A study of parameters of Kong-Fang fluid elastoplastic damage material model for Shandong granite

The establishment of the dynamic mechanical model of rock materials and the determination of the relevant model parameters are of great significance to the studies of rock’s dynamic mechanical properties and related simulation calculation. Taking granite in Shandong Province as the experimental object, based on the Kong-Fang fluid elastic-plastic damage material model (KF model), the model parameters are classified into three categories, and the test scheme is then correspondingly determined. The basic strength parameters of the KF model were measured by quasi-static uniaxial compression and unconfined splitting tests. The strength-surface related material parameters were fitted by the results of the conventional triaxial tests under five different confining pressure conditions. In addition, the dynamic split Hopkinson pressure bar (SHPB) tests under several strain rate conditions were carried out to determine the strain-rate related parameters, of which the effectiveness were then verified by the dynamic split Hopkinson pressure bar-Brazilian disk (SHPB-BD) tests results. According to the principle of reverse impact and the Rankine-Hugoniot equation, the plate impact experiments with different impact stress levels were conducted by using a single-stage light gas gun, the state equation parameters in the KF model were fitted according to the impact Hugoniot results of rock samples. To verify the applicability of the material model and the experimentally measured parameter values, the simulation of a penetration process is furtherly conducted. The granite penetration tests were carried out by using a $\varnothing$30 mm caliber gun. The $\varnothing$20 mm bullets penetrated the $\varnothing$1200 mm×800 mm rock targets vertically, which was used to characterize the semi-infinite thickness condition, at an approximately designed speed of 670 m/s. To avoid accidental errors, combined with the high-speed photographic images, three effective penetrate results were obtained. The penetration depth and crater size of the target failure surface were directly measured and scanned by 3D scanner, the experimental average penetration depth, maximum and minimum diameters of the penetration craters were approximately 80.62 mm, 381.47 mm and 263.01 mm, respectively. Using the parameter values obtained from the laboratory experiments, the KF model is then implemented into LS-DYNA through a user-defined material model and used to simulate the penetration test of granite. According to the simulation result of damage distribution and cratering parameters of the target, the calculated penetration depth, maximum and minimum diameters of craters are 80.02 mm, 400 mm and 300 mm, respectively, so the errors between the calculated and the test results are less than 15%, which is acceptable in dynamic problems. The agreement between the numerical and experimental results provides a support to the application of the KF model and the relevant parameter values.