Stretch and Rotation Tensors. Strain
Consider the (pointwise) polar decomposition
of the deformation gradient F into a rotation R and positive-definite symmetric tensors U and V; using terminology motivated in §7.3, we refer to U as the right stretch tensor and to V as the left stretch tensor. The tensors U and V, which have the explicit representations
are useful in theoretical discussions but are often problematic to apply because of the square root. For that reason, we introduce the right and left Cauchy–Green (deformation) tensors C and B defined by
Then, by (7.1),
For future reference,we list the properties of the stretch and Cauchy–Green tensors:
A tensor useful in applications is the Green–St. Venant strain tensor
Note that E vanishes when F is a rotation, for then F⊤F = 1. This property of E is often adopted as one necessary for a tensor to qualify as a meaningful measure of strain.
Next, by (M1) and (M2) on page 65 and (7.3) and (7.6),
(M3) U, C, andEmap material vectors to material vectors;
(M4) VandBmap spatial vectors to spatial vectors;
(M5) Rmaps material vectors to spatial vectors.
To verify (M3), consider F⊤F: (M1) implies that F maps a material vector fR to a spatial vector FfR and, by (M2), this spatial vector is mapped back to a material vector. Thus C = F⊤F maps material vectors to material vectors and since UU = C, with U symmetric, U must also map material vectors to material vectors.