Monday, October 17, 2005Volume 5, Issue 6
A Fracture in the Glass Community
What makes glass break? After 2,000 years of making and breaking glass, you might think we would have a definitive answer. But at the 3rd International Workshop on the Flow and Fracture of Advanced Glasses held at the Penn Stater Conference Center this week, fifty or so of the world's top glass scientists scratched their heads as researchers presented sharply conflicting views on the topic.
Why Does It Matter?
Glass is a versatile material that is ideally suited for any number of medical and optical uses in addition to its wide application in the building and automotive trades. Glass products, from microscope slides to optical fibers to space telescopes, are a $22 billion contributor to the U.S. economy. Glass is beautiful, but fragile. An understanding of the basic structure of glass, including how and why it breaks and how it can be strengthened to lessen its fragility, will extend the functionality of glass into new areas.
The French Got It Wrong
In the workshop's opening session, American Sheldon Wiederhorn of the National Institute of Standards and Technology disputed the findings of French glass scientists who, in 2003, published research proposing that glass fractures through nucleation and coalescence of submicroscopic cavities. Wiederhorn and colleague J.Guin compared the faces of fractured glass using atomic force microscopy and found no indication of the cavities that should appear along the fracture surface if the French researchers were correct. Wierderhorn argued in favor of the classical model: glass fractures through the stepwise "elastic rupture of interatomic bonds," and this process is accelerated by the condensation of water at the crack-tip.
Au Contraire
Not so, replied the program's next speaker Elizabeth Bouchaud of CEA in Saclay, France, who is a member of one of the groups who ascribe to the cavity model. She presented experimental evidence for cavity nucleation and coalescence ahead of the crack-tip through studies of both silicate and metallic glasses. The size of the cavities observed ranged from a few nanometers in very high velocity cracks, to hundreds of nanometers in ultra slow stress fractures, she said.
Wiederhorn interrupted. "If there are cavities, they must be found in high concentration along the fracture surface," he said. His atomic force microscope studies did not reveal cavities along the arbitrary fracture line measured by his probe.
"Our difference is in how we measure the fractures," Bouchaud riposted, suggesting a little more precision might set him straight.
A Mathematician to the Rescue?
"If experimentalists cannot solve their differences, then computer modelers and their simulations will have to come in," exclaimed Rajiv Kalia of the University of Southern California. Using video simulations of millions of atoms in molecular dynamics simulations on ultra fast computers, Kalia described how atoms under pressure slide across one another causing friction, giving rise to shear fractures. These wing cracks extend through nanovoids that are healed through lateral confinement. "Maybe there are cavities," or nanovoids, as Kalia calls them, but they close up under pressure.
Or is it another mechanism entirely, as J. J. Mecholsky, Jr., from the University of Florida, contends? He sees the fracture process as a series of bond reconfiguration events at the crack tip. Also using molecular dynamics simulations like Rajiv Kalia, he presented a video simulation of the glass atomic structure pulling apart like stretched rubber bands through the rearrangement of atoms "not bond rupture" to propagate the growing crack.
Who Wants to Know?
One group who would be very interested in discovering more about the fragmentation of brittle materials is the government, according to Rajan Tandon of Sandia National Laboratories. The military is interested in ceramic armor and wants a fundamental strategy to minimize their fracture. The government would also like to know how radioactive materials disperse under explosive pressure. Two additional areas that are ripe for application are how to get a material to fail irreversibly at a well-defined load, and predicting how materials fragment in size and distribution. Both require a clear understanding of the fracture process in brittle materials such as glass.
Thinking Ahead of the Crack!
Another important question was asked and considered at this workshop. Suppose we develop stronger glass, say 50 times stronger than the glass we have today (which is still below the maximum theoretical strength.) What would we do with it? What kind of products might emerge? Students in engineering, business and architecture were challenged to think outside the box and answer this question through a contest. The NSF International Materials Institute on "New Functionality in Glasses", working together with the Glass Manufacturing Industry Council, sponsored the attendance of ten undergraduate and graduate students to this workshop. All of those students presented posters at the workshop, and four of those students were awarded prize money for their innovative ideas about how to use stronger glass; the contest was first announced in March 2005, the entries were due in June 2005, and the awards were made at this 3rd FFAG Workshop:
1st place: Strengthened Glass for Applications in Hybrid Wind-Solar Energy Systems
2nd place: Ultra-High Energy Density Capacitors Through Improved Glass Technology
3rd place: The Flywheel Energy Storage System
The Fractures Begin to Heal
It is fortunate that a potential international fracas was averted during a coffee break when American scientist Wiederhorn approached the French scientist Bouchaud and complimented her on her eloquent presentation. Bouchaud, in turn, suggested collaboration between their groups to settle their dispute experimentally. Dueling microscopic crack-tips, so to speak, rather than rapiers at dawn.
This 3rd International Workshop on Flow and Fracture in Advanced Glasses (FFAG) was organized by Professor's Carlo G. Pantano and David J. Green of the Pennsylvania State University. The 4th FFAG is scheduled for 2007 in Japan.
