The effect of cracks on the natural frequencies of axisymmetric vibration modes of gas cylinders

The effect of transverse annular defects on the natural frequencies of axi-symmetric vibration modes of a commercial gas cylinder was investigated. Experimental modal analysis was carried out on gas cylinders containing thin annular slots machined into the external surface near the base. Finite element analysis was also carried out on models that simulated the real cylinders, but in addition the effect of both external and internal annular slots was investigated. The FE predictions of the reduction in natural frequencies produced by cracks were in excellent agreement with the experimental measurements. It was found that the reduction in natural frequencies of the vibration modes is approximately proportional to the square of the length of a perturbing crack or slot. Although absolute measurement of natural frequency cannot be used to detect the presence of cracks, it is concluded that the relative changes in the frequencies of different modes could be used to detect crack growth over time. (C) 1999 Elsevier Science Ltd. All rights reserved.