Trucos para pescados y mariscos: aplicaciones

ALCOA was not happy with the news in 1987 of a new product just at the moment that the product life cycle of the first one was starting. Expensive qualification work needed to be carried out on the Arall-3 material to be able to apply the material on the C-17 aft cargo door. This expensive qualification is necessary for each new type of aircraft material to guarantee production quality and to specify minimum material parameters like strength, which can be used by aircraft designers. Hundreds of specimens from different production runs have to be tested for tensile strength, compression strength, shear strength, and so on for statistical reasons. For Arall, this also meant different production runs for the ingredients: metal layers, metal pretreatment, fibres and adhesive, as well as the final autoclave cycle to bond the laminate and even the stretching process after the cure. The high costs required for qualification form a big obstacle for new aircraft materials, since qualification is only started when it is clear that the material will be applied, and aircraft designers are not inclined to design their structures from materials which are not yet qualified. This is a clear example of a chicken-and-egg situation. Qualification for metals is easier to obtain than for composite materials, whose strength strongly depends on the specific fibre orientations in the stacking sequence (lay-up) of the different fibre/plastic layers of the material. Arall-3 could be qualified for the C-17 according to the metal procedures, proving the advantage of this hybrid material. The ‘Metal Volume Fraction Approach’ was found to be a very strong approach for the qualification of Arall. This meant that one Arall-3 thickness could be qualified and that the strength properties of other thicknesses could be calculated from the relative fraction of the metal relative to the total thickness of the Arall laminate. A linear relationship was proven between the strength values and this metal volume fraction. However, even with this simplified qualification process, hundreds of specimens had to be tested for this single Arall type.

After qualification, the Arall-3 material could be applied in the C-17 cargo aircraft of the US Air Force. The tail section of this aircraft was too heavy and therefore a desperate search for possible weight savings in the back made the application of Arall possible. The aft cargo door of the C-17 consists of two parts: a lower section which hinges down and is used as a ramp for loading the cargo bay of the aircraft and a second part which lifts up to enlarge the size of the opening of the fuselage. The latter part had a size

of 5.6 x 9.7 metres and the outer skin of it was made from Arall. It is a fuselage structure but, unlike other parts of the fuselage, it is loaded in one direction only, i.e. in the length direction of the aircraft, since the door is free to move in the other direction and is not mounted to the rest of the fuselage structure. This unidirectional loading resembles the situation in a wing panel, the structure that was studied so extensively in Delft for the Arall development programme. Because of the unidirectional loading Arall, which had fibres in one direction only, could be applied here.

However, the Arall cargo door appeared to require an extremely elaborate production process. The structure is curved in two directions and therefore the Arall panels were stretch formed into the right contours.

Because of the enormous size of the door and the limited size of Arall sheets, the door had to be constructed from around twenty parts which had to be connected by titanium straps and by fasteners. The titanium parts also had to be manufactured to fit the contour and the drilling of the rivet holes through a package of Arall and titanium proved to be a delicate process. The limited size of the Arall was caused by the thin aluminium sheets in the laminate. These sheets of 0.3 to 0.4 millimetres were hot rolled to this thickness in the ALCOA plant in Davenport near Pittsburgh, and due to the high forces that had to be exerted on the material to reach this thickness, there was a limit to the length of the rolls and therefore to the width of the aluminium sheets. The rolls would bend too much when the width was too large and this would, in turn, create a varying thickness over the width.

Moreover, when the aluminium ingots are rolled down to the thin sheets cracks are created at the edges of the sheets which may even lead to rupture of the sheet and stop the rolling process. When this happens, the product is not accurate enough and has to be scrapped. The importance of the limitation on the size of Arall sheets will return later in our story, since it had to be solved to apply the material in the gigantic structure of the A380 super-jumbo.

In August 1990, Fokker got a license to produce the laminate free of charge because it was also seriously considering applying it in the Fokker 100. A glorious future seemed to be at hand for the material, but at the same time cracks appeared in the relationship between the American aluminium producer ALCOA and the Dutch aramid producer AKZO. The Wall Street Journal reported on the chilly relationship between the two just after Arall was used for the first time in a real aircraft, the C-17. Rivals seemed worried:

‘“When it was part of the big ALCOA, I didn’t consider it competition,” says

Marc Verbruggen, marketing director for AKZO’s rival product, Glare. “Now, he says he does.” AKZO’s Glare even became a competitor of ALCOA’S Arall for a while.