This laboratory is for studying the dynamic response of materials subjected to impulsive, high strain-rate loadings. The laboratory contains Kolsky (or split-Hopkinson) torsion and compression bars as dynamic loading devices. These apparatuses can produce a rapid rising, trapezoidal pulse of torsion, compression or tension loading, or a combination of these loading pulses. Both the delivery of dynamic loading to a test sample and the result of the sample material response to the loading are in the form of linear elastic stress waves propagating within two long metallic bars (elastic waveguides) and can be determined accurately by analyzing time-resolved measurements of the profiles of these waves in the bars. Such measurements are obtained using a state-of-the-art electronic system consisting of high-impedance, precision strain gauges, a 12-bit high-resolution digital oscilloscope with multi-channel differential amplifiers, and a PC workstation with control software for automated data acquisition. The experimental technique enables various materials of interest (including ceramics, metals, and polymer melts) to be examined under well-defined dynamic loadings. The following are some of the ongoing projects in the laboratory:
1. Transient Rheometry of Polymer Melts at High Shear Rates
A novel polymer melt rheometer has been developed by incorporating a cone-and-plate rheometric cell and a thermal chamber into the Kolsky torsion bar device. The impulsive loading delivered in the form of guided torsional stress wave pulse can drive the new rheometer to an angular sliding velocity as high as 1600 rad/s in a time less than 100 ms, thus enabling measurements of the transient, large-deformation rheological response of polymer melts at shear rates up to 10000 1/s, shear strains up to 10, and temperatures up to 300 °C. This new technique is currently used to characterize the viscoelastic response of a branched low-density polyethylene melt under high-rate and large-strain shearing deformations. The results are useful for improving the material modeling in computerized analysis and design of the manufacturing processes involving rapid flows of the material, e.g., injection molding and extrusion.
2. Dynamic Tribometry of Fracture Surfaces
A dynamic tribometer has been developed by adding a compression unit into the conventional Kolsky torsion bar device. The modified apparatus produces combined loadings of dynamic compression and torsion, thus providing a new technique for dynamic tribometric experiments at sliding velocities up to 10 m/s and compressive contact stresses up to 1 GPa. This technique is being used in the research work sponsored by the U.S. Army Research Office to investigate the dynamic frictional resistance between closed fracture surfaces. Tribo-pairs formed by pre-fractured specimens are tested under various contact stresses and sliding velocities. The tribometric results are studied in conjunction with the statistical characterization of the fracture surface topography in an attempt to develop an understanding of whether and how a micro- fractured material under high confining stresses can resist dynamic deformation. This scientific issue is important for material and structure designs of advanced armors, particularly those involving the use of hard and brittle solids such as ceramics and ceramic composites.