Internal tension (or internal stress) is a measure of the contact forces exerted between the internal parts of a continuous three-dimensional body through the relative separation surface. It is defined as the contact force per unit area, ie it is the limit of the ratio between the acting force and the area of the surface on which it acts.
It is a vector quantity and its unit of measure is the Pascal (symbol Pa). In technical practice, megapascals (MPa) or gigapascal (GPa) are more commonly used.
The concept of tension is based on the concept of continuous and was introduced around 1862 by Cauchy. It plays a fundamental role in all the mechanics of the continuous as it characterizes the state of internal forces (internal stresses) of a body and, consequently, the behavior of the material constituting the body, that is, how it deforms under the action of forces applied.
In general, the stress depends both on the point of the volume of the solid in which it is measured and on the direction in which it is measured. In some very simple cases, like in a prismatic beam loaded only with an axial force, the stress, far from the constrained sections, is constant and equal to the applied force divided by the area of the cross-section to the axis.
The stresses are therefore due to two causes: the need to preserve the balance of the body subjected to external loads (moment forces) and the need to preserve the congruence (ie the continuity) of the body.
The stresses related to equilibrium are independent of the characteristics of the material, depending exclusively on the geometry of the structure and the applied external loads. The congruency stresses, on the other hand, occur only if the structure is in hyperstatic equilibrium or if it is intrinsically hyperstatic, as in the case of our plants, which, since they are integrated, contract an undeformable constraint.