Numerical estimation of fatigue life of aeroengine fan blades

During every startup and shutdown of an aviation gas turbine, the fan blades are subjected to centrifugal, gas bending and vibratory loads. This repeated loading and unloading can reduce the life of fan blades. Previous works on fan blades have focused mainly on fatigue life estimation in the vicinity of foreign object damage. There is no evidence of fatigue life estimation from centrifugal, gas bending load and dynamic load.

As it is important to assess the fatigue life of the blade subjected to static and dynamic loads, the analysis is divided into two parts. In the first part, the blade is subjected to static loading that is centrifugal and gas bending load. Based on stress results, a decision is made whether to proceed with stress based fatigue life assessment or strain based fatigue life assessment. In case of dynamic loading, the blade is subjected to a tip load. Harmonic analysis is carried out to study the alternating stresses. The equivalent endurance stress obtained is checked against the S-N curve to obtain the HCF life under dynamic loading.

Results obtained are based on the above process. In case of static loading, it is observed that the dovetail regions will have a minimum life in LCF. And in case of dynamic loading, it is observed that the maximum speed of the fan blade is close to one of the blade passing frequencies. Hence, the deflections are predominantly similar. It is also observed that under dynamic loading, the aerofoil is having a minimum life in HCF.