Development of a dynamic true triaxial electromagnetic Hopkinson bar system

Heping Xie, Jianbo Zhu, Yulong Li, Tao Zhou, Shiwei Zhang, Weiyue Bao, Zhongbin Tang, Tao Suo, Xiaoyong Song, Jian Zhao

 Subsurface rock masses and rock engineering are typically subjected to multiaxial static stresses, often superimposed by multidirectional dynamic disturbances induced by seismic events, blast loading and mechanical vibrations, etc. A comprehensive understanding of the dynamic mechanical behavior and failure mechanisms of rocks under such coupled multiaxial or three-dimensional static and dynamic loading conditions is critical for the rational design, safe construction and long-term stability of rock engineering projects. However, there has been no three-dimensional dynamic theory of rock so far. A major reason is the lack of dynamic true triaxial apparatus, which is extremely difficult to develop using traditional techniques due to technical challenges such as precise and repeatable stress pulse generation, six-directional stress pulse synchronous loading, and triaxial static-dynamic coupled loading. To address this need, we developed a novel cutting-edge testing platform, i.e., dynamic true triaxial electromagnetic Hopkinson bar (DTEHB) system. It is the first apparatus in the world for studying dynamic responses of rock masses under coupled three-dimensional dynamic disturbances (with strain rates ranging from 101 s-1 to 103 s-1) and true triaxial static in situ stresses. The fundamental working principles, system configuration and key technical challenges overcome in the development of the DTEHB are introduced in detail in this paper. The system is comprised of the dynamic true triaxial Hopkinson bars, a triaxial/six-directional electromagnetic stress pulse synchronous loading system, a triaxial/six-directional static-dynamic coupling loading system, and a multivariate dynamic real-time data acquisition and analysis system. By combining electromagnetic energy conversion and synchronous control techniques, the DTEHB achieves highly repeatable (>99%) and precise true triaxial synchronous impact loading, and synchronous six-directional dynamic loading (time error ≤5 μs). The DTEHB system is also capable of simulating coupled triaxial/six-directional impact and static confining pressure loading, replicating 3D in situ stress conditions. Moreover, the system’s reliability and performance are validated through analyzing controlled and adjustable stress waveforms (repeatability, duration, amplitude and arrival time), static true triaxial confining pressures and synchronous generation and arrival six-directional stress pulses. Dynamic true triaxial synchronized impact tests were successfully conducted on rock specimens for the first time. The establishment of the DTEHB could facilitate experimental testing of rock and other materials under a range of dynamic disturbances, thereby advancing the theory of three-dimensional rock dynamic theories.