For brittle materials such as glass, the surface energy term dominates and G ≈ ≈ 2 γ γ = 2 J/m 2 . For ductile materials such as steel, the plastic dissipation term dominates and G ≈ ≈ G p = 1000 J/m 2 . For polymers close to the glass transition temperature, we have intermediate values of G between 2 and 1000 J/m 2 .