Aerospace is the winner in clash of metal and composites

Farnborough Air Show » 2006
November 14, 2006, 11:42 AM

What are the advantages and disadvantages of carbon-fiber reinforced plastics?

A key advantage of CFRPs is their strength-to-weight ratio. Their resistance to fatigue, damage and corrosion is also highly rated. They are still being improved–with higher-strength fibers, for example; however, some drawbacks remain. “For example, if electrical conductivity is needed–as it is to cope with lightning strikes–you have to ensure it by another means,” Rosner explained to Aviation International News. And production cost for CFRPs remains higher than that for metals, although it has been consistently decreasing.

The new Boeing 787 is viewed as the flagship for CFRPs; some 50 percent of its airframe is made of composite materials. “We elected CFRP for the fuselage after having considered the aircraft’s entire life cycle–design, production, in-flight life, maintenance and end of life,” said David Polland, a Boeing lead structures engineer.

On the Airbus A350 program, CFRPs are allowing the wing to be smarter. It is a passive load-adaptive wing. “Smart directions of fibers enable geometry changes and thus aeroelasticity,” Rosner said.

Dutch-based Stork Aerospace (Hall 3 Stand C6) is working to improve one kind of CFRPs, called fiber-reinforced thermoplastic. Using it, one can make a floor beam of the lowest possible weight in one manufacturing step. A light floor beam could therefore be available at acceptable cost. Although some applications have been found already, manufacturing load-carrying structures with this material still involves some development work.

What about glass-fiber reinforced plastics?

Both Airbus and Boeing representatives pointed to lower performance of GFRPs. “We usually prefer carbon for its lightness and higher strength,” Rosner said. Polland echoed Rosner’s opinion and mentioned stiffness of GFRPs as a weakness.

GFRP, however, has a higher damage tolerance than CFRP, and therefore, it is used on the Airbus A380’s vertical tailplane leading edges. Impacts are more frequent on this part of an airplane. “As a raw material, it is less expensive,” Polland added. It also is compatible with aluminum in terms of its galvanic properties (electrical conductivity), which makes interfaces simpler in terms of corrosion prevention.

How are alloys, such as aluminum- lithium, being used?

Threatened by the increasing use of composites, metals have struck back in recent years. “Aluminum-lithium [Al-Li] alloys are now the spearhead of metal technology,” Rosner said. He sees Al-Li as mature and therefore competitive with composites and Glare. Compared to standard aluminum alloys, Al-Li is said to have higher stiffness, lower density and better corrosion resistance.

“On the A380, the cockpit floor’s lower crossbeams use this alloy,” Rosner said. On the A350’s pressurized fuselage, the skin, stringers, frames and crossbeams will be made of Al-Li, he added.

The Al-Li used on the A380 and A350 is the third, and current, generation of the product. A fourth generation is in view, with an increased percentage of lithium. “Lithium decreases the alloy’s density but degrades its thermal stability at high percentage levels,” Rosner explained. Alloys with a poor thermal stability expand when heated, hence the need for a new approach to go beyond the current 2-percent level.

How important is titanium on an airframe?

It accounts for no less than 9 percent of the A380 and 15 percent of the 787 airframes. “In terms of strength and density, it is somewhere between aluminum and steel, the latter being the strongest and heaviest,” Polland explained. It has very good corrosion resistance, thermal stability and durability properties. It also can resist high temperatures. It is a good match with composites–from a galvanic standpoint.

“But it is costly,” Rosner pointed out, before adding that its cost must be carefully measured against its value to a specific application. In general, titanium is a heavier material than alloys or composites, but it offers great strength, as well as heat and corrosion resistance, which is particularly valuable when it comes to engine pylons.

Boeing and Airbus seem to agree on the use of titanium. On the 787 and A380 programs, they employ it on the engine pylon structures and landing gear. Its resistance to corrosion makes it useful, too, in areas prone to moisture caused by fuel and water leaks.

Recently, Russian firm VSMPO-Avisma inked titanium supply agreements with both Airbus and Boeing. According to French daily newspaper Les Echos, demand for titanium is increasing in the aerospace and defense sectors, creating problems in the procurement of the metal. Rosner disagreed, however: “Procurement is not a problem yet,” he said.

Are composites as easy to maintain as metals?

On this point, Airbus and Boeing are not united.

Boeing’s Polland commented: “Yes, we now have many years of experience. Airlines spend more and more time each year checking for corrosion, so composites are even better. And technology progressively brings more possibilities for bonding.”

However, according to Rosner of Airbus, composites are not competitive everywhere in terms of maintainability, hence the choice for metal on the A350’s fuselage. Similarly, the A320 has composite inner flaps but experience impelled Airbus to go back to metal on the equivalent part for some other programs, including the A380 and A350. “These are thin parts, exposed to damage,” Rosner said. He still sees composites preferred for the empennage or wing applications, where panels are thicker and far from the ground.

Could one single material combine he best of both worlds–metal and composites?

Glare is a candidate. It is a hybrid material, built of alternating layers of aluminum foil and unidirectional glass fibers, impregnated with an adhesive.

Stork Aerospace developed this new fiber laminate with Airbus. The density of conventional aluminum alloys is close to 2.8, whereas that of Glare is approximately 2.1 and CFRPs have a density of about 1.6.

Fatigue and damage resistance are superior to those of aluminum. The energy required to create a dent (of a given depth) on Glare is much higher than it is for aluminum. Crack propagation is so slow that a crack cannot reach a critical size in the entire aircraft’s life in service, the experts say.

The A380’s passenger version is Glare’s first application. Airbus chose the material for the upper fuselage in the forward and aft sections because of its fatigue performance and damage tolerance. It chose aluminum for the center section because that area is subjected to higher static loads because of the wing. On the freighter version, which will undergo less fatigue, a higher-static-strength Glare will provide even more weight gain.

So far, Glare has found application on only the A380. “We are still in the race for the A350,” said Giel Van der Kevie, Stork’s research-and-development manager for Glare and fiber metal laminates. Is this too optimistic? “We also are looking at new variants of Glare,” he said. A feature of this hybrid material is that it can accept new fibers and new alloys that are developed.  

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