# Stereo-DIC Measurements of a Vibrating Bladed Disk: In-Depth Analysis of Full-Field Deformed Shapes
## 6 Conclusions  
The present work describes an experimental setup for obtaining full-field 3D displacement maps of high-frequency vibrating targets, through Digital Image Correlation. Two low frame-rate cameras were used to assemble a stereo system. This optical setup, in opposition to high-speed cameras, has much lower cost and higher resolution, which allows for higher sensitivity. On the other hand, the main limitation of the proposed approach is the need for downsampling, which imposes the constraint that the displacement signal under measurement necessarily consists of just a single sinusoidal harmonic. However, this can be easily obtained in many testing setups by using an electrodynamic shaker which can produce an accurate sinusoidal excitation load. Additionally, the camera shutter time must be short with respect to the measured signal period. A ratio of at least 50 between the signal period and the shutter time was used in this work. The described acquisition system was applied to the vibrational measurement of a blisk, and this allowed for the investigation of its mistuning level in the high-frequency range. To this extent, harmonic response analyses were performed at 2568 Hz, 6239 Hz and 6357 Hz to assess and compare the shape of these response peaks. It is worth noting that these frequency values in combination with the low response amplitudes, in the range of \(10\) um, were not found in any previous research or in studies exploiting high-speed camera setups.  
The obtained 3D displacement maps highlighted that all the observed harmonic responses could be explained as excited modes featuring relevant mistuning. This conclusion would be much more challenging to obtain with conventional single-point measurement techniques, since a great number of measurement points would be needed to fully describe the complex mode shapes. An insight in terms of the deformed shapes was additionally given by analyzing the amplitude and the phase maps. This investigation highlighted that in the case of blisks it is really difficult, or merely impossible, to separate different mode shapes with a harmonic response analysis. The reason for this is the mistuning effect which splits the degenerate modes in two separate, asymmetric and close in frequency modes.  
In principle, the described measurement system could be used to estimate the blisk damping, by comparing the response and the load amplitude, provided that the excitation force is measured during the test. However, this phenomenon is generally quite low for this kind of structure, thus an accurate detection of the damping is expected to be a challenging task and can be considered for a future investigation.  
**Author Contributions:** Conceptualization, P.N., A.P. and C.S.; software, P.N.; validation, P.N., A.P. and C.S.; writing--original draft preparation, P.N.; writing--review and editing, A.P. and C.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** It is not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.