NATICK, Mass.—Joint research by the U.S. Army Engineer Research and Development Center (ERDC) in Vicksburg, Miss., and the U.S. Army Natick Soldier Research, Development & Engineering Center (Natick) has developed a family of new cloth-type structural components named “Hydrostatically Enabled Structural Elements,” or HESEs. U.S. Patent 8,209,911 was issued to Dr. Charles Welch, Dr. Kevin Abraham and Dr. Robert Ebeling of ERDC, along with Karen Buehler and Claudia Quigley of Natick, for the invention.
The HESE employs common materials such as sand or gravel inside cloth containers to form columns or beams that have expanded load-bearing capabilities. The element takes advantage of the same phenomena that causes vacuum-packed coffee to be rigid and strong. The HESE places the contents under stress through either external pressure, such as vacuum packing, or internal pressurization. The pressurization causes the material to have increased shear, tensile, and compressive strength.
An example of a HESE is a cloth bag filled with sand, then fitted with a pressurized internal bladder. The bag can then be used to form temporary structures, or for temporary beams or columns in damaged buildings, a technology particularly useful in emergency situations where more conventional materials are unavailable.
During joint testing by ERDC and Natick, a HESE consisting of a tubular bag of sand four inches in diameter and 28 inches long was fitted with an internal bladder pressurized to 100 pounds per square inch (psi). This HESE “sand column” was found to support a load of approximately 5,500 pounds.
Another form of the HESE was calculated to be able to support approximately 40 percent greater loads than wooden beams that are often used for supporting damaged structures.
The HESE grew out of ERDC research on carbon nanotube materials that would have much greater strength than existing fibers and cloths.
“We asked ourselves, ‘if you had such a strong material, how could you use it in austere field environments to construct temporary structures,’ said Welch. “The existing advanced textile materials from Natick provided surprisingly large load-bearing capability when used in a HESE, even when pressurized to only moderate levels.”
The Natick fabrics had tensile strengths of approximately 100,000 psi. The tests displayed load-carrying capabilities more than ten times that of unpressurized sand-filled cloth structures.
Ongoing research on carbon nanotubes, which is part of the ERDC Advanced Material Initiative, is directed towards using them to make fibers that have many times the tensile strength of the Natick fabrics. Welch and Dr. Charles Cornwell co-published articles in 2011 and 2012 that provide the design basis for carbon nanotube fibers that would have tensile strengths of up to 8.6 million psi. Such fibers would allow sand-filled HESE structures to carry much larger loads than those of the Natick tests, because these HESEs could be pressurized to much higher levels.
Another group of the Advanced Materials Initiative team, led by Dr. Charlie Marsh and including Ben Ulmen and Erik Wotring, is attempting to synthesize such super strong fibers. While doing so, they discovered a new structural form of carbon nanotubes, called a Self-Assembling Tube Structure (SATS). Both the 8.6-million psi fiber and the SATS are currently "patent pending.”
The Advanced Materials Initiative program, which funded the HESE development, is being conducted jointly by the ERDC Information Technology, Geotechnical and Structures, Construction Engineering Research, and Environmental Laboratories. The program often engages academic, private sector and federal partners such as Natick.
The ERDC is the premier research and development facility for USACE, with more than 2,500 employees, $1.2 billion in facilities, and an annual research program exceeding $2 billion. It conducts research in both military and civil works mission areas for the Department of Defense and the nation.