It has been observed that an external force acting on an equilibrium body is countered by internal forces created inside the substance.
Therefore, if a bar is subjected to a uniform tension or compression, also known as a direct force, that is evenly distributed across the cross-section, the internal forces created are also distributed uniformly, and the bar is said to be under a uniform direct or normal stress, with the stress being defined stress, .
Thus, depending on the kind of the load, which is expressed in newtons per square meter (N/m2) or multiples thereof, stress sigma may be either tensile or compressive. The stress at each location is determined by the limiting value of since goes to zero in some loading situations where the stress will vary over any particular segment.
A material's shape-changing is referred to as deformation.
Deformation must be connected to spatial variation or gradient of displacement as homogenous displacement of material locations does not result in deformation.
As a result, a displacement gradient tensor serves as a description of deformation.
But even though rigid body rotation has nothing to do with deformation, it is included in this displacement gradient.