The effect of grain size and cross-head velocity on the lower yield strength during inhomogeneous discontinuous yielding in iron and steel

Abstract

The validity of the relation,$\underline {Ag}$, has been checked with experimental results on the lower yield strength, σ _aF of various irons and steels determined at room temperature over wide ranges in cross-head velocity,V _c, and grain boundary intercept or grain diameter,l. Good agreement was found with the proposed relation form ^*=4, wherem ^* is the dislocation velocity-effective stress exponent, and α=1/3. σ _i , the internal stress, ranged between 5000 and 13,000 psi andN ^αθK’ between 4.3×10⁻¹⁵ and 19.3×10⁻¹⁵. In the original relation, the value of α was one andK’ was given asK, which was assumed to be a material constant. The modification which became necessary indicates that possibly the density of mobile dislocations and/or the resistance of grain boundaries to propagating Lüder’s bands depend on Lüder’s band velocity and strain. The validity of the relation at very high and very low cross-head velocities or strain rates is discussed. The relation should apply in an intermediate range of cross-head or dislocation velocities wherem ^* should remain constant and should show an apparent decrease at both higher and lower velocities. The basis for this prediction is discussed and results are presented at low strain rates.