Study on friction effect and damage evolution of end face in uniaxial compression test

The end friction effect in uniaxial compression test has a great influence on the deformation and failure characteristics of the specimen. Based on the Coulomb slip criterion,numerical tests of uniaxial compression under different end friction coefficients are carried out by using UDEC software. The deformation and failure of the specimens with different lithology and different aspect ratios are studied. The mechanism of the end friction effect is explored from the angle of the stress-strain relationship,crack propagation and stress distribution. The results show that:① The end friction effect can inhibit the initiation and propagation of micro cracks,and change the failure mode of brittle samples from splitting to shear or tension shear joint failure,which makes the strength of sandstone increase significantly,and has little effect on the strength of coal and mudstone;② There is a threshold value for the effect of end friction on the strength and failure characteristics of the samples. The threshold values of the end friction coefficient of the 50 mm×100 mm standard coal,mudstone and sandstone samples are 0.2,0.2 and 0.5,respectively;③ The results show that the size effect of specimen strength at different heights is mainly caused by "crack triangle inhibition zone" and local stress concentration zone caused by end friction. It is suggested that the friction reduction measures should be taken to avoid the end friction effect in the indoor uniaxial compression test.