CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
IMPACT INDUCED FAILURE ZONES
IN HOMALITE BARS
Rod Russell, Stephan J. Bless, and Tim Beno
Institute for Advanced Technology, 3925 W. Braker Ln., Ste. 400, Austin, TX 78759
Abstract. Impact tests were conducted on Homalite bars. Bars were impacted at 250 m/s with various
flyer plates. Bar behavior was observed with a high-speed digital camera. Homalite bars exhibited
repeatable failure modalities with little effective change coming from flyer plate or bar geometry.
Failure is characterized by early, late, and intermediate morphologies. Early failure exhibits a radial
damage cone near the impact event. Late damage adds a catastrophic failure zone near the bar end and
multiple wave front locations along the length of the bar. Intermediate time pictures indicate that
catastrophic failure starts as a series of spall-like planes in the catastrophic failure zone.
contain a degree of internal structure. Depending
on their degree of internal structure and loading
rate, brittle failure may be mainly intrinsic or
extrinsic. Rapid loading is usually necessary to
observe intrinsic phenomena, in which failure
occurs locally and is not influenced by structural
features. However, such rapid intrinsic failures do
not always require impact loading; rock burst and
BACKGROUND
Brittle materials can be characterized by their
degree of microstructure. There is a continuum,
ranging from polycrystalline ceramics to
homogeneous amorphous materials. In between are
materials, such as cubic polycrystals (A1ON, for
example) and some glasses, like Pyrex, that do
FIGURE 1. Failure zone in impacted Homalite bar typical of "early" failure mode. Homalite bar was ~200mm long by 12.7-mm
diameter. Fiducial lines are spaced -25.4 mm apart. Impacts from the left at 253 m/s. Time ~ 150 fis.
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wavespeeds, which causes difficulty with
instrumentation timing; and they are stronger and/or
higher impedance than witness bars, which
complicates the interpretation of measurements.
Use of a brittle plastic can circumvent many of
these difficulties.
Homalite has been used
frequently in the past for fracture-mechanics
studies, and it seems to have properties ideally
suited for bar impact studies as well. Homalite is a
thermoset manufactured by the Homalite Division
of Brandywine Industries. According to the
manufacturer, density is 1.23 g/cm3, compressive
strength is 0.18 GPa, and Young s modulus is 4.5
GPa. From these values, the bar speed, (E/p)1/2, is
1.9mm/jis.
the explosion of Prince Rupert s drops are
apparently examples of self-propagating intrinsic
failure.
Structure is also apparently a condition for the
manifestation of failure waves in brittle materials.
Across a failure wavefront, there is a loss of
shear strength but little or no change in the stress
component perpendicular to the wavefront.
Bar impact geometry provides a particularly
useful way to steady dynamic failure of brittle
materials. In this geometry, the loading near the
impact face is one-dimensional strain, but in most
of the sample the state is one-dimensional stress. In
this way, the sample loading mimics that associated
with many important engineering impact scenarios.
Use of Homalite as a Surrogate Material
EXPERIMENTAL DETAILS
Most experiments on brittle failure have been
conducted with ceramics and glass. However,
ceramics pose a number of experimental
difficulties: they can be relatively expensive; they
can be difficult to machine; they are often harder
than the striker, which leads to uncertainties in
impact geometry; they may have very high
Experiments were conducted with a 5.5-m-long,
56-mm-diameter single stage compressed gas gun.
Various flyer plate geometries - 5- and 10-mm
thick steel discs, 100-mm long aluminum bars, and
100- and 200-mm long Homalite bars - were used.
Velocity was measured by monitoring laser light
FIGURE 2. Failure zones in impacted Homalite bar typical of a "later" time failure. Note radial ejecta and numerous fracture planes.
Lines are -25.4 mm apart. Homalite bar was -200 mm by 12.7-mm diameter. Velocity 259 m/s, time -240 )J,s.
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from reflecting strips on the sabot using the
technique described by Simha (1).
Target bars were placed at the end of the gun
barrel and positioned using an alignment system.
Two bar geometries were investigated. Round bars
12.5 mm in diameter ranging in length from 170- to
200-mm, and rectangular prisms 25-mm tall and
12.5-mm thick were also used in lengths of 170 to
200 mm. Instrumentation consisted of a single
frame DYCAM CCD camera manufactured by
Cooke Corporation, using a 30-ns exposure.
Triggering was accomplished using 2ju,m tungsten
break-wires placed in front of the impact plane.
The first wire triggers a one millisecond flash, and
the second triggers the camera. Actual exposure
was delayed by a preset interval. The flash lamp
illuminated from above while images were taken
from the side. A laser alignment system was used
to maintain camera orthogonality. Various
backgrounds were used; best results were obtained
using a black background, against which fractured
plastic appeared white. Bright fiducial lines were
printed on the background for measurement.
Unlike earlier bar impact tests with glass and
ceramic bars, there was no indentation of the flyer
plates in the Homalite experiments.
around 150 (is and is characterized by an impact
cone extending about 25 mm from the flyer plate
and no visible damage ahead. Behind the failure
front, the plastic becomes opaque. The radius
expands 2-3 mm, and the edges are indistinct. This
appearance is consistent with a material that has
been turned to rubble by the fracture. There is little
additional change in radius over the next 10 mm or
so, which means that radial expansion velocity
behind the fracture is small or zero. Very near the
flyer plate, fractured material is violently splayed
out radially, as seen in Fig. 1.
Late time, ~250-|is, photographs reveal that a
zone near the distal end of the bar has also been
destroyed. The failed zone starts about a diameter
from the rear surface, and extends about 50 mm
toward the advancing projectile. The failed region
also appears to be experiencing radial expansion.
(See Fig. 2.) The free end of the bar has moved.
Assuming that motion begins with the arrival of the
1.9-mm/jis elastic wave, average velocity is -100
m/s. There are also several thin and stable damage
zones in the midsection of the bar.
Intermediate time, ~200-|is, pictures show an
expansion cone standing off from the flyer plate,
similar to its appearance at early time. Fracture
planes appear to be coalescing in the center of what
will become the catastrophic failure zone on the
distal end of the bar. (See Fig. 3.) These
"intermediate" phenomena are seen in both round
bars and prisms.
OBSERVATION AND DISCUSSION
One-Sided Compressive Loading
Three principal morphologies were seen in bar
failures. These morphologies are associated with
space along the bar and time after impact. The
first, or early, morphology is associated with times
Sympathetic Compressive Impact
The final test included in this paper consisted of
the rectangular prism geometry being loaded on
FIGURE 3. Failure zones in impacted Homalite bar showing "intermediate" failure mode. Note how multiple spall-like planes are forming
along bar's length. Lines are spaced -25.4 mm apart, Velocity 253 m/s, Time -200 jus.
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FIGURE 4. Intermediate time double ended compressive failure in sympathetically loaded prismatic bar. Projectile is impacting from the
left at 261m/s. Plasticity is indicated on distal end of the bar by the apparent distortion of the ~25.4-mm fiducial line located -100 mm to
the right from the mounting plate.
both ends. A 10-mm flyer plate was glued to the
distal end of the bar prior to the experiment. The
bar was impacted at -250 m/s with a 10-mm steel
flyer, and an intermediate time photograph was
taken. The resultant image, Fig. 4, shows the
expected compressive failure cone and radial ejecta
on the impact side. Compressive failure has
occurred near the back end without the unfailed gap
seen previously. The slight radial expansion
evidences a compressive condition. The failure
nucleates along the top and bottom of the bar.
These machined surfaces are rougher than the sides
parallel to the image plane. Unlike the free end
case, the rear surface of the bar has moved only a
few millimeters. It is our preliminary conjecture
that the absence of "intermediate type" fracture
planes suggests that they do not evolve in a
compressive environment.
We conjecture that these Homalite experiments
point to a complex set of wave interactions
inconsistent with simple shock and release wave
conditions. The nearly simultaneous formation of
multiple "spall-like" planes in the unbounded
fracture zone, and their suppression in the
restrained case, suggest that a complex series of
pressure and tension regions are evolving in the bar.
REFERENCES
1. Simha, H-M, "A Novel Technique to Measure In-Bore
Velocity in a Single Stage Light Gas Gun," 46th
Meeting of the Aeroballistic Range Association,
Minneapolis, MN, 1995.
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