Enabling high-strength composites to reach their full potential

  • 10-Mar-2014 09:51 EDT
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Four years ago, one of a B-2 bomber’s four engines caught fire during what the Air Force called a “routine” engine start. The fire was one of three that the Air Force studied “to identify new tools and techniques that will allow firefighters to more efficiently cut, penetrate, and extinguish burning aircraft made of composite materials.”

The use of composite materials for construction of aerospace components began in the early eighties and is now the material of choice for commercial and military airframe designers. Composite material properties such as stiffness, weight, strength, and corrosion resistance have invigorated the transfer of composites within aerospace to include interior components such as seats and service carts. Interior parts and components account for as much as 40% of a commercial airliner’s empty operating weight and represent a larger market (by volume) than airframe structures.

The same properties that attract aerospace designers to use composite material have also attracted other industries such as transportation, wind energy, and sporting goods equipment manufacturers to evaluate its use for their products. Composite materials offer a highly attractive alternative to metal, but the transfer of the material into high-rate production commercial products such as automobiles has been limited to high-priced luxury performance examples such as Lamborghini.

Composite materials and processes

In their infancy, the performance attributes of composite materials for aerospace parts outweighed the complex manufacturing processes and material cost as parts slowly replaced metal. The focus was on qualifying composite parts and understanding the complexities of replacing well-understood performance and manufacturing processes of metal materials with parts made from the new composite material.

Once composite material, its design criteria, engineering allowables, and manufacturing processes matured and were better understood, the focus turned to cost. The two primary targets for cost reduction to produce parts made from composite materials are manufacturing processes and material cost. The two manufacturing processes where research focus has led to promising results is out-of-autoclave cure (OAC) and advanced analytics to control the inherent part variability of composite materials part fabrication process that complicates the assembly process.

The autoclave cure of composite material is complex and costly. Elimination of the autoclave to cure composites reduces cost and manufacturing complexity, improves throughput, and enables lean production initiatives. Progress has been made in the last decade, and OAC processes such as vacuum assisted resin transfer molded (VARTM) parts are replacing parts formerly requiring an autoclave. VARTM and similar resin transfer molding (RTM) processes are lowering cost and incentivizing the extensibility of composite parts to use in other industries such as wind energy for the manufacture of large wind turbine blades and towers. VARTM and other RTM processes use materials that have a per-pound cost that is a fraction of the prepreg composite materials they replace.

The evolution of computer-aided design has led to a digital tapestry that is beginning to be tapped to mitigate the inherent variability of composite parts. Composite parts vary in thickness and require secondary processing to insure outer mold line (OML) continuity between parts when they are delivered to the assembly floor. The digital tapestry of data derived from discrete manufacturing data at the point of part manufacture provides assembly with the necessary information to adjust the assembly for rapid fit-up and ease of assembly.

It is apparent that the attributes of composite materials have created a market pull for their use, and industry is on the cusp of developing manufacturing processes and lowering material costs that will facilitate composite transfer to a wide range of consumer and commercial products.

Future challenges

The continued use of composite materials in aerospace has revealed other challenges that need to be addressed while the solutions to material and manufacturing cost are reduced. Before the widespread use of the material in transportation vehicles can be realized, safety challenges surrounding the material must be addressed.

Four years ago, one of a B-2 bomber’s four engines caught fire during what the U.S. Air Force called a “routine” engine start. The fire was one of three that the Air Force studied “to identify new tools and techniques that will allow firefighters to more efficiently cut, penetrate, and extinguish burning aircraft made of composite materials.”

Such fires are challenging because hidden interior fires are difficult to extinguish, composites smolder and reignite, and fuselage penetration is virtually impossible with an axe and difficult with a K-12 fire-fighting saw.

As a result, the past few years have seen the emergence of research directives to address the safety challenges of operating a vehicle made from composite material. They include developing a new flammability test method for composites; composite prepregs that display fire resistance; flame-retardant adhesive technology for interior applications; modified cyanate ester for air duct applications; and fire-resistant nano-coatings for foam and fabric using renewable and/or environmentally benign materials. In addition, emergency response teams must be trained and equipped with the tools to enable the rescue of individuals trapped in damaged vehicles.

When these issues are resolved, the attributes of composite materials will be fully realized through their extensibility to other products beyond aerospace.

George N. Bullen, President and CEO of Smart Blades Inc., wrote this article for Aerospace Engineering. He is author of the book “Automated/Mechanized Drilling and Countersinking of Airframes,” recently published by SAE International, and is a member of the organizing committee for the SAE 2014 Design, Manufacturing and Economics of Composites Symposium, to be held June 10-11 in Madrid, Spain.

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