This analysis was posted on GLG News October 18th, and is reproduced here for our readers.
Aluminum-Lithium alloys will likely become the material of choice over composites for the fuselages of the next generation of narrow-body aircraft. Lighter than traditional Aluminum, the trade-offs between composites and the new lightweight alloys appear to be favoring Aluminum-Lithium.
Carbon fiber composites have made tremendous inroads on commercial aircraft, primarily due to their light weight and high strength. The Boeing 787 wide body airliner will be the first example of an aircraft fuselage and key structural components being constructed with composites, to be followed by the forthcoming Airbus A350XWB.
But those materials appear to be taking a back seat to new technology metals, particularly Aluminum-Lithium alloys, such as Airware™ from Alcan, a division of Rio Tinto. These new alloys are significantly lighter than traditional Aluminum construction, have significantly improved corrosion resistance, and are lower in cost than composite materials.
Why is Al-Li overtaking composites for narrow-body aircraft? The answer lies in an analysis of the trade-offs between the two materials in airline operations.
Narrow-body airliners, unlike wide body aircraft, fly short-haul flights and operate many more take-offs and landings each day. This requires a more robust airframe to withstand the stress of multiple daily take-offs and landings, as well as the pressurization cycles and the inherent stresses on the aircraft.
Airframes are subject to ground damage, often from baggage loaders and carts that may inadvertently strike the airframe during routine operations. Carbon fiber composites, while high strength, can incur damage on the inside, rather than the outside, of the airframe from a strike. As a result, non-destructive testing, such as an x-ray device, is necessary to determine whether damage has occurred and if it is significant enough to require a repair process to ensure structural integrity of the airframe.
For Aluminum-Lithium structures, damage can be easily determined through inspection, as with traditional aluminum structures, and straightforward repairs undertaken using “scab patches” of additional material as necessary.
Of course, composite structures can be made stronger and more damage tolerant. One aircraft manufacturer, in evaluating the trade-offs, indicated that an additional ply of composites would be required to those used on the Boeing 787 to provide the level of damage tolerance of Al-Li alloys, and that additional ply would virtually eliminate the weight advantages of the composite material.
Composite materials require different manufacturing processes than more traditional aluminum construction. Composites require curing in ovens or autoclaves, and significantly different manufacturing processes to manufacture, and attach, components to an aircraft. Aluminum-Lithium alloys utilize the same manufacturing processes as aluminum, which are well known and already established.
Cost and Durability
Tradeoffs also exist between the cost and durability of these materials. The cost of composites is higher than that of aluminum, but is not subject to corrosion. While the new alloys are much more corrosion resistant than traditional materials, corrosion in certain operating environments remains possible. Composites, by contrast, may delaminate should moisture enter a flaw in the surface, as evidenced by delamination around fasteners in some composite tail structures on today’s aircraft. From a durability standpoint, the behavior of Al-Li is well known, while the long-term behavior of composites remains to be proven for aircraft structural applications.
Aluminum-Lithium alloys are a fully recyclable material, and at the end of an aircraft life can be recycles into a new aircraft. Carbon fiber composites include carbon fibers, as well as a metallic mesh to protect against lightning strikes, encased in cured resin. The high strength carbon fibers and metallic mesh are difficult to extract from the material, making recycling difficult if not impossible.
The behavior of Aluminum-Lithium and Composite components vary significantly in a crash. Aluminum is less rigid than carbon fiber composites, and tends to crush and absorb impact during a crash. Carbon fiber composites, by contrast, tend to shatter on impact, with the high strength carbon fibers separating from the resin upon impact. In a crash scenario, will carbon fiber composite provide an equivalent level of protection for passengers than aluminum alloys, particularly should a fire occur, given the toxicity of the materials and the potential for fire to reach a passenger cabin through a gap? The jury remains out.
After examining the trade-offs between carbon fiber and Aluminum-Lithium construction for the next generation of narrow-body aircraft, Bombardier selected Al-Li for the fuselage of its forthcoming CSeries. A senior executive at Airbus has also indicated that their narrow-body replacement would not necessarily utilize composite structures. The aluminum manufacturers are fighting back with innovative materials, and aircraft manufacturers now have a competitive choice.