Interfacial separation of fibres in fibre reinforced composites

An analytical model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding.

The analytical analysis is based on the principle of the compliance method in fracture mechanics with the presence of an interfacial crack at the fibre/matrix interface. The model is developed on the basis of the assumption that both the matrix and the fibre behave elastically and the matrix strain at a zone far from the matrix-fibre interface is equal to the composite strain. Furthermore, it is assumed that a complete bond exists between the fibre and the matrix and that the crack faces are traction free.

It is shown that the separation strain energy release rate for fibre-reinforced composites can be obtained for cases with and without the existence of an interfacial crack. Numerical examples are presented and compared with results obtained in the literature by finite element analyses and from experimental tests. The comparison demonstrates the accuracy and the convergence of the model.     

Numerical analysis of fibre bridging and fatigue crack growth in metal matrix composite materials

The analysis of bridged crack configurations in unidirectional fibre-reinforced composites is relevant to a variety of crack growth problems, including the fatigue of metal matrix composites and the study of fibre failure in the wake of a bridged matrix crack. Details of numerical procedures for predicting fibre stresses and their effect on crack tip stress intensity factors are presented here to provide a useful overview of how standard bridging calculations are done. Results are presented and discussed in the context of predicting fatigue crack growth with fibre failure in metal matrix composites.     

Amplitude distribution acoustic emission signatures of unidirectional fibre composite hybrid materials

The failure processes in fibre-reinforced materials can be arranged in order of ascending energies as matrix crazing, interfacial delamination and fibre fracture. In glass fibre-reinforced plastics, delamination plays a significant part in the failure process. In carbon fibre-reinforced plastics, failure normally occurs by catastrophic fibre fracture with very little acoustic activity before the event. When two composites are combined within a single matrix to produce a fibre composite hybrid material, it is expected that a mixture of failure modes will occur, although this will be dependent on stress and strain levels. Acoustic emissions are the sounds generated by materials under stress: in practice, the amplitude of the acoustic emission signal is most commonly used as a measure of energy. Thus if a histogram of the number of acoustic events within a specific range of energies is formed, then a characteristic series of ‘signatures’ can be expected for each type of composite. The results of amplitude distribution acoustic emission tests are reported in this paper for unidirectional carbon fibre with glass fibre hybrid reinforced plastics, tested to failure in bending.     

Analysis of composite failure mechanisms using acoustic emission and ultrasonic scanning techniques

Acoustic emission data gained during testing of glass fibre-reinforced plastic tubular specimens under internal pressurization loading are presented, along with ultrasonic scanning traces of the test specimens and photomicrographs of samples cut from the test specimens. The data obtained by these tests indicate that fibre-reinforced composites exhibit two levels of failure: the first is that of matrix cracking, the second level is fibre fracture.     

A micromechanical model for kink-band formation: Part II—Analytical modelling

An analytical micromechanical model for kink-band formation in an unidirectional fibre-reinforced composite is developed. This is supported by the conclusions of experimental and numerical programmes (presented in Part I of this paper) and is based on the equilibrium of an imperfect fibre laterally supported by an elasto-plastic matrix. The model predicts the longitudinal compressive strength of the composite (in closed form), the deflection and main stress fields in fibres and matrix at different stages of kink-band formation, the kink-band width, and the orientation of the fibres at the onset of fibre failure.     

Co-operative microbuckling of two fibres in a model composite

Cooperative fibre microbuckling, a compressive failure mechanism in unidirectional fibre-reinforced composites, was studied in a model system composed of two polyamide fibres in a transparent silicone matrix. The transparent matrix permitted direct observation of fibre microbuckling during compression. In all cases fibres buckled in a sinusoidal pattern with a critical wavelength characteristic of the fibre diameter and the modulus ratio of the fibre and matrix as observed previously with single fibre composites. At smaller separation distances, the two fibres microbuckled co-operatively in the common plane. At larger separation distances, the fibres microbuckled non-co-operatively in different planes. A stress overlap criterion based on the in-plane shear stress is proposed for co-operative fibre microbuckling.     

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.     

3D constitutive model of anisotropic damage for unidirectional ply based on physical failure mechanisms

A 3D anisotropic continuum damage model is developed for the computational analysis of the elastic–brittle behaviour of fibre-reinforced composite. The damage model is based on a set of phenomenological failure criteria for fibre-reinforced composite, which can distinguish the matrix and fibre failure under tensile and compressive loading. The homogenized continuum theory is adopted for the anisotropic elastic damage constitutive model. The damage modes occurring in the longitudinal and transverse directions of a ply are represented by a damage vector. The elastic damage model is implemented in a computational finite element framework, which is capable of predicting initial failure, subsequent progressive damage up to final collapse. Crack band model and viscous regularization are applied to depress the convergence difficulties associated with strain softening behaviours. To verify the accuracy of the damage model, numerical analyses of open-hole laminates with different lay-up configurations under tension and compression were performed. The numerical predictions were compared with the experimental results, and satisfactory agreement was obtained.     

The role of interfaces and matrix void nucleation mechanism on the ductile fracture process of discontinuous fibre-reinforced composites

The role of fibre morphology, interface failure and void nucleation mechanisms within the matrix on the deformation and fracture behaviour of discontinuous fibre-reinforced composites was numerically investigated. The matrix was modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. For the matrix, two materials exhibiting identical strength and ductility but having different void-nucleation mechanisms (stress-controlled and strain-controlled) were considered and fibres were assumed to be elastic. The debonding behaviour at the fibre interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that in the absence of interface failure, for a given fibre morphology the void nucleation in the matrix is the key controlling parameter of the composite strength and ductility, hence, of the fracture toughness. The weak interfacial behaviour between the fibres and the matrix can significantly increase the ductility without sacrificing strength for certain fibre morphology and for certain matrix void-nucleation mechanisms.     

Mechanical behaviour of chemical vapour infiltration-processed two- and three-dimensional nicalon/SiC composites

The effect of two different fibre architectures on the mechanical properties of the Nicalon fibre-reinforced SiC composites processed by chemical vapour infiltration has been investigated. The microstructure, flexural strength, fracture toughness and failure mechanisms of both two-dimensional woven laminate and three-dimensional braided composites were characterized. It was found that the fibre placement in the preform will not only affect the infiltration of the SiC matrix, but also the mechanical property and failure behaviour of the composite. A strong, tough and damage-tolerant SiC matrix composite can be fabricated through the combination of a three-dimensional braided integrated fibre network and chemical vapour infiltration processing.     

Tensile failure mechanisms in synthetic fibre-reinforced mortar

The ultimate tensile behaviour of fibre-reinforced cementitious composites is closely related to its failure mechanisms which in turn are dependent on reinforcement parameters such as fibre characteristics and the fibre/matrix interface properties. Based on the direct tensile tests of mortar specimens reinforced with various synthetic fibres, this paper attempts to explain such relationships and to indicate directions towards more effective fibre reinforcement.     

Characterization of thermally oxidized carbon fibre surfaces by ESCA,wetting techniques and scanning probe microscopy,and the interaction with polyethylmethacrylate. Development of a biocompatible composite material

The development of biocompatible weft knitted carbon fibre-reinforced thermoplastics needs optimization of each composite component: fibre, matrix and interface. The material investigated was a composite of polyethylmethacrylate reinforced with a knitted and sized T300 carbon fibre. After chemical removal of the fibre sizing, the fibres were thermally oxidized at temperatures between 400 and 600°C. Angle-resolved photoelectron spectroscopy (ESCA) and Wilhelmy surface energy measurements have been used to describe the modification of the surface chemistry by thermal oxidation. The surface morphology, visualized by scanning probe microscopy and scanning electron microscopy, indicates an increased surface roughness. The interaction between fibre and matrix was investigated by observing the microscopic wetting behaviour of the thermoplastic at sinter temperature by the solid-body wetting technique. It is found that the strength and failure mechanisms of the knitted fibre-reinforced composite are determined by the interface properties.     

Mechanische Eigenschaften der durch thermisches Spritzen hergestellten unidirektionalen Faserverbundwerkstoffe

Mechanical properties of unidirectional fibre-reinforced composites made by thermal spraying Basic mechanical and technological properties of continuous fibre-reinforced metal-matrix composites made by thermal spraying are presented. The simple model of continuum theory has been used for the calculation of composite properties parallel to the fibre direction [1]. Taking into account the results obtained the theoretical analysis of composite failure modes and difference between praxis and theory is carried out. Problems concerning the properties measurements of thermally sprayed composites are discussed and their influence on the results is analysed as well. The comparison between the theoretically estimated composite strength and the experimental results is presented. The directions of future research work to improve the composite mechanical properties are also given.     

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part II: Parallel and anti-parallel fibre orientations

Finite element (FE) micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Two cases were studied, i.e. a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results relating to the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a micro-environment as part of a macro-environment, the displacement coupling technique was introduced. The contact analysis operates on both the macro- and the micro-level, applying node-to-node contact elements. The contact results, especially the contact pressure distribution, can characterize the real fibre/matrix micro-system. Displacement and strain results lead to explanations of fibre related phenomena, matrix shear effects, and fibre/matrix debonding events. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composite. For parallel fibre orientation, fibre/matrix debonding as a result of shear stresses at the interface, matrix shear type failure and fibre thinning are the dominant sliding wear mechanisms. If an anti-parallel fibre orientation is considered, matrix shear, tension/compression type fibre/matrix debonding and fibre thinning, associated with fibre cracking events, are the most dominant wear mechanisms. To study the wear mechanisms experimentally, diamond tip scratch tests were carried out, showing that the predicted failure events occur also in reality.     

Compressive failure of fibre-reinforced composites: buckling,kinking, and the role of the interphase

Recent experimental studies of compressive failure in fibre-reinforced polymeric composites have been analysed. It is shown that the parametric basis for most compressive strength models, i.e. pure plastic buckling controlled by matrix shear strength and initial fibre misorientation, is probably incomplete. It is argued that, instead, failure is triggered by the initiation of an unstable kink band prior to buckling instability, and that additional parameters (interfacial shear stress/strain; fibre strength) are responsible for this transition in mechanisms.     

复合材料单钉接头三维逐渐损伤破坏分析

针对纤维增强复合材料螺栓双盖板接头, 发展了面内静拉伸三维逐渐损伤模型。并对损伤累积过程中出现的四种基本损伤机理类型(纤维断裂、纤维-基体剪切、基体开裂、分层) 及其之间的相互关联性进行了分析模拟, 并能成功预测其接头层合板静拉伸强度、破坏模式及损伤与扩展的整个过程。同时用参数化设计方法, 对任意铺层、任意尺寸单钉接头进行建模, 使分析工作更加方便。用本模型计算的结果与文献[ 4 ]提供的9 种不同铺层尺寸接头实验结果进行了比较分析, 对比结果非常令人满意。      

Micro-mechanical theory of macroscopic stress-corrosion cracking in unidirectional GFRP

A micro-mechanical theory of macroscopic stress-corrosion cracking in a unidirectional glass fibre-reinforced polymer composite is proposed. It is based on the premise that under tensile loading, the time-dependent failure of the composite is controlled by the initiation and growth of a crack from a pre-existing inherent surface flaw in a glass fibre. A physical model is constructed and an equation is derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for mode I. Emphasis is placed on the significance of the size of inherent surface flaw and the existence of matrix crack bridging in the crack wake. There exists a threshold value of the stress intensity factor below which matrix cracking does not occur. For the limiting case, where the glass fibre is free of inherent surface flaws and matrix crack bridging is negligible, the relationship between the macroscopic crack growth rate and the apparent crack tip stress intensity factor is given by a simple power law to the power of two.     

Morphological investigations of injection moulded fibre-reinforced thermoplastic polymers

Fibres affect the matrix morphology in fibre-reinforced composites. Especially in semi-crystalline melts the fibres can act as nucleation rods causing a structure known as transcrystallinity. Transcrystalline structures are also found in injection moulded parts. They affect the structure of fracture surfaces especially in the case of long fibre-reinforced polymers with good fibre/matrix adhesion. Transcrystalline structures are usually generated and investigated in a microscope with a hot stage. For injection moulded parts this is an inadequate method. The morphological results presented here were obtained from plasma-etched inner surfaces and thin sections by means of light and contrast interference microscopy accompanied by scanning electron microscopy of fracture surfaces. Plasma etching is a well suited preparation technique to reveal both morphological superstructures and damage such as voids, debonding and fibre cracks in composites.