If resistance to crack growth always increased with crack velocity, then its measure at nil velocity, Gc or GIc, would always represent minimum toughness and thus a safe criterion. The practical difficulty arises if an increased crack speed, or loading rate of a fixed crack, results first in a decrease in R, possibly to some minimum, prior to its increase for the “stable” propagation balance. Arrest of such a crack will require reduction of the crack driving force, G, at least to minimum R. In strain-rate and temperature-sensitive materials this minimum can lie far below initiation levels. Even in relatively high-strength materials, speed-thermal effects can be large.
The onset and arrest of rapid fracture provide relatively abrupt measurement points suitable for crack toughness evaluation. An understanding of these behaviors can be sought through study of the influence of plastic flow properties. Use of strain-rate sensitive materials over a wide range of temperatures and strain rates permits study of the influences of flow properties without alteration of the inherent flaws. A correlation of rising-load KIc values with the strain-hardening exponent, n, suggests that the onset of fast fracture is controlled to a substantial degree by a tensile instability with a simple relationship to the strain-hardening exponent. The limited information available suggests that crack-arrest conditions can be predicted on the basis of adiabatic values of n at high strain rates.