Crack identification in beams by coupled response measurements

Abstract

In this paper a new method for the determination of the depth and the location of a transverse surface crack in a beam is presented. Simulating the crack by a local compliance matrix of six degrees of freedom has been used for crack modelling in the last decade. This matrix contains diagonal and non-diagonal terms, and thus coupling among different modes of vibration occurs. A harmonic force or a moment of known amplitude and frequency is used to dynamically excite the beam. The exciting frequency should be far from a system eigenvalue to avoid the effect of the damping near resonance. Two response measurements at a point are required by the method. The first measurement is taken in the direction of the excitation, while the second one in the direction where coupling effect occurs due to the crack. The crack is considered to always remain open. The identification of the existence of the crack will be shown to be feasible, if a response on a degree of freedom, other than the one of the excitation, exists. A Timoshenko beam finite element of six degrees of freedom per node, is used. This method can be applied in structures in air as well as under water.