Non-contact vibration tests with detection-free input based on pulsed-laser ablation for underwater structures

This paper proposes a method of a non-contact experimental vibration analysis for underwater structures by pulsed-laser ablation. The impact testing with impulse hammer is used widely for vibration analysis due to the simplicity of the apparatus. However, the impact testing has limitations in use in underwater condition because of contact method by the experimenter. The proposed method yields the frequency response functions by applying apulsed-laser- ablation excitation force to the structure and measuring the output using a laser Doppler vibrometer. Since the direction, strength and effective duration of the pulsed-laser-ablation force are essentially constant, this force can be calibrated by measuring these properties in advance. Therefore input-detection-free frequency response function measurements can be realized. The method is validated by comparing frequency response functions obtained using laser excitation with FEM. Additionally, the frequency response function obtained in underwater conditions shows that the natural frequency decreases and the amplitude is attenuated by comparing with frequency response function in air conditions.