Effects of martensite morphology on dynamic torsional behavior in dual phase steels

Investigations on the dynamic torsional characteristics of the two dual phase steels reveal that fracture of the IQ-specimen proceeds a typical ductile fracture mode irrespective of strain rates. The SQ-specimen, on the other hand, is predominated by cleavage fracture under quasi-static loading, but shows ductile fracture with varying deformation and fracture mechanisms under dynamic loading. The SQ-specimen's ductile fracture under dynamic torsional loading is associated with deformation of martensites themselves, which releases the deformation constraint of ferrites and leads to an increase of fracture shear strain. It can be inferred from the dynamic shear stress-shear strain curves that adiabatic shear bands are easily formed in the IQ-specimen, whereas they are not in the SQ-specimen. This is because the initiation of microcracks at the martensite/ferrite interfaces and their propagation are caused by the difference in the deformation behavior between martensites and ferrites.