In each of the first three load cases a uniform compression deformation of 0.01 mm is applied. All nodes on the two boundary surfaces which limit solid in x-dimension are connected with couplings in their y-displacement (y(0,y,z) = y(ux,y,z)). Amount and direction of their y-displacements are thus equal. The same nodes have to be connected by constrain equations in their x-displacement (x(0,y,z)+x(ux,y,z) = 0), resulting in an equal amount of displacement in the opposite direction.
All nodes on the boundary surfaces in which limit the solid in y-dimension are coupled in x-displacement and connected by constrain equations in y-displacement.
Furthermore, the nodes are coupled in z-displacement, but the nodes at the upper and lower surfaces of the unit cell are not coupled.
These periodic boundary conditions according to the homogenisation theory ensured that the deformed surfaces would still fit perfectly together. The homogenisation theory assumes that the unit cell is imbedded within an infinite array of identical unit cells and displacements or forces are imposed at infinity, then every unit cell will deform identically.
