A novel technique to measure the biaxial properties of materials at high strain rates by electromagnetic Hopkinson bar system

Abstract

A new electromagnetic biaxial Hopkinson bar system is developed for the first time to investigate the true dynamic mechanical properties of materials under biaxial loadings. Four stress waves along two axes are generated by two independent sets of electromagnetic loading system. Each set of the loading system consists of two loading devices which are drove by LC circuits. The variation of the loading ratios between two axes can be achieved by adjusting the charging voltages of capacitors corresponding to each axis. The synchronism of the incident stress waves is controlled by a digital delay generator. A cruciform specimen with slits and thickness reduction is proposed for biaxial Hopkinson bar test and the stress distribution in gauge area is optimized with finite element method. Data process methods using stress wave signals on the four elastic bars are studied. Results show that a reasonable estimation of stress is obtained based on the bar signals. The first true biaxial tensile experiment is performed on oxygen-free copper at high rates of strain. Real-time images captured by a high-speed camera reveal reasonably homogeneous strain distribution in the gauge area. The stress and strain curves of the test piece under equi-biaxial stress condition are obtained and compared with the results of uniaxial test.