Frequency response function measurements of rotational degrees of freedom using a non-contact moment excitation based on nanosecond laser ablation

Abstract

Frequency response functions (FRFs) are generally measured by investigating the input–output relationship of a target structure. An input is applied to the target structure and the output is measured. Typical vibration tests measure inputs as excitation forces using load cells or similar devices and outputs as velocities or accelerations using laser Doppler vibrometers or accelerometers. Each input or output has three translational degrees of freedom (DOF). Consequently, experimental models established based on such tests have three translational DOF. However, numerical models using finite element analysis (FEA) for existing structures have six DOF at each node (three translational and three rotational DOF). Therefore, numerical models using FEA and experimental models have gaps in their DOF, leading to diverse problems. This study realizes a non-contact measurement method for rotational DOF FRFs using an impulse excitation force generated by laser ablation (LA) where the input is a moment and the output is the velocity. A laser beam radiated from an Nd:YAG pulse laser emitter is divided using a half mirror, halving the laser pulse energy. Then, these laser beams are radiated on two points near a target structure's excitation point. The simultaneous occurrence of LA at these two points realizes a moment that acts on the target structure in a non-contact manner. This experiment uses a 5052 aluminum alloy plate where one end is fixed as a test piece. Both the auto FRFs and cross FRFs for the rotational DOF between two arbitrary points on the test piece are measured. The FRFs obtained by our method agree well with those obtained by FEA, demonstrating its efficacy.