A modified Hopkinson bar system for testing ultra-soft materials under intermediate strain rates

An innovative Hopkinson bar system for testing ultra-soft materials under intermediate strain rates has been developed. We employ a striker to impact on a pulse shaper, tipped on the aluminum incident bar end to generate an ultra-long loading pulse (5 ~ 50 ms). Because of the overlap of incident wave and reflection wave and low amplitude of the strain signals, we can not use the conventional strain gauges that are mounted on the bars to carry out measurements. In our design, we use two piezoelectric force transducers that are sandwiched between the specimen and two bars respectively to directly measure the weak dynamic loads. Furthermore, a laser gap gauge is employed to monitor the deformation of the sample directly. The slope of the long incident pulse is small enough to ensure both the dynamic force balance of the sample and the intermediate to lower strain rates of the sample deformation. The annular-shaped foam rubber specimen is used in our experiment to demonstrate the feasibility of our new Hopkinson bar system. Experimental results show that this new method is effective and reliable for determining the compressive stress-strain responses of ultra-soft materials at intermediate strain rates