Abstract
Despite nearly 100 years of turbine engine design and development, blade vibrations still remain an important engineering challenge. For rotating turbine blades, their vibrations lead to cyclic oscillations — these results in alternating stress and strain in destructive environments of high temperature and pressure. Alternating stress amplitudes can exceed the safety endurance limit, what accelerates the high cyclic fatigue leading quickly to crack propagation and catastrophic failure of the blade. For the classical turbine blade design philosophy, the goal for dynamic design of the blading was so-called resonance-free solution (avoiding resonances for characteristic rotational speeds) achieved by eigenfrequency tuning. To meet current market demand for higher performance, modern engines need: to operate with larger mass flow, operate at higher firing temperatures, to startup and shutdown more frequently. Therefore, the rotating blade must be more often designed as the resonance-proof component under circumstances of the variable rotational speed and varying thermal conditions. A century of turbine engine development has provided many solutions for the improvement of High Cycle Fatigue lifetime of the blading. One of them is damping optimization through advanced design of parts. There are few main damping mechanisms occurring during blade vibrations: material damping (for Ni-based alloys does not exceed 0.02% [5]), aerodynamical damping (usually below 0.3%) and frictional damping (depending on the design). Nowadays, Additive Manufacturing (AM) and especially Laser Powder Bed Fusion (LPBF) allows to manufacture multifunctional and complex components with high structural integrity and extended lifetime. An example of an uncooled turbine blade design of a jet engine has been chosen for the study. Two designs have been modelled and manufactured using LPBF technology: a baseline design (‘Solid Blade’) and a new design where the airfoil was filled with a matrix of pockets with pins and lattice bars surrounded by non-fused powder (‘Lattice Blade’). Then, the damping ratio has been assessed for both designs using hammer-test for non-constrained parts, where vibration decay was measured by a laser vibrometer. Except material damping occurring in the baseline design, the new design has additional damping mechanisms: the wave propagates through different media (changes of wave propagation speed, wave reflections), energy dissipates in the non-fused metal powder (friction between powder particles), solid pins in the pockets vibrate independently (act as dynamic dampers and improve energy dissipation in the powder), lattice bars in the pockets transfer the vibration wave to the powder (activate energy dissipation in the whole volume of the non-fused powder). The results of the hammer-tests (excitation by impact) show significant damping ratio increase for all 6 modes investigated in this study. For example for Mode 1, the modal damping ratio changed from 0.011% for the baseline ‘Solid Blade’ design to 0.919% for the new ‘Lattice Blade’ design, giving improvement nearly over two orders of magnitude (82 times). Additionally, the LPBF approach might have a multifunctional character — except significant improvement of damping ratio, the mass can be reduced (in this case decreased by about 6%), eigenfrequency can be tuned to avoid resonance (here reduction of 1st eigenfrequency by about 8%), the stress concentration factors can be reduced (which is planned for next studies), etc. The proposed new design has not been optimized so far, giving wide margin for further improvements of the damping performance.
