A body subjected to a state of general load exhibits macroscopic and measurable displacements.
Knowing of the relationship between applied loads and the induced displacements is of great interest in mechanical engineering. This knowledge is made possible through the definitions of the concepts of stress and strain. The existing relationship between the states of stress and strain is material dependent and characteristic for each material. It is calculated through an expression known as material constitutive equation.
Stress and strain are both tensorial (tensile) magnitudes. Generally speaking, they are described by means of tensors (stress and strain) which are defined by a given number of independent components (3 normal components and 3 shear components).
In order to simplify the study of materials behaviour and to work with relatively simple constitutive equations, it is common to speak in terms of equivalent stresses and strains. These are scalar magnitudes obtained from the tensorial variables when the material is tested under controlled and simplified conditions (for example, uniaxial testing).
The real behaviour of the materials can be very complex. Approximate or ideal models are used to simplify the resolution of the problems.