PSI - Issue 5

Pedro J. Sousa et al. / Procedia Structural Integrity 5 (2017) 1253–1259 Pedro J. Sousa et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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2.2. Approach

The main goal of this work is to develop and implement a method for image acquisition of rotating targets with apparent stillness, in order to be able to calculate displacements and strains using 3D digital image correlation. Each experiment is divided in two main phases: data acquisition and data processing. Data acquisition is the proper physical experiment and involves the acquisition of three types of images: calibration, reference and loaded. Before the images were acquired, the high-speed cameras were positioned and their focal distance, aperture and exposure time were adjusted to obtain maximum focus and contrast. After the camera setup was completed, calibration images were acquired, using a symmetrical circle pattern, which is the pattern used by the VIC-3D software. The reference images were captured manually, with the helicopter blade placed in a position close to the one that corresponded to the camera trigger. Afterwards, images of the rotating blade were captured using the photodetector signal to trigger the cameras at a specific position. This procedure was performed for two different rotation velocities, the RC helicopter’s minimum and maximum speeds, around 600 and 1200 rpm respectively. Afterwards, each experiment involved the following data processing steps:  Calibration of the stereo camera system;  Calculation of the 3D shape of the surface and its displacement field between the reference state and the deformed state;  Analysis and validation of the results. Most of these steps were performed using Correlated Solutions’ VIC -3D software. However, the final step also involved the use of MATLAB and an open-source solution for point-cloud comparison.

3. Results

Over 600 frames were obtained in this experiment, one per each rotation of the rotor, excluding calibration and reference images. Fig. 3 shows one of the acquired reference images, as seen from both cameras. It can be seen that a region of the blade (lower left side in Fig. 3a) is not visible in the left camera and therefore cannot be considered for the calculations. Furthermore, the acquired shape will never include the connection to the rotor, since it is not visible.

Fig. 3. Example reference images acquired by (a) the Left Camera; (b) the Right Camera.

For each of the acquired frames, VIC-3D was able to calculate shape and displacement information. However, X and Y direction displacements (U and V) showed mostly the rigid rotation relationship from the reference image to the moving images due to the difficulty of exactly positioning the stopped blade for reference acquisition. On the other hand, Z direction displacements (W), Fig. 4, are able to highlight the difference in deformation for both tested speeds.