Dynamic mechanical testing encompasses many types of mechanical tests such as Charpy impact tests, automotive crash tests, and Hopkinson bar tests. Mechanical tests are often classified by their duration which sets their strain rate. In traditional quasi-static tensile and compression testing, this strain rate is often prescribed and used as a control for the test which can last anywhere from a few seconds to many hours. On the other hand, dynamic experiments typically last under one second.
Hopkinson Bar Testing
Hopkinson bars (also known as Kolsky bars or more colloquially “Hoppy bars”) are dynamic testing machines that test material samples in tension, compression, or torsion. Typically, they use a pneumatically fired rod or tube which creates a stress wave that deforms a specimen. Hopkinson bar tests directly measure these stress waves usually with strain gages; then, by utilizing one dimensional wave equations, the stress vs strain curve of the material can be developed at strain rates up to 2000 /s. The illustration below shows a compression Hopkinson bar experiment along with the strain gage voltages. The initial square wave, the “incident wave,” shows the amount of energy supplied by the striker bar to the incident bar. The incident wave passes through the incident bar and into the specimen where some of its energy is reflected back, the “reflected wave,” some of the energy passes through, the “transmitted wave,” and some of the energy is absorbed by the plastic deformation of the specimen. Force vs displacement data is derived by analyzing these three waves, and the stress vs strain data is then found from the specimen geometry.