Measurement of vibrational modal parameters using laser pulse excitation techniques

The attention is focused on the experimental evaluation of uncertainty sources in modal parameter measurement, when high-energy laser pulses are used to excite the vibration of the structure. The tests were performed on a cantilever beam excited firstly with a hammer and then with laser pulses from a Nd-YAG source (532 nm, 100 mJ/pulse). This paper consider the problem of the lack of knowledge on laser-induced input force that makes the precise quantitative measurement of the mobility functions (FRF) and of the damping ratio difficult. To calculate the FRF an “equivalent” input force was estimated by solving an inverse problem: the “equivalent” force is useful to determine the features of the laser-induced impulsive excitation. The achieved results are in complete agreement with other characterisations presented in previous studies, in particular in terms of specific impulse, time duration and characteristic frequencies. The paper reports several suggestions useful for minimising the effect connected to the use of laser pulse excitation, in particular concerning the limited energy given to the structure under investigation. Two typical applications (modal analysis and vibrational analysis) are reported as example.