The operation period of man-made structures is limited and is not eternal. Structural health monitoring (SHM) provides a process for obtaining information about the existence of damage, location and severity of the probable failure, and the remaining service life of the structure. Therefore, in recent decades, much research has been carried out in the field of health monitoring. One of the methods of damage detection is originated in vibration and is based on the theory of modal analysis, which is used to eva‎luate the failure and monitor the damages on the surface. In the present study, the identification of cracks in concrete beams has been studied using the modal analysis method. For this purpose, 5 reinforced concrete beams with dimensions of 120x140x1000 mm and 4 unreinforced concrete prism beams with dimensions of 100x100x350 mm were made and tested. Furthermore, some voids were drilled in one of the reinforced concrete beams. One of the unreinforced concrete beams was used as control without any pre-crack while the other three specimens had pre-determined cracks. The specimens were subjected to the four-point bending test. During testing, flexural and shear cracks occurred naturally. Modal analysis were conducted on the specimens in the laboratory as well as through the finite element method using non-linear software. Both cracked and non-cracked specimens were tested using the modal analysis method and the effect of various parameters such as natural frequency and mode shape on the specimens were investigated. The results of the experiments indicated that the use of the frequency response function alone is not responsible for determining the location and intensity of cracks, and the results of the mode shape should also be used. The results also showed that the greater the crack depth across the beam, the greater the rate of change in natural frequency and the location of predetermined cracks in unreinforced concrete beams can be determined by curvature mode shape method, that identified with a difference of about 2.8%.The results obtained from reinforced concrete specimens and different loading states on the beams showed that the closer the force applied to the sample to the final load (beam failure), the more frequency changes is observed. Accordingly, from the moment of the first crack to the flexural failure of the beam, the natural frequency parameter is reduced by about 4% to 41% compared to the unloaded specimen, and this difference for the shear rupture of the beam is about 20%. Based on numerical and laboratory results, a good agreement of about 0.3 to 3.7% was observed between the results.

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عليرضا سلجوقيان اصفهاني، علي لقماني