M. McElhone, D. Rugg, Rolls-Royce plc, Derby, United Kingdom

Summary:

The dovetail root fixing is a common method used for attaching aero-engine fan and compressor blades to their corresponding disc. To maximise component life, bedding flanks can have a combination of surface treatments and surface coatings. This paper will discuss the use of these treatments on the fan blades of the Rolls-Royce Trent family of engines.

During service operation, the fan assembly is subject to a complex loading system, consisting of centripetal load, gas-bending and vibration. In addition, to complicate matters further, the relative magnitude of these loads varies throughout a flight-cycle. The resultant relative displacement between bedding flanks, combined with the centripetal loading, leads to wear of the applied low-friction coating. This can result in wear and fretting damage to the underlying material, potentially reducing component life.

Full-scale component testing involves extremely costly, long lead-time programmes and is therefore used for final validation of a new design. Conversely, at the design-optimisation stage, simple laboratory specimens provide a much more cost-efficient alternative. However, these rarely replicate the complex loading regime encountered in the real engine environment.

At Rolls-Royce, an experimental method, utilising ‘sub-element’ testpieces, has been developed, which can be used to replicate the loading regime seen in the engine. This method has been used to evaluate the fatigue performance of various dovetail geometries and the effect of improved coatings and surface treatments. As part of this testing, the effects of laser shock peening (LSP) on dovetail fatigue performance has been investigated. The test method and results of these experiments will be discussed, in detail, and hypotheses proposed, which explain the observed behaviour.