Failure experiments were carried out through a stretch-bending test system for advanced high strength steels,i.e.dual-phase(DP)steels and martensitic steels(MS).The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries.Two failure modes were observed during the experiments.As a result,critical relative radii(the ratio of inner bending radius Rto sheet thickness t)for DP590 and DP780 steels were obtained.The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels,and curve of DP980 in different thicknesses does not coincide well.High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force.The unique character of high strength martensitic steel(1 500MS)is that no shear fracture is found especially over small bending radius(R=2mm)under the same experimental conditions.Microstructure analysis indicates that there are obviously elongated grains on shear fracture surface.It shows smaller diameter and shallower depth of the dimples than the necking failure.
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