A body subjected to a general load state exhibits macroscopic and measurable displacements.
Knowing the relationship between the applied loads and the induced displacements is of significant interest in mechanical engineering. This is possible thanks to the definitions of the concepts of stress and strain. The existing relationship between the states of stress and strain is material dependent and characteristic of each material, and it is given by an expression, more or less complex, known as material constitutive equation.
Stress and strain are both tensorial (tensile) magnitudes, so in general they are described by means of tensors (stress and strain) that 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).