Conch Shell Gives Nano Insights Into Composite Materials

David Williamson and Bill Proud review how these organisms build such tough shells from such a seemingly weak substance. They discover that the key to conch strength lies in the small size of the calcium carbonate crystals from which it is formed by the sea snail. The crystals are below a threshold size known as the Griffith flaw size, any bigger and the crystals would be large enough for cracks to propagate through them under stress, the team explains. This makes the shells tough enough to cope, to some extent, with the crushing jaws of predatory turtles and the vice-like grip of crab claws. Weight for weight the shells are as tough as mild steel.

In the early twentieth century, engineers were preoccupied with the premature failure of materials used in shipping and railways. Concepts such as stress magnification and the propagation of tiny cracks that grow to form big cracks were beginning to be understood. Civil engineer Charles Edward Inglis Inglis devised a mathematical equation to help explain the process. And, in 1920, Alan Arnold Griffith built on the Inglis work to explain for the first time that the reason materials in the real world are not as strong as theoretical calculations would suggest is that the presence of tiny flaws magnify the applied stress in a manner according to the Inglis analysis leading to premature failure.

"Griffith pointed out that the effective strength of technical materials might be increased many tens of times if these flaws could be eliminated," explain Williamson and Proud. Little was known at the time of biomaterials and how their properties might one day copied to create biomimetic materials of much greater strength than their industrial counterparts. Griffith's work has now been used to improve our understanding of conch shells and other biomaterials to allow scientists to produce novel composite biomimetic materials. Research in this area has seen almost exponential growth in the last decade.

The team explains that in the archetypal conch shell material, the queen conch (Strombus gigas) uses a crossed layered, or lamellar, structure. At the smallest length scale the shell is made from tiny crystals of calcium carbonate in the so-called orthorhombic polymorphic form of aragonite. Each single crystal is a mere 60 to 130 nanometres thick and about 100 to 380 nanometres across, although they can be several micrometres long. A nanometre is a billionth of a metre; a micrometre is a thousand times bigger, a millionth of a metre. These dimensions, the Cambridge team explains are below the critical flaw size described by Griffith almost a century ago.

To make a biomimetic material, researchers might first adopt the small crystal size for their composites as well as the crossed layered structure of the conch shell. However, to be truly biomimetic, such materials will also have to incorporate another critical feature of the living material: the ability to self-heal. Attacked by a hungry turtle the shell of a queen conch might be strong enough to deter the predator, but damage will occur, but living tissue can carry out repairs. Materials scientists have discovered that certain polymers can be heat treated so that they undergo self-healing, extended research might allow crystalline composites that mimic conch shell to be made that have the same property.

The team concludes that, it is important to treat these biomaterials as sources of inspiration, rather than prototypes to be replicated in exquisite detail. After all, if nature had access to a modern, high-tech material like the extremely tough ceramic titanium boride used in aluminium smelting equipment and electrical discharge machining, would seashells look the same as they do now?

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Only the Weak Survive? Self-Healing Materials Strengthened by Adding More 'Give'

Conventional rules of survival tend to favor the strongest, but University of Pittsburgh-based researchers recently found that in the emerging world of self-healing materials, it is the somewhat frail that survive.

The team presents in the journalLangmuira new model laying out the inner workings of self-healing materials made of nanoscale gel particles that can regenerate after taking damage and are being pursued as a coating or composite material. Moreover, the researchers discovered that an ideal amount of weak bonds actually make for an overall stronger material that can withstand more stress.

Although self-healing nanogel materials have already been realized in the lab, the exact mechanical nature and ideal structure had remained unknown, explained Anna Balazs, corresponding author and Distinguished Professor of Chemical Engineering in Pitt's Swanson School of Engineering. The team's findings not only reveal how self-healing nanogel materials work, but also provide a blueprint for creating more resilient designs, she said. Balazs worked with lead author and Pitt postdoctoral researcher Isaac Salib; Chet Gnegy, a Pitt chemical and petroleum engineering sophomore; German Kolmakov, a postdoctoral researcher in Balazs' lab; and Krzysztof Matyjaszewski, a chemistry professor at Carnegie Mellon University with an adjunct appointment in Pitt's Department of Chemical and Petroleum Engineering.

The team worked from a computational model Gnegy, Kolmakov, and Salib created based on a self-healing material Matyjaszewski developed known as nanogel, a composition of spongy, microscopic polymer particles linked to one another by several tentacle-like bonds. The nanogel particles consist of stable bonds -- which provide overall strength -- and labile bonds, highly reactive bonds that can break and easily reform, that act as shock absorbers.

The computer model allowed the researchers to test the performance of various bond arrangements. The polymers were first laid out in an arrangement similar to that in the nanogel, with the tentacles linked end-to-end by a single strong bond. Simulated stress tests showed, however, that though these bonds could recover from short-lived stress, they could not withstand drawn out tension such as stretching or pulling. Instead, the team found that when particles were joined by several parallel bonds, the nanogel could absorb more stress and still self-repair.

The team then sought the most effective concentration of parallel labile bonds, Balazs said. According to the computational model, even a small number of labile bonds greatly increased resilience. For instance, a sample in which only 30 percent of the bonds were labile -- with parallel labile bonds placed in groups of four -- could withstand pressure up to 200 percent greater than what could fracture a sample comprised only of stable bonds. A film shows that as this sample is stretched, the labile bonds (red) rearrange themselves to hold the material together.

On the other hand, too many labile linkages were so collectively strong that the self-healing ability was cancelled out and the nanogel became brittle, the researchers report.

The Pitt model is corroborated by nature, which engineered the same principle into the famously tough abalone shell, Balazs said. An amalgamation of microscopic ceramic plates and a small percentage of soft protein, the abalone shell absorbs a blow by stretching and sliding rather than shattering.

"What we found is that if a material can easily break and reform, the overall strength is much better," she said."In short, a little bit of weakness gives a material better mechanical properties. Nature knows this trick."

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High-Tech Concrete Technology Has a Famous Past

Almost 1,900 years ago, the Romans built what continues to be the world's largest unreinforced solid concrete dome in the world -- the Pantheon. The secret, probably unknown to the Emperor Hadrian's engineers at the time, was that the lightweight concrete used to build the dome had set and hardened from the inside out. This internal curing process enhanced the material's strength, durability, resistance to cracking, and other properties so that the Pantheon continues to be used for special events to this day.

But it is only within the last decade or so that internally cured concrete has begun to have an impact on modern world infrastructure. Increasingly, internally cured concrete is being used in the construction of bridge decks, pavements, parking structures, water tanks, and railway yards, according to a review of the current status of the new (or old) concrete technology just published by the National Institute of Standards and Technology (NIST).

The virtues of internally cured concrete stem from substituting light-weight, pre-wetted absorbent materials for some of the sand and/or coarse aggregates (stones) that are mixed with cement to make conventional concrete. Dispersed throughout the mixture, the water-filled lightweight aggregates serve as reservoirs that release water on an as-needed basis to nearby hydrating cement particles.

According to one study cited in the review, bridge decks made with internally cured, high-performance concrete were estimated to have a service life of 63 years, as compared with 22 years for conventional concrete and 40 years for high-performance concrete without internal curing.

"As with many new technologies, the path from research to practice has been a slow one, but as of 2010, hundreds of thousands of cubic meters" of the lighter and more durable material have been successfully used in U.S. construction, write the report's co-authors, NIST chemical engineer Dale Bentz and Jason Weiss, Purdue University civil engineering professor.

Compared with conventional varieties, internally cured concrete increases the cost of a project by 10 to 12 percent, Bentz and Weiss estimate on the basis of bridge-building projects in New York and Indiana. The increased front-end cost, they write, must be evaluated against the reduced risk of cracking, better protection against salt damage, and other improved properties that"should contribute to a more durable structure that has a longer life and lower life-cycle costs," they write."Further, this could have substantial benefits in a reduced disruption to the traveling public, generally producing a more sustainable solution."

The 82-page report summarizes the current practice and theory of internal curing, reviews project experiences and material performance in the field, and describes opportunities for research that could lead to enhancements in the material.

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Materials Identified That May Deliver More 'Bounce'

The alloys could be used in springier blood vessel stents, sensitive microphones, powerful loudspeakers, and components that boost the performance of medical imaging equipment, security systems and clean-burning gasoline and diesel engines.

While these nanostructured metal alloys are not new -- they are used in turbine blades and other parts demanding strength under extreme conditions -- the Rutgers researchers are pioneers at investigating these new properties.

"We have been doing theoretical studies on these materials, and our computer modeling suggests they will be super-responsive," said Armen Khachaturyan, professor of Materials Science and Engineering in the Rutgers School of Engineering. He and postdoctoral researcher Weifeng Rao believe these materials can be a hundred times more responsive than today's materials in the same applications.

Writing in the March 11 issue of the journalPhysical Review Letters, the researchers describe how this class of metals with embedded nanoparticles can be highly elastic, or"springy," and can convert electrical and magnetic energy into movement or vice-versa. Materials that exhibit these properties are known among scientists and engineers as"functional" materials.

One class of functional materials generates an electrical voltage when the material is bent or compressed. Conversely, when the material is exposed to an electric field, it will deform. Known as piezoelectric materials, they are used in ultrasound instruments; audio components such as microphones, speakers and even venerable record players; autofocus motors in some camera lenses; spray nozzles in inkjet printer cartridges; and several types of electronic components.

In another class of functional materials, changes in magnetic fields deform the material and vice-versa. These magnetorestrictive materials have been used in naval sonar systems, pumps, precision optical equipment, medical and industrial ultrasonic devices, and vibration and noise control systems.

The materials that Khachaturyan and Rao are investigating are technically known as"decomposed two-phase nanostructured alloys." They form by cooling metals that were exposed to high temperatures at which the nanosized particles of one crystal structure, or phase, are embedded into another type of phase. The resulting structure makes it possible to deform the metal under an applied stress while allowing the metal to snap back into place when the stress is removed.

These nanostructured alloys might be more effective than traditional metals in applications such blood vessel stents, which have to be flexible but can't lose their"springiness." In the piezoelectric and magnetorestrictive components, the alloy's potential to snap back into shape after deforming -- a property known as non-hysteresis -- could improve energy efficiency over traditional materials that require energy input to restore their original shapes.

In addition to potentially showing responses far greater than traditional materials, the new materials may be tunable; that is, they may exhibit smaller or larger shape changes and output force based on varying mechanical, electrical or magnetic input and the material processing.

The researchers hope to test the results of their computer simulations on actual metals in the near future.

The Rutgers team collaborated with Manfred Wittig, professor of Materials Science and Engineering at the University of Maryland. Their research was funded by the National Science Foundation and the U.S. Department of Energy.

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NASA Readies for World's Largest Can Crusher Test

It's similar to what a team of NASA engineers will do to an immense aluminum-lithium rocket fuel tank in late March; their hope is to use data from the test to generate new"shell-buckling design factors" that will enable light-weight, safe and sturdy"skins" for future launch vehicles.

Testing for this innovative study is under way at NASA's Marshall Space Flight Center in Huntsville, Ala., where engineers are supporting the test led by the NASA Engineering and Safety Center, or NESC, based at NASA's Langley Research Center in Hampton, Va.

The aerospace industry's shell buckling knockdown factors are a complex set of engineering data that dates back to Apollo-era studies of rocket structures -- well before modern composite materials, manufacturing processes and advanced computer modeling. The hope is for the new test data to update essential calculations that are typically a significant cost, performance, and safety driver in designing large structures like the main fuel tank of a future heavy-lift launch vehicle.

The large-scale test follows a series of smaller scale tests, all aimed at reducing the time and money spent designing and testing future rockets. And by incorporating more modern, lighter high-tech materials into the design and manufacturing process, rockets will save weight and carry more payload.

This week, technicians moved a 27.5-foot-diameter and 20-foot-tall space shuttle external tank barrel-shaped test article into place at Marshall's Engineering Test Laboratory. Once installed, the section will be sandwiched between two massive loading rings that will press down with almost one-million pounds of force on the central cylindrical test article forcing it to buckle.

"Spacecraft structures, especially fuel tanks, are designed to be as thin as possible, as every pound of vehicle structure sacrifices valuable payload weight and can dramatically increase the cost of flying a rocket," said Mark Hilburger, a senior research engineer in the Structural Mechanics and Concepts Branch at Langley and the principal investigator of the NESC's Shell Buckling Knockdown Factor project."Looking toward future heavy-lifters, our goal is to provide designers greater confidence in how buckling happens in structures so we can develop lighter-weight tanks."

Research to date suggests a potential weight savings of as much as 20 percent.

Leading up to the big crush in late March, the shell buckling team has previously tested four, 8-foot-diameter aluminum-lithium cylinders to failure. In preparation for the upcoming test, hundreds of sensors have been placed on the barrel section to measure strain, local deformations and displacement. In addition, advanced optical measurement techniques will be used to monitor tiny deformations over the entire outer surface of the test article.

"This unique test rig was essential to developing the lightweight space shuttle external tank that is flying today. Our sophisticated testing capability is back in action to better understand design factors for next-generation metallic launch vehicle structures," said Mike Roberts, an engineer in Marshall's Structural Strength Test branch and the center lead for this test activity."Months of preparation for the facility, test article, high-speed cameras and data systems are all in place and ready to support this major test."

The Shell Buckling Knockdown Factor Project is led and funded by the NESC; Marshall is responsible for the test including the engineering, the equipment design, the hardware facilities and safety assurance. Lockheed Martin Space Systems Company fabricated the test article at Marshall's Advance Weld Process Development Facility using state of the art welding and inspection techniques.

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