Boron Composites Under the Top

Boron Composite Structures in Aviation

The purpose of this paper is to discuss, and analyze the development and application of boron composites in airframe structures.

BORON'S HISTORY

Compounds of boron, most notably from unfinished borax ore, known as Tincal, were exported from Tibet in olden times. Historically, boron has been used for refining gold and silver in Arabia, ceramic glazes in China, and embalming in Egypt.

During the 13th Century A.D., "Regular imports of Tincal (name derived from tincana, the Sanskrit word for borax), from the Far East into Europe began along trade routes taken by Marco Polo's caravans. The source of Tincal (Tibet) and its methods of production remained a secret closely guarded by Venetian traders for four centuries."

Boric acid, a mild antiseptic, was isolated in the laboratory by chemistry professor William Homberg in 1720 and was discovered also occurring naturally from steam vents in Tuscany, Italy. Sir Humphrey Davy, a British chemist, and two French chemists, Joseph Gay-Lussac and Baron Louis Thenard, discovered boron in 1808 concurrently. However, it took another 155 years before science turned its focus from borax to boron, and began to reveal the secrets of this mysterious and valuable element.

"The recovery of pure crystalline boron did not succeed until the 1950s…Because of its ability to absorb neutrons the lighter isotope is important as an alloying component of boron steels used in nuclear reactors" (Stark, H.C). The revival of interest in boron in the late fifties and into the sixties was driven by the ability of the U.S. To introduce large quantities into a potentially lucrative global market. The elemental properties of boron were now well-known, and chemical variants began to find application in: grinding materials, lightweight armor, rocket motor propellant, and tools for metalworking.

Aerospace industries constantly seek new or improved hard, lightweight, heat resistant materials that can be introduced cost effectively into the manufacturing process. The introduction of high strength boron filaments and boron composite epoxy in the early 1960s, although expensive, lead to the rise of composites technology in aerospace manufacturing. Ceramic fibers including boron carbide were developed for use in heat-resistant composite materials such as ceramics.

Many components of helicopters, military aircraft, civil aircraft, missiles, and spacecraft, including satellites and space shuttles, are made from these high-strength, lightweight composites. Different kinds of fabrics and widths of woven composite filaments (some up to 60" wide) substantially increased the speed of hand lay-up processes. Boron's superior draping ability was enormously helpful in the manufacturing of complex and concave shaped products.

The emerging business of boron was clearly on the rise. Industrial consumption of boron in the world grew from 239,000 metric tons in 1964, to 443,000 metric tons in 1970.

Driving this growing market (especially in aerospace) were boron's inherent properties. It has an exceptionally high melting point, which is one reason it is very difficult and expensive to extract. It has the strength of steel, but is as light as aluminum.

Boron composite epoxies are very strong adhesives, corrosion and temperature resistant. Components bonded with boron adhesives require less maintenance. NASA found they could reduce the weight of the space shuttle by using these materials. "By using boron fiber-reinforced materials, there has been a saving of 20% of the take off weight of space shuttles" (Attallah).

RECENT DEVELOPMENTS OF BORON COMPOSITES

In the 1980s boron-reinforced aluminum composites were becoming an industry standard. Improvements in the mechanical properties, resistance to crushing of the filaments, the general robustness of the composite material, and the capability of being reliably shaped and joined led to the favorable reception of these composites.

Since the 1970s boron epoxy and boron fiber composites have been successfully demonstrated on various aircraft airframes including: the horizontal stabilizer of a General Dynamics F-111, the rudder on a selection of 50 McDonnell Douglas F-4 Phantoms, and the tail unit of the F-15A in twin fins and rudders

In the late 1980s the Department of Science and Technology Organization (DSTO) a division of The Department of Defense developed a new crack patching system for aircraft, ships, and other structures. Similar in concept to repairing car body panels with fiberglass, DSTO used a boron composite material, Bortex, to repair cracks in critical airframe and structural components, such as the wings of the C-130 Hercules transport aircraft, and aileron hinges on the Hornet fighter, with patches which are even stronger and stiffer than carbon fiber.

An Australian-based company, Helitech, signed a licensing agreement with DSTO in 1993 and has applied the Bortex crack patch to ageing U.S. Air Force C-141 Starlifter transports and an F111 wing-torque box (see fig. 1). With the Department of Defense under increasing pressure to save money and become more efficient, helping improve its customer's bottom line is now an important justification for much of the R&D, which DSTO carries out.

Research in the long-term exposure of boron epoxy composites in the tropics, completed in 2000, produced in-depth knowledge of the composite's shear capabilities in an environment where moisture uptake of the material was as high as 85%. The experiment, which lasted three and half years, showed there was no significant change to the composites bonding ability.

APPLICATION OF BORON COMPOSITES IN AIRFRAME MANUFACTURING

Boron composites were first employed in airframe manufacturing in Grumman Corporation's F-14A twin-engine, carrier-based jet fighter engineered for the U.S. Navy. "The F-14 will be the first aircraft designed from inception to utilize boron composite outer skins on the horizontal stabilizer over an aluminum honeycomb core (See fig. 2).

To avoid the weight penalty of mechanically fastening the composite fibrous skin to metal, the company uses a boron composite-titanium subassembly that is bonded to the honeycomb core… Structural testing of flight hardware will involve two complete static test airframes and various key components, including titanium wing box beams and boron epoxy stabilizers."