Research on eigenfrequency shifts due to cracks in cylindrical structures and the application in non-destructive testing

This paper presents numerical and experimental studies on modal behavior of cylindrical, beam-like lightly damped structures having circumferential cracks with variable position, width and depth. In the numerical investigation utilizing Finite-Elements a discretization strategy is worked out that enables a three-dimensional crack representation. The experimental setup uses a Laser Scanning Vibrometer in the frequency range between 0 - 40 kHz. The major challenges to achieve adequate precision of the experimental data exist in the realization of a broadband impulse excitation and support conditions for repeatable measurement conditions. Numerically as experimentally the first 15 mode shape pairs with the corresponding eigenfrequencies are identified. The model updating is performed for the elastic parameters and appropriate boundary conditions to minimize the deviation between measured and calculated eigenfrequencies. The acquired data serves for the implementation of a damage identification procedure in which the geometrical crack properties are identified. The deviation between real and determined crack positions is lower than 0.3 % for crack cross-section ratios less than 7 %.