There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

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Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Particulate reinforced metal matrix composites — a review

The impact resistance of composite materials — a review

Phase structure and adhesion in polymer blends: a criterion for rubber toughening

Inspired by the unique and efficient wound healing processes in biological systems, several approaches to develop synthetic polymers that can repair themselves with complete, or nearly complete, autonomy have recently been developed. This review aims to survey the rapidly expanding field of self-healing polymers by reviewing the major successful autonomic repairing mechanisms developed over the last decade. Additionally, we discuss several issues related to transferring these self-healing technologies from the laboratory to real applications, such as virgin polymer property changes as a result of the added healing functionality, healing in thin films v. bulk polymers, and healing in the presence of structural reinforcements.

Self-healing polymers and composites

Epoxy resins are toughened significantly by a dispersion of rubber precipitates. Microscopic examinations of propagating cracks in epoxy-rubber composites reveal that the brittle epoxy matrix cracks, leaving ligaments of rubber attached to the two crack surfaces. The rubber particles are stretched as the crack opens and fail by tearing at large, critical extensions. This fracture mechanism is the basis of a new analytical model for toughening. An increase in toughness (ΔGIC) of the composite is identified with the amount of elastic energy stored in the rubber during stretching which is dissipated irreversibly (e.g. as heat) when the particles fail. The model predicts the failure strain of the particles in terms of their size. It also relates the toughness increase to the volume fraction and tearing energy of the rubber particles. Direct measurements of the tearing strains of rubber particles, and toughness data obtained from epoxy-rubber composites, are in good agreement with the model. The particle-stretching model provides a quantitative explanation, in contribution to existing qualitative theories, for the toughening of epoxy-rubber composites.

A model for the toughness of epoxy-rubber particulate composites

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

The fracture micromechanics and underlying physical processes of fracture in Al2O3-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique The toughness is found to increase with grain size Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening The conclusion is supported by hysteresis measurements

R-curve behaviour of Al2O3 ceramics

The processes of debonding and pull-out in fibrous composites are described. Models predicting the debond length and the probability distribution of pull-out lengths of fibres and bundles are derived. These lengths are functions of the fibre, matrix and interface properties. Prediction is then compared with experiment and a simple relationship between pull-out and debond lengths is found. An understanding of the debonding and pull-out processes is important because they affect the fracture toughness of fibre composites.

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Peter W. R. Beaumont

Papers

A model for the toughness of epoxy-rubber particulate composites

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

R-curve behaviour of Al2O3 ceramics

Debonding and pull-out processes in fibrous composites

Damage mechanics of composite materials: I— Measurements of damage and strength