Pre-Compression Kolsky Bar Tension System for Brittle Material Testing

The objective of this effort is to develop a tensile Kolsky bar technique capable of being utilized to measure the response of brittle materials, such as rocks and concretes, in a state of uniaxial dynamic stress and strain-rate equilibrium.  Due to the low-tensile-strength and strain-to-failure behavior of these materials, it is difficult to achieve dynamic stress and strain-rate equilibrium in a traditional Kolsky tensile test before specimen failure is achieved.  Since these physical metrics have not been achieved during testing, the validity of the experimental data is therefore questioned.  Specifically, the state-of-stress and the strain-rate at failure metrics are difficult to determine in a traditional tensile Kolsky set-up.  To address this challenge, the technique developed in the Phase I effort and to be manufactured in this proposed Phase II effort will apply an elastic axial compressive pre-load to the test specimen before a dynamic load is applied through a modified tensile Kolsky bar system.  During the initial tensile loading of the test specimen from the incident bar, the pre-compressive load on the specimen will be negated while allowing the loading pulse and strain rate to equilibrate load at lower mean values and therefore earlier into the experiment.  The design allows the preload to be adjustable to account for different material types and can be applied at a sufficiently low-level to restrict the test specimen from being plastically damaged before the dynamic tensile load is applied.   Finally, novel pulse-shaping and specimen grip designs were developed and demonstrated in the Phase I effort to address potential risk features of this technique.  During the proposed Phase II effort, DSR/SMU staff shall complete the development, manufacturing, installation, and training of US Army personnel with this pre-compression tensile Kolsky bar system. Pulse-shaping enhancements from Phase I testing will also be completed along with any modifications to the specimen grip designs developed and demonstrated in this Phase I effort.  The proposed 24-month effort will consist of a 12–16-month manufacturing phase followed by 8-12 months of installation certification and testing support.  It is expected that the results from initial testing of concretes in this device will aide in hardened structures constitutive modelling efforts for shock environments from penetration and blast events for systems such as hyper-sonic missiles.