and subsequent growth of cracks in structures such as bridges, aircraft,
and oil pipelines have been studied and modeled for years. In contrast,
cracks and failures of parts driven by high-explosive detonations
are less well understood and poorly modeled. Much more complete
information is needed in these cases because the Laboratorys
defense-related mission requires an understanding of how metals
respond to the sudden shock waves and subsequent high-strain-rate
deformations caused by high explosives.
one of the challenges facing the National Nuclear Security Administrations
(NNSAs) Stockpile Stewardship Program is using computational
models to predict dynamic material failure relevant to nuclear weapon
safety and reliability. Changes in material properties caused by
aging nuclear warheads must be represented in computer simulations
that accurately reflect the particular metals internal structure.
Our ability to account
predictively for dynamic material failure is inadequate and, in
some cases, primitive, says Livermore engineer Richard Becker.
We want our computational models to reflect in detail how
cracks form, evolve, and lead to the failure of a part, he
says. Becker is a member of Livermores code development team
that supports NNSAs Advanced Simulation and Computing (ASCI)
program. He is using results from experiments conducted at Livermore
and elsewhere to construct advanced computer models of how metals
crack and ultimately fail.
The new models will help
to assure nuclear weapon safety and reliability as well as to advance
nonnuclear military applications used to design equipment such as
shaped charges and armor-defeating projectiles. The new models will
also likely benefit a number of industrial processes, such as explosive
welding and shock processing, which use high explosives.
Physicist Elaine Chandler,
an associate division leader in the Laboratorys Defense and
Nuclear Technologies Directorate, was the original architect of
the combined modeling and experimental effort on dynamic failure,
which now spans several Livermore directorates. The goal, she says,
is to couple theory, simulation, and experiments to yield a much
better predictive capability for the behavior of ductile metalsmetals
such as copper and aluminum that bend before breakingunder
extreme conditions of high pressures and high strain rates (deformation).
We need real physics underpinnings for models of how materials
fail, she says.
|(a) This micrograph of a tantalumtungsten alloy
cylinder driven by a gas gun shows that the material breaks
along shear bands (darker diagonal line). (b) The crack tip
at a higher magnification. (Micrograph produced by Anne Sunwoo.)
The experimental effort consists
of several multidisciplinary projects, some supported by Laboratory
Directed Research and Development funds, that investigate different
aspects of dynamic failure.
The experiments use well-characterized
ductile metals, experimental tools such as gas guns and scanning
and transmission electron microscopes, and advanced facilities such
as the Laboratorys Janus laser and High Explosives Applications
Facility, the University of Rochesters Omega laser, and the
Stanford Synchrotron Radiation Laboratory. Together, the experiments
cover a wide range of strain rates and pressures. An important focus
of the experimental effort is the development of novel diagnostics
to illuminate the microsecond-by-microsecond details of material
fracture and failure.
The experimental results
are being incorporated into Beckers advanced computational
models. Becker says that traditional codes provide only simple characterizations
of the dynamic fracture behavior of ductile metals. Often, they
prescribe just the minimum pressure at which the metal fails. We
need to more accurately capture the complex underlying processes
so that we can better account for the influence of microstructure,
strain rate, and pressure on the failure of ductile metals. We also
need to simulate the orientation of cracks and the recompression
of material that is possible following severe cracking.
Becker sees a significant
drawback to current models in that they do not take into account
a metals microstructure, which is known to control its mechanical
properties. Metals are composed of microscopic grains that have
different orientations and, inevitably, contaminants. Some aspects
of the subgrain microstructure change dramatically when subjected
to a strong shock from a high-explosive detonation. In particular,
a strong shock induces numerous dislocations within a metals
crystalline lattice, which changes the metals mechanical properties
such as its strength, ductility, and resistance to cracking.
In addition, shocked ductile
metals are known to develop cracks by nucleation (formation), growth,
and linking up of microscopic voids, and a metals microstructure
also affects void nucleation and growth. For example, impurities
and inclusions often act as void nucleation sites. What makes the
current suite of Livermore experiments so important, says Becker,
is that the metals under investigation have their microstructure
characterized both before and after being subjected to different
strain rates and pressures.
The Stanford Synchrotron Radiation Laboratory is used
to obtain three-dimensional x-ray tomographic images of experimentally
produced incipient spallation. The images are from a 6-millimeter
region in the center of the spall plane in (a) single-crystal
aluminum and (b) polycrystalline aluminum.
Shocking Samples with Lasers
Geoff Campbell is looking at the connection between shear (displacement
across a narrow band of material) and fracture in shocked metals.
He notes that when metals fail at high rates, the behavior is often
associated with shear that is confined within narrow bands. These
shear bands are typically precursors to the formation of cracks.
The metal deforming within the shear bands becomes hot and softens,
which makes it susceptible to failure.
To gain a fundamental understanding
of shear localization and fracture, Campbell conducts experiments
in which he determines the mechanical properties of shocked metals.
He creates the shocked microstructure with laser-shock processing,
a method that is considerably easier and less expensive than high-explosives-driven
methods. The solid-state, high-energy (50-joule), neodymium-doped
glass laser was developed at Livermore as part of a method, now
commercialized, to improve the fatigue performance of metals by
imparting intense shocks. (See S&TR, March
and Stressed, Metals Get Stronger.)
In Campbells experiments,
the laser pulses the metal sample several times to achieve conditions
similar to explosively shocked material. Each laser shot lasts only
20 to 50 nanoseconds, compared to a high-explosive detonation that
typically lasts about 1 microsecond.
Campbells focus is
on three metals: copper, tantalum, and a tantalumtungsten
alloy. These are popular, well-understood materials at the
Laboratory, and they allow different experimental teams to compare
results, he says.
Following laser shocking,
Campbell determines traditional mechanical properties and the degree
to which the metals are susceptible to crack propagation and ultimate
failure, information that is critical to the development and calibration
of Beckers computational models. The information is obtained
with tests that measure the materials strength as it is being
deformed and the strain energy release required to propagate a crack.
The same tests are also performed on unshocked samples as controls.
Campbell notes that understanding
the real response of materials has always been important to national
security as well as industry. During World War II, Liberty Ships
were manufactured using welds for the first time instead of rivets.
It was not appreciated at the time that the welds could become brittle
below a certain temperature, and several ships broke in two. Some
sank right after launch, while others were lost suddenly at sea.
||In this simulation of a gas-gun
experiment, (a) is the initial configuration of aluminum striking
a copper target, and (b) shows formation of spall. The green
area on the left is the aluminum plate that strikes a 5-millimeter-thick
cylindrical disk target. The targets two spall rings can
be seen on the disk. The formation of voids (red) is seen in
the center of the disk.
Gas Guns Create Spall
Physicist Jim Belak is looking at the microstructural
origins of dynamic fracture in ductile metals to obtain a better
understanding of beginning damage from spallation, the scab that
forms near the metal surface during high-explosive detonations.
We lack a detailed model of spall fracture, says Belak.
He explains that spall fractures occur when a shock wave reflects
from a surface and produces extreme tension inside the solid. When
this tension exceeds the materials internal rupture strength,
the solid fails by rapidly nucleating voids, which quickly link
to form fractures. The origin of the voids is tied to the solids
microstructure, especially weak points such as inclusions and boundaries
between metal grains. Improving our understanding of spall requires
correlating the observed incipient damage with the well-characterized
Belak and colleague James
Cazamias use the Livermore gas-gun facility to create spall in samples
of aluminum, copper, titanium, and vanadium, metals with crystal
structures of interest and that are well characterized. He also
uses some samples containing engineered contaminants that have been
prepared by Livermore metallurgists Adam Schwartz and Mukul Kumar.
The metallurgists engineer grain-boundary and inclusion microstructures
and examine sample microstructures both before and after the gas-gun
experiments with transmission and scanning electron microscopes.
The gas gun shoots a metal
flyer at velocities ranging from 150 to 210 meters per second. Though
higher velocities are possible, the slower velocity is used to create
incipient damage. The flyer hits a 25-millimeter-diameter thin metal
target of the same material. The target has outside rings that reduce
unwanted effects associated with the specimens edges. At impact,
the rings break off, and the 16-millimeter-diameter center of the
target flies into a catch tank, where it is recovered with minimal
Belak and physicist John
Kinney take the target pieces containing incipient spall to the
Stanford Synchrotron Radiation Laboratory to obtain three-dimensional
(3D) x-ray tomographs in 700 orientations. The images, which have
a resolution of about 5 micrometers, are combined to compute the
3D size and space distribution of the voids that have been created
during spallation fracture. The data are essential input to spallation
models. After the tomographic data are taken, the samples are sectioned
to make detailed comparisons with traditional two-dimensional microscopy.
The synchrotron imagery
that Jim is obtaining is quite a breakthrough, says Chandler.
We obtain data of the 3D void distribution just from the images
and without having to take thin slices of the material and count
the number of voids in each slice.
Belak and physicists Robert
Rudd and Eira Seppälä are also performing 3D simulations
at the atomic level that track how voids grow and link. The simulations
feature 1 to 10 million atoms representing the crystal structure
of aluminum or copper. When tensile forces are applied in different
directions, the simulations reveal the dislocation mechanism by
which microscopic voids grow. The spall recovery experiments using
single-crystal copper and aluminum will enable direct validation
of these dislocation mechanisms.
|This photomicrograph of a
copper disk used in a gas-gun experiment shows the formation
of voids in the spall layer.
Closing Up Voids
In some cases, layers of
a spalled material can collide as the pressure from the high explosive
continues to drive one of the surfaces. The result can be recompression
of the spalled material, which closes the voids created by the original
shock. Under these conditions, the damaged material could jet out
from pores, continue deforming, have localized heating, and even
Currently, simulations do
not include experimentally based models of recompression behavior.
Including such models is necessary for accurate stockpile stewardship
calculations, says Becker. We want to determine the material
response as these two pieces meet, obtain estimates for the strength
of the recompressed region, and insert a recompression model in
our ASCI code.
Becker and his colleagues are performing
recompression experiments on recovered metal disks that contain
well-characterized spall damage. They use a gas gun and copper targets
the same size and shape as those used in Belaks experiments.
The targets are soft-recoveredthat
is, captured using soft materials that do not further damage themand
small specimens containing spall are excised from them. The samples
are then compressed at various rates to close the voids. Becker
monitors the microstructure evolution and the manner in which the
damage is being closed. Then the targets are sectioned and micrographs
taken of them to examine the recompressed microstructure and track
the evolution of the voids.
The data obtained from these
experiments will be used to construct a model describing the material
behavior during recompression and the residual strength in the damaged
samples. The recompression component of an overall model will provide
a more accurate representation of material behavior for explosively
This is a first-cut
model based on limited data, but it is a major step along the way
toward developing an accurate and robust simulation capability for
recompressed damaged materials for stockpile stewardship,
||Example of void formation
and spall fracture in a copper sample after the passage of a
powerful shock wave from the Janus laser at Livermore. Direct
laser irradiation generates a high-pressure shock that causes
the formation and coalescence of voids, which in turn create
with X Rays
Using lasers, physicist Dan
Kalantar has also demonstrated the recovery of shocked single-crystal
copper samples about 500 micrometers thick. The experimental results
are helping to refine the development of models of void growth and
The laser experiments provide
pressuresin some cases exceeding 100 gigapascalsgreater
than those produced by related laboratory experiments using high
explosives. The laser pulse lasts 2 to 5 nanoseconds and exerts
a maximum pressure at the driven side of the sample. The pressure
wave decays as it propagates into the material, resulting in a range
of pressures accessed in a single experiment.
One series of experiments
is devoted to developing a technique called time-resolved, dynamic
x-ray diffraction. This technique uses a high-intensity laser beam
focused on a thin metal foil (such as vanadium or iron) to create
a source of x rays. The x rays diffract from a single-crystal sample
that is shocked by direct laser irradiation with a separate laser
The diagnostic x rays provide a means for recording
the response of the metals lattice as the shock from the laser
pulse passes through. The x rays are diffracted simultaneously from
multiple planes within the metals crystalline lattice. Kalantar
has developed a large-angle film detector that records the diffracted
x rays. In addition, optical and electron microscopes are used on
recovered shocked targets to determine the metals altered
Kalantar has also demonstrated,
with the Omega laser, the recovery of shocked single-crystal copper
samples about 500 micrometers thick. Direct laser irradiation generates
a high-pressure shock that causes the formation and coalescence
of voids, and this void formation and coalescence in turn create
spall. Optical and electron microscopy are used on thin slices of
the targets to investigate the final structure. The effect of the
dislocation microstructure on the x-ray diffraction pattern is compared
with the dynamic x-ray diffraction pattern.
Kalantar is working to extend
the dynamic diffraction experiments using the two beams of the Janus
laser. In addition, to expand the experiments that Becker is performing,
he is designing two-beam experiments to shock materials, create
voids and incipient spall with one beam, and then recompact them
with the second beam.
||Researchers at the gas-gun
facility. In the front row, from left, Keith Lewis, Sam Weaver,
and Erica Nakai. In the back are James Cazamias, Jim Belak,
and Rich Becker.
Metal Cylinders Solve Part of the Puzzle
Physicists Ted Orzechowski, Omar Hurricane, and colleagues
are exploding cylindrical samples of metals and monitoring
how they fracture and then fly apart. The researchers
analyze the failure of the metal cylinders through high-speed
images and characterize the fragments that are explosively
produced. Were missing a fundamental understanding
of material failure. Just having a big computer, without
the correct physics models, is not going to help,
collaboration with Lalit Chhalabildas and his group
at Sandia National Laboratories, is looking at the failure
of metals at high strain rates caused by 2.5-centimeter-long,
LexanTM flyers fired from a gas gun and traveling at
about 2 kilometers per second. The flyer slams into
another piece of Lexan inside a metal cylinder about
5 centimeters long, with an inner diameter of 1.2 centimeters,
and 1, 3, or 5 millimeters thick. The cylinder materials
are 1045 steel (a common steel formulation), nitinol
(nickeltitanium alloy), and tantalumtungsten
alloy. Upon impact, the Lexan behaves a bit like
a working fluid, driving the cylinder radially
outward, says Hurricane.
cylinder experiments provide a direct way to quantify
differences in material failure. Even under identical
drives, differences in cracking and failure are obvious.
experiments are more controlled and compact than high-explosives
experiments, and researchers do not have to contend
with smoke obscuring the high-speed cameras. The shock
wave from the LexanLexan impact sweeps through
the surrounding metal cylinder with a pressure of about
2.4 gigapascals. Although there is a shock, it
is the rapid radial expansion that causes the material
to fracture, says Hurricane.
are heavily monitored with diagnostics that record the
strain rate at different positions on the cylinder.
Optical cameras allow Hurricane to watch stop-action
movies as cracks form, spread, and quickly tear apart
the cylinder. In the case of the tantalumtungsten
alloy, the cracks are associated with shear bands, which
tend to form at 45-degree angles from one another.
In what Hurricane
likens to a forensic examination, metallurgist Anne
Sunwoo cuts up the soft-captured fragments (that is,
fragments captured with light materials to prevent further
damage) and examines them with a transmission electron
microscope to study the metals altered microstructure.
gas-gun cylinder experiments provide a direct way to
document differences in failure according to the changing
microstructure of the metal cylinder. Although identical
Lexan projectiles are used, there are obvious differences
in cracks, fragment size and number, and microstructure
of the failed pieces, depending on the metal.
Explosives Increase the Pressure
and colleagues are conducting experiments similar to
Hurricanes, but they are using high explosives
to study the dynamics of fragmenting cylinders. These
pipe bomb experiments involve pressures
some 10 times greater (about 20 gigapascals) than those
generated in the gas-gun experiments, but the different
pressure regimes complement each other, Orzechowski
are providing the data required to develop, improve,
and validate material failure models for different kinds
of weapons. We want to improve the understanding
of failure and fragmentation of metals and alloys subjected
to explosive force, Orzechowski says. In addition
to stockpile stewardship applications, the research
is relevant to understanding material failure in conventional
weapons. The research is funded by Laboratory programs
and a Memorandum of Understanding with the Department
of Defenses Office of Munitions.
measure about 5 centimeters in outside diameter and
20 centimeters long. Preliminary experiments were conducted
by John Molitoris at Livermores High Explosives
Applications Facility. Physicist Peter Bedrossian is
continuing the experiments at the Laboratorys
remote Site 300. The cylinders, made from 1045 steel,
Aermat 100 steel, or a uranium alloy, are detonated
from one end. The high-explosive detonation front sweeps
along the axis, with the shock lasting for several microseconds.
The metal fragments that are violently produced are
soft-captured with glass wool or other light materials.
A wealth of
information is provided by diagnostics, including FabryPerot
interferometry (which provides time-dependent surface
velocity measurements), high-speed optical imaging,
and conventional radiography. In addition, a series
of proton radiograph experiments, using smaller scale
pipes, was conducted at the Los Alamos National Laboratory
Neutron Science Center by Livermore physicists Bedrossian
and Hye-Sook Park. The proton radiography provides sequential
radiographs that show the details of cracks evolving
and the cylinder disintegrating into many fragments.
(See S&TR, November
Reveal the Inside Story.) Metallurgist Sunwoo also
characterizes the cylinder metal before the experiment
and examines the recovered fragments to help determine
their mode of failure.
Like the gas-gun
experiments, shear bands are found where the cylinder
rips apart. As with the experiments conducted by other
researchers, the tests show that a materials microstructure
may affect its performance. For example, the experiments
reveal differences between steel cylinders that are
heat-treated to increase hardness and those that are
using high explosives to study the dynamics of fragmenting
cylinders are well-diagnosed. Diagnostic instruments
include high-speed optical imaging, metallography, radiography,
and FabryPerot interferometry.
Putting It All Together
As experimenters across the
Laboratory acquire data, Becker incorporates them into his evolving
models of how materials fracture and fail under extreme conditions.
The data from material characterization, metallurgical analysis,
and dynamic experiments are helping to constrain and guide our 3D
code development, says Becker. In particular, the code development
effort is being aided by insights gained from examining different
material microstructures both before and after experiments. Initial
simulations employing the advanced models are encouraging, but much
work remains to be done.
notes that the modeling effort is aided by simulation advances made
by other Laboratory researchers. Geophysicists such as Lew Glenn
have long sought to accurately model the way rocks fracture. Because
rocks are brittle, simulations of their fractures are not directly
applicable to ductile metals, but methods to account for crack orientation
and certain numerical techniques can be applied to modeling ductile
metal fractures. Also, some metals important to stockpile stewardship,
such as beryllium, are brittle. And glass, a highly brittle material,
is vitally important to scientists preparing to operate the National
Ignition Facility, now under construction at Livermore to serve
the stockpile stewardship mission.
is looking forward to offering scientists a robust, flexible model
that can simulate different metals under a wide range of extreme
pressures and strain rates. The payoff will be increased confidence
in the nations nuclear stockpile.
Key Words: Advanced
Simulation and Computing (ASCI), FabryPerot interferometry,
gas gun, high explosives, High Explosives Applications Facility,
Janus laser, Office of Munitions of the Department of Defense, Omega
laser, proton radiography, Site 300, spall, Stanford Synchrotron
Radiation Laboratory, stockpile stewardship, three-dimensional x-ray
For further information contact Richard Becker (925) 422-1302 (email@example.com).