A case study is presented in which a composite driving shaft of a motor-bike was loaded with different increasing numbers of load-cycles in a pulsating testing machine to reach different stages of damage. For each number of load-cycles an experimental modal analysis was performed by an iterative, global, multi-degree-of-freedom, frequency-domain modal testing technique and the eigenfrequencies and modal damping ratios were identified. From the results it is shown that the damping increases with the number of load-cycles i.e. the state of damage. However, it turned out that a more sensitive indicator for the state of damage is the splitting and shifting of the ‘double’ eigenfrequencies of the bending modes of the driving shaft with the ‘axisymmetric’ circular ring cross section. This fact is shown on the basis of several related experimental data.

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