Abstract
The strain rate sensitivity index, m, of Al-33%Cu and Al-27%Cu-5%Si alloys has been measured by the two methods, (a) the differential cross-head speed method and (b) the strain rate change method. the m-value by the method (a) is dependent strongly on strain and decreases with increasing strain. The m-value by the method (b) is dependent slightly on strain, but strongly on the width and the sense of the change from a given strain rate, \dotε1, to another one, \dotε2. The dependence of m on the width, J=∂m⁄∂\sqrt(lnN)2 where N=\dotε2⁄\dotε1, is positive or negative depending on whether the strain rate is lower or higher than the strain rate having the maximum m-value on the m-ln\dotε curve when N=2(the reference curve of m). At the strain rate having the maximum m-value, the value of |J*| shows the minimum value (|J*|∼0). Since the m-value is defined for the strain rate and strain rate change, it is advisable to measure m by the method which excludes the effect of strain on flow stress, and it is suggested that the strain rate change method is the one in accordance with the definition of m. When specimens are deformed under the condition of the same m-value on the reference curve of m, the fracture strain and the uniformity of deformation in the strain rate region where J*>0 are higher than those in the region where J*<0.
