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[Sandia Lab News]

Vol. 54, No. 10        May 17, 2002
[Sandia National Laboratories]

Albuquerque, New Mexico 87185-0165    ||   Livermore, California 94550-0969
Tonopah, Nevada; Nevada Test Site; Amarillo, Texas

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Sandia researchers seek better understanding of building glass Labs' rapid prototyping could aid in back surgeries Acceleration data recorders New explosive-detection portal unveiled

A painstaking pursuit of better glass

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By John German

For a fraction of a second following a bomb blast, window panes hundreds of feet away become sails in the blast's pressure wave. Then they shatter into thousands of flying, razor-sharp daggers.

That's why shattered glass is often among the most dangerous hazards in a terrorist bombing. During the 1995 Oklahoma City federal building attack, for instance, 200 injured people at locations other than the Murrah building at the time of the blast reported they were hurt by falling or flying glass.

A team of Sandia ceramics experts now is studying glass in a quest to develop windows that are stronger, that survive and fail when they're supposed to, and that shatter into smaller pieces, like grains of sand rather than shards, when they break.

They also want to develop a standard set of methodologies to test samples of many different glass types and configurations so architects and builders have more information at hand when they select glass for a building.

The project is supported by Sandia's Architectural Surety® program and funded through the Laboratory Directed Research and Development (LDRD) program, with past support from the US Air Force Blast Mitigation Action Group and Halliburton Energy Services, a Labs industrial partner.

"Glass is everywhere, and until now looks and energy efficiency have been everything," says project leader Jill Glass (1843). "Few building designers have considered the security and safety advantages better glass might offer."

Building Glass 101

Glass is a brittle material, she says, and its strength is highly sensitive to surface defects that often are too small to see. Once stress on the glass exceeds that required to activate a flaw, the pane fails, usually at the spot of the most severe defect.

"It's like when your windshield has a tiny chip or crack in it -- one day it's fine, then the next day it goes zing," she says.

Every piece of glass is different, she adds. Its age, the way it is framed into a building, its position relative to the sun, or microscopic dings from wind-borne particles can significantly affect its strength.

That's why building glass fails over a wide range of stresses.

A better understanding of glass behavior under dynamic loading conditions (sudden pressures, like those from a blast), and an ability to modify the fracture properties of glass, will ultimately bring stronger and safer windows, she says.

What we don't know

In the lab, Jill and her team have subjected various glass types to dynamic and static (constant or slowly increasing pressures) loads to characterize their strengths and behaviors in blast and other environments.

They've studied when and where cracks begin, how large flaws must be to reduce a pane's strength, what forces are at work within a pane when it fails, what size fragments are created when panes shatter, and how far and fast they fly in varying overpressure environments.

They're looking into how water affects glass strength.

They're examining behaviors of several variations of glass compositions, thicknesses, configurations, and engineered features, including glasses manufactured with strength-enhancing stresses stored within the material, such as automobile safety glass.

"What we've discovered is that there is a lot we don't know about glass behaviors in a blast environment," she says. "Architects are making modifications and decisions about glasses based on little or no supporting scientific evidence that they are safety improvements." (See "Rushing headlong into the glass problem" right.)

An oxymoron, reliable glass

One needed improvement, says Jill, is glass that survives and fails very reliably.

"Reliable and glass haven't often been used in the same sentence," she says.

Toward that end her team and researchers at Penn State University and the Universita' di Trento in Italy are experimenting with a specialized family of glasses called Engineered Stress Profile (ESP) glass that offers high strength, reliability, and control of fragment size through modifications to the glass following production.

The team is refining the two-step ion exchange process used to make ESP glass to carefully manipulate the glass composition at the surface. The process creates glass with peak compressive stresses 10-20 microns under the surface rather than right at the surface, like automobile safety glass.

(A pane of auto safety glass incorporates compressive stresses at the pane's surface that are counteracted by tensile stresses at the pane's center. A surface crack or bending strain destabilizes these competing stresses, and the pane fails catastrophically. These tensile stresses cause the "dicing" behavior associated with the fracture of safety glass.)

This new processing trick results in a stress profile within the ESP glass that initially arrests penetration and growth of networks of surface cracks, then releases the stored compressive and tensile stresses when a critical stress value is reached.

The result is crack tolerance and failure at predictable, narrowly defined stress loads. (Safety glass offers control of fragment size but is highly sensitive to surface flaws and, thus, fails over a wide range of stress loads.) In addition, the surface crack density increases dramatically prior to failure, serving as a warning mechanism, says Jill.

Sandia, Penn State, and Halliburton have filed several patents on the processes to produce modified ESP glass and its applications.

"For the first time we have glass types that are both strong and dependable, that crack noncatastrophically, and that fracture into small fragments," she says.

Scientifically valid comparisons

Currently no standards exist for selection of blast-resistant glasses for nongovernment buildings, says Jill.

She has worked with the US Air Force's Force Protection Battle Lab in San Antonio to compare the blast performances of large panels of ESP glass with those of conventional glasses under varying overpressure conditions.

The tests were conducted in a shock tube, providing carefully controlled test environments. Free-field blast tests at Sandia's Explosives Components Facility are planned for this summer using smaller-scale glass samples.

"Even under nominally similar conditions, glasses perform differently from one day to the next," she says. "The goal is to come up with a set of protocols that allow engineers, manufacturers, researchers, and end users to make scientifically valid comparisons under rigidly controlled, standardized conditions."

"I believe the research being accomplished here and at Penn State with the ESP glass will revolutionize the application of glass as a structural material for buildings and infrastructures," says Rudy Matalucci (5862), Sandia Architectural Surety® program manager and chairman of the Engineered Glass Committee of the American Society of Civil Engineers' Architectural Engineering Institute.

"I find a new excitement within the industry for using glass differently as a construction material now that the internal stress profile can be altered to meet a variety of strength requirements," he adds. "Many of the architects and engineers I talk with daily are encouraged to know that Sandia is investigating the basic performance of ESP glass."

And building glass is just one market for better glass, adds Jill.

"There are many other uses, including weapons and automobile applications, that would benefit from a glass you can trust," she says. - - John German

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Sandia's rapid prototyping expertise could help surgeons alleviate chronic back pain

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By Bill Murphy

Who doesn't know someone with a bad back?

It's one of those universal physical complaints doctors deal with all the time, a condition experienced by millions of Americans. For most of those so afflicted, occasional back pain is a fact of life, something to tough out and endure. For thousands of others it is literally unendurable, a pain so intense and relentless as to become incapacitating.

When it gets that bad, physicians will try "a," they'll try "b," they'll try "c"-- and then, if nothing else helps, they'll look at surgery as an option.

Back surgery involving spinal fusion, which is called for in the case of intractable back or neck pain related to disc degeneration and/or instability that doesn't respond to other therapies, is problematic: it can and does succeed in reducing pain and restoring mobility for lots of patients. But it's a tricky, difficult, unforgiving procedure that doesn't always work.

Now, with the help of Sandia rapid prototyping expertise, these spinal fusion operations may become a more consistently successful and effective treatment, thus delivering thousands of people from lives of chronic pain.

Maintaining proper alignment

As an orthopedic surgeon at University Hospital in Albuquerque, Dr. George Brown has done his share of back surgeries. Dr. Brown explains that the biggest challenge in the surgery is maintaining proper alignment of the so-called pedicle screws. These are, as the name suggests, actual screws that are inserted into the spine. These screws are used to correct deformity, and/or treat trauma. Similar to other bone screws, pedicle screws may be used in instrumentation procedures to affix rods and plates to the spine. The screws may also be used to immobilize part of the spine to assist fusion by holding bony structures together.

Many techniques -- including lots of hi-tech computer-assisted processes -- have been developed in recent years to help docs get the best possible alignment of pedicle screws. Brown realized, though, that none of the techniques were as effective as surgeons would like them to be. He wanted something better.

Clint Atwood turned him on

About the time Dr. Brown was pondering the challenges of pedicle screw alignment, he heard a talk from Sandia rapid prototyping evangelist Clint Atwood (1314) and soon became a true believer. (If you've heard Clint talk about his passion, you will understand how he wins converts.) Rapid prototyping is an advanced manufacturing technology that enables you to generate real-world 3-D models from virtual 3-D computer renderings. In the post-Cold War era, with DOE downsizing its manufacturing capabilities, rapid prototyping was seen as a way to streamline the development cycle for certain weapon components. Dr. Brown wasn't interested in the weapon application, of course. He immediately saw rapid prototyping as a solution to the pesky pedicle screw alignment problem.

To appreciate the difficulty, visualize a spinal column. It's not solid bone. Rather, it's hollow in the center where the spinal cord travels from the brain down through the spine. The column is a series of articulated vertebrae. The front and back sections of each vertebra are connected by bone structures called pedicles. When you perform a fusion surgery, you're trying to rigidly link these front and back portions of the column together. Where are you going to put the screws? Why, through the pedicles. Straightforward -- except that the pedicles aren't really all that big, and they're canted at an angle. They didn't evolve with a surgeon's needs in mind.

How could they not miss

And consider this: when you're operating on the patient, you can't see the pedicles. You're drilling down through them from back to front, not into them from the side. A surgeon does a prodigious amount of planning before going into a fusion surgery, devising the angle for the screw placement based on MRI imagery. That's helpful, but on the operating table, it's still in the surgeon's hands to get the screws in the right place. And sometimes, even the very best surgeons miss. When you weigh the technical complexity of the operation, you have to ask yourself, "How could they not miss from time to time?"

Back to Dr. Brown: He envisioned a way to use rapid prototyping to make a 3-D model -- a jig -- in which the pedicle screw placement would be almost foolproof. Armed with some R&D funding from UNM, he came to Sandia with his concept, hoping to partner with the Labs on an effort to apply this industrial technology to the healing arts.

Alan Parker, Manager of Mechanical Engineering Dept. 14184, thought Brown's concept sounded like a good challenge for his team. Researcher Mark Ensz and technologists Daryl Reckaway and Terry Litts were assigned the project. They're experts at turning computer models into real objects.

Mark explains how the process works: The doctor orders an MRI or CAT scan for the patient. He or she then works with the data from the scan, tweaking the bit-mapped MRI images (to compensate for certain limitations in MRI imagery) and highlighting the region of interest. The process is done layer by layer, as the MRI data come in swaths of 2 mm to 5 mm -- you stack them together to get the whole picture. After the doc has cleaned up the image to his or her satisfaction, the layers are combined together as a 3-D computer model (using off-the-shelf Mimics software from Materialize, Inc.). This model, virtually a perfect image of the area of the spine where the surgery will occur, enables the physician to pre-plan the trajectory of the pedicle screws.

A perfect replica of the spine

So far, there is nothing unfamiliar here; similar techniques have been in use for some time. However, it's at this point that things start to depart from current practices. Up to now, the MRI/Mimics data has been used as a visualization tool. In the UNM/Sandia approach, the doctor takes the process to the next step: he or she actually "drills" the holes through the pedicles in the computer model of the spine, carefully indicating on the image precisely where the holes will be. Mark then takes these images with the "holes" drilled in them, and converts those holes -- i.e., the trajectories through the pedicles -- into separate, mathematically defined files. Then, the original 3-D file of the spine and the new file of the trajectory are combined into a rapid prototype-compatible file. This file is then "printed," either on a stereolithography machine or a selective laser sintering machine, as a solid real-world model of the spine, with the pedicle screw trajectories in place. (The machines build, layer by layer, a solid model of the computer file, using an epoxy or nylon material.) The model is a perfect replica of the patient's spine at the area where the surgery will occur. Now, the next step is where the real beauty of Brown's original concept shines through. Mark, Daryl, and Terry make a second model, a reverse image of the spine model, one that slips over the vertebrae of the spine as snugly as a dainty foot in a glass slipper. This model also has trajectory holes in it -- and they align perfectly with the holes in the first model. In other words, what you have is a perfect jig. It can fit in only one place on the patient's spine and, because it has trajectory holes in place, it guides the surgeon's drill with computer-aided precision. When you drill through the jig, you have to have perfect alignment, because it's already been worked out in the 3-D model.

More precise, less invasive surgery

The process, in addition to allowing more precision than any alternative method, should also make spinal fusion surgeries much less invasive -- the surgeon needs to open only a small area in the immediate area of interest.

Brown and his Sandia partners have been perfecting the technique over the past couple of years. The tests done with cadavers, Mark says, have been highly successful and the interest level in the orthopedic community has been high. Brown is pursuing a National Institutes of Health grant to further refine the technique, including doing the procedure on real patients, alleviating real pain with a real-world 21st century solution.

Sometime in the next few years, if Brown's research continues to advance as successfully as it has so far, every major hospital in the nation could have its own rapid prototyping shop, manufacturing not just spines, but hips, shoulders, and other body parts that wear out due to age or trauma. - - Neal Singer

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Testing acceleration data recorders leaves Sandia department feeling like it's the Fourth of July

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By Chris Burroughs

Every time instrumentation developed by Telemetry and Instrumentation Dept. 2665 is fired in a projectile or bomb into concrete, rock, water, or earth targets to support a customer's test, it's like the Fourth of July.

So says Dept. 2665 Manager Mike Partridge.

"It's both exciting and nerve- wracking," Mike says. "The team works hard to get the instrumentation right, but the worry is that something might go wrong. And when the experiment is a success, everyone feels a sense of elation."

That feeling of exuberance was evident April 30 when the Penetrator Instrumentation Team participated in an experiment at Sandia's rocket sled track. They put instruments designed to monitor acceleration inside a casing of a test unit, which was rocket propelled along the track into a water target -- Styrofoam blocks containing a two-foot-deep water trough. When the test unit hit the target, it was traveling at more than Mach 2.

The instrumentation survived the test.

Acceleration data gathered from the instrumentation will be compared with computer simulations to determine how accurate the models are.

For nearly three decades Dept. 2665 has been designing, building, and fielding flight instrumentation data recorders that are put into various size projectiles and used to measure acceleration and velocity as a projectile is fired and penetrates targets.

What's new is the development of recorders that are smaller, more sensitive, and more rugged than any of their predecessors.

"Our new acceleration recorders use a microcontroller-based architecture for test flexibility, while maintaining electronics and packaging techniques developed over many years of penetrator testing at Sandia," Mike says. "Because of their small size, they can go where no recorders have gone before."

There are several versions of the new small recorders: the 2.5-inch-diameter-by-6-inch AdPen2, the 2-inch-by-3.5-inch MinPen2, and the 1-inch diameter-by-1.8-inch MilliPen. The design team for these versions was led by Tedd Rohwer, former member of the Earth Penetrator Instrumentation Team who is now in Dept. 2131. A next-generation MilliPen is being developed by Tony Mittas (2665).

The small size and weight of these modern acceleration recorders allow their use in smaller and more compact projectiles, which greatly reduces testing costs.

Tom Togami of Applied Engineering and Technical Development Dept. 15414 says his department uses the instrumentation "on the majority of our penetration tests," which "usually means a few full-scaled tests and multiple mid-scale tests per year."

"When we obtain data they are always compared with pretest simulations and are also used to further the development of our predictive models," Tom says. "We have performed suites of experiments with the MilliPen that provide comprehensive data sets for the DOE/DoD community to benchmark their modeling and simulation efforts."

Tom's colleague in Dept. 15414, Danny Frew, says, "Much of the test data collected over the past few years has been outstanding, and we are using this data to benchmark existing computer codes."

And, he adds, Dept. 2665 is the only organization at Sandia that can do this work.

Another customer that frequently uses Dept. 2665's instrumentation is B61 System Engineering Dept. 2111.

"We have used instrumentation packages from Mike's department in B61 testing over the past eight to ten years," says Steve Pink (2111). "We have been extremely happy with the performance of their recording systems and the responsiveness of Dept. 2665's personnel in meeting aggressive schedules and supplying quality data. During the B61 Alt 354 program, for example, we flew 13 flight tests and did not lose so much as one channel of data during the test series."

Ed Henry (2665), who leads the Earth Penetrator Instrumentation Team, says among the biggest challenges in building the small and highly sensitive measuring instruments is to make them rugged enough to survive impacts of as much as 40,000 Gs. Over many years of field experience, his team has learned numerous "tricks."

One advantage to the new recording technologies is that they are designed to operate with minimal support equipment in remote test areas.

"Only a small interface box and a laptop computer are needed to initialize the recorder for the test or download and plot the data after the unit is recovered," Ed says.

After the projectile is dropped, it is retrieved and the recorder is removed so that data can be downloaded. Researchers in Dept. 2665 have designed a version using radio frequencies -- still in the experimental stage -- that will allow for real-time data recovery without having to wait for penetrator recovery.

The small recorders have been used in supporting a wide range of deployment methods. They have been fired from a cannon, flown on rocket sleds, and dropped from aircraft like F-16s, B-2s, B-52s, helicopters, and even a hot air balloon.

Most of the time Ed and his crew are given a full definition of mission requirements and objectives, but sometimes they are not.

"Occasionally a customer will come in with a request for instrumentation for a project they can't talk about" Ed says. "We're given specs -- like the instrumentation must be built to withstand certain levels of Gs -- and that's all we know."

Over the years Dept. 2665 has had many successes. One they like to talk about was an experiment conducted several years ago when a projectile containing an acceleration data recorder was fired into 18 feet of concrete.

"It was interesting to see the 'before scene' when the target was six concrete blocks solidly stacked together and afterwards when it was all in pieces," Mike says. "The amazing part is that we can design electronics that routinely survive such extreme environments."

Working on the penetrator instrument team are Ed, Tony, John Heise, Phil Reyes, Dave Faucett, and Randy Lockhart (all 2665). -- Nancy Garcia

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Labs' licensee unveils explosives-sniffing security portal in Washington, D.C.

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By John German

Explosives-detection technology originally developed at Sandia is part of a new security portal unveiled last week in Washington, D.C.

Barringer Instruments (Warren, N.J.) showed off its new walk-through explosives-detection portal, the SENTINEL II, to members of the news media and potential customers May 9.

The portal noninvasively screens about seven people per minute for explosives and a variety of other chemical residues and can be used at airports, office buildings, sports arenas, and other high-

traffic areas, according to Barringer.

Sandia developed and licensed to Barringer the sample preconcentrator used in the SENTINEL II. The technology traps particles and vapors from a large volume of air, then directs the concentrated chemical sample to Barringer's IONSCAN® detector for analysis. The preconcentrator makes possible the detection of very low concentrations of chemical compounds of interest, says project leader Kevin Linker (5848).

The company says the device can detect

narcotics and some chemical warfare agents as well.

"The recent shoe bomber incident is just one example of the necessity to screen people," says Barringer President Ken Wood. "However, the traveling public expects screening to be quick and nonintrusive. The SENTINEL II solves all of these issues."

Bob Eagan, VP for Energy, Information, and Infrastructure Surety Div. 6000, represented

Sandia at the news media demonstration.

"All of us at Sandia are proud to be a part of this effort, and to see what began at Sandia . . . evolve into a public safety product that will give us all peace of mind when we travel," he said.

A Barringer spokesperson says the com-pany hopes for FAA approval to use the device as an airport screening tool within the next few months. -- John German

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