Properties for Composite Materials

Improving the properties and performance of materials by reinforcing them with different types of stiffer and/or stronger phases, such as particles or fibres, leads to the class of material known as composites. This approach was first exploited during natural developments (both living plants and organisms) and later by mankind e.g. the use by Egyptians when making clay building bricks reinforced with straw to improve handling and performance, and when using gravel to reinforce cement forming a much stronger concrete material. Over the centuries more and more sophisticated composites have developed, responding especially to the advent of higher performance fibres (for high stiffness, strength and/or high temperature resistance). Even greater benefits canarise by the manufacture of composites reinforced with hollow and/or multi-coated inclusions, and nanotubes. The simplest inclusion geometry for matrix reinforcement is a set of spherical particles which might exhibit a range of radii. When the matrix and reinforcement is homogeneously mixed, the resulting material has improved properties which are usually considered to be isotropic. Another simple geometry uses aligned continuous fibres to reinforce an isotropic matrix forming a material that is anisotropic such that the properties in the fibre (or axial) direction differ from transverse properties in the plane normal to the fibre direction. This type of material is known as unidirectionally fibre reinforced composite where the transverse stiffnesses and strengths are usually much lower than the stiffness and strength in the fibre direction. To overcome this significant practical problem, composite laminates are considered where a stack of unidirectional composites known as plies are bonded together where the fibres in each ply in the laminate are aligned in a direction that varies from ply to ply. Laminates are often weak under compression because of a damage mode known as delamination where debonding occurs at or near the interfaces between the various plies. To overcome this practical problem, woven or stitched fibre architectures are used. Such composites can be analysed effectively only if numerical methods are used. This topic will not, therefore, be considered in this book as the intention is to focus here on the development and use of analytical methods. Composites, often made where the reinforcement is a set of short fibres which are either aligned in a given direction or are randomly oriented, will also not be considered in this book.--