Split-Hopkinson pressure bar

The Split-Hopkinson pressure bar, named after Bertram Hopkinson, sometimes also called a Kolsky bar, is an apparatus for testing the dynamic stress-strain response of materials.

The Hopkinson Pressure Bar was first suggested by Bertram Hopkinson in 1915 as a way to measure stress pulse propagation in a metal bar. Later, in 1949 H. Kolsky refined Hopkinson's technique by using two Hopkinson bars in series, now known as the split-Hopkinson bar, to measure stress and strain, incorporating advancements in the cathode ray oscilloscopes in conjunction with electrical condenser units to record the pressure wave propagation in the pressure bars as pioneered by RM Davies a year earlier in 1950.

Later modifications have allowed for tensile, compression, and torsion testing.

Although there are various setups and techniques currently in use for the Split-Hopkinson pressure bar, the underlying principles for the test and measurement are the same. The specimen is placed between the ends of two straight bars, called the incident bar and the transmitted bar. At the end of the incident bar (some distance away from the specimen, typically at the far end), a stress wave is created which propagates through the bar toward the specimen. This wave is referred to as the incident wave, and upon reaching the specimen, splits into two smaller waves. One of which, the transmitted wave, travels through the specimen and into the transmitted bar, causing plastic deformation in the specimen. The other wave, called the reflected wave, is reflected away from the specimen and travels back down the incident bar.

Most modern setups use strain gages on the bars to measure strains caused by the waves. Assuming deformation in the specimen is uniform, the stress and strain can be calculated from the amplitudes of the incident, transmitted, and reflected waves.

For compression testing, two symmetrical bars are situated in series, with the sample in between. The incident bar is struck by a striker bar during testing. The striker bar is fired from a gas gun. The transmitted bar collides with a momentum trap (typically a block of soft metal). Strain gauges are mounted on both the incident and transmitted bars.

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