Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamic conditions, aimed at elucidating the fundamental mechanisms of shear localization underlying both adiabatic shear localization and fracture processes. Experiments were also devised to study the effect of hydrostatic pressure and austenitizing temperature on the critical strain to localization. Experimental evidence strongly suggests that strain localization in the steels investigated is driven by microvoid softening controlled by nucleation at 100 nm scale particles. This is supported by the observed pressure dependence of the instability strain, enhanced resistance to shear instability with particle dissolution, and direct observation of microvoids at these particles in deformed material. For the steels investigated with approximately equivalent strength levels, a direct correlation between the crack extension force and shear instability is demonstrated. Consequently, both fracture toughness and shear localization are dependent on the size, type, and distribution of second phase particles.
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