Structural Dynamic and Inherent Damping Characterization of Additively Manufactured Airfoil Components

The push for low cost and higher performance/efficient turbine engines have introduced a new demand for novel technologies to improve robustness to vibrations resulting in High Cycle Fatigue (HCF).  There have been many proposed solutions to this, some passive and some active.  With the advent of Additive Manufacturing (AM), new damping techniques can now be incorporated directly into the design and manufacture process to suppress the vibrations that create HCF.  Recent work performed by Scott-Emuakpor et al. used powdered filled pockets within the beam structure to act as a damping mechanism to increase the structural damping.  This method has shown success, with up to 95% forced response reduction when using 1-3% unfused powder within the pocket volume.  The corresponding strain limit due to the increased damping was found to be between 300 and 700με.  Also noted was that the damping effectiveness does not return after the strain is reduced, but was observed to be relatively steady in the measured damping quality factor after the initial loss.  The next step in this investigation is to apply the technology to blade structures.  In this study, this novel unfused pocket damping technology is applied to a blade structure and the resulting damping effectiveness is quantified.  The application of this technology to complex geometries will provide insight into both the underlying damping mechanism and its overall effectiveness.  The blades are manufactured using a laser powder bed fusion process with Inconel 718.  An intentional void is left in each blade to serve as the internal pocket which remains filled with unfused powder as part of the regular AM building process.  The finished blades are then computed tomography (CT) scanned to determine the as manufactured fill volume and to verify initial powdered locations. In this paper, first the damping quality is investigated when this technology is applied to a fan-like blade.  Second, the strain limitations are investigated to determine if they are similar to that previously observed by Scott-Emuakpor et al. and if they are affected when the blades are put in representative operating temperatures for both sub-sonic and supersonic post inlet conditions.  To accomplish the tests, both solid and pocketed blades at room temperature are tested followed by testing at elevated temperatures.  All damping tests were conducted using an electrodynamic shaker and a convection furnace for the elevated fan temperatures.  Post-test, the blades were CT scanned again and compared to the pre-test scans, to ascertain the state of both the powder and the blades as a whole.  This research allowed an initial evaluation of the effectiveness of this technology to turbine engine fans as well as highlighting the overall effectiveness of this type of passive damping.  If successful, this technology will allow for rapid manufacturing of components with a highly effective inherent passive damping capability.