By Heather Newman

n the material world, there’s always room for improvement.

Torex fibers add unparalleled tensile strength and ductility to concrete under stress.

Working from that premise, scientists at the College of Engineering are developing concrete that can withstand earthquakes, and metals that defy conventional rules of behavior. In fact, some of these metals have acquired properties that make them useful in biomedical applications such as the replacement of damaged bones and organ transplants.

With this kind of ongoing research, the College of Engineering has become a brave new world where inquiring minds and inventive engineering have made discovery a daily occurrence.

Fiber-Reinforced Concrete Shear Walls

Antoine E. Naaman, professor, Civil and Environmental Engineering, is designing walls that are used to brace buildings and keep them from swaying out of bounds under severe and repeated earthquakes. To understand the complexity of this work, it’s necessary to know how traditional building materials work.

Usually, concrete shear walls reinforce framed buildings. When severe shaking and/or impact hits the building, the concrete wall, typically reinforced with steel bars (rebar), keeps the drift of the structure within acceptable limits thus reducing the probability of collapse. But traditional concrete shear walls have their limitations and are also very difficult to put into existing buildings.

Naaman attacked those problems in two ways: by creating a hybrid concrete-and-steel wall system that resists shaking better than existing shear walls, and by incorporating fiber-reinforced concrete into the design.

In the former method, two steel columns “sandwich” a series of roughly X-shaped reinforced concrete units stacked to the height of the structure. As the two units grind against each other under seismic excitations, the action absorbs and dissipates energy. This is particularly so, because the central part of the X-shaped units is made weaker than the rest of the unit in order for it to fracture slowly and dissipate energy under severe cyclic motions.

Naaman also experimented with reinforcing the concrete—not only with the large metal bars that reinforce traditional reinforced concrete, but also with tiny fibers that, under stress, created hundreds of microscopic fissures. These fissures absorbed energy and kept the units from crumbling under large drifts. However, this method of reinforcement also had a failure point.

The original fiber-reinforced concrete used tiny metal wires that looked like straight needles. When stresses became too extreme, the wires held for a while and then easily pulled out, allowing the concrete to break apart. New and improved wire fibers had tiny hooks molded into the ends to better grip the concrete—they worked well, but only to a point. Another advanced fiber was Spectra, the same type of material that goes into bulletproof vests. This polymeric material is exceptionally strong, but it, too, eventually pulled out.

Naaman realized that the number and type of fibers in the mix determine the strength of fiber-reinforced concrete—the greater the number of the fibers (up to a limit), the stronger the concrete. He addressed this problem by first putting piles of fibers into the molds, then pouring concrete slurry over them. That was an effective solution for his small-scale tests, he said, but it wasn’t attractive to people who construct large-scale walls and want their concrete delivered premixed. So, Naaman has been working on mixable fibers with more elastic-plastic pull-out properties; that is, these fibers offer an almost constant resistance as they slip out of the concrete, simulating the yielding of steel yet providing more flexibility.

One of the best designs he’s tested in the lab involves twisted wires of triangular shapes. The wire is cut off into mixable fibers of a standard length: about 1 to 2 inches. Reshaping and twisting the wire has had two effects: It increases the surface area of the fiber to do a better job of bonding to the concrete, and the twist gives the fibers an unexpectedly strong grip and plasticity. So, when put under stress, the fibers have to untwist before they’ll pull out and, in the process, provide a constant resistance under large slips. Naaman calls these Torex fibers.

The tensile strength and ductility added to concrete are remarkable and offer characteristics that have never been possible before. Whereas previous materials that could be used for fibers, such as carbon, had been too brittle to be stretchy, the Torex fibers in their new configuration offered many advantages: They’re as strong as or stronger than existing wire systems, they grip concrete better and they can handle much more stress during the “stretching” phase before they pull-out or fail. The only major drawback so far: No one has stepped up to put the fibers into commercial production.

Right now, the Torex fibers used in Naaman’s test are generated (by a small laboratory machine that he has built with his students) at a rate of about one a second, a rate that’s far too slow for large-scale tests. In fact, Naaman couldn’t even use his own fibers in the steel- and reinforced-concrete-wall tests because it would have taken months to produce enough fibers for one test.

Naaman admits that his finished walls are too expensive to be used in standard new construction. They’re also much stronger than necessary for standard earthquakes and should resist several earthquakes with little damage. But where traditional shear walls aren’t possible, Naaman’s walls are a feasible, even advisable solution for existing buildings that need to be rehabilitated for resistance to earthquakes or other severe motions.

A further advantage: Naaman’s units can be removed and replaced with new ones if there’s damage after a serious quake.

Designed Metals

Carmakers would like make lightweight parts out of designed metals such as the nickel-chromium, above, which has unique expansion-contraction properties and an extremely high melting point.

Jyoti Mazumder, the Robert H. Lurie Professor of Engineering, might be able to lend a hand with Naaman’s search for new fibers. Mazumder, who’s a professor in both the Materials Science and Engineering department and the Mechanical Engineering department, is working on designed metals: alloys created from multiple metals and constructed so precisely that they have properties which, until now, have defied conventional behavior.

Once again, necessity mothered invention. Manufacturers were selecting metals from an existing supply that could be used for a variety of products. This limited manufacturers’ abilities to design new products that required metals with very specific properties.

Mazumder started his work by concentrating on creating entirely new metallic structures. That quickly led him to his current research, which combines theoretical structures in mesoscale (from Homogenization Design Method, developed by Professor Noboru Kikuchi, Mechanical Engineering) with extremely precise construction methods that use self-correcting laser-aided assemblies to place and fuse the metals.

According to Mazumder, materials science was at an impasse—until recently. Using mathematical models, researchers could create structures that on paper appeared to provide new benefits to common products and parts. But in many cases, researchers had no good way to create the physical product that they had mathematically proven could exist.

Now, however, with the methods Mazumder and other scientists in the departments of Mechanical Engineering and Materials Science and Engineering developed, researchers working for product manufacturers have the opportunity to design specialized metals or other compounds for very particular

Scafolding team (l-r): Jason Carroll, graduate student research assistant, MSE; Huan Qi, graduate student, ME; Paul Krebsbach, associate professor, BME, School of Dentistry; Jyoti Mazumder, Robert H. Lurie Professor of Engineering, MSE, ME (not pictured, Scott Hollister, associate professor, BME).

needs. Their work has raised a lot of eyebrows—major manufacturers are already working with several members of the departments to find out how the technology could be applied in their own businesses. Business cards from employees of major American car manufacturers are scattered around Mazumder’s office, and there are examples of items already in production—these include a variety of parts that are necessary to make everything from running shoes to telescope mirrors.

Here’s how Mazumder’s method works.

Using existing metals, he designs an original metal with unique characteristics required by a product. For example, aircraft and car engine parts are a major headache for manufacturers because they have to fit together so precisely, resist enormous heat and pressure and be as light as possible to increase fuel economy. Because metals expand dramatically at the high temperatures inside an engine, construction can be tricky.

In the case of airline engines, air comes in at ambient temperatures, rises to 1200 degrees Celsius, then drops back to much lower temperatures—all within the space of three feet. So putting metals that expand and contract at different rates next to one another in an engine can spell disaster.

Carmakers would like to put more aluminum parts in engines because the parts are so light. But aluminum melts at 660 degrees C, too low for the 1000 degrees C found inside the average engine. The present solution is to line the cylinder with cast iron and put ferrous or copper inserts in the valve seats, which can survive the temperature. But iron expands at a different rate than aluminum. “You get a teenage driver trying to hot rod the engine and there it goes,” Mazumder said.

Mazumder—along with professors Noboru Kikuchi and Deba Dutta, Mechanical Engineering, and Professor Amit Ghosh, Materials Science and Engineering—tackled that metal-mix problem as part of research for a grant from DARPA. Using a complex mathematical model, he was able to design a nickel and chromium mix that actually shrinks instead of expanding when heated. The next problem was turning that model into computerized assembly information that a manufacturing device could use. Traditional computer-aided design (CAD) programs can handle only one material at a time, not a

Above: Three-dimensional model of scaffolding, which is molded into the shape of the tissue that is being replaced. Below: A scaffold loaded with cells is placed into a subcutaneous pouch in a mouse.

mix of several that must be laid down with extreme precision. Fortunately, Dutta had already designed a “heterogeneous CAD system,” which allows the use of multiple metals in the same design. Mazumder translated his model into mathematical directions that the system could understand. Then he laid down the materials in powdered form using a concentric nozzle; a laser at the center of the nozzle melts the powder. Mazumder said it’s “as if you were using a fountain pen to write with real metal.”

The powdered metals are deposited individually, with the laser fusing them together and attaching them to the substrate. Sensors examine the light being emitted during the process for minute changes in dimension, and correct the laser’s heat, target and powder-mix flow rate. The result is a precisely constructed physical copy of the structure that Kikuchi created as a mathematical model.

Mazumder is now working with associate professors Scott Hollister, Biomedical Engineering, and Paul Krebsbach, Biomedical Engineering and School of Dentistry, to take the design and construction system into a whole new area: biological hybrid materials.

These structures contain a mix of organic tissue and precisely designed inorganic materials. Medical technicians could use the system to design replacement bones, for example. Or, using genes from the patient’s bone marrow, they could grow new cells on “scaffolding” that’s molded into the shape of the anatomical part that doctors want to replace. The scaffolding simply serves as a “trellis” on which the cells grow.

The team is now working on making spinal columns that have a titanium frame. So far they’ve created sample vertebrae for mice.

“Early indications are encouraging,” Mazumder said.

The advantages of using a method like this are many: Patients won’t reject parts as often if they’re built with their own tissues. And the parts—bone, for example—grow and develop naturally. This means that, in the case of a child in need of a bone replacement, the implant can be made once and will grow right along with the child. In previous scenarios using a metal substitute for damaged bone, doctors had to replace metal inserts periodically.

Mazumder hopes that someday his research will lead to metals that can biodegrade inside the body, being replaced as the person’s own tissue takes over. That can happen now with polymers, he says, but those compounds aren’t strong enough to withstand the heavy loads put on the human body in places like joints or bones. He theorizes that someday doctors could use designed metals to replace soft tissues.

“Our plan is to go into organs after bone,” he said. “Once you learn how to grow tissues in the presence of inorganic materials, you open up a whole new world. You can make all sorts of things.”—E

Heather Newman is a technology columnist for The Detroit Free Press.