22 Imager-Based Characterization of Viscoelastic Material Properties 223 20 90 0 0.05 0.1 0.15 80 70 60 50 40 30 20 10 40 60 80 Deflection (mm) 100 120 Fig. 22.6 Deflection vector diagram An advantage of the imager-based method is that deformations of the specimen can be calculated as well. These displacements are found from the superposition of the mode shape vectors multiplied with the time series. Then, these values are converted into metric displacements using spatial resolution calculations. Due to the high spatial resolution, displacements at each pixel which has detected motion, is calculated. With this approach, displacements along different areas on the specimen can be determined, which is not possible using traditional contact-based methods. Figure 22.6 shows an example plot of the deflection vectors for a single frame of time within a single test. The average displacement calculated from the active pixels for this test was 0.01 mm or 0.00043 in. 22.5 Conclusion Phase-based, full-field imager-based modal identification techniques are able to extract modal parameters of viscoelastic materials at intermediate strain rates. Imager techniques offer valuable capabilities for identifying modal properties, as they possess high spatial resolution. Due to this, displacements and damping ratios of the object can be calculated from the video data, providing further information on a material’s response. This work involved oscillating a specimen at a known resonant frequency and then turning off the shaker input to allow the specimen to damp out while recording it. Through a signal processing technique, modal properties were extracted from the video. Successfully extracting the damping ratio and specimen deformations in the intermediate strain rate was accomplished through this study. Future studies may focus on using averaging across several tests to reduce noise. Further work may address exploring the use of imager-based modal analysis at a smaller scale. This study has shown that cameras are able to detect extremely small displacements, and it is possible that this sort of technology could be applied to micro-scale structures as well. Reduction of the specimen size is another improvement to be explored in order to more accurately represent the PBX explosive. Modeling specimen in a finite element software will aid in verifying the results. Acknowledgements Los Alamos National Laboratory (LANL) is operated by Los Alamos National Security LLC, for the National Nuclear Security Administration of the U.S. Department of Energy, under DOE Contract DE-AC52-06NA25396. This project was supported by LANL under the Engineering Institute Dynamics Summer School (LADSS) fellowship program. This project was mentored by Bridget Martinez, Nathan Miller, Trevor Tippetts, and David Mascarenas. A special thanks to Gregory Taylor for his previous work and contributions. References 1. Shim, J., Mohr, D.: Using split Hopkinson pressure bars to perform large strain compression tests on polyurea at low, intermediate and high strain rates. Int. J. Impact. Eng. 36(9), 1116–1127 (2009)
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