# what is the infinitesimal strain theory?

In the field of continuum mechanics, the infinitesimal strain theory (also referred to as the small displacement theory, the small deformation theory, and the small displacement-gradient theory) is a mathematical description of the deformation of solid substances. Here, the displacements of the particles that constitute the solid are assumed to be extremely small – much smaller than any dimension of the body which is relevant. Therefore, the constitutive properties of the solid material and the geometric properties of the solid material (examples of which include the stiffness of the solid and the density of the solid) and any given point in space can be assumed to not experience any change as a consequence of the deformation of the solid.