11 Vibro-Acoustic Modulation of a Spinning Apparatus for Nondestructive Evaluation 113 Graphic not to scale (a) (b) Mean Est. Standard Deviation Actual Radial Position 144.8mm 15.5mm 142.9 Angular Position 18° 12° 20° Secondary Angular Position 243° 16° - Fig. 11.12 Spin-down VAM results representation (a) and tabulated results (b) Radial data count was too small for a full statistical characterization, but the data appears centered, minus one outlier. The angular position data presents a bimodal, approximately Gaussian distribution, with a mean matching each the primary and secondary damage types. The obtained results (see Fig. 11.12) represent a successful radial and angular location of a simulated damage source despite using a noisy surrogate air-bearing. The secondary damage source—due to extra motion in the damage mechanism— complicates the location, but a more complete constitutive model would eliminate this. 11.5 Conclusions This study was conducted to investigate whether it was possible to detect damage non-destructively using a novel method of VAM. The new method entailed spinning the test article to provide the low frequency pumping signal, rather than excitation with a piezoelectric device or shaker table as in conventional methods. The damage was detectable in this testing paradigm due to the nonlinearity it caused, which resulted in a visible modulation of the high frequency probing signal on an acoustic emission spectrogram, occurring at frequency off-bands equivalent to the pumping frequency at the time of modulation. This method yielded an angular and radial position with corresponding uncertainties that formed a statistical area that correctly encapsulated the improvised damage source added to the system. This method relies on the use of a constitutive model of how the damage will behave as a way to categorize and predict how certain types of damage will appear in the acoustic emission response. For example, in the case of this study, the damage was modeled as a mass free to move in the radial direction, encumbered by friction, so that gravitation and inertial forces acting upon it could reach an easily computable equilibrium angular velocity. This velocity was used as an estimate of at what speed the damage would become visible, and pumping frequency interval was set based off of that prediction. Also, it is important to mention that this study was conducted to augment a pre-existing NDE method using spin-down on an air bearing to determine the presence of damage. However, it was shown in this study that an air bearing is not expressly necessary to perform spin-down NDE. This information could be value for a reader interested in reproducing this method for other NDE needs. All that is required to test an article with this method is a valid constitutive model of how the damage will behave, and an apparatus that can accelerate and decelerate the test article with acceptable precision and accuracy. Acknowledgments This work is approved for public release and distribution is unlimited under LA-UR-16-28176. This project was funded through the Los Alamos Dynamics Summer School program of the Los Alamos National Laboratory under the direction of Charles Farrar.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==