Ultra-stiff and lightweight: Carbon-fiber epoxy honeycombs mimic material performance of balsa wood

 

Like other manufactured products that use sandwich panel construction to achieve a combination of light weight and strength, turbine blades contain carefully arrayed strips of balsa wood from Ecuador, which provides 95 percent of the world's supply.

For centuries, the fast-growing balsa tree has been prized for its light weight and stiffness relative to density. But balsa wood is expensive and natural variations in the grain can be an impediment to achieving the increasingly precise performance requirements of turbine blades and other sophisticated applications.

As turbine makers produce ever-larger blades -- the longest now measure 75 meters, almost matching the wingspan of an Airbus A380 jetliner -- they must be engineered to operate virtually maintenance-free for decades. In order to meet more demanding specifications for precision, weight, and quality consistency, manufacturers are searching for new sandwich construction material options.

Now, using a cocktail of fiber-reinforced epoxy-based thermosetting resins and 3D extrusion printing techniques, materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering have developed cellular composite materials of unprecedented light weight and stiffness. Because of their mechanical properties and the fine-scale control of fabrication, the researchers say these new materials mimic and improve on balsa, and even the best commercial 3D-printed polymers and polymer composites available.

A paper describing their results has been published online in the journal Advanced Materials.

Until now, 3D printing has been developed for thermo plastics and UV-curable resins -- materials that are not typically considered as engineering solutions for structural applications. "By moving into new classes of materials like epoxies, we open up new avenues for using 3D printing to construct lightweight architectures," says principal investigator Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS. "Essentially, we are broadening the materials palette for 3D printing."

"Balsa wood has a cellular architecture that minimizes its weight since most of the space is empty and only the cell walls carry the load. It therefore has a high specific stiffness and strength," explains Lewis, who in addition to her role at Harvard SEAS is also a Core Faculty Member at the Wyss Institute. "We've borrowed this design concept and mimicked it in an engineered composite."

Lewis and Brett G. Compton, a former postdoctoral fellow in her group, developed inks of epoxy resins, spiked with viscosity-enhancing nanoclay platelets and a compound called dimethyl methylphosphonate, and then added two types of fillers: tiny silicon carbide "whiskers" and discrete carbon fibers. Key to the versatility of the resulting fiber-filled inks is the ability to control the orientation of the fillers.

The direction that the fillers are deposited controls the strength of the materials (think of the ease of splitting a piece of firewood lengthwise versus the relative difficulty of chopping on the perpendicular against the grain).

Lewis and Compton have shown that their technique yields cellular composites that are as stiff as wood, 10 to 20 times stiffer than commercial 3D-printed polymers, and twice as strong as the best printed polymer composites. The ability to control the alignment of the fillers means that fabricators can digitally integrate the composition, stiffness, and toughness of an object with its design.

"This paper demonstrates, for the first time, 3D printing of honeycombs with fiber-reinforced cell walls," said Lorna Gibson, a professor of materials science and mechanical engineering at the Massachusetts Institute of Technology and one of world's leading experts in cellular composites, who was not involved in this research. "Of particular significance is the way that the fibers can be aligned, through control of the fiber aspect ratio -- the length relative to the diameter -- and the nozzle diameter. This marks an important step forward in designing engineering materials that mimic wood, long known for its remarkable mechanical properties for its weight."

"As we gain additional levels of control in filler alignment and learn how to better integrate that orientation into component design, we can further optimize component design and improve materials efficiency," adds Compton, who is now a staff scientist in additive manufacturing at Oak Ridge National Laboratory. "Eventually, we will be able to use 3D printing technology to change the degree of fiber filler alignment and local composition on the fly.

The work could have applications in many fields, including the automotive industry where lighter materials hold the key to achieving aggressive government-mandated fuel economy standards. According to one estimate, shedding 110 pounds from each of the 1 billion cars on the road worldwide could produce $40 billion in annual fuel savings.

3D printing has the potential to radically change manufacturing in other ways too. Lewis says the next step will be to test the use of thermosetting resins to create different kinds of architectures, especially by exploiting the technique of blending fillers and precisely aligning them. This could lead to advances not only in structural materials, but also in conductive composites.

Previously, Lewis has conducted groundbreaking research in the 3D printing of tissue constructs with vasculature and lithium-ion microbatteries.

Primary support for the cellular composites work came from the BASF North American Center for Research on Advanced Materials at Harvard.

Additional support was provided by the Materials Research Science and Engineering Center at Harvard, funded by the National Science Foundation (DMR 0820484).

Video: https://www.youtube.com/watch?v=pnGPYwNM4rE

Weekly Drug News Round Up - June 25, 2014

 

Sivextro Approved for Serious Skin and Skin Structure Infections

Sivextro is available for intravenous and oral use Read More...

The U.S. Food and Drug Administration (FDA) has approved Cubist Pharmaceutical’s Sivextro (tedizolid phosphate), an antibacterial drug for serious and possibly life-threatening skin infections. Sivextro, available in oral and intravenous forms, is approved to treat patients with acute bacterial skin and skin structure infections (ABSSSI) caused by bacteria such as Staphylococcus aureus (including methicillin-resistant strains), various Streptococcus species, and Enterococcus faecalis. In clinical trials, Sivextro was as effective as linezolid for skin and skin structure infections. Common side effects include nausea, headache, diarrhea, vomiting and dizziness. On May 23, the FDA also approved Dalvance (dalbavancin) to treat patients with ABSSSI.

 

FDA Safety Update: Olmesartan Cardiovascular Risks for Diabetics Not Conclusive

Recommendations for use of olmesartan, an angiotensin II receptor antagonist, remain the same Read More...

U.S. Food and Drug Administration (FDA) has completed an investigation into the heart safety risks linked with olmesartan (Benicar, Benicar HCT, Azor, Tribenzor, and generics). The FDA believes the benefits of olmesartan in diabetic patients with high blood pressure continue to outweigh the potential risks. The FDA safety review was prompted by concern that use of olmesartan could result in an increased risk of cardiovascular death, an unexpected finding in the ROADMAP trial evaluating olmesartan use to delay kidney damage in diabetics. To evaluate these findings, FDA reviewed additional studies, including a large study in Medicare patients. Patients should not alter their olmesartan regimen without talking to their doctor.

 

FDA Alert: Docetaxel Treatment Can Lead to Alcohol Intoxication

FDA is revising the labels of all docetaxel drug products to warn about this risk Read More...

The U.S. Food and Drug Administration (FDA) is warning that the intravenous chemotherapy drug docetaxel (Taxotere) contains ethanol (alcohol), which may cause patients to feel intoxicated during and after treatment. Docetaxel is a prescription chemotherapy drug used to treat different kinds of cancer, including cancers of the breast, prostate, stomach, head and neck cancers, and non-small-cell lung cancer. Health care providers should consider the risk of alcohol intake in patients receiving docetaxel, especially those who should avoid alcohol intake or minimize its use due to possible drug interactions.

 

New Class Labeling for Testosterone Products Warns of Generalized Clot Risk

Venous blood clots as a possible consequence of polycythemia is already included in the labeling of testosterone products Read More...

Manufacturers of all approved testosterone products are now required to include a warning in product labeling about the risk of blood clots in the veins, also known as venous thromboembolism (VTE), including deep vein thrombosis (DVT) and pulmonary embolism (PE). This new warning, a class labeling change, is not related to an ongoing FDA evaluation of the possible risk of stroke, heart attack, and death in patients taking testosterone products. FDA is currently evaluating the potential risk of these cardiovascular events, which are related to blood clots in the arteries and are described in a January 31, 2014 MedWatch Safety Alert.

 

Bee Pollen Weight Loss Products Linked with Health Risks

FDA has received more than 50 reports of problems caused by tainted bee pollen weight loss products, including at least one death Read More…

As noted by the U.S. Food and Drug Administration (FDA), many bee pollen products make unproven claims about treating or preventing certain diseases, including obesity, allergies, high blood pressure and cholesterol. According to the FDA, the undeclared ingredients sibutramine and/or phenolphthalein have been found in many bee pollen weight loss products, including Zi Xiu Tang, Ultimate Formula, Fat Zero, Bella Vi Amp'd Up, Insane Amp'd Up, and others. Sibutramine was removed from the U.S. market in 2010 due to an increased risk of heart attack and stroke. Phenolphthalein, a laxative and possible cancer-causing agent, is not approved in the U.S.

Morphable surfaces cut air resistance: Golf ball-like dimples on cars may improve fuel efficiency

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Researchers made this sphere to test their concept of morphable surfaces. Made of soft polymer with a hollow center, and a thin coating of a stiffer polymer, the sphere becomes dimpled when the air is pumped out of the hollow center, causing it to shrink.

There is a story about how the modern golf ball, with its dimpled surface, came to be: In the mid-1800s, it is said, new golf balls were smooth, but became dimpled over time as impacts left permanent dents. Smooth new balls were typically used for tournament play, but in one match, a player ran short, had to use an old, dented one, and realized that he could drive this dimpled ball much further than a smooth one.

Whether that story is true or not, testing over the years has proved that a golf ball's irregular surface really does dramatically increase the distance it travels, because it can cut the drag caused by air resistance in half. Now researchers at MIT are aiming to harness that same effect to reduce drag on a variety of surfaces -- including domes that sometimes crumple in high winds, or perhaps even vehicles.

Detailed studies of aerodynamics have shown that while a ball with a dimpled surface has half the drag of a smooth one at lower speeds, at higher speeds that advantage reverses. So the ideal would be a surface whose smoothness can be altered, literally, on the fly -- and that's what the MIT team has developed.

The new work is described in a paper in the journal Advanced Materials by MIT's Pedro Reis and former MIT postdocs Denis Terwagne (now at the Université Libre de Bruxelles in Belgium) and Miha Brojan (now at the University of Ljubljana in Slovenia).

Shrinking leads to wrinkling

The ability to change the surface in real time comes from the use of a multilayer material with a stiff skin and a soft interior -- the same basic configuration that causes smooth plums to dry into wrinkly prunes. To mimic that process, Reis and his team made a hollow ball of soft material with a stiff skin -- with both layers made of rubberlike materials -- then extracted air from the hollow interior to make the ball shrink and its surface wrinkle.

"Numerous studies of wrinkling have been done on flat surfaces," says Reis, an assistant professor of mechanical engineering and civil and environmental engineering. "Less is known about what happens when you curve the surface. How does that affect the whole wrinkling process?"

The answer, it turns out, is that at a certain degree of shrinkage, the surface can produce a dimpled pattern that's very similar to that of a golf ball -- and with the same aerodynamic properties.

The aerodynamic properties of dimpled balls can be a bit counterintuitive: One might expect that a ball with a smooth surface would sail through the air more easily than one with an irregular surface. The reason for the opposite result has to do with the nature of a small layer of the air next to the surface of the ball. The irregular surface, it turns out, holds the airflow close to the ball's surface longer, delaying the separation of this boundary layer. This reduces the size of the wake -- the zone of turbulence behind the ball -- which is the primary cause of drag for blunt objects.

When the researchers saw the wrinkled outcomes of their initial tests with their multilayer spheres, "We realized that these samples look just like golf balls," Reis says. "We systematically tested them in a wind tunnel, and we saw a reduction in drag very similar to that of golf balls."

Now you see it, now you don't

Because the surface texture can be controlled by adjusting the balls' interior pressure, the degree of drag reduction can be controlled at will. "We can generate that surface topography, or erase it," Reis says. "That reversibility is why this is pretty interesting; you can switch the drag-reducing effect on and off, and tune it."

As a result of that variability, the team refers to these as "smart morphable surfaces" -- or "smorphs," for short. The pun is intentional, Reis says: The paper's lead author -- Terwagne, a Belgian comics fan -- pointed out that one characteristic of Smurfs cartoon characters is that no matter how old they get, they never develop wrinkles.

Terwagne says that making the morphable surfaces for lab testing required a great deal of trial-and-error -- work that ultimately yielded a simple and efficient fabrication process. "This beautiful simplicity to achieve a complex functionality is often used by nature," he says, "and really inspired me to investigate further."

Many researchers have studied various kinds of wrinkled surfaces, with possible applications in areas such as adhesion, or even unusual optical properties. "But we are the first to use wrinkling for aerodynamic properties," Reis says.

The drag reduction of a textured surface has already expanded beyond golf balls: The soccer ball being used at this year's World Cup, for example, uses a similar effect; so do some track suits worn by competitive runners. For many purposes, such as in golf and soccer, constant dimpling is adequate, Reis says.

But in other uses, the ability to alter a surface could prove useful: For example, many radar antennas are housed in spherical domes, which can collapse catastrophically in very high winds. A dome that could alter its surface to reduce drag when strong winds are expected might avert such failures, Reis suggests. Another application could be the exterior of automobiles, where the ability to adjust the texture of panels to minimize drag at different speeds could increase fuel efficiency, he says.

John Rogers, a professor of materials research and engineering at the University of Illinois at Urbana-Champaign who was not involved in this work, says, "It represents a delightful example of how controlled processes of mechanical buckling can be used to create three-dimensional structures with interesting aerodynamic properties. The type of dynamic tuning of sophisticated surface morphologies made possible by this approach would be difficult or impossible to achieve in any other way."

Video: http://www.youtube.com/watch?v=_86mIMPbcDg

The research was supported by the National Science Foundation, MIT's Charles E. Reed Faculty Initiatives Fund, the Wallonie-Bruxelles International, the Belgian American Education Foundation, and the Fulbright Foundation.