2. Investigation of the Discrepancy Between Contact Force and Measured Force in a Hopkinson Tube

Hopkinson tubes can be used similarly to Hopkinson bars and undergo larger deformation than a solid Hopkinson bar of equal outer diameter. This is advantageous for loading that is small in magnitude, where the resulting strain in the bar may be too small to be measured with strain gages. The challenge with using Hopkinson tubes is that the force determined from the traveling stress wave in many cases does not match the contact force causing the deformation. This is due to the relatively complicated dynamics of the Hopkinson tube in comparison to the Hopkinson bar. In this work, simulations of the dynamics due to an impulse hammer striking a Hopkinson tube are used to investigate why the force measured from the impulse hammer does not match the force derived from measurements on the tube. Simulations are first validated with experimental data, and then used to demonstrate that the inertia of the end cap and tube length caused the significant force discrepancies in the experiment.