Behaviour of composite plates with drilled and moulded hole under tensile load

Drilling of composite material causes damages of different natures. To avoid machining in composite material, it is proposed to include holes while moulding. In the literature, there is few works concern the behaviour analysis of specimens with moulded holes. All the literature works concern a specimens with moulded holes made of woven fabric. Moreover no literature is available on local behaviour of specimens around the hole. In this paper, several mechanical tests are carried out on specimens with quasi-isotropic stacking sequence made from UD prepreg with drilled holes and with moulded holes. To analyse the behaviour of the specimens around the hole, the strain field is measured using three-dimensional Digital Image Correlation technique (DIC). The pictures analysed given by the CCD camera reveal that, the damage mechanisms are different between the plates with drilled holes and those with moulded holes. The SEM observation have shown that, the fibre content and porosity content near the hole is around ≈ 8% higher compared to the values obtained far the hole. Moreover the tensile strength of the plates with moulded holes is 28% higher than with drilled holes.     

Local fibre reinforcement of holes in composite multilayered plates

For various technical reasons, cutouts such as holes in thin-walled structures are inevitable and are of significant technical relevance. Unfortunately holes lead to an undesired stress concentration at the hole vicinity and a reduced strength of the structure. Therefore in practice a local reinforcement in the form of a ring is usually applied around the hole. The increasing requirements for modern structures in terms of low weight and high strength lead to the question of an optimal reinforcement design. The present paper addresses the new but well-approved techniques of the use of curved fibre format to determine the aforementioned optimal design of the reinforcement. The optimization of cutouts in laminated composite plates under bi-axial tensile loading conditions has been investigated using two approaches: the finite element and the Rayleigh-Ritz method. The used methodology implemented successfully theoretical results based on the complex potential theory. For the considered problems, the proposed methods were shown to successfully produce a constant objective function around the hole boundary under biaxial loading. The optimal reinforcement of holes in laminated composite plates illustrated that the optimum depends on the degree of orthotropy. Significant reduction of stress concentrations were demonstrated. The results obtained illustrate the necessity and usefulness of the applied optimization procedure.     

Impact of Nd-YAG laser drilled holes on the strength and stiffness of laminar flow carbon fibre reinforced composite panels

The effect of Nd-YAG laser drilled holes on the strength and stiffness of carbon fibre epoxy composite panels has been investigated. The holes, approximately 50 μm in diameter and 500 μm apart, are required to produce a porous skin for a Hybrid Laminar Flow application on an aircraft. Scanning Electron Microscope inspection indicated resin damage around the holes due to the heat of the laser. The damage area was elliptical in shape and measured approximately one hole diameter on either side of the drilled hole in unidirectional carbon fibre panels. The perforations reduced the static strength between 2 and 54% compared to non-drilled specimens, depending on the material type (unidirectional or weave), drilling method (single or multi-pulse) and hole pattern.     

Dynamic stress concentration studies by boundary integrals and Laplace transform

The dynamic stress field and its concentrations around holes of arbitrary shape in infinitely extended bodies under plane stress or plane strain conditions are numerically determined. The material may be linear elastic or viscoelastic, while the dynamic load consists of plane compressional waves of harmonic or general transient nature. The method consists of applying the Laplace transform with respect to time to the governing equations of motion and formulating and solving the problem numerically in the transfomed domain by the boundary integral equation method. The stress field can then be obtaind by a numerical inversion of the trasformed solution. The correspondence principle is invoked for the case of viscoelastic material behavious. The method is simplified for the case of harmonic waves where no numerical inversion is involved.     

Thermal conductivity and thermal diffusivity analyses of low-density polyethylene composites reinforced with sisal, glass and intimately mixed sisal/glass fibres

The thermal conductivity and thermal diffusivity of sisal-reinforced polyethylene (SRP), glass-reinforced polyethylene (GRP) and sisal/glass hybrid fibre-reinforced polyethylene (GSRP) has been evaluated at cryogenic to high temperature (120–350 K). It has been observed that the variation of thermal conductivity with temperature is almost the same for LDPE and SRP containing perpendicularly oriented sisal fibres. The difference between the values of thermal conductivity shown by LDPE and GRP is greater than that of SRP and LDPE. The enhanced thermal conductivity of glass fibre is due to the presence of Fe²⁺ ions in the glass fibres. The linear variation in thermal conductivity with fibre loading is explained with the help of a model suggested by Agari. The difference between the thermal conductivity properties in directions parallel and perpendicular to the applied flux is a maximum for SRP owing to the anisotropic nature of sisal fibre. The difference is marginal for GRP on account of its isotropic nature. The position of GSRP is found to be intermediate. It can been observed that the variation of thermal diffusivity with temperature is just opposite to that of thermal conductivity. This may be due to a reduction in the mean free path of phonons. An empirical equation is derived to explain the variation in thermal conductivity and thermal diffusivity with temperature.     

Investigation of cladding effects on optical guidance of microstructured holey fibres based on FDM and FDTD methods

Effects of the hexagonal air-hole cladding layer of a microstructured holey fibre on propagation characteristics are investigated based on the two-dimensional (2D) finite difference method and the 2D finite-difference time-domain technique. A holey fibre with parameters of r = 0.6 µm and Λ = 2.0 µm is designed and analyzed numerically by changing the number of hexagonal cladding layers. It is found that the layers beyond which fields become sufficiently small in the transverse plane (air-hole layers out of the fifth hexagonal cladding layer in the designed holey fibre) have little influence on propagation characteristics around the wavelength range of 1.3 and 1.55 µm for general optical communications, which means the air-hole layers close to the core region play an important role on its performance. Chromatic dispersion of the holey fibre with five cladding layers of air holes is approaching zero around 0.9 µm wavelength.     

Viscoelastic composite materials for noise reduction and damage tolerance

This paper describes the concept of viscoelastic composites and how their dynamic physical characteristics may be exploited for ship applications where high vibrational damping and damage tolerance is a requirement. A mathematical model is used to describe the effect of fibre alignment on viscoelastic composite dynamic Young's Moduli and loss factors. The model, supported by experimental validation, requires the dynamic physical properties of the matrix resin for the range of operational temperatures and frequencies of interest. The model predicts that the resulting loss factor of a fibre reinforced viscoelastic resin composite behaves in an anisotropic fashion, however, damping optimisation may be achieved in two different ways dependant on the angle of fibre alignment. The implications of these findings and how the fibre alignment in a viscoelastic composite may be exploited for noise free ship and submarine machinery rafts are also discussed. Empirical studies on viscoelastic composites of this type using a drop weight impact tester have shown that their damage tolerance is superior to conventional glass reinforced polyester GRP. Although a theoretical analysis has not been made, it is implied that the viscoelastic nature of the matrix resin and its optimisation over the typical damaging impact frequencies is an important factor in absorbing the impact energy in much the same way as it is for absorbing vibrational energy. In this case the impact impluse is considered to be one particular extreme form of vibrational source. Provided the impact force can do work on the matrix, the energy will be absorbed by it if the viscoelastic nature of matrix is correct with little or no resulting damage to the composite.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

Boundary element analysis for viscoelastic solids containing interfaces/holes/cracks/inclusions

With the aid of the elastic–viscoelastic correspondence principle, the boundary element developed for the linear anisotropic elastic solids can be applied directly to the linear anisotropic viscoelastic solids in the Laplace domain. Green's functions for the problems of two-dimensional linear anisotropic elastic solids containing holes, cracks, inclusions, or interfaces have been obtained analytically using Stroh's complex variable formalism. Through the use of these Green's functions and the correspondence principle, special boundary elements in the Laplace domain for viscoelastic solids containing holes, cracks, inclusions, or interfaces are developed in this paper. Subregion technique is employed when multiple holes, cracks, inclusions, and interfaces exist simultaneously. After obtaining the physical responses in Laplace domain, their associated values in time domain are calculated by the numerical inversion of Laplace transform. The main feature of this proposed boundary element is that no meshes are needed along the boundary of holes, cracks, inclusions and interfaces whose boundary conditions are satisfied exactly. To show this special feature by comparison with the other numerical methods, several examples are solved for the linear isotropic viscoelastic materials under plane strain condition. The results show that the present BEM is really more efficient and accurate for the problems of viscoelastic solids containing interfaces, holes, cracks, and/or inclusions.     

An original approach for mechanical modelling of short-fibre reinforced composites with complex distributions of fibre orientation

In this paper, an original and effective model of behaviour for short-fibre reinforced composites is presented. In particular, complex fibre distributions of orientation can be dealt with in a very easy way, without orientation averaging or additional homogenisation steps. The matrix material has elastoplastic damage behaviour with non-isochoric plastic flow. Ductile damage can be fully anisotropic depending on the reinforcement characteristics. The model is validated for the case of a polypropylene reinforced with short flax fibres. In addition, simulations are performed to investigate the influence of key parameters like fibre length and interfacial shear strength, as well as the impact of progressive debonding at the fibre tips.     

考虑混杂效应时纤维混杂缠绕筒三维等效弹性模量的理论估算和试验研究

给出了多种纤维混杂多向缠绕筒三维等效弹性模量的理论估算方法, 应用该方法计算了玻璃-碳纤维混杂缠绕筒的三维等效弹性模量。计及混杂效应的影响, 考虑了碳纤维体积分数、 铺层方式、 纤维分散度等因素, 对玻璃-碳纤维混杂缠绕结构, 引入混杂效应系数对该方法进行修正。试验结果表明, 该方法预测的三维等效弹性模量的精度较高。采用该方法可将复杂的纤维混杂缠绕结构等效为具有各向异性性质的均质单一材料, 极大降低了应力分析的工作量。     

Fibre structure and anisotropy of glass reinforced thermoplastics

The fibre structure and orientation distribution of two commercially available glass mat thermoplastics reinforced by continuous glass fibre was studied to investigate the anisotropic behaviour under compression moulding and mechanical loading, and to investigate the influence of the fibre structure and orientation on the anisotropic behaviour. Circular samples were deformed into ellipses when moulded, due to the anisotropic fibre orientation. The fibre content and orientation were examined in different locations of the elliptically deformed specimens.X-ray pictures were taken of the material in order to develop images of the fibres, before and after compression moulding. In another procedure, the matrix was burned off and the fibre network structures were studied in each case. A CCD camera was used to scan the fibres as digital images to measure the orientation distribution functions of the fibres. The fibre orientation measuring process was facilitated by subroutines implemented in the source code of the public domain NIH-image analysis software using simulated Fraunhofer diffraction.     

Temperature dependence of concentrations and mobilities in thin bismuth films

The resistivity, Hall coefficient and magnetoresistance of thin bismuth films were measured at various temperatures (80–300 K) and thicknesses (0.1–3 μm). The mobilities and concentrations of the free electrons and holes were calculated from the experimental data using an anisotropic two-carrier model. Four approaches to the calculation were applied and the results were compared with one another. It was found that the electron and hole concentrations are approximately equal.     

Structure of round-shaped methylnaphthalene-derived mesophase pitch-based carbon fibres prepared by spinning through a Y-shaped die hole

Structures of as-spun, stabilized, carbonized and graphitized fibres prepared by spinning a methylnaphthalene-derived mesophase pitch through a Y-shaped die hole at 295 °C, was examined by combining optical, scanning electron and transmission electron microscopy from the macro- and microscopic view points. The prepared round-shaped fibre spun through a Y-shaped spinning die hole at 295 °C exhibited excellent tensile and compressive strengths of 410 and 70 Kg mm^–2, respectively, after graphitization at 2500 °C. The stabilized fibre consisted of densely packed anisotropic domains in very random alignment, of which transverse domains and longitudinal features appeared as bent, multi-bent and looped, and endless thin stripes, respectively. The size of domain in the transverse section ranged above 100 nm in length and below 100 nm in thickness, respectively. Further heat treatment (carbonization and graphitization) slightly reduced the dimension and deformed the shape of domains to shrink and to have more sharp edges at their bends according to the graphitic growth; however, the shapes and distribution of domains in transverse section were basically unchanged. High-resolution SEM and TEM observations of the domain confirmed the existence of smaller units of graphitic layers in their assemblies which were more closely arranged in the domain. Such a sub-unit was defined as a micro-domain. TEM revealed that the micro-domain was composed of more than one unit of graphitic layers in the graphitized fibre. Most of them were around 10 nm thick and 10–100 nm long. The thickness of micro-domains was observed to be smaller than the value ofL _c (002), 23 nm, in the same graphitized fibre. Micro-domains have not yet been identified in the stabilized fibre, while TEM suggested some stackings of hexagonal planes. A number of voids (micro- and meso-voids) up to 40 nm diameter were formed at the intra- or inter-domain locations, due to the graphitic shrinkage and evolution of volatile matter by the heat treatments. Micro-voids of around 5 nm diameter were formed within a domain. The better mechanical performances of the present fibre spun through a Y-shaped die hole were ascribed to the homogeneous distribution of looped or bent domains in the transverse section (random nature of transverse alignment). Such a random alignment may also lead to the least number of macro-voids and cracks in the fibril.     

Fabrication and characterization of Ge-doped flat fibres

In this paper, we demonstrate fabrication and characterization of Ge-doped flat fibres. The Ge-doped flat fibre was fabricated from a 6 mol% Ge-doped preform. The flat fibre was successfully fabricated using dual stage drawing. Near field characterization was done and results show similar modal profile patterns as simulated using the software COMSOL. The results confirmed that with dual stage drawing and optimized drawing parameters, flat fibres with no defect holes can be fabricated.     

Optimal curved fibre orientations of a composite panel with cutout for improved buckling load using the Efficient Global Optimization algorithm

Composite structures with cutouts (like panels with holes) are a challenge to design because discontinuities of this kind provoke stress concentrations and become critical regions. With curved fibres, the effect of these discontinuities can be decreased by choosing the fibre paths properly. In this article, fibre-path optimization to improve the buckling load of laminated composite panels with cutouts is studied. Two fibre path parameterizations are tested: the usual curvilinear Cartesian and the radial one, proposed in this article, in which the fibre orientations vary linearly with the Euclidean distance from the centre of the panel. To reduce the simulation costs associated with the optimization, the Efficient Global Optimization (EGO) algorithm is used. EGO is a technique based on a stochastic process approach (Kriging) that approximates expensive-to-evaluate functions and sequentially maximizes the expected improvement to update the surrogate at each iteration. A stiffened panel with a cutout subjected to compression and in-plane shearing loads is analysed. The results show that the buckling load when curved fibres are used is substantially higher than the buckling load for straight-fibre laminates. In addition, the optimization framework indicates a low final computational burden.     

Stress concentration analysis of fibre-reinforced multilayered composites with pin-loaded holes

The problem of stress concentrations in the area of pin-loaded holes is of particular significance in the design of multilayered fibre-reinforced composite structures. For the purpose of simulating such problem zones in anisotropic multilayered composites, analytical methods offer decisive advantages since they, in comparison to numerical methods, allow weighting of influencing parameters and in this way permit a physical interpretation of complicated notch phenomena. At the Institut für Leichtbau und Kunststofftechnik (ILK) sophisticated analytical solution methods for the stress concentration problem in multilayered composites with pin-loaded holes were developed on the basis of layer-related solutions and have been confirmed in numerical calculations.     

Highly Anisotropic Conductors

Composite materials with ordered microstructures often lead to enhanced functionalities that a single material can hardly achieve. Many biomaterials with unusual microstructures can be found in nature; among them, many possess anisotropic and even directional physical and chemical properties. With inspiration from nature, artificial composite materials can be rationally designed to achieve this anisotropic behavior with desired properties. Here, a metallic wood with metal continuously filling the wood vessels is developed, which demonstrates excellent anisotropic electrical, thermal, and mechanical properties. The well‐aligned metal rods are confined and separated by the wood vessels, which deliver directional electron transport parallel to the alignment direction. Thus, the novel metallic wood composite boasts an extraordinary anisotropic electrical conductivity (σ_||/σ_⊥) in the order of 10¹¹, and anisotropic thermal conductivity (κ_||/κ_⊥) of 18. These values exceed the highest reported values in existing anisotropic composite materials. The anisotropic functionality of the metallic wood enables it to be used for thermal management applications, such as thermal insulation and thermal dissipation. The highly anisotropic metallic wood serves as an example for further anisotropic materials design; other composite materials with different biotemplates/hosts and fillers can achieve even higher anisotropic ratios, allowing them to be implemented in a variety of applications.     

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.