Abstract
The resistance to shear instability and subsequent flow localization in ultrahigh strength (UHS) steels is dependent upon second-phase particle dispersions and the matrix strain hardening. The effect of the interparticle spacing λ to the geometric mean particle radius R ratio on the shear instability strain of UHS AISI 4340 steel is discussed. Experimental results indicate that a linear relationship exists between shear instability strain and this λ/ R ratio. Microvoid nucleation softening associated with second-phase particles appears to be the dominant destabilizing event leading to fracture. The effect of the hydrostatic stress is also discussed. Experimental results of mode I and II fracture toughness testing are compared. A high hydrostatic tension field was found to be the cause for the lower mode I critical stress intensity factor ( K Ic ) than mode II ( K IIc ). The high hydrostatic tensile stress field induced early microvoid nucleation which promoted flow localization leading to fracture. However, both mode I ( K Ic ) and mode II ( K IIc ) critical stress intensity factors directly relate to the critical particle parameter λ/R 1 2 .
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