ANALYTICAL AND EXPERIMENTAL CRACK IDENTIFICATION OF BEAM STRUCTURES IN AIR OR IN FLUID

In this paper a new method for crack identification in beams is presen ted. The transverse surface crack is considered to remain always open. Using this method the crack location and depth can be determined. The method is based on the basic observation that the eigenmodes of any c racked structure are different from those of the uncracked one. The di fferences are due to the slope discontinuity of the vibration eigenmod es at the crack location. The basic idea in this paper is to correlate the mode differences with the crack depth and location. These correla ted differences are chosen to be (a) the ratio of two amplitude measur ements in two positions and (b) the distance of the node of the vibrat ing mode from the left end, while the structure is vibrating under har monic excitation in resonant condition. The analytical results present ed here are also experimentally proven. In a free-free beam the two fi rst eigenmodes are measured and the node distance of the uncracked and the cracked beam are used in pre-plotted diagrams to verify the crack location and depth. Here, it is also theoretically proved that the ab ove results for the beam are the same in both the air and the water. T his is valid irrespective of whether the damping effect is taken into consideration or not. (C) 1997 Elsevier Science Ltd.