Fan blades for high by pass aero-engines were, for many years, manufactured from solid titanium alloy forgings and were designed with mid span snubbers to control vibration. However, snubbers impeded airflow and reduced aerodynamic efficiency, penalising fuel consumption. Modern designs have deleted the snubber to provide a more aerodynamically efficient aerofoil, and increased the blade chord for mechanical stability, reducing the number of blades by approximately one third. This has been achieved at reduced weight with a hollow construction and an internal core.
For both snubbered and wide chord blades, a conventional fine grain titanium alloy - 6% aluminium and 4% vanadium (Ti6Al4V) is used. It is simple in terms of chemistry, with the aluminium offering strengthening and low density, and the vanadium making hot working of the material easier. It is used for discs and compressor blades up to about 350°C, but excellent superplastic forming and solid state diffusion bonding capabilities make it particularly suitable for the wide chord blade.
The low density core for the hollow design is an integral part of the structure. The two external skins are separated by either honeycomb filler or a superplastically formed corrugation which carries a share of the centrifugal load. Both panel-to-panel and core-to-panel joints must achieve parent material properties to withstand the effects of foreign body impact and fatigue.
For the first generation design the joints are made by a transient liquid phase diffusion bonding process, whereas the second generation employs solid state diffusion bonding in association with superplastic forming of the assembly. The cavity of the bonded construction is inflated at elevated temperatures between contoured metal dies using an inert gas to expand the core and simultaneously develop the blade's external aerodynamic profile.
The reliability of these wide chord blades has been second to none. The step in technology produced a major competitive advantage and ten years passed before an equivalent design appeared from a manufacturer other than Rolls Royce. This service record was the result of thorough development testing. Fatigue testing in both low and high cycle modes was essential. Groups of blades were repeatedly accelerated to maximum speed in vacuum to establish low cycle endurance, and high cycle fatigue was investigated on a static vibration rig up to the maximum stress levels likely to be encountered in service.
With a large forward facing area, resistance to bird ingestion is required. Ingestion of a number of medium size birds has to be demonstrated by running an engine at take-off power and requiring it to ingest four birds within the space of one second. The engine continued to deliver power, accelerating and decelerating for a total period of thirty minutes to simulate the likely operating procedure following a severe ingestion incident.
In the very unlikely event of a blade mechanical failure, the engine has to be shown to be structurally sound and to contain all the debris, even if the failure occurs at maximum power. Containment in modern engines is achieved with aluminium or titanium casings through which the blade fragments can penetrate, to be caught in external windings of Kevlar.
As an indication of the benefits of materials development and design enhancements, engines incorporating the wide chord blade have fan modules that are approximately 24% lighter and an engine which is 7% lighter (typically the Trent 800 engine as used in the Boeing 777).
Looking to the future, some believe that carbon composite materials can be used to reduce weight. At present these materials limit the speeds for which the blade can be designed, requiring a greater diameter for a given thrust. It may be that this alternative approach will converge with the hollow design because airworthiness requirements have led to the incorporation of titanium sheathing around a large part of the composite blade with, of course, some weight penalty. The composite can be considered as an alternative core to the titanium honeycomb or corrugation. Rolls-Royce is studying the future possibility of titanium based metal matrix composites with selective reinforcement provided by silicon carbide monofilaments to control blade untwist.