CRACK LOCALIZATION BY SENSITIVITY ANALYSIS AND ACCURATE FREQUENCY MEASUREMENT

Vibration measurements can be used to detect fatigue cracks in structures. In fact, a damage alters the dynamic characteristics of the structure. In this report, a parameter called Local Modal Crack Sensitivity (LMCS) is used for crack localisation. Its analytical expression is explicitly written for a flexural vibrating beam. For more complex structures, FEM analysis can be used. LMCS is the coefficient of proportionality between the relative frequency shift, dwr/wr, of r-th eigenmode and the actual bending stiffness variation dK of damaged beam. It is a measure of how much local damage affects each eigenfrequency of the undamaged beam, as a function of crack location. This concept can obviously be generalised for other structures besides a beam in bending. From the well known expression of the modal stiffness kr for a beam in pure bending, the following expression of LMCS for a beam in bending can be deduced: