The Use of Composite Materials in the Military and Aerospace Industry
By Jenny Bieksha, Bishop & Associates Inc.

Composite materials have revolutionized a number of industries and gained popularity in high-performance products that need to be lightweight, yet strong enough to take harsh loading conditions. The expected benefits of economical, high-performance military/aerospace designs are being realized through the development of lightweight, high-temperature composite materials to reduce weight, fuel consumption, and operating costs.

Modern aviation, both military and civil, would be much less efficient without composites. In fact, the demands made by that industry for materials that are both light and strong has been the main force driving the development of composites. It is common now to find wing and tail sections, propellers, and rotor blades made from advanced composites, along with much of the internal structure and fittings. The airframes of some smaller aircraft are made entirely from composites, as are the wing, tail, and body panels of large commercial aircraft.

Driven by market and application-specific requirements, composite interconnect devices are replacing those manufactured from brass, nickel, aluminum, bronze, or stainless steel. Composite interconnect components are ideally suited for use in environments where resistance to high temperatures, electromagnetic compatibility, outgassing, and halogen-free operation are required. Composite materials are generally stronger than steel, provide excellent resistance to corrosion, and offer greater durability; yet weigh substantially less than their steel counterparts.

Making a Composite
Composites are made up of individual materials referred to as constituent materials. The purpose of a composite is to create a substance that combines the constituent parts in some beneficial way. There are two categories of constituent materials: matrix and reinforcement. At least one portion of each type is required. For the matrix, many modern composites use thermosoftening or thermosetting plastics (also called resins). The plastics are polymers that hold the reinforcement together and help determine the physical properties of the end product.

Thermosoftening plastics are hard at low temperatures, but soften when heated. Although less commonly used than thermosetting plastics, they do have some advantages, such as greater fracture toughness, long shelf life of the raw material, capacity for recycling, and a cleaner, safer workplace because organic solvents are not needed for the hardening process.

Thermosetting plastics (thermoplastics) are liquid when prepared, but harden and become rigid (cure) when heated. The setting process is irreversible, so these materials do not become soft under high temperatures. When the plastic matrix is enhanced with glass fibers, the thermoplastics resist wear and attack by chemicals, making them very durable, even when exposed to extreme environments. Such materials also provide design flexibility and high dielectric strength.

If classified by matrix, then there are thermoplastic composites, short and long fiber thermoplastics, or long fiber-reinforced thermoplastics. The most common are known as polyester, epoxy, phenolic, polyimide, polyamide, and polypropylene. Ceramics, carbon, and metals are used as the matrix for some highly specialized purposes. For example, ceramics are used when the material is going to be exposed to high temperatures and carbon is used for products that are exposed to friction and wear. Cermet (ceramic and metal) is an example of a ceramic matrix composite.

Polymers are not only used for the matrix, they also make a good reinforcement material in composites. For example, Kevlar is a polymer fiber that is immensely strong and adds toughness to a composite. Although glass fibers are the most common reinforcement, composites can also use metal fibers reinforcing other metals, as in metal matrix composites (MMC). In comparison with conventional polymer matrix composites, MMCs are resistant to fire, can operate in wider range of temperatures, do not absorb moisture, have better electrical and thermal conductivity, are resistant to radiation damage, and do not display outgassing. These are usually more expensive than the more conventional materials they are replacing and used where improved properties and performance can justify the added cost. Today these applications are found most often in aircraft components and space systems.

Strength and thermal resistance are the most sought-after characteristics in polymers used in high-performance applications. Products intended for commercial and military aerospace applications must be produced using “engineering plastics” or other specialized, high temperature polymers. Engineering plastics such as Polyetherimide (PEI), polyphthalamide (PPA), polyphenylene sulfide (PPS), and Polyamide-imide (PAI) are designed specifically for use in high operating temperature environments. Resins such as polyetheretherketone (PEEK) and various liquid crystal polymers (LCP) are also capable of withstanding extremely high temperatures. These later, high-performance plastics also meet stringent outgassing and flammability requirements.

The Benefits to Using Composites
We all depend on composite materials in some aspect of our lives. Fiberglass, developed in the late 1940s, was the first modern composite and is still the most common. Comprising about 65% of all composites produced today, you may well be using something made of fiberglass without knowing it. Composites are less likely than metals to break up completely under stress. A small crack in a piece of metal can spread very rapidly with very serious consequences (especially in the case of aircraft). The fibers in a composite act to distribute the stress and block the widening of any small cracks.

In any composite, fibers carry the load and their type, amount, orientation, and straightness determine their effectiveness. Fiberglass is used for applications where toughness, electrical non-conductivity, or abrasion resistance is required. Carbon fiber is used for applications requiring high strength and stiffness. The composite resin transfers loads between fibers, protects them, and holds them in the correct location and orientation. Resin type determines water and chemical absorption and sensitivity, mechanical properties at elevated temperatures, and compressive strengths and stiffness. In addition, the resin type determines the method of fabrication of the final structure and its cost relative to alternate resin types and fabrication methods.

An increasing number of products use composite materials as more manufacturers discover the benefits, including:

• Composites are incredibly lightweight and increasingly being specified in interconnect systems as a weight reduction measure. For most applications, the typical weight reduction for composites over aluminum is approximately 40%, and 80% over brass and stainless steel.

• Composite materials are extremely strong. An example of this are the high tenacity structural fibers used in composites, which are widely used in body armor. Due to high strength composites, soldiers are well protected from blast and ballistic threats.

• Composites are highly resistant to chemicals and will never rust or corrode. This is why the marine industry was one of the first to adopt the use of composites.

• Polymer plastics are less subject to harmonic resonance, so threaded components made from these materials are less likely to loosen when exposed to extensive vibration and shock.

• Certain composites are non-conductive. This is important because often a structure is needed that is strong, yet will not conduct electricity.

• Composites offer a reduction of magnetic, corrosion-related magnetic and acoustic signatures, which is critical in the development of stealth applications.

Use of Composites in Mil/Aero Applications
The greatest advantage of composite materials is strength and stiffness combined with lightness. It is more difficult to design composite components that take advantage of these properties while still meeting the requirements of the application for form, fit, and function. By choosing an appropriate combination of reinforcement and matrix material, manufacturers can produce properties that fit the requirements for a particular structure for a particular purpose.

Electronic connectors for power and data in military and aerospace are shrinking in size and weight in response to demand for soldier-worn electronics, unmanned vehicles, and other applications where small size and weight are solid requirements. Many military customers are seeking smaller, lighter, more dynamic solutions that meet the industry’s stringent performance and durability specifications. Recent design and material developments have enabled advancements in connector technology that meet these demanding performance and environmental parameters.

Composite thermoplastics are at the heart of many advanced stealth application development projects. One area where this is particularly apparent is in the field of unmanned aerial vehicles (UAVs). Composite materials have been used extensively in the construction, the net result being proven close surveillance at an affordable price.

Thermoplastic composites provide a high level of durability and toughness, making them very suitable material systems for many avionic structures. These materials offer light weight, high strength and operational robustness that is able to surpass many metals and thermo-set composite materials.

Atmospheric conditions require components used in space applications to include non-outgassing and non-magnetic materials. Carbon composite is a key material in today's launch vehicles and heat shields for the re-entry phase of spacecraft. It is also widely used in antenna reflectors, yokes of spacecraft, payload adapters, inter-stage structures, and heat shields of launch vehicles.

The fact that composites are increasingly being designed and specified in interconnect systems, despite the complications of the design and manufacturing process, is an indication of the true value the materials provide. The downside of composites is usually the cost. Although manufacturing processes are often more efficient when composites are used, the raw materials are expensive. Composites will never totally replace traditional materials like steel, however the significant advantages of composites represent real, out-of pocket savings in fuel consumption and lifetime system maintenance for a broad range of military and aerospace applications. No doubt, we have yet to see all that composites can do.

Jenny Bieksha Director, Renewable Energy, Medical, and Military, Bishop & Associates Inc. Jenny Bieksha joined Bishop & Associates in 2008 as its market segment director for the renewable energy, and the test, measurement, and instrumentation markets. She is currently a management consultant specializing in strategic business planning, with an emphasis on the development of program, market, and product plans. Bieksha has more than 20 years of experience in the electronics industry, with a background in market management, business development, channel sales, product management, and operations for ITT Corporation, Delphi Connection Systems, and Hughes Aircraft Company. Bieksha has a bachelor of science degree in marketing from the University of Wyoming, and has since received heralso holds a certificate as a project management professional.