When colliding and grounding of ship, fracture may occurs in the hull structural member, which may cause serious accidents, such as environmental pollution and the loss of lives/cargo due to damage of the ship. To examine the safety of the ship for the collision using experiments will take much time, cost, and effort. Therefore, it is necessary to confirm it by a numerical calculation.

In the collision analysis using the finite element method (FEM), it is judged that the rupture occurs if the equivalent plastic strain of an element exceeds the critical value, and then the element is generally erased. In the meantime, it is known that the equivalent plastic strain at the ductile crack initiation will decrease with an increase of stress tri-axiality. Therefore, the accuracy of fracture analysis is expected to be improved by taking account of the tri-axial effect on fracture strain.

In this paper, to investigate the effect of stress tri-axiality on the equivalent plastic strain at the ductile crack initiation, the tensile tests of round bar with notch of different acuity were carried out for the three kind of steel with different strength. And, the rupture calculations for the specimens with a stress concentration part were performed with and without taking account of the fracture strain under tri-axial stress condition. Moreover, the calculation accuracy and the effectiveness of such calculations were clarified by comparison of experimental results.