In mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are produced between the particles in the body as a reaction to external forces applied on the body. In materials without microstructure (these are materials whose microstructure does not play an important role in the mechanical deformation), these internal forces are distributed continuously within the volume of the material body, and result in deformation of the body's shape. Beyond certain limits of material strength, this can lead to a permanent change of shape or physical failure. The dimension of stress is that of pressure, and therefore the SI unit for stress is the pascal (Pa). A three-dimensional stress eld in a material can be represented as a sym- metric matrix of the following form:

where the diagonal terms represent tensile or compressive stresses and the o-diagonal terms represent shear stresses.