Are composite materials dangerous?

Aviation Rescue Services

As aircraft manufacturers strive to improve the efficiency of their aircraft, they look increasingly to using new materials in their manufacture. In order to increase the range of aircraft, they strive to reduce the weight of the aircraft and maintain or increase the strength of their aircraft. By incorporating components made from composite fibre into the construction, they are able to achieve these goals. Many aircraft incorporate composite fibres into some portion of their airframe. These are often used as body panel rather than structural members; but with evolving technology, this too is changing.

History of Composite materials
Composite fibres can be traced back to the late 1800s when Edison made carbon filaments from natural cellulose fibres. Eli Whitney patented a process for coating carbon fibres made from cellulose with pyrolytic graphite by flashing it at temperatures of up to 4,000 degrees Celsius. The use of carbon fibres in lamps declined after the introduction of tungsten filaments. In the 1950s, the search for new structural materials for use in rockets and missiles caused an upsurge in interest in carbon and others fibres. Early work at this time on pyrolysed viscose rayon produced relatively strong and flexible fibres. This early process was not easily reproducible, but reliable fibres and fabrics could be produced. These materials were used as heat shields in strategic missiles for the re-entry vehicles and in rocket nozzles during the early 1960s.

The major breakthrough in the manufacture of high strength composite fibres occurred during research conducted from 1963 -1965. During this research, it was found that by subjecting the precursor fibre (starting material) to a controlled continuous tensile stress during high temperature treatment, a very high strength carbon filament could be produced. It is the high specific strength and elasticity of the fibres
produced by this process that made them desirable and useful as structural materials. Aspects of this material allowed designers and manufactures to create and manufacture aircraft that could only be dreamed of before. The forward swept wing is a classic example of the application of this technology in conjunction with aircraft design.

Carbon and graphite fibres used in commercial applications today are made from a precursor which can be any carbonaceous, fibrous, raw material that pyrolyses to a char, does not melt, and leaves a high carbon residue. These same characteristics have an impact on the material’s response when exposed to a fire. The actual physical characteristics of the fibre produced depend on the manufacturing process used to make it. There are a number of manufacturing variables, such as: rates of heating; maximum baking temperature; time at maximum temperature; and the strain applied to the fibre during pyrolysis that can all have an impact on the fibre. The precursors of carbon fibres are usually a continuous and either single or multi-strand filament. In the early 1970s, most US producers used rayon precursors; but by the end of the decade they had switched to a polyacrylonitrile precursor. Regardless of the precursor material used, all carbon fibres are produced basically the same way. The precursor material is pyrolysed at temperatures between 1,100 and 1,200 degrees Celsius. The fibres that result
from this process are essentially amorphous carbon networks.

Graphite fibres
Graphite fibres, on the other hand, are produced by the application of stress to a carbon fibre during the heat treatment at temperatures exceeding 2,200 degrees Celsius. This application of heat and stress creates a crystalline microstructure in the fibre. This structure results in a tenfold increase in the elastic modulus and a simultaneous increase in the electrical and thermal conductivity along the fibres’ axis. This electrical conductivity has resulted in the carbon graphite fibres being used in everyday items such as the spark plug wires in your car. The conductivity of graphite fibres have resulted in their use in military aircraft such as the stealth fighter and bomber to aid in reducing the
radar signature of these aircraft and dispersing the heat from exhaust gases. While technically there are many differences in carbon and graphite fibres from a design and engineering standpoint, from the emergency responder’s perspective these make no difference. There are also other materials used to manufacture composite materials such as tungsten and boron, but for the remainder of this report the term
composite material will be used to refer to carbon and/or graphite composite materials.

Aircraft construction
Making aircraft components out of these fibres can be done in several ways and is handled much the same as fibreglass. Fibres will often be woven in cloth on looms just as any other cloth is made. These cloths are then layered onto moulds with fibres running in predetermined directions to provide the strength needed, just as they layer wood in manufactured wood products. Once the layers of cloth are in place, they have special epoxy binders forced through them and are subjected to pressure and heat to cure the epoxy. The strength of the finished material is dependent upon the strength of millions of individual fibres acting as a single structure. Also, they can use the same basic process but with individual fibres instead of woven cloth being used to produce the desired strength. In the past, this process has been used to produce body panel and some structural components for aircraft; but now as technologies involved in the process have advanced entire aircraft can be produced from composite fibres. The most radical examples are many of the small experimental aircraft being produced by hobby enthusiast and in several smaller corporate aircraft. The extreme example is that of the MV-22 Ospry being produced in the United States for its military. Over 90 per cent of the aircraft and 50 per cent of the aircraft weight is made of composite fibres. The aircraft fuselage itself is manufactured in two halves that are then glued together, much as a plastic model aircraft is assembled. The strength on the
aircraft fuselage is not dependent on structural ribs, but on the honeycomb composite structure itself.

Why Composite fibres?
Aircraft designers and manufacturers are constantly looking for new materials to use that make their aircraft more efficient. The inherent strength and flexibility of components made from composite fibres make them especially attractive. For the same weight, composite fibre structures offer several times the strength of metal components. This strength to weight ratio allows designers to reduce the weight of
components while maintaining their strength. Another important characteristic is the elasticity of the material. Components made of
composite fibres can flex to greatly and return to their original shape. This ability of composite structures has allowed the construction of aircraft with forward swept wings and other radical wing designs. While these designs have only appeared in research aircraft, expect to
see the more successful of the technologies adopted into the regular air fleet. For military applications, the ability of composite fibres to disperse the electromagnetic radiation from radar has large-scale application. It is this characteristic of composite fibres that gives the stealth aircraft a portion of their low visibility. These same characteristics have resulted in composites invading our daily lives as parts of automobiles, tennis rackets, and golf clubs, to name just a few applications.

Hazards to responding personnel
Composite fibres present some very specific hazards for emergency response personnel when involved in a mishap. Actually it is not the fibres themselves, as they are simple carbon, but the residues and other materials they may pick up or the products of combustion from the epoxy binders that produce the direct hazards to responders’ health. The risk of structural failure of composite fibres following fire exposure presents additional risks to personnel. The direct threats posed by composite fibres are either respiratory or skin irritation. The various types of agents used to bond the fibres together can produce many toxic products when exposed to fire. Many of these resins are made up of compounds of hydrogen, chlorine, oxygen and nitrogen. When burned, the various resins can produce smoke containing formaldehyde,
toluene, ammonia, and carbon monoxide; but they can also produce such highly toxic gasses as hydrogen chloride, nitrogen dioxide and hydrogen cyanide. These gasses can remain present in the area as the aircraft debris smoulders and will present a hazard to unprotected personnel for substantial period of time. A secondary respiratory hazard is that of inhaling microscopic fibre components. Again the hazard is not from the fibres themselves but from items that may be carried by the fibres. The best defence for personnel from both of these hazards is the use of self-contained breathing apparatus (SCBA). Given the high number of hazards present in any craft as either part of the aircraft or potentially as cargo, SCBA should be considered an essential part of any emergency response, not only for firefighters but for all personnel required to operate in the immediate vicinity of a aircraft mishap (i.e., medical personnel). Composite fibres can also present a skin irritation hazard to personnel. This is very similar to that encountered when working with fibreglass. The fibres themselves do not appear to present any long-term health concerns; but the materials, which can be carried on them (fuel, hydraulic fluids, and residue from the binders), can cause some longer lasting irritation. The irritation from the fibres will eliminate itself as the fibres are either forced from the body or
absorbed. There is also concern about infection when any foreign body enters the body. The use of protective clothing will reduce or eliminate this hazard. Protective clothing including self-contained breathing apparatus worn by firefighters will provide them with protection during emergency response. During salvage and overhaul operations, disposable coveralls can provide protection to the wearer from fibres.

A third hazard to response personnel is the loss of strength in composite structures when exposed to fire. Many of the bonding agents that hold most composite structures together will break down when exposed to temperatures in excess of 370 degrees Celsius. These temperatures can easily be reached in most aircraft mishaps involving fuel fires. As a result of this exposure, the fibres can turn into a carbon powder with virtually no strength. Composite structures can also be fractured by impact. While they may appear to still be intact, they can have fractures
running throughout the structure, changing it from a single piece into numerous smaller pieces. Any external force-exerted on this structure can cause it to fail.

Response Procedures
Responders to incidents involving composite fibres need to obey some basic rules to ensure their safety and that of the public. These are the same rules they should use when responding to any aircraft incident, because there are many other portions of the aircraft that pose a greater risk than those of composite fibres. The most important action is to wear your full protective clothing including SCBA. This affords you the
greatest level of protection, not only from composite fibres but also from fuel and other hazards present. An approach from up-wind will not only carry any airborne fibres away from responders: but also other products of combustion and fuel vapours. Limit the number of personnel in the accident scene to the minimum number of personnel needed to accomplish rescue and firefighting. This is perhaps the most difficult action to control, since everyone wants to help at the scene of a disaster. It can only be accomplished through pre-planning and training. This training has to involve all agencies that will respond to an aircraft incident. While the emergency response will differ little in incidents involving composite materials, you may not even know they are involved until well into the incident. The post response period is when a number of protective actions can be taken. A simple method to reduce the release of composite fibres is to keep the material wet. This involves gently applying either water or foam to the portion of the fibre damaged by the incident, whether from fire, impact or both. Simply keeping the material damp will prevent most fibres from being released. For longer-term protection, water emulsion or acrylic floor wax can
be applied with sprayers on the affected areas to bond the materials together. Areas treated with wax can then be wrapped in plastic to further reduce the release of fibres during handling. Protection of personnel conducting salvage operations has to also be considered. While emergency response personnel will usually have personal protective clothing and SCBA, many of the personnel involved in aircraft removal will not. Measures that can be taken to protect personnel from these hazards, include the use of Tyvek or other similar type coveralls, rubber
gloves with leather out gloves and full face respirators with appropriate cartridges for any vapours that may be present. Protective clothing should be removed upon exit from the site and disposed of in accordance with local policies for the other hazards present at the scene. Personnel should be allowed to shower to ensure that no contaminants are taken away from the work place.

Composite fibres in and of themselves should not pose any special problems or risk to emergency response personnel. They have some unique characteristics that responders need to know about, but by taking some simple precautions that should be followed at any incident –
regardless of the type of construction in the aircraft – you can ensure the safety of your personnel. The first line of defence against composite fibres, just as it is against other hazards, is the proper use of adequate personal protective equipment to include self-contained breathing apparatus.

Published in IAFPA Bulletin October 2004

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