Fig. A-1.
The deformation of an infinitesimal 2-D element at a point in a solid body illustrated in the reference configuration and deformed configuration. X1 and X2 are the coordinate’s axes for the reference configuration. v1 and v2 are the principal axes of this 2-D element in the reference configuration. The original size of this element is L1 by L2 along the principal axes. After deformation, the element was reshaped into (L1 + ΔL1) by (L2 + ΔL2). The principal axes are rotated into N1 and N2 in the deformed configuration.