Vibration control evaluation for underwater structures using non-contact laser excitation

A system that uses non-contact laser excitation based on pulsed-laser ablation is proposed to evaluate vibration suppression of fully submerged underwater structures. Although impact testing with an impulse hammer is commonly used in vibration analysis due to the simplicity of the apparatus, impact testing has limitations in underwater applications. Additionally vibration testing in dangerous conditions (e.g., deep oceans, under ice, and nuclear reactor plants) is difficult. The proposed method yields the frequency spectrum by applying a pulsed-laser-ablation excitation force to the structure and measuring the output using a laser Doppler vibrometer. Because the direction, strength, and effective duration of the pulsed-laser-ablation force are essentially constant, it is possible to obtain the dynamic characteristics of an underwater structure using only the output frequency spectrum. The validity of the proposed system is demonstrated by comparing the frequency spectrum suppressed by passive vibration control to that without a control.