Abstract :
In this series of papers, we investigate
themechanics and physics of necking and fragmentation
in ductile materials. The behavior of ductile
metals at strain rates of about 10,000 per second is
considered. The expanding ring experiment is used
as the vehicle for examining the material behavior
in this range of strain rates. In the present paper,
the details of the experiment and the experimental
observations on Al 6061-O are reported. Specifically,
the design of the expanding ring experiment
is evaluated through an analysis of the electromagnetic
and mechanical aspects of the problem.
Then, through an innovative use of high-speed,
high-spatial resolution imaging we determine the
sequence of deformation and failure in the expanding
ring. In particular, the high speed photographs
reveal that multiple necks nucleate along the circumference
of the ring near a critical strain level;
this is followed by a sequence of fractures, and
eventually the fragments are unloaded and move
as a rigid body. The strain at the onset of localized
deformation, the time of fracture initiation,
and the sequence of fragmentation are all quantified
in these experiments. These experimental results
facilitate detailed comparison to analyticaland numerical models of the fragmentation process.
Following this, quantitative interpretation of
the experimental observations is pursued. First, the
uniform expansion of the ring is considered; the
observed radial expansion is shown to agree well
with an analytical solution of the problem based
on a strain-rate-independent plasticity model. The
evolution of the strain in the specimen and the
onset of necking are evaluated quantitatively and
shown to exhibit no dependence on the applied
strain rate for this material; the strain at final fracture,
averaged over the entire ring, is shown to be
an inadequatemeasure of the ductility of the material.
The fragmentation process is modeled with finite
element analysis, incorporating the concept of
the Mott release waves; this simulation provides a
detailed numerical characterization of the experimental
observations. Finally, the statistics of the
necking and fragmentation are evaluated; these are
interpreted both with the predictions of the linear
perturbation analysis and a Weibull/Mott model
of necking and fragmentation. In the sequel, we
will explore the effect of material ductility, strain
rate dependence, the effect of geometry and constraint,
and finally the effect of a compliant cladding
or coating on the development of necking and
fragmentation.
