The 4-node square linear elements seen in Figure 2 have a distance between nodes j and j + 1 equal to Δ Δ a . Consider a crack with its tip located at node j . Similar to the nodal release method, if the crack were to propagate one element length along the line of symmetry (parallel to the x 1 -axis) the crack opening displacement would be the displacement at the previous crack tip, i.e. u j and the force at the new crack tip ( j + 1 ) would be F j + 1 . Since the crack growth is assumed to be self-similar the displacement at node j after the crack propagates is equal to the displacement at node j − − 1 before the crack propagates. This same concept can be applied to the forces at node j + 1 and j . Utilizing the same method shown in the nodal release section we recover the following equations for energy release rate: