Properties of materials include mechanical properties (such as strength, hardness, toughness), thermal properties (conductivity), optical properties (refractive index), electrical properties (resistance) etc. Here, however, we shall concentrate only on mechanical properties which are most important in manufacturing processes and also in everyday life and we use these terms quite often. To understand the mechanical properties, it is useful to first understand the behaviour of the material when subjected to a force which causes deformation; this could be understood with the ‘stress-strain diagram. If the specimen is stressed beyond point B, permanent set takes place and we enter plastic deformation region. In the plastic deformation region, the strain does not get fully removed even with the removal of the force causing it. If the force is increased further, point ‘C’ is reached where the test specimen stretches even when the stress is not increased. This point is called yield point. Infact, there are two yield points C and D which are called upper and lower yield points respectively.