Recent indications of a direct proportionality between plane strain fracture toughness KIc and the strain hardening exponent n provide a new basis for comparing the initiation with the propagation of a crack. A mild steel from the University of Illinois wide plate studies was employed because of its extensive crack propagation data. With this, the proportionality KIc/n was established at low temperatures. The n values were then measured at service temperatures and also at extremely high strain rates, to 6000/sec, with bar impact loaders. The additional speed range delineates a series of minimum fracture toughness levels even at temperatures above the NDT or CV15. A plot of adiabatic minima KIc(Q) as pertinent to the crack arrest temperature and isothermal KIc(T) to crack initiation permits a more quantitative interpretation of the Pellini fracture diagram.

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