Effect of Stress Triaxiality and Strain Rate on Ductile Fracture Initiation in Steel

Abstract

It is widely known that the critical strain to ductile crack initiation depends on the stress triaxiality. And the ductile crack may occur at the strain concentration region in the structural member unless initial crack by large deformation. Such large deformations occurs during ship crash, seismic condition, etc. And then, strain rates are usually high. But, the strain rate effect to the ductile crack initiation is not known.
To study the effect on ductile crack initiation condition, the axisymmetrically notched bar tensile tests were conducted under various tensile speed for SM400B, SM490A, SM490B and SN490B steels. And critical strains to initiate ductile crack were evaluated. To estimate the stress - strain states during dynamic tensile test, the thermal mechanically coupled FE-analysis (MARC K7.2) were used.
The results showed the strain rate effect on the critical strain to initiate ductile crack is small. And the results also suggested that if the load is controlled by displacement, the higher yield ratio steel is advantageous for the ductile crack initiation.