resin matrices were tested using the instrumented falling weight method. The force/time or force/displacement curves have been characterized by the displacement on loading (gradient), the peak force, the energy to the peak force and the total impact energy to failure. The results suggest that the polyether etherketone composite (APC-2) is much tougher than the epoxy composites, requiring larger energies and forces to initiate and propagate cracks thereby resulting in much greater impact energies to failure.     

Effective elastic moduli of two-phase transversely isotropic composites with aligned clustered fibers

Summary In this paper, we address the issue of the effective elastic moduli of transversely isotropic composites reinforced with aligned clustered continuous fibers. Clustering implies that there are portions of the matrix with a dense reinforcement of fibers and other portions with a sparse reinforcement. The clustering effect is characterized by a probability density distribution in local fiber volume fractions, obtained from the Dirichlet tessellation of a microstructure. Using a combination of Christensen and Lo's solution of a 3-phase boundary value problem and Hill's self-consistent method, the effective moduli are derived in terms of the probability density distribution function. It is shown that a unimodal distribution (representative of a random microstructure) has a modest effect on the effective moduli whereas a bimodal distribution (representative of a clustered microstructure) has a significant effect over a wide range of inclusion/matrix properties. A parametric study demonstrates that clustering has a significant effect on the shear moduli and the plane strain bulk modulus of the transversely isotropic composite and has a negligible effect on the longitudinal Young's modulus and the major Poisson's ratio. The theory has been compared with the Hashin-Rosen [1] bounds (appropriately modified for the clustered microstructure) and the classical Hashin-Shtrikman [2] bounds, and the theoretical predictions have been found to be bracketed by both bounds. In addition, the plane strain bulk modulus of a sample clustered periodic microstructure is computed by the developed theory and also by the finite element analysis, and the modulus computed by both approaches demonstrates a sensitivity to clustering.     

The effect of fibre aspect ratio on the stiffness of discontinuous fibre-reinforced composites

The mechanics underlying the stiffness of discontinuous fibre-reinforced composites are well understood. In particular the critical fibre aspect ratio is known to affect the resultant deformation of the composite. This paper will explain how the Cox theory can be used to determine the critical aspect ratio for a given fibre/matrix combination at a given volume fraction. Supporting experimental evidence for the key dependencies influencing the critical aspect ratio are shown at the microscopic level (by a Raman spectroscopic approach) and at the macroscopic level (by a tensile creep approach) for a series of different composite materials.     

Fracture process of thermally shocked discontinuous fibre-reinforced glass matrix composites under tensile loading

Fracture mechanisms of discontinuous carbon-fibre-reinforced glass matrix composites were experimentally studied for specimens with initial damage induced by thermal shock. First, matrix cracking due to thermal shock was observed using both optical microscopy and scanning acoustic microscopy (SAM) to reveal the damage state. Secondly, tensile stress-strain behaviour and acoustic emission during tensile tests were measured for specimens with and without thermal shock. The progress of microscopic damage during tensile loading was also investigated using both replica and in-situ SAM techniques. Finally, macroscopic transient thermal stresses during thermal shock were calculated using finite-element analysis. It is proved that the fracture process of the composite specimen with thermal-shock-induced cracks is different from that of the virgin specimen. This difference in fracture processes is attributed to the difference in the evolution of matrix cracking, which is affected by pre-existing microcracks in the matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the trade-off between damping and stiffness in the design of discontinuous fibre-reinforced composites

Quasi-static models are first developed, by using a forced balance approach, to define the effects of selected microstructural parameters, e.g. fibre aspect ratio, fibre off-axis angle and fibre volume fraction, on the damping and stiffness of a class of polymeric, discontinuous fibre-composite systems. Simultaneous optimization of damping, stiffness and weight of a class of such material is then carried out by using the so-called inverted utility function method. The obtained results show that discontinuous fibre-reinforced composites have superior design flexibility as compared with those pertaining to continuous fibre-reinforced composites.     

Enhanced thermal conductivity in polymer composites with aligned graphene nanosheets

We developed highly aligned graphene nanosheets (GNSs) in epoxy composites with incorporating magnetic GNS–Fe₃O₄ hybrids under a magnetic field with the aim to take full advantage of the high inplane thermal conductivity of graphene. GNS–Fe₃O₄ hybrids were fabricated by a simple coprecipitation method, and their morphology, chemistry, and structure were characterized. GNS–Fe₃O₄ hybrids were found to be homogenously dispersed and well aligned through the direction of the magnetic field in the epoxy matrix, as confirmed by SEM observation and Raman spectra analysis. The resulting epoxy/GNS–Fe₃O₄ composites possessed high thermal conductivity in a parallel magnetic-alignment direction at low GNS–Fe₃O₄ loadings, which greatly outperformed the composites with randomly dispersed bare GNSs. The obtained results indicated that the magnetic alignment of magnetic-functionalized GNSs is an effective way for greatly improving the thermal conductivity of the graphene-based composites.     

Pseudo-ductility in intermingled carbon/glass hybrid composites with highly aligned discontinuous fibres

The aim of this research is to manufacture intermingled hybrid composites using aligned discontinuous fibres to achieve pseudo-ductility. Hybrid composites, made with different types of fibres that provide a balanced suite of modulus, strength and ductility, allow avoiding catastrophic failure that is a key limitation of composites. Two different material combinations of high strength carbon/E-glass and high modulus carbon/E-glass were selected. Several highly aligned and well dispersed short fibre hybrid composites with different carbon/glass ratios were manufactured and tested in tension in order to investigate the carbon ratio effect on the stress–strain curve. Good pseudo-ductile responses were obtained from the high modulus carbon/E-glass composites due to the fragmentation of the carbon fibres. The experimental results were also compared with an analytical solution. The intermingled hybrid composite with 0.25 relative carbon ratio gave the maximum pseudo-ductile strain, 1.1%, with a 110 GPa tensile modulus. Moreover, the initial modulus of the intermingled hybrids with 0.4 relative carbon ratio is 134 GPa, 3.5 times higher than that of E-glass/epoxy composites. The stress–strain curve shows a clear “yield point” at 441 MPa and a well dispersed and gradual damage process.     

Thermal conductivity of aligned CNT/polymer composites using mesoscopic simulation

Thermal conductivity of CNT/polymer composites depends on alignment, dispersion, volume fraction and size of CNTs as well as polymer size. By coupling smoothed particle hydrodynamics and dissipative particle dynamics, thermal conductivities of random and aligned composites along with their meso morphologies are studied in detail. Thermal conductivity along the alignment of CNT can be significantly enhanced to 16 times that of polymer by increasing volume fraction, dispersion degree and length of CNT, meanwhile thermal conductivity perpendicular to the alignment of CNT is affected modestly by these factors. Enhancement of thermal conductivity of random composites could only be efficiently achieved by increasing the volume fraction of CNT. Particularly, thermal conductivity $\kappa$ is proportional to the square of volume fraction of CNT v in well dispersed random and aligned composites, i.e. $\kappa \propto v^2$.     

A review of the tensile and fatigue responses of cellulosic fibre-reinforced polymer composites

ABSTRACT

The use of polymer-based composites has been gaining popularity in the industry over the last few decades. Their high strength to weight ratio and high fatigue resistance make these composites the preferred materials for a wide variety of applications. The current trend has inclined towards hybrid fibre reinforced composites owing to their outstanding characteristics compared to non-hybrid composites. Numerous research works have been conducted to study the fatigue life behaviour of such composite materials. This study addressed the monotonic and dynamic performance of non-hybrid and hybrid natural fibre based composite materials, and the factors that influence their fatigue performance, along with the stiffness decay of each composite material. Most studies have shown the superior potential of using natural fibres in place of synthetic fibres in those critical applications that involve tensile and cyclic loading.