Low-cycle fatigue life prediction of a variable thickness disk

Abstract

The turbomachines are characterized by complex geometry of the main parts and extreme operation conditions. The review of the testing results indicates that in most cases the failure of turbine disks and blades is caused by low- and high-cycle fatigue. We included the experimental dependences of the material properties of temperature, time and number of loading cycles in the fatigue life calculation. This enabled us to observe the kinetics of the part stress-strain state which is present even under constant external load. The observed increase of strain (decrease of stress) at the constant external load indicates the process of cyclic damage accumulation in the disk material. The plastic strains determine the damage fields (distribution of damage in the part material) which in turn allow predicting the area of initiation and probable growth direction of a fatigue crack in a part. We chose a material damage model, and numerically calculated the damage field by taking into account the change of the part stress-strain state with the number of cycles. The damage fields allow determining the point life (time or number of cycles to failure since the moment when a crack passes through a given point), the number of cycles before crack initiation, number of cycles to failure with a crack, crack propagation speed in the damaged and not damaged material. Finally, we determined the permissible number of cycles, time to failure and crack length with a given safety factor. The numerical investigations of the cyclic failure of machine parts should be conducted taking into account the kinetics of the accumulated damage fields.