PSI - Issue 13

Francisco Barros et al. / Procedia Structural Integrity 13 (2018) 1993–1998 Author name / Structural Integrity Procedia 00 (2018) 000–000

1995

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2. Methodology 2.1. Test setup and image acquisition

A stereo rig was set up to perform acquisition for DIC on a tensile test and calibrated using a planar pattern using the Zhang calibration algorithm [6]. An elastomeric flat specimen prepared with a speckle pattern for DIC was set up for the tensile test, and objects where printed speckle patterns had previously been applied were placed near the specimen, to provide reference points for recalibration. It was ensured that the patterns were positioned so as to have different orientations and occupy as much of the camera images as possible, while remaining within the depth of field of both cameras and avoiding contact with moving parts. An image pair was acquired before any load was applied, to serve as a reference state. Another pair was acquired after the specimen was loaded imposing a displacement of 5 mm on the top grip. The load was held as the camera rig was removed from its location and repositioned at an arbitrary point close to the original one. The positions of the cameras relative to each other were also changed. They were then realigned and refocused in order to accommodate the new positions. A new image pair was then acquired of the loaded specimen with altered camera positions. The acquired image pairs can be seen in Fig. 1. The camera repositioning and refocusing intended to emulate DIC image acquisition in cases where the complete image acquisition setup is removed and measurements are resumed at a later time.

Fig. 1. Acquired image pairs: (a,b) Reference state; (c,d) Deformed state; (e,f) Deformed state after repositioning and refocusing the cameras.