Carbon fiber epoxy honeycombs mimic performance of balsa

Harvard engineers use new resin inks and 3-D printing to lightweight cellular composites.

In wind farms across North America and Europe, sleek turbines equipped with state-of-the-art technology convert wind energy into electric power. But tucked inside many of their  blades is a decidedly low-tech core material: balsa wood.

Like many other manufactured products, wind turbine blades use sandwich panel construction to achieve a combination of light weight and strength. Balsa wood as a core material offers light weight and stiffness, but natural variations in the grain can be an impediment to achieving the increasingly precise performance requirements of wind turbine blades and other applications.

As turbine makers produce ever-larger blades — the longest now measure 75 m (246 ft), almost matching the wingspan of an Airbus A380 jetliner — they must be engineered to meet increasingly more demanding specifications for precision, weight, and quality consistency while operating virtually maintenance-free for decades. Thus, manufacturers are searching for new sandwich construction material options.

Using fiber-reinforced epoxy resins and 3-D extrusion printing techniques, materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS, Cambridge, Mass., USA) and the Wyss Institute for Biologically Inspired Engineering (also in Cambridge) 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.

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

Until now, 3-D printing has been used mostly with unreinforced thermoplastics and UV-curable resins — materials that are not typically considered for structural applications. “By moving into new classes of materials like epoxies, we open up new avenues for using 3-D 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 3-D 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. “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.

A video of the 3-D printing is available at: 
http://www.youtube.com/watch?v=pnGPYwNM4rE&feature=youtu.be

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