Performance of Gen IV LSP for Thick Section Airfoil Damage Tolerance

Abstract

The General Electric Aircraft Engines Gen IV (low energy) laser shock peening (LSP) process is evaluated along a thick section of Ti 6Al-4V fan blade airfoil leading edge. Simulated foreign object damage (FOD) was imposed to assess the FOD tolerance improvement of the LSP treatment during high cycle fatigue (HCF) testing using an air siren. Data are presented for a total of 21 blade subarticles which were HCF tested to determine: (1) untreated baseline material capability without simulated FOD, (2) untreated material capability with simulated FOD and (3) LSP-treated material capability with simulated FOD. LSP-treated subarticles showed restored HCF capability and performed at the mean of the undamaged/untreated material capability with minimal variation within the population. Post-test fractography of the LSP test articles revealed fatigue crack initiation at the notch with crack propagation consistent with the fatigue strength measured. Crack initiation and propagation modes were further validated via optical and scanning electron microscopy. A further evaluation criterion was airfoil distortion due to the imposed residual stress. Post-LSP distortion data was evaluated via coordinate measurement machine inspection of all treated blades. The LSP-treated fan blades satisfied the distortion requirement. Conclusion of the study was that Gen IV LSP was able to provide the necessary notched HCF capability while still meeting airfoil distortion requirements.