The three – dimensional state of stress at point (Figure 5.2a) may be separated into two parts. The state in Figure 5.2b is associated with the volume changes, so – called dilatations. In the figure Qm represents the mean stress or the octahedral stress Qoct, defined in section 3.16. On the other hand, the shape changes ,or distortions, are caused by the set of stresses shoe in Figure 5.2c. The dilatational strain – energy density can be obtained through the use of Eq.(5.6)by letting