The mechanical properties of composite materials usually depend on structure. Thus these properties typically depend on the shape of inhomogenities, the volume fraction occupied by inhomogenities, and the interfaces between the components.The strength of composites depends on such factors as the brittleness or ductility of the inclusions and matrix.
For example, failure mechanisms in fiber-filled composites include fracture of the fibers; shear failure of the matrix along the fibers; fracture of the matrix in tension normal to the fibers or failure of the fiber-matrix interface. The mechanism responsible for failure depends on the angle between the fibers and the specimen's axis.
If a mechanical property depends on the composite material's orientation, the property is said to be anisotropic. Anisotropic composites provide greater strength and stiffness than do isotropic materials. But the material properties in one direction are gained at the expense of the properties in other directions. For example, silica fibers in a pure aluminum matrix produce a composite with a tensile strength of about 110,000 psi along the fiber direction, but a tensile strength of only about 14,000 psi at right angles to the fiber axis. It therefore only makes sense to use anisotropic materials if the direction that they will be stressed is known in advance.
Isotropic material are materials properties independent of orientation. Stiff platelet inclusions are the most effective in creating a stiff composite, followed by fibers, and then by spherical particles.
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