Increasing the fatigue resistance of fan blades

The initiation and propagation of fatigue cracks can be suppressed by the introduction of compressive residual stresses. The most commonly used method is to fire “shot” at the surface (Shot Peening), which generates compression to a depth of about 100–500 µm. In recent years, however, Rolls-Royce has introduced a new method called Laser Shock Peening (LSP). In LSP, a high-power laser pulse irradiates the surface of the material, producing plasma that generates shock waves. These induce compressive stresses on and beneath the surface. This method has the advantage that it improves the depth of compressive residual stress fields (up to several millimetres) in the material while maintaining a smooth surface finish.

Around 10 years ago Rolls-Royce was one of the first companies to use LSP commercially, applying it to aero-engine fan blades. “It is absolutely critical that fan blades resist the effect of fatigue and fretting fatigue over many thousands of flying hours,” explains Phil Withers, professor of materials science at the University of Manchester and leader of the collaboration with Rolls-Royce and the ESRF. “The University of Manchester supports Rolls-Royce in qualifying the LSP production process,” he says.

Lab X-rays require metal removal and measurement correction to determine stress profiles and are therefore destructive. The highly penetrating, non-invasive, hard X-ray beams of the ESRF come into the picture when the researchers try to understand the structural changes taking place in the fan blades during the service throughout their lives. Rolls-Royce has techniques that can simulate a very wide range of extreme service conditions. “Synchrotron radiation is the only means of characterising the way that these protective stresses evolve over the life of the component without destroying the part,” Withers explains.

At the ESRF, scientists probe the samples with a very high spatial resolution on beamlines like ID11, ID15 and ID31 where they carry out diffraction experiments using the poly-crystal structure as an atomic strain gauge. “ID31 is especially convenient to investigate the material close to the surface,” says Withers, “but then on ID11 and ID15 you have higher energies and we can study larger components.” Neutrons are a complementary tool in this research: they allow full-size engine assemblies due to their even higher penetrating power, although they provide less spatial resolution. In addition to the ESRF, the team also uses the Institut Laue-Langevin in Grenoble as well as the ISIS neutron source, and most recently Diamond (UK).

Figure shows the in-plane (x and y) and out of plane (z) stresses with depth from the surface in the root of the fan blade a) as-laser peened and b) after in-service conditions. Fretting has reduced the near surface compression somewhat but a significant compressive stress is maintained.

The benefits of this research go from underpinning the safety of the blades and their resistance to fretting fatigue and foreign object damage to extending the method to other applications: so far, the team has tested titanium alloys (normally used in fan blades and biomaterials too), but also stainless steel for applications in power plants. “In the case of aero engines, they are increasingly operating at higher temperatures and there is a continuous drive to make aircraft lighter. We need to find ways to make the materials and develop manufacturing processes that are up to the task and the ESRF helps us ensure that the science keeps up with technology,” says Withers.

M Capellas

This article appeared in ESRFnews, December 2010. 

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Top image: The unique titanium wide chord fan blades, seen here on a Trent 900 (Image copyright: Rolls-Royce plc 2010).