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.
Standard Mechanics uses a combination of traditional and custom equipment and fixtures to ensure the highest quality data for any application:
- Custom Hopkinson bar / Kolsky bar load frames can test materials at the upper end of strain rates
- Ultra high speed videography allows for optical strain measurements and fracture analysis at frame rates up to 10,000,000 frames per second
- Various temperature ranging from -50C up to 250C
- Custom techniques that can be used to test materials down to strain rates of 100/s
- We have many years of experience in the design, development, and use of Hopkinson bars at their lowest speeds
