13th International Conference on Fracture June 16–21, 2013, Beijing, China 9 paper took the transformation as follow: True stress True Stress (12) True strain True Strain (13) The above equations could directly measure the responses in the range of plastic-flow, the true stress and strain could be calculated by the equations before the strain developing to necking. However, after necking, the material exceeded the elastic limit, which caused the geometric size was great different from the initial condition. The plane of the specimen was in a complicated three dimensional stress states, and the strains were nonuniform in the gauge length of the specimen. In this situation , it would bring some errors to calculate the true stress-strain. To get more accurate true stress-strain curve, it generally used the semi-empirical and semi-analytical Bridgman formula, which was deduced to average longitudinal stress correction formula as follows: average = 2R a + In + a 2R σ σ （1 ） （1 ） （14） where, R denotes the radius of the necking surface, a denotes the minimum radius after necking and average σ denotes the average longitudinal stress of the minimum cross section. The present paper provided the CTOD toughness assessment for the high strength steel plate, the more accurate results need more accurate true stress and strain while there are several method to get the true stress-strain curve after necking as Photographs method[11], FEM[12], etc. 6 Conclusions As the steel plates applied in the ocean engineering structure becomes more and more thicker, the acceptable CTOD value of different Standards are applied to prevent the structural safety and stability. Such as the minimum acceptable value is 0.15 mm in the DNV-OS-C401-2008, while it is 0.13 mm in the API RP2Z standard. This paper takes the single edge fatigue precrack of CTOD specimen as a flaw under the guidance of BS7910 Standard, plots the FADs of the the welding seam center. The failure boundary value is 0.576, which are applied in the acceptable value of CTOD. The average critical CTOD value is 0.06 mm, which is calculated by r = / mat δ δ δ Ι and far lower than the value specified by the Standards. As the true stress-strain curve is calculated within the elastic stage, which would cause the errors. If the true stress and strain are measure by photography with single specimen, the true stress and strain values would larger than the values obtained by the equations. It would caused the critical value of CTOD would larger than 0.06 mm, the results should be corrected. In the meanwhile, the present CTOD test ensures the straightness of the front crack with the partial compression method, which would cause the results of test much more conservative[13]. According to the critical CTOD value calculated above, it suggests that the CTOD acceptable value of heavy plates could be modified lower appropriately based on value specified by Standards.
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