The ductile fracture mechanism of a round bar of S 15 CK with a hole is investigated in order to obtain an understanding of the extension behavior of a ductile surface crack from a hole. A ductile crack starting from a hole grew stably with mixed mode fracture which combined tensile mode and shear mode fracture, and the nondimensionalized growth rate of crack for strain increment was proportional to nondimensionalized crack size l/D independent of the hole shape, which can be written as d(l/D)/dε∝l/D where l is crack length, D is the diameter of a specimen and ε is the true strain. Hence, the extension behavior of a ductile crack depends on crack size in this investigation. The ductile fracture originated in a surface is different from the cup-and-core type in macro-scopic features.
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