A modified servo-hydraulic machine for testing at intermediate strain rates

Assessing the mechanical properties of materials at intermediate strain rates (1–200/s) is one of the major concerns in impact engineering. Servo-hydraulic machines are usually used at this range of strain rates. However, detrimental oscillations appear on the measured load for strain rates above 10/s. In this paper, we present a modified servo-hydraulic machine. The load is measured by a Hopkinson bar-like technique. Besides, the BCGO wave separation method is used to overcome the limited test duration problem. Moreover, the strain in the sample is measured using Digital Speckle Photographs (DSP) which is captured via high-speed video camera. This modified machine is an equivalent and alternative solution to the slow bar technique. It is applied to aluminium 2017 T4 alloy at strain rates of approximately 100/s. Compared to the piezoelectric force sensor, the Hopkinson bar-like method gives less-oscillating signals. The sample attaching device is also taken into account. It is considered as a rigid mass. This simple assumption is valid up to strain rates of some hundreds per second.