单轴压缩湿砂样局部及整体体积应变的数字图像相关方法观测

Measurements of local and global volumetric strains for wet sand specimens under uniaxial compression using digital image correlation method

确定体积应变局部化区域对于涉及孔隙流体的地质灾害研究具有重要意义。体积应变局部化区域可能比最大剪切应变局部化区域更接近于孕灾地点。在单轴压缩位移控制加载条件下,测试了3个湿砂样(含水率为12.7%~17.1%)的应力–纵向应变曲线,在微裂纹出现之前及稍后,利用自主开发的基于粒子群优化的数字图像相关(DIC)方法计算了砂样的局部体积应变的变化规律。研究发现,在加载过程中,局部体积应变由均匀分布(近似线性阶段)向不均匀分布(硬化阶段)转变,直至出现局部体积应变局部化现象。在剪切带切向上,和最大剪切应变的相对均匀分布相比,局部体积应变的分布具有多峰性,这应与周期性的微裂纹出现有关。在微裂纹出现稍前,剪切带的变形是以膨胀为主,剪切带已较为显著,带内的最大剪切应变和局部体积应变分别比带外多2倍和4倍,但在此前,剪切带的膨胀和收缩现象共存。计算了所有测点所围区域的整体体积应变随纵向应变的演变规律。基于DIC方法的整体体积应变计算方法的优越性在于:具有亚像素精度,考虑了应变二阶量的影响。

Determination of volumetric strain localization regions in shear bands is significant for studies on geological hazards related to pore fluids. Possibly, compared with those of the maximum shear strain localization, the positions of the volumetric strain localization are closer to sites for the source of hazards. Stress-longitudinal strain curves of three wet sand specimens (water contents=12.7%~17.1%) under uniaxial displacement-controlled loading are measured. Using a developed digital image correlation (DIC) method based on the particle swarm optimization algorithm, the distribution of local volumetric strain in sand specimens is calculated before and slightly beyond microcracks are initiated. It is found that as deformation proceeds, the uniform distribution of the local volumetric strain at the approximately linear stage is changed to the nonuniform one at the strain-hardening stage until the local volumetric strain localization occurs. In the tangential direction of a shear band, compared with the relatively uniform distribution of the maximum shear strain, the distribution of the local volumetric strain exhibits several peaks, which is related to the periodic occurrence of microcracks. When microcracks will be initiated, the deformation in the shear band is basically dilatational; the shear bands are more apparent; and the maximum shear strain and local volumetric strain in the shear band are two and four times larger than those outside the band, respectively. However, at the earlier stage, the dilatational and contractive regions are coexistent in the shear band. Evolution of the global volumetric strain of the region surrounded by all points with the longitudinal strain is calculated. The proposed DIC method to calculate the global volumetric strain has the sub-pixel accuracy and the influence of the second-order strains is considered.