Abstract: This paper discusses the energy release of crack and cut in propagation under general boundary conditions and points out that the use of force potential can result in an error under these conditions. Then, from the global energy integration with no additional hypothesis, formulae by Griffith and by Sih and Liebowitz are proved strictly and directly. Therefore, it is proved that Eftis in ref. 2 refuted Griffith formula and returned to Griffith former formula in 1921 wrongly. And it is shown that the not ve...