The normal velocity FRFs at the web centre obtained from the measured data are shown in Fig. 10. The multiple measured velocity FRFs of each case keep consistent with each other, indicating that the test is robust. The coherence coefficient at the resonance frequency is larger than 0.9, showing that the tested dynamic response is caused by the impact force. From these figures, the sharp resonant peaks are flattened by the viscoelastic treatment. Among all the tested cases, the resonant peaks in Case 6 are lowest.The non-dimensional velocity FRF is: