Promising aspects of COD concept are stressed in comparison with the limited applicability of Griffith-Irwin criterion. Overall displacement of a deeply notched tension coupon is when unloaded nearly equal to the crack tip plastic opening. The critical COD-value φ_c, can, therefore, be approximately deemed as the crosshead displacement at the initiation of brittle fracture, if the elastic component is subtracted. On this basis, a graphical method is proposed for determining φ_c on the curve of load vs. crosshead displacement. The values of φ_c determined in this way on various stages beyond the general yielding show an Arrhenius-type temperature dependence, which is also applicable to the theoretically estimated values of φ_c for the fracture initiation below the general yielding. A proposal of basic approach to the problem of thickness effect is supplemented.