Session: VIB-01/MSNDC-08-03

Paper Number: 147846

147846 - Strain-Dependent Damping Measurement in Bending Mode Vibration 

All real materials dissipate some energy, no matter how little, during cyclic deformation regardless of the precise physical mechanisms involved. Such effects are often highly nonlinear, so detailed analysis of response with such damping mechanisms is usually very difficult. However, experimental measurements of behavior of samples of specific materials can be qualitatively understood in terms of the measured specific damping energy for various strain levels [1]. Internal friction (1/Q), or damping (zeta), has been used to study the inherent vibrational energy dissipation due to microstructural and micromechanical phenomenon in materials for many years. Consequently, several test techniques have been developed to measure damping in test materials, all of which require the vibration of the specimen. Each technique has frequency and vibratory strain amplitude ranges, and damping can be measured either during free decay or during a continuous driving force at a given frequency and/or strain amplitude. Because damping is often measured in different modes of vibration, namely extensional, torsional, and flexural, careful attention must be given to the data from each test in order to obtain valid data comparison [2].

In this study, among those damping measurement systems developed from prior research [2, 3, and 4], a flexural bending mode vibration damping measurement system was chosen, due to the vibration mode that is mostly used in naval application. Furthermore, the system is simple to establish, and the specimen is easy to fabricate from ingot, which we can be made at the Navy. However, there is a limitation of accuracy of the damping performance evaluation in this mode. As indicated in the reference [2], the strain for flexural mode vibration is localized, and there is possibly an extra damping effect from the sample support and airflow around the sample. For minimizing the boundary condition effect from the measurement, a solid material based magnetostrictive actuator was established in the system for initiating the vibration. This excitation methodology can be applied to non-magnetic materials as well. For the vibration measurement, non-contact laser sensor was implemented for avoiding any extra effects on the beam vibration as shown in Figure 1. We assumed the airflow around sample is negligible for small strain excitation levels and for small area of the samples.

As shown in Figure 2, two different technique were evaluated including (a) modal curve fit method and (b) sharpness of resonance method using 3-dB bandwidth calculation for the damping coefficient from the measurements for a legacy structural material, HSLA-100. The damping results from modal curve fit method showed better consistency and less variation compared with the 3-dB bandwidth calculation results for various excitation levels. Considering the dependence of excitation level for the damping performance, a new way to define the damping performance of metallic material is suggested in this study. The damping parameter will be compared with respect to the sample thickness and excitation level. Further discussion on the results will be included in the presentation.

Presenting Author: Jin Hyeong Yoo Naval Surface Warfare Center at Carderock Division

Presenting Author Biography: Dr. Yoo is a research engineer in the Physical Metallurgy and Fire Performance branch at Naval Surface Warfare Center at Carderock Division (NSWC-CD), and currently works on application development using Magnetostrictive Materials. Prior to joining to NSWC-CD, he was a Research Scientist at Army Research Laboratory. In his academic career, he has studied modal testing on vibration systems and vibration isolation control using smart materials.

Authors:

Jin Hyeong Yoo Naval Surface Warfare Center at Carderock Division
Nicholas Jones Naval Surface Warfare Center at Carderock Division
Aphrodite Strifas Naval Surface Warfare Center at Carderock Division