Blunt Answer: Cracking the puzzle of elastic solids’ toughness

April 23, 2003 at 11:48 am

Soft, springy materials like rubber and skin don’t tear easily because they stretch before breaking apart. For decades, however, detailed understanding of how that stretchiness toughens these materials has eluded researchers.

TENSE SCENE. In the polymer PVB (left), a horizontal crack transforms into a hole (white). In a similarly elastic but weaker polymer (right), the crack elongates. Colors indicate stress levels. Hui et al./Proceedings of the Royal Society of London A

Now, researchers from the DuPont Company in Wilmington, Del., and Cornell University have come up with a comprehensive explanation–based on both experiments and computer modeling–for the toughness of soft solids.

The new insights have “massive implications,” says team member Stephen J. Bennison of DuPont. The findings could potentially influence development of materials ranging from adhesives and solid rocket propellants to artificial biological tissues and foods, he says.

The essence of what the team presents in an upcoming Proceedings of the Royal Society of London A is this: Cracks that begin in soft solids, having a ratio of stiffness to strength of about one-third or less, stop moving forward through the material. Instead, says Cornell theorist Chung-Yuen (Herbert) Hui, the crack “just sits there and opens up”–a phenomenon known as blunting.

While materials investigators had previously noted blunting at crack tips in some soft solids, the DuPont-Cornell team has identified a new mechanism of crack propagation that incorporates blunting. “It’s really a fundamental change in [comprehending] the way a crack runs in a material,” Bennison says.

Kenneth R. Shull of Northwestern University in Evanston, Ill., agrees. “I really do think it is potentially one of the key ideas . . . that’s been largely missed before,” he says.

In materials-speak, a tough substance is one that absorbs a lot of energy as it breaks apart. Consider the transparent polymer called plasticized polyvinyl butyral, or PVB, that DuPont makes to cement sheets of glass together in car windshields. PVB puzzled DuPont scientists, Bennison notes, because it was tougher than could be accounted for by the prevailing ideas of how materials fail.

In studies that began in 1995, the DuPont-Cornell team made the startling experimental observation that a slit the length of a paperclip in stretched PVB would open into a hole rather than tear the fabric further.

Adding to this unexpected, macroscopic demonstration of crack blunting, computer simulations of fracturing in even more elastic materials pointed to blunting as a barrier to crack growth on a microscopic scale.

To understand what makes these materials give way when they finally do, the team found X-ray evidence that thousands of minuscule voids–the result of blunting of submicroscopic cracks–set the stage for the failure. The scientists contend also that stretching around blunted microcracks eventually stiffens the material to the point that it can’t deform any more, leading to the massive breaking of chemical bonds that sums into ordinary fractures (SN: 1/4/03, p. 3: Fracture Protection: Nanotubes toughen up ceramics).

Manoj K. Chaudhury of Lehigh University in Bethlehem, Pa., calls the new work the first quantitative theory to show that blunting can account for the toughness of rubbery materials. Nonetheless, cautions Shull, many more experiments are needed to see if it’s possible to poke holes in this new theory.

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