Finite element analysis of vessels to study changes in natural frequencies due to cracks

When significant damage occurs in structures, there is a change in stiffness, which in turn affects the natural frequency. To study this, a study was conducted to analyse the effect of cracks on natural frequencies in two vessel structures. Finite element analysis has been used to obtain the dynamic characteristics of intact and damaged vessels for the first eight modes of these structures. Two kinds of vessel, boilers and storage tanks, were chosen and through-thickness cracks were analysed. Different cases were examined by changing the size and locations of cracks with the help of a FEM (Finite element model). Natural frequencies and mode shapes were analysed. The natural frequencies for different modes have been used as input pattern of ANN (artificial neural network) model. The output of the ANN model is a crack size for a particular location. It was found that as the crack size increased, natural frequency changed to a large extent, but the frequency was not reduced in the same manner for every position of damage for the same size of crack. It was also found that the reduction in natural frequencies depends upon the mode shapes of the structures