In the case of prescribed displacement, the strain energy can be expressed in terms of the specified displacement and the crack surface Ω Ω ( q , s ) , and the change in this strain energy is only affected by the change in fracture surface area: δ δ Ω Ω = ( ∂ ∂ Ω Ω / ∂ ∂ s ) δ δ s . Correspondingly, the energy release rate in this case is expressed as