Materials are commonly classified as:
(1) Homogeneous and isotropic material: A homogeneous material implies that the elastic properties such as modulus of elasticity and Poisson's ratio of the material are same everywhere in the material system. Isotropic means that these properties are not directional characteristics, i.e., an isotropic material has same elastic properties in all directions at any one point of the body:
(2) Rigid and linearly elastic material: A rigid material is one which has no strain regardless of the applied stress. A linearly elastic material is one in which the strain is proportional to the stress.
(3) Plastic material and rigid-plastic material: For a plastic material, there is definite stress at which plastic deformation starts. A rigid-plastic material is one in which elastic and time-dependent deformations are neglected. The deformation remains even after release of stress (load).
(4) Ductile mid brittle material : A material which can undergo 'large permanent' deformation in tension, i.e., it can be drawn into wires is termed as ductile. A material which can be only slightly deformed without rupture is termed as brittle.
Ductility of a material is measured by the percentage elongation of the specimen or the percentage reduction in cross-sectional area of the specimen when failure occurs. If L is the original length and L’ is the final length,
% increase in length = L-L/L x 100
The length l' is measured by putting together two portions of the fractured specimen. Likewise if A is the original area of cross-section and A’ is the minimum cross sectional area at fracture, then.
% age reduction in area = A-A/A x 100
A brittle material like cast iron or concrete has very little elongation and very little reduction in crosssectional area. A ductile material like steel or aluminium has large reduction in area and increase in elongation. An arbitrary percentage elongation of 5% is frequently taken as the dividing line between these two classes of material.
