Experimental evidence of macroscopic plastic flow nonhomogeneity development in uniaxial tensile test samples

An experimental study of the macroscopic plastic flow nonhomogeneity in the course of a uniaxial tensile test is conducted on several aluminum alloys, nickel and 4340 steel. It was observed that the plastic flow initiates throughout the entire gage length in a nonuniform fashion, so that the growth of the deformation in the middle of the gage is faster than it is at the edges. That initial strain rate gradient almost disappeared shortly after its evolution, and the strain rate through the entire gage length became about uniform. The plastic flow nonuniformity emerged again upon further stretching, producing a gradual acceleration in the middle of the gage with corresponding slowdown toward the edges. That final development of the strain rate gradient commenced well in advance of the load maxima and was the cause of the consequent neck formation in the middle portion of the gage. It is demonstrated that the origin of plastic flow nonhomogeneity stemmed from the second elastic strain component in the transverse direction and its gradient evolution along the reduced section upon loading. It is found empirically that acceleration in the strain rate in the middle part of the reduced section was accompanied by a reduction in the apparent strain-hardening exponent, n, calculated from the stress/strain chart. The maxima in the apparent strain-hardening exponent, n, obtained from the common stress/strain charts can be used to indicate the strain rate gradient onset.