Strain Energy Release Rate (G)

How do I calculate the strain energy release rate (G1) for an edge crack by using strain energy (U). Using the formula G1=d(U)/d(Area) and with constant thickness and plane stress case: G1=(1/thickness)xd(U)/d(a) where: U- strain energy a- crack length G1-strain energy release rate for mode 1 crack propagation (opening mode).

Thanks!!