Hydraulic machine tests for compression of a quasi-brittle material at medium strain rate

Abstract
This paper describes an experimental device used to determine the dynamic compressive behaviour of quasi-brittle material at medium strain rates (1 to 100 s ^{- 1}). The tool combines a servo-hydraulic machine with a high-speed photography. Tests consist in compressing a sample between a dynamic jack and an instrumented anvil according to the direct impact method. The main difficulty of brittle material testing is to achieve dynamic equilibrium in the sample before failure because of their low failure strains. Furthermore, oscillations phenomena disturb load measurement. In this paper, we present adequate methods in order to carry out homogeneous testing and to simplify data interpretation. Two experimental configurations are developed. We use firstly the anvil as a load cell for low impact velocity and secondly the wave propagation in the anvil for medium impact velocity. Finally, in order to investigate experimentally the strain uniformity, axial strain measurements are quantified by image processing. Results are compared with experimental ones obtained on a crossbow system.