PSI - Issue 17

Francisco Barros et al. / Procedia Structural Integrity 17 (2019) 986–991 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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In particular, restrictions imposed by accessibility problems can be overcome by the use of unmanned aerial vehicles (UAVs) to acquire DIC images. Reagan et al. (2018) used a previously calibrated camera pair mounted on a UAV to detect cracks in concrete structures under bridges prepared with a speckle pattern and fixed reference points using stereo DIC. Khadka et al. (2019) used a similar method to analyse the modal behaviour of a scaled down model of a wind turbine. This work proposes a method for 3D-DIC to measure deformations under constant loads or detect long term displacements or shape changes in structures through the use of a single camera mounted on a UAV. The UAV would acquire images of the structure and its surroundings, obtaining a set of images which can be used to calibrate the camera using a structure from motion (SfM) algorithm, as well as images of the region of interest viewed from two directions for use in DIC analysis. At a later point in time, or after a load is applied, a similar system would acquire a second image pair showing the speckle pattern and its surroundings. Fixed objects in the surroundings and their locations across different images are detected using a feature detection and matching algorithm. The obtained matches are then input into a SfM algorithm, which computes the positions of the detected points and the position of the camera in each image, up to a scale factor, as well as the camera’s intrinsic parameters. The basic principles of the idealised method are demonstrated in this paper through a laboratory test where the behaviour of a UAV is emulated by a manually held camera.

2. Methodology

2.1. Setup and video acquisition

A rectangular acrylic plate prepared with a speckle pattern was mounted on a structure which keeps it in a fixed vertical position, clamped at the midpoints of both sides (Fig. 1). Behind the plate, a post attached to a linear positioner was roughly aligned with the middle of the plate in order to apply a load perpendicular to the speckled surface. A 2x3 checkerboard pattern with a square size of 10 mm was placed near the plate in order to provide two easily detectable points with a known distance between them.

Fig. 1. Test setup, showing the plate prepared with a speckle pattern, the checkerboard pattern for size reference and the load application system.

Before the load was applied, a video sequence was recorded using a manually held camera which, alongside the speckle pattern, shows objects found in the background viewed from different camera positions. The load, imposing a displacement of 8 mm, was then applied by acting on the linear positioner. A second video sequence was recorded with the sole objective of obtaining images for the loaded state to use for the DIC analysis.