Hydro-pneumatic machine (HPM)

The hydro-pneumatic machine (HPM) is a device for tensile material testing and are operating in the strain-rate range between 1-200s-1. The diameter of the shafts is in the range 8–12 mm. The facility is operated by gas and water and has a lightweight movable piston made of steel or aluminium. The movement of the piston is controlled by the difference in pressure between the two chambers.

Hydro-pneumatic machine


Prior to testing, both chambers are brought to equal pressure by introducing nitrogen gas in one chamber and water in the other. The pressure difference is established by firing a rapid valve located in the exhaust line to the water chamber causing a rapid evacuation of the water through an orifice, thus allowing the piston to move at a constant velocity and stress the test specimen to fracture. The piston velocity and the hence the rate of loading is controlled by the size of the orifice. The load applied to the specimen is measured by using strain gauges on the bars which behave elastically during testing. The specimen elongation is measured by means of a displacement transducer.

The facility can be operated at low and high temperatures with the same instrumentation as for the SHTB. So far the test rig has been used to characterize steel and aluminium alloys at elevated rates of strain and temperatures.


Tarigopula V., Albertini C., Langseth M., Hopperstad O.S., Clausen A.H.: A hydro-pneumatic machine for intermediate strain-rates: Set-up, tests and numerical simulations. 9th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, Brussels, Belgium 7-11 September. DYMAT2009 (2009) 381-387. (doi)