Cylindrical specimens are commonly used in Split Hopkinson pressure bar (SHPB) tests to study the uniaxial dynamic properties of concrete-like materials. In recent years, true tri-axial SHPB equipment has also been developed or is under development to investigate the material dynamic properties under tri-axial impact loads. For such tests, cubic specimens are needed. It is well understood that static material strength obtained from cylinder and cube specimens are different. Conversion factors are obtained and adopted in some guidelines to convert the material strength obtained from the two types of specimens. Previous uniaxial impact tests have also demonstrated that the failure mode and the strain rate effect of cubic specimens are very different from that of cylindrical ones. However, the mechanical background of these findings is unclear. As an extension of the previous laboratory study, this study performs numerical SHPB tests of cubic and cylindrical concrete specimens subjected to uniaxial impact load with the validated numerical model. The stress states of cubic specimens in relation to its failure mode under different strain rates is analyzed and compared with cylindrical specimens. The detailed analyses of the numerical simulation results show that the lateral inertial confinement of the cylindrical specimen is higher than that of the cubic specimen under the same strain rates. For cubic specimen, the corners are more severely damaged because of the lower lateral confinement and the occurrence of the tensile radial stress which is not observed in cylindrical specimens. These results explain why the dynamic material strengths obtained from the two types of specimens are different and are strain rate dependent. Based on the simulation results, an empirical formula of conversion factor as a function of strain rate is proposed, which supplements the traditional conversion factor for quasi-static material strength. It can be used for transforming the dynamic compressive strength from cylinders to cubes obtained from impact tests at different strain rates.

圆柱形试样通常用于分裂霍普金森压力杆 (SHPB) 测试，以研究类混凝土材料的单轴动态特性。近年来，真正的三轴SHPB设备也已经开发或正在开发中，用于研究三轴冲击载荷下的材料动态特性。对于此类测试，需要立方试样。众所周知，从圆柱体和立方体试样获得的静态材料强度是不同的。在一些指南中获得并采用了转换因子来转换从两种类型的试样获得的材料强度。以前的单轴冲击试验也表明，立方试样的失效模式和应变率效应与圆柱形试样有很大不同。然而，这些发现的机械背景尚不清楚。作为先前实验室研究的延伸，本研究使用经过验证的数值模型对承受单轴冲击载荷的立方体和圆柱形混凝土试件进行数值 SHPB 试验。分析了立方体试样在不同应变率下的应力状态与其失效模式的关系，并与圆柱试样进行了比较。数值模拟结果的详细分析表明，在相同应变率下，圆柱试件的横向惯性约束高于立方体试件。对于立方试样，由于侧向约束较低，并且在圆柱试样中没有观察到径向拉伸应力的发生，角部的损坏更为严重。这些结果解释了为什么从两种类型的试样获得的动态材料强度不同并且取决于应变率。基于仿真结果，提出了应变率函数换算系数的经验公式，补充了传统的准静态材料强度换算系数。它可用于将动态抗压强度从圆柱体转换为在不同应变率下的冲击试验中获得的立方体。