A simple scheme for classifying composite materials consists of three divisions:
1. Particle-reinforced composites.
Particle-reinforced composites are subdivided into large-particle reinforced and dispersion-consolidated. This distinction is based on the consolidation or reinforcement mechanism.
The term “large” is used to indicate that the matrix-particle interactions cannot be described at the atomic or molecular level, but by continuum mechanics. In most composite materials the dispersed phase is harder and tougher than the matrix and the reinforcing particles tend to restrict matrix motion in the vicinity of each particle. In essence, the matrix transfers some of the applied stress to the particles, which carry some of the load. The degree of strengthening or enhancement of mechanical behavior depends on the cohesive strength at the matrix-particle interface. A composite material with large particles is concrete, consisting of cement (matrix) and sand or gravel (particles). The smaller the particles are and the better distributed they are in the matrix, the more effective the reinforcement is. In addition, the volume fraction of the two phases influences the behavior; mechanical properties increase with increasing particle content.
The particles of dispersion-consolidated composites are usually much smaller: the diameters range from 10 to 100 nm. The matrix-particle interactions that lead to consolidation occur at the atomic or molecular level. While the matrix bears most of the applied load, the small dispersed particles hinder or prevent the displacement of dislocations. This restricts plastic deformation in such a way that yield strength, tensile strength and hardness are increased.
2. Structural composites
They combine composite and homogeneous materials whose properties depend, in addition to the materials they are made of, on the geometry of the design of the structural elements. They can be classified as follows:
Sandwich-type structures: composed of core and caps, they allow to improve mechanical properties, but without an excessive increase of their weight. This type of structure improves thermal and acoustic insulation.
Monolithic structures: parts with a more or less complex geometry, formed by superimposed fabrics with certain orientations that allow specific characteristics to be obtained. These types of parts are designed to withstand the highest structural loads.
3. Fiber-reinforced composites
Technologically, composite materials with dispersed phases in the form of fibers are the most important. Fiber-reinforced composites are often designed with the aim of achieving high strength and stiffness at low density. These characteristics are expressed by the parameters specific strength and specific modulus, which correspond, respectively, to the ratios of tensile strength to specific weight and modulus of elasticity to specific weight. By using low-density materials for both the matrix and the fibers, fiber-reinforced composites with exceptionally high specific strengths and moduli are produced. Fiber-reinforced composites are subclassified by fiber length.
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