The usual criterion for cavitation is that cavities will form in a liquid when and where the pressure falls below a critical value. In the ideal case, the cavitation threshold is the vapor pressure. The pressure in an incompressible viscous liquid is not a thermodynamic or material property; it is the average stress (actually the negative of the average stress, which is positive in tension). The viscous part of the stress is proportional to the rate of strain, which has a zero average with positive and negative values on the leading diagonal in the principal coordinates. It follows that in motion the liquid will develop stresses that are both larger and smaller than the average value. The theory of stress-induced cavitation seeks to relate the fracture or cavitation of a liquid to its state of stress rather than to its average stress. This kind of theory requires that the state of stress be monitored in the evolving field of motion to determine when and where the liquid will fracture. The theory can be thought of as an application area for Navier–Stokes fluid dynamics that can be studied by VPF when the flows are irrotational or nearly irrotational. The link between the theory of stress-induced cavitation and VPF is the fact that viscous stresses can be computed on irrotational motions.