Dynamic strain field for granite specimen under SHPB impact tests based on stress wave propagation

The full-field strain of rock material under dynamic compression load was studied using the high-speed three dimensional digital image correlation (3D-DIC) method. The dynamic test was conducted on Laizhou granite using a split Hopkinson pressure bar (SHPB) method. Wave propagation, dispersion and radial inertial effect on the specimen were found by DIC results. A recovery of strain in the post-peak stage was detected on the specimen by DIC, which was unrevealed in the traditional one-dimensional theory method. It can be found that the strain measured by strain gauge was a calculated average one, whereas the strain measured by 3D-DIC could reflect more variation details. Specifically, the testing principle with impact loads and rock dynamic behavior was re-examined using stress wave propagation theory. The theoretical results showed that the specimen reached equilibrium after a series of wave reflections and transmissions and its stress was infinitely close to the initial value of 109.2 MPa. Moreover, the specimen had the calculated maximum strain of 0.52% and strain rate of 15.11 s(-1), improving the reasonable agreement with the experimental results and requirements of rock mechanical properties measured by SHPB technology.