Abstract :
In this series of papers, we investigate the mechanics of deformation localization
and fragmentation in ductile materials. The behavior of ductile metals at strain rates between
4000 and 15,000 s−1 is considered. The expanding ring experiment is used as the primary
tool for examining the material behavior in this range of strain rates. In Part I, the details
of the experiment and the experimental observations on Al 6061-O were reported. Statistics
of necking and fragmentation were evaluated and the process was modeled through the
idea of the Mott release waves both from necking and fragmentation. Finally, it was shown
that the strain in the ring in regions that strained uniformly never exceeded the necking
strain. In the present paper, Part II, we address the issue of strain hardening and strain-rate
sensitivity. Specifically, we examine different materials—Al 1100-H14, and Cu 101—in
order to determine the role of material constitutive property on the dynamics of necking.
These experiments reinforce the conclusion presented in Part I that the onset of necking
essentially terminates the possibility of further straining in other parts of the ring and even
more importantly that there is no influence of material inertia on the strain at the onset of
necking in this wide range of materials. Furthermore, the effect of aspect ratio of the specimen
is evaluated; this reveals that as the aspect ratio increases beyond about five, in addition to
or instead of diffuse necking, localization into the sheet necking mode is observed; in this
mode, the effect of ring expansion speeds is demonstrated to result in an increase of the strain
at the onset of localization. In addition, an absolute size effect is observed: larger specimens
exhibit localization at larger strain levels. These observations are explained in terms of plastic
wave propagation and reproduced with finite element simulations. In future contributions as
part of this sequel, we will explore the effect of other geometrical constraints and the effect
of a compliant cladding or coating on the development of necking and fragmentation.
