Void Growth at the Neck of Tensile Specimen of Low Carbon Steel

Abstract

It is well known that ductile fracture occurs by the void coalescence, but few experimental studies of void growth have been published to date. In this paper, we describe an experimental study of the increasing rate of volume fraction of voids with deformation in low carbon steel. Many drawn or annealed round specimens whose initial shapes were smooth or notched were deformed in tension at room temperature. Some of the specimens, interrupted tensile deformation, were either ground by a small grinder to increase the radius of curvature of the neck or annealed at 700°C to remove strain hardening, and deformed again. Densities of the necked region of these specimens were measured. The results lead to the following conclusions: (1) Except in the early stage of deformation in which it is expected for microcracks to grow into voids, the increasing rate of volume fraction of voids (v) with true strain (ε) can be expressed by \dfracd(lnv)dε=1. This equation can be understood that each void is elongated equal to matrix in tensile direction with no reduction of its cross-sectional area. Otherwise in the early stage of deformation, v increased more rapidly than expected from the above equation. These experimental results can be explained well by the model proposed by Gurland et al. (2) In the early stage of deformation, v of the notched specimen increased more rapidly than that of the smooth one, but except in this stage v of the former increased as expected from the above equation, too. It is reasonable to consider from these results that the radial stress has a greater effect on void formation than on void growth. (3) After the true strain measured from annealing state exceeded 0.5∼0.7, the increasing rate of v was accelerated. Therefore we conclude that strain hardening has direct effects on void growth.