不同尺寸砂岩动态力学性质和应力平衡性的试验研究

采用大直径分离式霍普金森压杆系统获得的不同尺寸试样的实验冲击动态力学参数有差异，因此在直径100 mm压杆上进行了3种直径（50、75和100 mm）和5种长径比（0.4、0.5、0.6、0.8和1.0）的砂岩试样冲击试验，分析了不同尺寸试样应力-应变曲线和应变率曲线随尺寸的变化，提出了用于比较波形对齐重合度的波形叠加系数，并与应力平衡因子共同构建了动态应力平衡性研究体系，由此确定大直径霍普金森压杆试验的试样建议尺寸。同时，利用高速摄影机监测试样的动态破坏状况。结果表明:当长径比相同时，直径75与100 mm岩石试样的动态抗压强度测试值相近，直径50 mm试样具有更明显的长度效应；随着试样直径的增大，应变率曲线从单峰变为双峰；小尺寸试样更易发生轴向劈裂破坏，大尺寸试样受内部应力波叠加影响产生了较大的拉应力，易发生层裂拉伸和轴向劈裂的复合型破坏；对直径75 mm且长径比0.3~0.4的试样，波形对齐后重合度较高，在起始破坏前拥有充足的应力平衡时间，应变率加载效果较好。获得了砂岩试样冲击压缩试验的尺寸效应，可为大直径岩石试样的尺寸选择提供参考。

Rock dynamic mechanical properties and dynamic stress balance of sandstone specimens with different sizes

Aiming at a clarification of the differences in dynamic mechanical properties of rock specimens with different sizes when characterized by a large diameter split Hopkinson pressure bar system, sandstone specimens with three different diameters (50, 75 and 100 mm) and five kinds of length-diameter ratios (0.4, 0.5, 0.6, 0.8 and 1.0) were employed for impact experiments on a pressure bar of diameter 100 mm. The variations of stress versus strain and strain rate versus time of specimens with different sizes were analyzed. The concept of a superposition coefficient for comparing waveform alignment overlap was then proposed, and together with the equilibrium factor it was used to study dynamic stress equilibrium. Thus, the recommended size range of specimens was determined for large-diameter split Hopkinson pressure bar tests. Also, a high-speed camera was used to observe the dynamic damage of the specimens. The results show that when the length-diameter ratio of specimen remains the same, the tested dynamic compressive strengths are close for the specimens of diameter 75 mm and 100 mm, but it is affected by more pronounced specimen length for the specimens of diameter 50 mm. With the increase of specimen’s diameter, the curve of strain rate versus time changes from single peak to double peak. The small-size specimen is more prone to axial splitting failure, and the large-size specimen produces larger tensile stress due to the superposition of internal stress waves, which is prone to the composite failure of spallation tension and axial splitting. When the specimen with a diameter of 75 mm and the length-diameter ratio of 0.3–0.4 is used, the coincidence degree after waveform alignment is better, sufficient stress balance time is achieved before initial failure, and the strain rate loading is more effective. It is helpful to reveal the size effect on the rock dynamic compression mechanical properties with different sizes of specimens, as it can provide a good reference for the specimen size selection in large-diameter SHPB tests.