Abstract: The yielding spread mechanism and the variations of fracture parameters in loading are investigated by means of wide plate tests and bursting experiments on cracked vessels. The relationships between the stresses, strains and COD (σ) on these plates and vessels are presented and explained. In order to study assessment criteria the test results have been assessed with COD design curve in accordance with ⅡW. Doc. 741-74, in which the values of three point bending COD test were used. The conclusions are that the significance of defects have to be directly related to the overall behaviour and assessment of overall behaviour can be based o the gross strength and gross ductility.

Key words: 裂纹张开位移, 临界裂纹尺寸, 强度, 韧度