Short fatigue crack behavior of carbon structural steel under tension-torsion loading

Fatigue short crack replica tests of a carbon structural steel, i.e., LZ50 steel, were carried out under tension-torsion loading using MTS electro hydraulic servo machine. Totally eight specimens with smooth axial hourglass shape were tested. The results show that short fatigue cracks generally initiate from the ferrite grain boundary or in the ferrite grain. In the microstructural short crack (MSC) stage, the crack grows slowly, but the growth rate will decrease when the crack encounters microstructural barriers, such as the ferrite grain boundary and the rich pearlite banded structure. In the physical short crack (PSC) stage, the crack breaks through the rich pearlite banded structure and meanwhile the dominant effective short fatigue crack (DESFC) has established its dominant position. As a result, the size and the growth rate of the crack increase rapidly. Based on the fatigue failure mechanism and the safety in tail region predictions, the statistical evolution of the DESFC size and the life fraction was analyzed respectively by comparing six commonly used statistical distributions. The statistical results show that the extreme minimum value (EMV1) distribution is a good assumed distribution for both the DESFC size and the life fraction.