Boron composite structures in aviation history

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).

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."

The McDonnell Douglas F-15 Eagle's airframe is partially constructed using boron composites. The twin tail and rudders are boron composite skin over a honeycomb metallic structure. Like the F-14, boron composite materials make up a small percentage of the complete airframe.

CONCLUSION

The use of boron composite epoxies in aerospace manufacturing has far surpassed boron composite fabrics. Although boron composites have successfully tested in many types of aircraft and have proven superior structural replacement for certain metal alloys, their use has been constrained to test articles, the empennage and small non-load bearing components of the Grumman F-14 Tomcat, and the McDonnell Douglas F-15 Eagle.

Boron fiber composites are also finding usage and acceptance in the marketplace of golf clubs, tennis rackets, ski poles and other sporting equipment.

In today's competitive world of aerospace technology, cost ratios of boron composite materials to other composites must be addressed if boron composites are to be practical. Boron composites continue to be almost five times more expensive than carbon/carbon or graphite composites. The cost of refining boron and the creation of the actual fiber needs to be reduced and aligned with current industry financial abilities, if the use of boron-based composites in the manufacturing of aircraft structures is to be adopted.