Abstract
The plastic extension of zinc crystals in the region just beyond the elastic limit is found to obey the empirical law, η=btm in which η is the resolved shear strain, t is the time, m is a constant with a value close to 0.5 for initially unstrained crystals, and b is dependent on the resolved shear stress. Application of a theory due to Koehler shows that b should be related to stress (σ) in such a way that a plot of ln(mbσ2/Δσ) against 1/σ should be a straight line of slope −cG. In this, Δσ is the stress increment from one creep curve to the next, G is the shear modulus, and c is the concentration of anchor points which determines the lengths of segments of dislocation lines which generate according to the Frank-Read mechanism. The experiments are in fair agreement with this theory and values of c obtained are roughly equal to the concentrations of metallic impurities. The small amount of dissolved gases appears to play no significant part in the creep process.
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