Simulation of fatigue failure in composite axial compressor blades

Centrifugal forces are generated by a spinning impeller, of magnitudes that create large stresses. Aerodynamic forces are also imparted on an impeller blade, which varies with time and position. These two forces play different roles during compressor events. Damage accumulated from these events results in the fatigue failure of impeller material and structure. Therefore, it is important to design an impeller against dynamic and fatigue failure. The finite element method has been used in the study of impeller fracture mechanics and is regarded as an important tool in the design and analysis of material and structures. Novel axial composite impellers, manufactured through filament winding technology, were invented and studied at Michigan State University. These impellers will be used to compress water vapor so as to use it as a refrigerant. In this study dynamic and fatigue behavior of two types of composite impellers 8B and 8C, were analyzed using commercial code ANSYS. Firstly, load cases were identified calculated and evaluated. Static analysis was then performed with a full 3-D finite element model. The critical zone where fatigue failure begins was extracted and used to determine life assessment positions. Secondly, aerodynamic forces imparted on the blade were obtained from FLUENT, so that damage from dynamic stresses could be calculated. Finally, based on the FEM and FLUENT simulation results, a linear damage accumulation model was employed as a damage estimation rule to predict life of the two composite impellers. A conservative life of 6498 h for 8B was calculated by this method, as was a life of 5435 h of 8C. Based on the method, safe fatigue life reduces for about 32% in case of 8B and 40% in case of 8C compared with conventional approaches where aerodynamic forces are omitted.