PSI - Issue 5

V. Epin et al. / Procedia Structural Integrity 5 (2017) 620–626 R. Tsvetkov / Structural Integrity Procedia 00 (2017) 000 – 000

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Under service loads, cracks appear in the stretch zones of the longitudinal beams. In Fig.1, crack localization areas are shown by a wave-like symbols. A deformation monitoring system was developed, which, on the one hand, registers the local deformation processes concentrated in the vicinity of the crack and, on the other hand, controls the changes in the macroscopic parameters of the structure (vertical displacements in a few transverse cross-sections along the beam length).

3. The deformation monitoring system

The proposed automatic deformation monitoring system comprises sensors of our own design, data acquisition and storage system, data processing software, and web-based graph visualization system. The monitoring system can notify specialists by SMS about the probable dangerous situations, i.e. the structure conditions at which the strain criteria reach the value corresponding to the pre-critical stage of failure. The sensors of the monitoring system are based on various measurement principles and aimed at recording various-scale deformation processes: local deformations (crack opening) and changes in the macroscopic parameters of the structure (vertical displacement of beams, beam deflection). The process of measurement is implemented in a non-contact way by using IP cameras, which transfer images to the server by cable or wireless technologies. Interrogation of sensors is carried out in automatic mode at a given frequency. Decoding of the received images by photogrammetry techniques yields deformation parameters.

Fig. 2. Diagram of sensor location on the beams of the air bridge (top view).

3.1. Crack width measurements

The monitoring system is based on the non-contact method of crack opening width measurements with the use of photogrammetry technique. The images of the crack- containing area are recorded by the IP camera. Control of the crack opening width is accomplished by measuring the distance between the markers – special marks placed on the opposite crack edges. The marker represents a square, which is separated by a diagonal into black and white-colored zones. The algorithm for searching markers in the images is based on comparing the contrast ratio of adjacent regions. Here, the known quantity is the mutual arrangement of two markers in the initial state. Its change can be used to fix a change in the distance between the crack edges and relative displacement of the crack edges in the plane of the crack. Resolution of such sensor depends on the resolution of the camera and the dimensions of the visible area with the introduced markers. For a single measurement the resolution is 30µm, for a series of 10 measurements it is equal to 15 µm. Sensors testing has shown that cameras with the diagonal field of view (angle of view) up to 45º allow measuring the crack opening up to 3 mm with an accuracy of ±50 µm until the effect of distortion appears and up to 6mm with an accuracy of ±100µm. Measurements can be made up to the crack opening of 12 mm, until the markers are in the field of view of the camera.