应变率对含裂隙红砂岩裂纹扩展模式及破碎特征的影响

Strain rate effect on crack propagation and fragmentation characteristics of red sandstone containing pre-cracks

以含不同倾角预制裂纹的长方形板状红砂岩为研究对象,采用分离式霍普金森压杆沿试样宽度方向施加冲击荷载,使用高速摄像机记录裂纹扩展过程,获得试样的裂纹路径特征以及动态压缩强度和动态弹性模量,利用筛分统计法分析试样碎块分布特征,结合分形理论定量描述试样破碎程度及特点,探讨中应变率条件下含裂隙试样裂纹扩展模式与动态力学性质和破碎程度的相互关系。研究结果表明,应变率较高时试样会更多地出现远场裂纹和离层裂纹,并且相比相关低应变率实验结果,中应变率范围内试样破坏模式及裂纹分布情况随应变率的变化规律是不同的。随着应变率的提高,试样大体上从1条拉伸裂纹的临界破坏演变成X形剪切裂纹为主的复杂裂隙网络,并且不同角度预制裂隙对于这种裂纹扩展模式的演变有重要影响。在预制裂纹倾角一定的情况下,岩样动态压缩强度和动态弹性模量表现出明显的应变率效应,不同角度预制裂纹对于试样的应变率敏感性有显著影响。随裂纹倾角的增大,试样的动态强度、动态弹性模量和分形维数表现出的变化趋势具有一定的相似性,大体呈现先减小后增大的趋势,裂纹倾角为45°的试样的动态压缩强度、动态弹性模量和分形维数均为最小。随应变率的升高,不同预制裂纹倾角的试样碎块分布更加分散,应变率越高,预制裂纹倾角对于岩石冲击破碎程度、分形维数的影响越显著。

In this experiment,finite size red sandstone containing pre-existing single crack was taken as the research object.The ratio of length to width of the samples was set about 0.65.The inclination angle of the pre-crack includes 0°,30°,45°,60°and 90°.A split Hopkinson pressure bar was used for impact test,and a high-speed camera was used to record the crack propagation.The dynamic loads were applied along the width of the samples.Velocities of striker in impact tests were set as 6,8 and 10 m/s by adjusting the pressure of the air gun.Acquisition frequency of the high-speed camera was set as 75000 s−1.The characteristics of crack propagation,dynamic compressive strength and dynamic elastic modulus of the samples were obtained.The fractal theory was used to describe the fragmentation characteristics of the samples.The relationship between dynamic mechanical properties,fragmentation characteristics and crack propagation under medium strain rate was discussed.The findings show that when the strain rate is high,more far-field cracks and separation cracks appear in the sample.In the range of medium strain rate,the failure mode and the number of cracks change differently with strain rate compared with the experimental results of low strain rate.The strain rate and the angle of pre-existing crack have a great influence on the crack propagation and failure mode of the samples.The crack propagation of the samples with different pre-existing crack is different.With the increase of strain rate,the failure mode of the sample becomes more complex,gradually evolving from critical failure with a tensile crack to complex failure mainly with X-shaped shear crack.When the angle of the pre-existing crack is fixed,the dynamic compressive strength and dynamic elastic modulus of the samples show obvious strain rate effect,and the pre-existing crack with different angles have significant influence on the strain rate sensitivity of the samples.With the increase of pre-existing crack angle,the variation of dynamic compressive strength,dynamic elastic modulus and fractal dimension of the samples show a certain similarity.In all types of samples,the dynamic compressive strength,dynamic elastic modulus and fractal dimension of the sample containing cracks with the inclination angle at 45°are the smallest.With the increase of strain rate,distribution of fragments becomes more dispersed.The higher the strain rate,the more significant the effect of the pre-existing crack on the fracture degree and fractal dimension of the samples.