基于高速3D-DIC技术的砂岩动力特性粒径效应研究

利用分离式霍普金森压杆（SHPB），对粗砂岩、中等粒径砂岩和细砂岩进行了应变率为69～83 s–1的动态单轴抗压实验，研究了粒径尺寸效应对砂岩动力特性的影响。通过三维数字图像相关（3D-DIC）技术分析高速摄像图像，获得了砂岩的实时应变场，据此分析了动态荷载下3种粒径砂岩的动力变形特性和裂纹开展行为。结果表明，砂岩弹性应变储能可逆释放的临界应变率随着粒径的减小而升高，动态压缩强度随着粒径减小而增大，动态强度应变率敏感度则与强度规律相反。相较于静态条件下，中等粒径砂岩和细砂岩的动态弹性模量增长了2～3倍，粗砂岩的动态弹性模量增长达5倍以上。细砂岩的动态泊松比相较于静态提高了约25%，中等粒径砂岩的动态泊松比约为静态时的70%。动态裂纹首先出现于试件内部，然后传播至表面，呈现出应变局部化，动态荷载下岩石裂纹的孕育和扩展相比静态条件下均有所提前，其中细砂岩在动态荷载条件下的归一化裂纹起裂阈值仅为峰值强度的10%。微观分析表明，矿物粒径大小和黏土矿物含量分别在砂岩的动力力学性质和裂纹开展行为方面发挥主要作用。

Grain-size effect on dynamic behavior of sandstone based on high-speed 3D-DIC technique

The grain size effect on the dynamic behavior of sandstone was investigated through the compression tests on coarse-grained (CG), medium-grained (MG) and fine-grained (FG) sandstones by split Hopkinson pressure bar (SHPB) tests under the strain rates of 69–83 s–1 based on the thin section and electron scanning microscopic (SEM) images analysis, the CG, MG and FG sandstone were mainly composed by quartz with the average grain size of 200–500, 90–500 and 55–120 μm, respectively. With the increasing grain size, the percentage of clay mineral was decreased correspondingly from 8% to 1%. During the dynamic compression, two high-speed cameras were applied to capture the deformation of sandstone at frame rate of 2×105 s–1 and resolution of 256×256. The real-time strain fields of rock were obtained by high-speed three-dimensional digital image correlation (3D-DIC) technique, the dynamic deformative properties, particularly the lateral strain of the specimen, were extracted by averaging the lateral strain field by pixels. The fracturing behavior of three sandstones was analyzed through the strain localization evolution within the strain fields. Results show that the critical strain rate for reversible release of elastic strain energy increases with the decreasing grain size. The dynamic strength ascends along with the reduction of grain size, while the strain rate sensitivity to the dynamic strength has an opposite trend. Compared to the quasi-static case, the dynamic elastic modulus increases by 2–3 times for MG and FG sandstone, particularly 5 times for CG sandstone. The Poisson’s ratio under dynamic loading in FG sandstone is grown by 25%, but drops at 70% of the static one in MG sandstone. The crack primarily generates inside the specimen and propagates to the surface of the specimen afterwards. The crack development is advanced under dynamic loadings, where the normalized stress threshold for crack initiation in FG sandstone is only 10%. Based on the microscopic analysis, mineral structure and clay percentage dominate the dynamic property and fracturing behavior of sandstone, respectively.