PLANE-STRAIN COMPRESSION OF ROCK-LIKE MATERIALS

A new apparatus for determining the constitutive response of rock and concrete, named the University of Minnesota Plane-Strain Apparatus, wa s designed and built on a passive stiff-frame concept. The biaxial dev ice, with U.S. Patent number 5,063,785, is unique, because it allows t he failure plane to develop and propagate in an unrestricted manner, a s opposed to conventional systems where the material is constrained by the testing apparatus. By placing the upper platen on a low friction linear bearing, the prismatic specimen, subjected to confining pressur e and compressed axially, has the freedom to translate in the lateral direction once the deformation has localized across the entire specime n. In addition, homogeneous deformation is prompted by the use of a st earic-acid based lubricant, which is placed on the four surfaces of th e specimen contacting hardened-steel platens. Thus, the apparatus comb ines the positive features of a conventional triaxial compression test and a direct shear test. Some 30 experiments on sandstone and mortar indicated that even though localization of deformation was detected by locations of acoustic emission prior to peak load, the failure plane was not fully formed at the peak. For the mortar, one more or less pla nar shear band, the orientation of which is well predicted by plastici ty theories, was observed. A kinked rupture zone with two main portion s, one steep and one less inclined, appeared in the sandstone; the ste ep portion may be a result of fracture phenomena. The dilatancy charac teristics of rock-like materials may dictate the type of failure mode, either shear banding for ductile behavior or crack propagation for br ittle behavior. Copyright (C) 1996 Elsevier Science Ltd