Stringent mission requirements of Aero Gas Turbine engines result in severe aerodynamic loads on engine components. Structural integrity and durability of blades is an important aspect with fatigue being a major failure mode and especially High Cycle Fatigue being a critical design concern. HCF failures in blades are mainly attributed to resonant excitations where the dynamic stress amplitude in the blade increases as the exciting frequency approaches the resonant speed. These excitations could be due to integral orders, i.e. as multiples of rotational speeds and also due to nozzle or blade pass frequencies. The frequency of these excitations is very high and under these excitations, the blades are subjected to highly complex modes or deformation patterns resulting in a multi-axial state of stress. Other causes for multi-axial state of stress may be attributed to anisotropy in material, presence of stress raisers and end fixity/ support mechanism of the blade in context. Assessing the severity of this multi-axial stress state in the blade from HCF is very important from the designer’s perspective.
This paper describes the methodology employed in a compressor stator blade to assess the HCF damage by distortion energy based multi-axial fatigue failure criteria with a modification to include the mean static stresses. Based on this method, safe operating strain limits are established and are used as guide lines to monitor the stator blades during engine testing. When this methodology is checked for a pure bending mode of a compressor stator blade, i.e. for a state of stress which is predominantly unidirectional, both the Goodman approach and the distortion energy based multi-axial method yield the same assessment under HCF, whereas for a complex mode with a multi-axial state of stress, considerable difference in results is observed.
Theoretical model of creep fracture under tri-axial state of stress
