Preliminary Analysis of the End Friction Effect on Dynamic Compressive Strength of Rocks

Accurate measurement of dynamic behaviors of rocks under complex static stress conditions can provide valuable insight into the design of underground rock engineering. The end friction between the specimen and the loading device becomes more severe in the measurement of rocks under static stress conditions because the end friction effect may be amplified by the static stress on the specimen ends. However, there are few studies on quantifying the end friction effect on the dynamic test for deep rocks. Thus, three groups of Fangshan marble (FM) specimens were prepared in this study. Specimen S1 and S3 are cylinders, and specimen S2 is a tube. The diameter of specimen S1 is the same as the outer diameter of specimen S2, and the cross-sectional area of specimen S3 is the same as that of specimen S2. The dynamic and total uniaxial compressive strength (UCS) of three types of specimens under various preloads were measured using the modified SHPB system. The splitting failure can be observed for these three types of specimens, and the center portion of specimen S1 was relatively intact, which can be attributed to the end friction effect. The end friction inhibits crack propagation, and thus the total UCS of rocks continuously increases with the axial preload. The increment of total UCS of FM at the same loading rate in terms of the preload differs for these three types of specimens due to the end friction effect. The difference in the dynamic UCS between lubricated and dry conditions is slight when the preload is higher, but the lubrication effect is more severe when the preload is lower. A formula is proposed to quantitatively assess the end friction effect and predict the total UCS of rocks under various dynamic loading rates and static axial preloads. Appropriate specimen geometries are suggested to ensure the precision of measurement of the dynamic UCS of deep rocks. The total UCS of rocks increases with the axial preload due to the inhibition of microcrack propagation.The effect of lubrication at different preloads is analyzed.A formula is proposed to quantitatively assess the end friction effect.The total UCS of rocks under various dynamic and static loadings can be predicted.Appropriate specimen geometry is recommended to minimize the end friction effect.