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.
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.
Cylindrical test samples were compressed statically and dynamically at temperatures ranging from +100 to -195°C and the yield stress, form of yield and surface markings observed. A Hopkinson bar and throw -off rod and electric resistance strain gauges were used for stress determinations under dynamic loads. The stress necessary to in intiate yield rises steeply with fall in temperature and is accompanied by a change in mechanism of deformation from normal slip to the formation of Neumann lamellae at a critical temperature. The critical temperature is higher under dynamic loading, but there appears to be a critical stress for the formation of Neumann lamellae, whether caused by high rates of loading or low temperatures.
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