冲击荷载下层状砂岩变形破坏及其动态抗拉强度试验研究

针对层状砂岩的各向异性,探究了冲击荷载作用下层理角度对层状砂岩变形破坏的影响规律。加工制作了含软弱层理的砂岩标准试件,利用霍普金森杆试验系统进行了不同层理倾角下的砂岩动态巴西圆盘试验,并结合数字图像相关方法获得了圆盘试件变形场的演化云图。从破坏结果看,层理面与加载轴线之间的夹角对层状砂岩的变形破坏有显著影响。当软弱层理平行于加载轴线时,圆盘试件在加载端处首先产生应变集中,并随着冲击加载的作用迅速沿层理扩展,最终表现为从圆盘试件加载端向非加载端呈弧线形断裂的特征;当软弱层理垂直于加载方向时,圆盘试件中间首先形成多个应变集中区,表现为在加载轴线与软弱层理相交处萌生多个微裂纹,并在冲击加载的作用下微裂纹沿加载轴线不断相互贯通,最终形成径向扩展的宏观裂纹;当软弱层理面与加载方向成45°时,圆盘试件在加载端处首先沿层理方向形成显著的拉剪应变集中区,由于层理介质的抗拉强度和抗剪强度均低于砂岩基质体,因而表现为试件在拉、剪复合应力的共同作用下从加载端处产生多条沿层理面扩展的裂纹。从试验结果中还可以看出,在相同加载速率下,垂直层理试件的强度最高,水平层理试件的强度最低,倾斜层理试件的强度介于水平层理试件和垂直层理试件之间。随着加载速率的提高,不同层理方向的砂岩动态抗拉强度均呈线性增长的特征,但与无层理砂岩相比,含软弱层理砂岩的动态抗拉强度对加载速率的敏感程度较低。此外,层理角度对砂岩的开裂应变有较大影响,受剪应力的影响,倾斜层理砂岩的开裂应变高于垂直层理砂岩。

Experimental study on the failure and dynamic tensile strength of layered sandstone under impact loads

Due to the anisotropy of layered sandstone,the deformation characteristic of layered sandstone with different bedding angle under impact loads is investigated.The standard sandstone samples with weak layer were fabricated,the dynamic Brazilian disc test of sandstone with different bedding angles is conducted using the split Hopkinson pressure bar (SHPB),and the evolution of strain field of specimen during fracturing is obtained combined with digital image correlation method.From the failure of sandstone,it shows that the angle between bedding plane and loading direction significantly affects both the deformation and the failure of layered sandstone.When the weak plane is parallel to the loading direction,a strain concentration zone is first generated near to the loading end,and rapidly propagate along the weak plane under impact loads,which eventually presents an cambered shape crack.When the weak plane is perpendicular to the loading direction,multiple strain concentration areas are first formed at the middle of the disc specimen,which indicates that micro cracks first initiate on the middle of the specimen,then grow continuously and coalesce,and eventually form a macro crack parallel to the loading direction.When the loading direction is at 45° to the weak plane,the significant mixed tensile and shear strain concentration is formed along the weak plane at the loading end of the specimen,and induces micro cracks initiation from the loading end and propagates along the weak plane due to a relatively lower tensile stress and shear stress of layered media compared with the rock media.Under the same loading rate,the strength of specimen with vertical weak plane is highest,while the strength of specimen with parallel weak plane is lowest,and the strength of specimen with oblique weak plane falls between the strength of specimen with horizontal weak plane and the strength of specimen with vertical weak plane.The dynamic tensile strength of layered sandstone increases linearly with the loading rate,which shows less sensitive to the loading rate compared with the sandstone without weak plane.Besides,the failure strain of layered sandstone presents significant change with the increase of bedding angle,the crack strain of sandstone with oblique weak plane is higher compared with the sandstone with vertical weak plane.