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In most contemporary theories the deformation of metals is treated as a time-dependent or viscous process. A quasi time-independent character is sometimes introduced by means of an athermal back “stress” from long-range obstacles, but only at T = 0 K or in other limited temperature ranges is the deformation predicted to show time-independent behavior. Several experimental results, mainly on pure face-centered-cubic metals, are in disagreement with this theoretical approach. The most important results are that (1) time-independent strain (loading strain) has been measured on sudden increase of stress during creep deformation, even for very small stress increments; and (2) the strain rate ratio measured in a load relaxation experiment, R = ˙εr/˙εℓ, is consistently less than unity and increases toward unity as the stress approaches saturation. These results are reviewed and new data from load relaxation on lead and the superplastic Pb-Sn eutectic are presented. It is the author's conclusion from these data that time-independent deformation may be more important than is normally acknowledged. Thus, in future efforts at modeling plasticity, the possibility of a dual time-dependent, time-independent nature ought to be considered.
plastic flow, dislocation theory, load relaxation, recovery creep, viscous creep, thermal activation, mechanical activation
Professor, University of British Columbia, Vancouver, British Columbia