Imager-Based Characterization of Viscoelastic Material Properties

In this work we explore the use of emerging full-field, high-resolution, modal identification techniques from video to characterize the viscoelastic properties of a material. Currently, there are no cost-effective methods to directly measure viscoelastic material properties at intermediate strain rates. These properties can be measured at low strain rates using quasi-static loading techniques, while Split-Hopkinson’s bar tests are used at high strain rates. Determining material properties at the intermediate strain rate regime is challenging as it requires large, expensive testing apparatuses and involves complex experimental protocols. An imager-based technique would provide a simpler, more affordable method for measuring viscoelastic material properties at these strain rates. To obtain measurements for intermediate strain rates of viscoelastic materials, we develop a testing protocol that involves creating a simple structure from the material-under-test and measuring its vibrational response using an imager. A finite-element model of the viscoelastic testing structure is also constructed. We extract full-field, high resolution mode shapes and modal coordinates from the video measurements of the structure as it vibrates in the desired strain regime. The frequencies of oscillation and the damping ratios are then identified. This information is intended to be used to perform model updating on the viscoelastic material properties of the finite-element model, resulting in an improved estimate of the material properties. Imager-based techniques are particularly attractive for explosive testing applications because the optics can be adapted to address the small sample sizes necessary for explosive testing. In addition to advancing viscoelastic material modeling, this work points toward the development of an imager-based modal analysis technique for identifying the structural dynamics of micro-scale structures. At this small scale, conventional contact-based sensors would result in mass-loading effects, yielding inaccurate measurements. As a solution, our full-field, high-resolution imager-based technique provides a method to characterize viscoelastic material properties while also demonstrating potential for future work in identifying structural dynamics of micro-scale structures.