Figure 3 shows the von Mises yield surface in the three-dimensional space of principal stresses.
12.
The principal stresses and principal directions characterize the stress at a point and are independent of the orientation.
13.
Isochromatics are the loci of the points along which the difference in the first and second principal stress remains the same.
14.
Basics of their mechanisms are similar except the principal stress orientation applied is rotated from perpendicular to parallel to the ground.
15.
Thus the maximum principal stress in the beam may be neither at the surface nor at the center but in some general area.
16.
One set of such invariants are the principal stresses of the stress tensor, which are just the eigenvalues of the stress tensor.
17.
However, when one of the principal stresses becomes smaller ( or larger ) than the others the material is subject to shearing.
18.
The principal stresses can be combined to form the stress invariants, I _ 1, I _ 2, and I _ 3.
19.
Functions of the principal stresses, such as the yield function, can be represented by surfaces in "'stress space ".
20.
A common improvement of this model is to combine Coulomb's friction hypothesis with Rankine's principal stress hypothesis to describe a separation fracture.
