In order to investigate the dynamic behavior of frozen soil under compression, the Split Hopkins Pressure Bar (SHPB) was used. The SHPB set-up consists of a striker bar, an incident bar and a transmission bar. The incident and the transmission bars are instrumented with strain gages to capture elastic waves generated by the striker bar. A frozen soil sample is placed between the incident and the transmission bars and the striker bar is launched by compressed air. An oscilloscopic fast data acquisition system captures the wavelets in the incident and the transmission bars at a rate of 5 million samples per second per channel.

The elastic compression wave was generated by impacting the striker bar into the incident bar. The pressure wave was transmitted through the specimen and partially reflected at the interface, between the specimen and the incident bar, if the impedance of the specimen is less than that of the bars. Due to the low acoustic impedance of the frozen soil, the reflected and transmitted signals registered to strain gages have low signal to noise ratio.

To increase the signal to noise ratio, 7075-T6 Aluminum was used as the material for the incident and transmission bars and the high output semiconductor strain gages were used to measure pressure waves. A low-temperature enclosure was designed to enclose a frozen soil specimen under constant freezing temperature, while testing the specimen. To achieve an equilibrated stress state and homogeneous deformation throughout the specimen, the load at the incident-bar interface was controlled by pulse shaping.

Collaboration with Moo Y. Lee

Low Temperature SHPB Set-up