Incompressibility I cant understand some parts in a book The

Incompressibility

I can\'t understand some parts in a book.

\"The pointwise incompressibility condition i.s. the condition of vanishing dilatational strain, provides certain constraints to the assumed displacement finite element formulation. As an example the displacements for a four node rectangular plane strain element can be assumed as

These three conditions of constraint will limit the element motion to only two possible deformation modes which, of course, cannot represent more general state of strain.\"

My questions: I understand that these constraints limit the element motion, but why just two possible deformation modes are available?

How can I know which of them?

Solution