The heart of crystalline materials

When considering rocks in the natural environment, we do not tend to talk of them in terms of materials. But as soon as a rock is used to build a wall for example, we term it a material. Talking about a material is to evoke a function, if only implicitly. When the primary function is mechanical, for example regarding the strength required in relation to a load or a moment, then the material is known as structural. For all other functions, we will refer to a functional material.

Understanding the mechanical behaviour of materials demands knowledge of the laws that link stress (\(\sigma\)) to deformation (\(\epsilon\)) according to a conventional model of the relation of cause (\(\sigma\)) to effect (\(\epsilon\)).

Deformations are either time dependent or independent. They can be reversible or irreversible. The structural materials that we will study in greater detail therefore deform either elastically or plastically. They can even, when subjected to moderate loads below that they can statically sustain, fracture via processes linked to their inner microstructure and the defects they contain. The table below provides a simplified summary of the different types of deformations encountered in practice.