Crack identification in a cantilever beam using rotational laser vibrometer-based coupled of bending, axial and torsional vibrations

Abstract

In this paper, coupled response measurements are suggested for detection of a transverse surface crack in a cantilever beam. The crack has been modelled by a local compliance matrix of six degrees of freedom and simulated for detection. The eigenfrequencies are obtained using finite element method (FEM) and are compared with those of experiments. However, the small changes in eigenfrequencies are not enough for crack detection. The flexibility matrix contains non-diagonal terms, and thus indicating the coupling among the different modes of vibrations. A harmonic force of known amplitude and frequency is used to dynamically excite the beam in bending mode. But due to the presence of crack, the bending excitation induced the axial and torsional vibrations also. The measurements of direct (bending) response and coupled (axial and torsional) responses are carried out using triaxial accelerometer and rotational laser vibrometer (RLV). The experiments are carried out for different bending frequencies, crack depths and locations. The results indicate that the coupled responses are very sensitive to crack; hence these measurements are suggested for crack detection.