Experimental and analytical investigations into Crack Tip Opening Displacement, CTOD, were conducted to demonstrate the relationship between BS7448-CTOD and ASTM E1290-CTOD. CTOD test results showed that ASTM-CTOD was occasionally much lower than BS-CTOD both in single edge notch bend specimens and in CT specimens for low-strength structural steels, and this tends to be more remarkable in CT specimens. In addition, the analytical results of simplified elastic-plastic facture parameter calculation using the Electric Power Research Institute scheme demonstrated that the ratio of ASTM-CTOD to BS-CTOD was not constant but varied according to CTOD changes. Material factors such as the yield stress, the strain hardening exponent, specimen size and configurations influenced the CTOD ratio, and low strain hardening exponent in the Ramberg-Osgood relation and CT specimen configuration significantly decreased the CTOD ratio. An equation that transforms BS-CTOD into ASTM-CTOD is proposed in this study. This equation gives a good estimate of ASTM-CTOD from BS-CTOD.

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