VIBRATION is the enemy of all aircraft, as the fatigue it induces in crucial components eventually consigns them to the scrapyard. Now a 鈥渟mart鈥� material which damps that vibration could preserve airframes 鈥� and even make planes less noisy.
The vibrations are generated when vortices resulting from the plane鈥檚 motion through the air pass over the aircraft鈥檚 skin. Over time, this 鈥渇lutter鈥� stresses the skin and shortens the life of the plane. 鈥淐ommercial aircraft are designed for 40,000 to 60,000 flights,鈥� says Christian Boller of the University of Sheffield in the UK. 鈥淭oo much vibration can easily reduce that to 20,000.鈥�
How can the these vibrations be reduced? The frequency at which a material resonates depends in part on its Young鈥檚 modulus 鈥� a measure of its stiffness. Simply placing stiff metal wires in the composite material to strengthen it will not work because it needs to be tuned to cope with different vibrational frequencies experienced in flight. 鈥淐ontrollable stiffness is what you need,鈥� says Philip Irving, an aircraft structures engineer at Cranfield University in the UK.
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So engineers at the Fraunhofer Institute for Structural Durability in Darmstadt, Germany, and Boller鈥檚 team at Sheffield set about producing a reinforced carbon composite material that can vary its Young鈥檚 modulus 鈥� and therefore its resonant frequency. To do this, they report in the journal Smart Materials and Structures () they turned to a material called a shape memory alloy (SMA), which can switch between two shapes at a specific temperature. Crucially for the engineers, this switch changes the SMA鈥檚 stiffness too. They made a composite of epoxy resin reinforced with the usual Kevlar fibres 鈥� but unusually they also added a carpet of hair-thin wires made from a nickel, titanium and copper-based SMA.
From this composite they constructed a half-metre-square aerofoil in which the SMA strands are connected to an electrical circuit. Sending a current through the SMA wires heats them to their switching temperature of 65 掳C.
With the aerofoil mounted on a test rig they vibrated it at its resonant frequency of 90 hertz. When the current was turned on, the SMA stiffened, reducing the amplitude of the vibration. 鈥淥nly 7 per cent of the composite is SMA, yet we change the stiffness by a factor of three,鈥� Boller says, adding that the approach could also be used to reduce aircraft noise. Now the challenge is to scale up the technology to aircraft-sized parts.
鈥淭he degree of variation in resonant frequency of these structures is impressive,鈥� says engineering expert Andrew Flewitt at the University of Cambridge. 鈥淚 am sure there will be applications for this well beyond the aerospace sector.鈥�
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