| Summary: | 碩士 === 國立成功大學 === 土木工程學系碩博士班 === 97 === Resonant ultrasound spectroscopy (RUS) has been developed for measuring the elastic constants of materials. The RUS techniques require only slightly corner contact force to mount specimens between two transducers. One transducer gives an amplitude and frequency excitation to a sample, and the other as a resonant receiver. In this thesis, we focus on the linear viscoelastic damping of materials (such as Al, Sn, SS & Cu) and their behavior at high frequency. RUS provides a large frequency range of scientific interest. We adopted various specimens to demonstrate the linear viscoelastic damping. For the ECAP (Equal-Channel-Angular-Process) sample, the sub-micro grain specimens showed similar shear modulus, but larger damping, when compared with their large grain counter part. The loss tangent damping of ECAPed aluminum is about 2.5 times as large as the damping of UnECAPed one, they are 13.10×10-4 and 5.18×10-4, respectively. The shear modulus of Zr-based metallic glass is 42.37 GPa, and we get the linear viscoelastic damping which is 2.79×10-4. In order to eliminate residue stress from samples, stress relief annealing is required. There are no obvious changes in resonant frequencies and modulus between annealed & un-annealed samples, it is 3.52 % for resonant peak shift in maximum, and the damping of annealed ones decrease compared with un-annealed, indicating dislocation density affects damping. Lower dislocation density reduces damping. We also detect the lowest several modes for aluminum cube, there is frequency-split phenomenon due to lack of symmetry of the material, and ABAQUS simulation is performed for conformation. The linear viscoelastic damping is found decreasing as frequency increase. This is a strong evidence for dislocation-based damping mechanics. Small copper and tin cubes are combined by glue to be a lattice-like cube which compared with full cube, we observed that is the glue provides too much damping, the damping of combined copper cube is about 79 times as large as the damping of full cube which is from 12.23×10-2 to 15.49×10-4, and so occurred in tin sample that is 74 times which from 13.97×10-2 to 18.98×10-4. Lorentzian curve fit is widely used to fit the resonant peak for data in experiment, it is almost perfect consistency.
|
|---|
