Split-Hopkinson bar testing is a tried and tested method in materials testing. Unlike quasi-static testing machines a split-Hopkinson bar enables material properties to be determined in dynamic conditions. The method involving the use of a split-Hopkinson bar enjoys increasing popularity in many applications thanks to ever more powerful test and measurement technology.
A split-Hopkinson bar is used to dynamically determine material constants, for example Young's modulus or mechanical stress. Young's modulus is a material constant that is a measure for how much a component is deformed when a force is applied to it.
British electrical engineer Bertram Hopkinson first suggested such measurements in 1914. The setup used today is based on a modification developed by Herbert Kolsky in London in 1949. It is sometimes also called split-Hopkinson Kolsky bar.
The material sample is positioned between two bars in the split Hopkinson bar: the incident bar and the transmission bar. A so-called striker - for example, a projectile accelerated by compressed air - strikes the incident bar causing an elastic wave pulse.
This elastic wave pulse runs through the first bar. Part of the pulse is reflected at the bar end, the other part runs through the material sample into the transmission bar.
Strain gauges (SG) installed on the surfaces of the incident bar and the transmission bar measure the strains caused by the elastic wave pulse. The strain gauges enable the amplitudes of the elastic wave pulse applied to the incident bar, the reflected pulse and the transmitted pulse to be determined.
Which requirements does test and measurement technology neet to meet when a split-Hoplinson bar is used?
Successful use of a split-Hopkinson bar requires strain gauges installed on the surfaces of both the incident bar and the transmission bar and, in addition, a powerful data acquisition system. This is confirmed by latest research findings: „Generally speaking, the minmum frequency response of all the components in the data acquisition system should be 100 kHz. (Cheng / Song, Split Hopkinson Bar, S. 9)”.
The Genesis HighSpeed series offered by HBM Test and Measurement is the perfect data acquisition system for use on a split-Hopkinson bar. In addition, HBM provides you with strain gauges developed and produced 'in-house' for installation on a split-Hopkinson bar.
What is the difference between using a split-Hopkinson bar and a static material testing machine?
Young's modulus is usually determined from a stress-strain-curve created in a testing machine under quasi-static conditions - i.e. with (very) small strain rates. However, material behavior may differ substantially with dynamic loads. Depending on whether dynamic loads, too, occur in a structure, the design engineer needs to know the material's dynamic properties as well.
Normally a simple material testing machine is not able to apply the required high strain rates.
A comprehensive explanation of the functioning and use of a split-Hopkinson bar is given in the reverence book by Weinong Chen, Bo Song: Split Hopkinson (Kolsky) Bar- Design, Testing and Applications.
You can also contact Bundesanstalt für Materialprüfung in Berlin, Germany. They offer the determination of material properties as a service.