In uniaxial compression the end constraint has a large influence on the geometry effect of rocks. However, the relationship between the end constraint and the geometry effect and their combined effects on the post-peak behavior of rocks are poorly understood. In the present study, a numerical experiment of rock specimens with different geometries in 3D uniaxial compression was carried out. Specimen geometry (slenderness and shape) and contact properties (loading stiffness and friction) were considered in the simulation. It is found that in the absence of the end constraint, the post-peak behavior of rocks depends on the specimen slenderness, implying that the slenderness effect can be an intrinsic geometry effect of Mohr-Coulomb material such as rocks. Next, it is shown that the end constraint enhances the post-peak strength of rocks by inducing confined zones in the specimen, and the size of the confined zones increases as the specimen slenderness decreases. Furthermore, it is observed that it is the slenderness rather than the cross sectional shape of a rock specimen that is closely related to the end constraint; therefore, the slenderness dominates the geometry effect of rocks in laboratory test results. Finally, it is confirmed that there is a critical value of H/W ratio where the specimen is least affected by the end constraint.
Obtaining complete stress-strain curves of rocks is important for rock strength characterization in laboratory and pillar design in deep underground mines . End constraint is initiated under compression at specimen-platen contacts in rock laboratory tests due to the elastic mismatch between the rock specimen and the steel loading platen [2, 3]. Although different testing apparatus and approaches have been employed to minimize the influence of the end constraint on rock property testing [4-6], it is practically impossible to exclude the end constraint from rock tests . Therefore, it is important to study the influence of end constraint on the post-peak behavior of rocks in compression.
End constraint at the specimen-platen contacts mainly consists of two aspects: loading machine stiffness along the normal direction of the contact and friction across the surface of the contact. In most previous studies on the investigation of the friction effect on peak strength of rocks, a plane stress condition was usually assumed, which is inconsistent with the boundary condition in laboratory tests [7-10]. Furthermore, the effects of loading machine stiffness and friction on the post-peak behavior of rocks were not considered together . Hence, the relationship between the end constraint and the geometry effect and their combined effect on the post-peak behavior of rocks are poorly understood. Therefore, a three dimensional (3D) model that can realistically consider the stiffness and friction properties of the specimen-platen contact is needed to better capture the post-peak behavior of rocks in compression.