Academic Reading — Test 29

Reading passage

For most of the twentieth century, the bodies of large passenger aircraft were assembled from sheets of aluminium alloy, riveted together over a metal skeleton. In recent decades, however, manufacturers have increasingly turned to carbon-fibre reinforced polymer, a composite material that combines remarkable strength with comparatively low weight. The fuselage, which is the central tube-shaped body of the aircraft, is one of the structures that has benefited most from this shift. Reducing the weight of the fuselage lowers fuel consumption across the working life of the aeroplane, and the savings over many years of service can be considerable. The material also resists the corrosion and metal fatigue that gradually weaken aluminium, which means inspection and maintenance schedules can sometimes be relaxed. The starting point for any carbon-fibre component is the fibre itself. Carbon fibres are extremely thin filaments, each far narrower than a human hair, produced by heating a precursor material until almost everything except the carbon has been driven off. The most common precursor is a synthetic polymer, and the heating process is carried out in stages at carefully controlled temperatures. Thousands of these filaments are gathered together into bundles known as tows. On their own the fibres are stiff and strong in the direction along their length, but they are brittle and offer little resistance to forces applied across them. For this reason they are never used alone; they must be held within a surrounding material that binds them together and transfers loads between them. That surrounding material is a polymer resin, usually an epoxy, which forms what engineers call the matrix. In many aircraft factories the fibres are supplied already coated with partially cured resin, in a form known as prepreg. Because prepreg contains its resin in a precise, pre-measured ratio, it removes much of the guesswork involved in mixing and applying resin by hand, and it produces more consistent results from one panel to the next. Prepreg must be stored at low temperature to stop the resin from hardening prematurely, so it is typically kept in freezers until shortly before it is needed on the factory floor. The shaping of a fuselage section begins with a mould, or tool, that defines the final geometry of the part. Layers of prepreg are laid onto this mould one at a time, a process traditionally carried out by hand but now frequently performed by computer-guided machines that place narrow strips of material with great accuracy. The orientation of the fibres in each layer is chosen deliberately: by rotating the direction of the tows from one layer to the next, engineers can tailor the strength of the finished part to match the loads it will carry in flight. A single fuselage panel may contain dozens of such layers stacked in a planned sequence. Modern designs frequently aim to produce the fuselage as a small number of large, barrel-shaped sections rather than as many small riveted plates, which reduces the number of joints and fasteners required. Once the layers are in place, the assembly must be consolidated and the resin cured, or hardened, through the application of heat and pressure. This is most often achieved in an autoclave, a large sealed vessel resembling a pressurised oven. The part is sealed inside a flexible bag from which the air is removed, so that atmospheric and applied pressure press the layers firmly together while the temperature is raised. The heat triggers a chemical reaction that permanently sets the resin, locking the fibres into their intended shape. Controlling this stage precisely is essential, because trapped air or pockets of vapour can leave tiny voids that weaken the structure. After curing, the component is allowed to cool slowly before it is removed from the mould. The finished section is not ready for service immediately. It is trimmed to its final dimensions, and holes are drilled where it will be attached to other parts of the aircraft. Crucially, every major component undergoes thorough inspection, since flaws hidden beneath the surface cannot be detected by eye. Engineers commonly use ultrasonic scanning, in which sound waves are passed through the material and the returning echoes reveal internal defects such as voids or areas where layers have failed to bond. Only once a part has passed these checks is it cleared for assembly. Although manufacturing composite fuselages demands costly equipment and tightly controlled conditions, the weight savings and durability of the result have made the approach central to the design of the latest generation of airliners.